U.S. patent application number 11/698551 was filed with the patent office on 2007-08-23 for method and apparatus to process substrates with megasonic energy.
This patent application is currently assigned to P.C.T. Systems, Inc.. Invention is credited to Ahmed A. Busnaina, Sharyl L. Maraviov, Henry R. Miranda, Garry L. Montierth.
Application Number | 20070197138 11/698551 |
Document ID | / |
Family ID | 33556855 |
Filed Date | 2007-08-23 |
United States Patent
Application |
20070197138 |
Kind Code |
A1 |
Montierth; Garry L. ; et
al. |
August 23, 2007 |
Method and apparatus to process substrates with megasonic
energy
Abstract
A variety of techniques may be employed, alone or in
combination, to enhance contact between a processed substrate and
applied megasonic energy. In accordance with one embodiment of the
new invention, the vibration plate is brought into intimate contact
with one surface of the substrate, while cleaning or processing
fluid contacts the other. In accordance with an alternative
embodiment of the present invention, a reflecting surface may be
provided to cause emanated energy to be reflected back into the
near field and make it more uniform. In accordance with another
alternative embodiment of the present invention, energy may be
transferred across a substrate bounded on both sides by liquid with
incidence of megasonic energy that is either normal to the
substrate surface or within a critical range of incident angles. In
yet another embodiment, generated dilatational waves may be
converted to surface waves prior to contacting the substrate.
Inventors: |
Montierth; Garry L.;
(Fremont, CA) ; Miranda; Henry R.; (Fremont,
CA) ; Maraviov; Sharyl L.; (Pleasanton, CA) ;
Busnaina; Ahmed A.; (Ashland, MA) |
Correspondence
Address: |
TOWNSEND AND TOWNSEND AND CREW, LLP
TWO EMBARCADERO CENTER
EIGHTH FLOOR
SAN FRANCISCO
CA
94111-3834
US
|
Assignee: |
P.C.T. Systems, Inc.
Fremont
CA
|
Family ID: |
33556855 |
Appl. No.: |
11/698551 |
Filed: |
January 26, 2007 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10861793 |
Jun 4, 2004 |
7238085 |
|
|
11698551 |
Jan 26, 2007 |
|
|
|
60546383 |
Feb 20, 2004 |
|
|
|
60530194 |
Dec 16, 2003 |
|
|
|
60528941 |
Dec 10, 2003 |
|
|
|
60525435 |
Nov 26, 2003 |
|
|
|
60517255 |
Nov 3, 2003 |
|
|
|
60510054 |
Oct 8, 2003 |
|
|
|
60476845 |
Jun 6, 2003 |
|
|
|
60476527 |
Jun 6, 2003 |
|
|
|
Current U.S.
Class: |
451/36 |
Current CPC
Class: |
H01L 21/67057 20130101;
H01L 21/67051 20130101; H01L 21/67046 20130101; A61B 8/546
20130101; B08B 3/12 20130101; Y10S 451/91 20130101 |
Class at
Publication: |
451/036 |
International
Class: |
B24B 1/00 20060101
B24B001/00 |
Claims
1-91. (canceled)
92. A method of processing a substrate comprising: varying at least
one of frequency, power, and pulse width of ultrasonic energy
applied from a first energy source to a substrate in contact with a
processing fluid, such that a uniformity of energy in near field
regions is improved by at least one of moving high energy node
points and low energy null points, minimizing a difference between
a magnitude of the high and low energy points, and retarding
formation of high and low energy points.
93. The method of claim 92 wherein at least part of the substrate
is contacted by a processing fluid comprising one of a liquid, a
vapor, a gas, and a solid.
94. The method of claim 92 further comprising: applying sonic
energy from a second energy source to the substrate in contact with
a processing fluid, such that a far-field of the energy from the
second energy source overlaps near field regions from the first
energy source.
95. The method of claim 92 wherein a frequency of a driving voltage
applied to at least one vibration member is cycled about a
frequency set point.
96. The method of claim 95 wherein cycling the driving voltage
frequency varies a magnitude of a resulting sonic energy wave
within a process tank.
97. The method of claim 95 wherein the frequency of the driving
voltage is cycled about the frequency set point according to at
least one of a sinusoidal signal, a randomly fluctuating signal,
and a predetermined stepwise sequence.
98. The method of claim 92 wherein a magnitude of a driving voltage
applied to at least one vibration member is varied about a voltage
magnitude set point according to at least one of a sinusoidal
signal, a randomly fluctuating signal, and a stepwise sequence.
99. The method of claim 92 wherein a pulse width of a driving
voltage applied to at least one vibration member is varied by at
least one of cycling the pulse width of the applied voltage about a
pulse width set point by means of at least one of a sinusoidal
signal, a randomly fluctuating signal, and a stepwise sequence.
100. The method of claim 92 wherein at least one of a magnitude and
a character of the resulting sonic energy wave within a processing
tank is varied by varying the pulse width of the driving voltage
applied to at least one vibration member by varying the pulse width
of the applied driving voltage about a pulse width set point by
means of at least one of a sinusoidal signal, a randomly
fluctuating signal, and a predetermined stepwise sequence.
101. The method of claim 92 wherein the processing is conducted at
least one of less than, greater than, and equal to atmospheric
pressure.
102. The method of claim 92 wherein at least two individual
vibration elements are sequentially energized.
103. The method of claim 92 wherein at least two individual
vibration elements are energized simultaneously.
104. The method of claim 103 wherein a phase angle between the
driving voltage applied to the vibration elements varied about a
phase angle set point according to at least one of a sinusoidal
signal, a randomly fluctuating signal and a stepwise sequence.
105. The method of claim 104 wherein the phase angle set point is
greater than 0.degree. and less than 180.degree..
106. The method of claim 105 wherein the phase angle set point
changes at a rate which it is greater than one-tenth hertz and less
than 10 megahertz.
107. The method of claim 105 wherein at least one of a rate of
change of frequency of the applied driving voltage, and a rate of
change of the pulse width of the applied driving voltage, is
greater than one-tenth hertz and less than 10 megahertz.
108. The method of claim 92 wherein the pulse width is greater than
one picosecond.
109. The method of claim 92 further comprising applying microwave
energy at least one of prior to, during, and subsequent to the
application of sonic energy.
110. A method of processing a substrate with megasonic energy, the
method comprising: disposing a substrate in contact with a
processing fluid; applying megasonic energy to the substrate to
establish points of constructive and destructive interference
proximate to a substrate surface; and changing a position, a
magnitude, or a character of the points of constructive and
destructive interference in order to enhance uniformity of exposure
of the substrate to sonic energy.
111. The method of claim 110 wherein the position of points of
constructive and destructive interference is changed by varying at
least one of frequency, power, phase angle, and pulse width of the
applied megasonic energy.
112. The method of claim 111 wherein at least one of a magnitude, a
frequency, a phase angle, and a pulse width of a driving voltage of
a source of the megasonic energy is varied about a set point
according to at least one of a sinusoidal signal, a randomly
fluctuating signal, and a stepwise sequence.
113. The method of claim 111 wherein a rate of change of at least
one of the frequency, power, phase angle, and pulse width of the
megasonic energy is varied by a rate greater than about 0.1 Hz and
less than about 10 MHz.
114. The method of claim 110 wherein the position, magnitude, or
character of points of constructive and destructive interference is
changed by varying an orientation of the substrate relative to a
source of the applied megasonic energy.
115. The method of claim 110 wherein the position, magnitude, or
character of points of constructive and destructive interference is
changed by varying a distance of the substrate relative to a source
of the applied megasonic energy.
116. The method of claim 110 wherein the position, magnitude, or
character of points of constructive and destructive interference is
changed in a near field region of a source of the megasonic
energy.
117. The method of claim 110 wherein the position, magnitude, or
character of points of constructive and destructive interference is
changed in a far field region of a source of the megasonic
energy.
118-208. (canceled)
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The instant nonprovisional patent application claims
priority from the following provisional patent applications, each
of which is also incorporated by reference herein for all purposes:
60/476,845, filed Jun. 6, 2003; 60/476,527, filed Jun. 6, 2003;
60/530,194, filed Dec. 16, 2003; 60/510,054, filed Oct. 8, 2003;
60/546,383, filed Feb. 20, 2004; 60/517,255, filed Nov. 3, 2003;
60/528,941, filed Dec. 10, 2003; and 60/525,435, filed Nov. 26,
2003.
BACKGROUND OF THE INVENTION
[0002] The application of megasonic energy to substrate wet
processing has become widely accepted, especially in semiconductor
fabrication. As device/feature size has shrunk, and as substrate
structures have become more vulnerable to damage, the frequency of
megasonic systems have increased and have trended towards the
megahertz range.
[0003] The application of sonic energy having frequencies
approaching and exceeding one megahertz are often referred to as
megasonic processing. These higher frequencies are used in an
attempt to dislodge smaller contaminant particles and to reduce the
localized energy release associated with bubble formation/collapse
(cavitation and possible microcavitation) that some have theorized
can lead to substrate damage as has been observed with lower
frequency ultrasonic cleaners.
[0004] Historically, batch-processing systems have been designed to
introduce megasonic energy parallel to substrate surfaces. In the
quest for faster processing, particularly with the push for single
wafer processing, megasonic designs have tended towards high-energy
systems with introduction of energy perpendicular to substrate
surfaces. This has led to concerns regarding megasonic damage to
sensitive structures.
[0005] Therefore it is desirable to find gentler ways of applying
megasonic energy that will rapidly remove ever-smaller
contaminants.
BRIEF SUMMARY OF THE INVENTION
[0006] A variety of techniques may be employed, alone or in
combination, to enhance contact between a processed substrate and
applied megasonic energy. In accordance with one embodiment of the
new invention, the vibration plate is brought into intimate contact
with one surface of the substrate, while cleaning or processing
fluid contacts the other. In accordance with an alternative
embodiment of the present invention, a reflecting surface may be
provided to cause emanated energy to be reflected back into the
near field and make it more uniform. In accordance with another
alternative embodiment of the present invention, energy may be
transferred across a substrate bounded on both sides by liquid with
incidence of megasonic energy that is either normal to the
substrate surface or within a critical range of incident angles. In
accordance with yet another embodiment of the present invention,
generated dilatational waves may be converted to surface waves
prior to contacting the substrate. In accordance with still another
embodiment of the present invention, generated dilatational waves
may be converted into surface waves after contacting the substrate,
or within the substrate itself.
[0007] An apparatus in accordance with an embodiment of the present
invention, configured to process a substrate with megasonic energy,
comprises a processing region configured to receive a processing
fluid, a megasonic energy source, and a vibration member in
physical contact with the megasonic energy source and with at least
a portion of an element proximate to the processing region. A
combined thickness of the element and the vibration member being
about +/-30% of an odd one-quarter wavelength of a megasonic energy
applied by the source in order to transfer the megasonic energy
across the element.
[0008] An embodiment of a method in accordance with the present
invention of processing a substrate, comprises, placing at least
one substrate into a process vessel, making contact between at
least a part of the substrate and at least a part of a vibration
member, and introducing at least one processing fluid into the
processing vessel. At least a part of the substrate is contacted
with at least one of the processing fluids and megasonic energy is
applied, wherein the applying megasonic energy step occurs at least
one of before, during and after the introducing processing fluid
step.
[0009] An embodiment of an apparatus in accordance with the present
invention configured to process a substrate with megasonic energy,
comprises, a processing region configured to receive a processing
fluid, a megasonic energy source, and a vibration member in contact
with the megasonic energy source and oriented within a critical
angle range of between about 18-58.degree. relative to an element
positioned within the processing region, in order to transfer the
megasonic energy across the element.
[0010] An embodiment of a method in accordance with the present
invention for processing a substrate, comprises, placing at least
one substrate into a processing vessel, introducing at least one
processing fluid into the processing vessel to contact at least a
part of the substrate, and applying megasonic energy at between
about 18-58.degree. relative to a surface of the substrate, such
that a substantial portion of the megasonic energy is transferred
across the substrate. The introducing of a processing fluid step
may occur before, during, or after the applying megasonic energy
step.
[0011] An alternative embodiment of an apparatus in accordance with
the present invention configured to process a substrate with
megasonic energy, comprises, a processing region configured to
receive a processing fluid, a flow member configured to control a
path of the processing fluid within the tank, and a megasonic
energy source configured to apply megasonic energy to the tank,
such that a direction of the megasonic energy conforms to the
path.
[0012] An alternative embodiment of a method in accordance with the
present invention for processing a substrate with megasonic energy,
comprises, flowing a processing fluid within a tank containing a
substrate, and applying megasonic energy to the tank such that a
direction of the megasonic energy conforms to a path of the
processing fluid.
[0013] Another alternative embodiment of an apparatus in accordance
with the present invention configured to process a substrate with
megasonic energy, comprises, a processing region configured to
receive a processing fluid, a megasonic energy source, and a wedge
vibration member having a first face in contact with and configured
to receive energy from the megasonic energy source. The wedge
vibration member having a second face oriented at an angle relative
to the first face and configured to emit energy received from the
megasonic energy source to a substrate positioned within the
processing region.
[0014] An alternative embodiment of a method in accordance with the
present invention for processing a substrate, comprises the steps
of placing a substrate into a processing region, introducing at
least one processing fluid to the substrate, and contacting at
least a part of a vibration member comprising a plate to at least
part of a first face of a wedged shaped vibration member. Megasonic
energy is applied to the substrate from a second face of the wedge
shaped vibration member, wherein the applying megasonic energy step
occurs at least one of before, during and after the introducing a
processing fluid step.
[0015] Another alternative embodiment of an apparatus in accordance
with the present invention configured to process a substrate with
megasonic energy, comprises, a processing region configured to
receive a processing fluid, a vibration member in physical contact
with a megasonic energy source and configured to support a
substrate in the processing region within a near field of megasonic
energy incident in a first direction from the vibration member, and
an element configured to direct megasonic energy to the near field
from a second direction different from the first direction.
[0016] Another embodiment of an apparatus in accordance with the
present invention configured to process a substrate with megasonic
energy, comprises, a processing region configured to receive a
processing fluid, and a megasonic energy source configured to
output megasonic energy having at least one of a user-controlled
and variable frequency, power, and pulse width, to a substrate
present within the processing region.
[0017] An embodiment of a method in accordance with the present
invention for processing a substrate, comprises, varying at least
one of frequency, power, and pulse width of ultrasonic energy
applied from a first energy source to a substrate in contact with a
processing fluid, such that a uniformity of energy in near field
regions is improved by at least one of moving high energy node
points and low energy null points, minimizing a difference between
a magnitude of the high and low energy points, and retarding
formation of high and low energy points.
[0018] An embodiment of a method in accordance with the present
invention for processing a substrate with megasonic energy,
comprises, disposing a substrate in contact with a processing
fluid, applying megasonic energy to the substrate to establish
points of constructive and destructive interference proximate to a
substrate surface, and changing a position of the points of
constructive and destructive interference in order to enhance
uniformity of exposure of the substrate to sonic energy.
[0019] An embodiment of a method in accordance with the present
invention for processing a substrate, comprises the steps of
supporting a substrate in a holder, positioning the substrate
adjacent to a portion of a processing member, and causing relative
motion between the substrate and the processing member at least one
of before, during, or after a substrate processing step. The
substrate is brought into at least one of into contact with and
closely spaced apart from, the processing member surface. One of a
solid, a fluid, and a mixture is between the substrate and the
processing member, and energy is transmitted to an interface
between the substrate and the processing member to modify
processing of a surface of the substrate.
[0020] An embodiment of a processing apparatus in accordance with
the present invention comprises a vessel configured to contain an
electrochemical fluid, a voltage supply, and a support configured
to position a substrate in contact with the electrochemical fluid,
one of the support and the substrate in electrical communication
with a first terminal of the voltage supply. An electrode is in
electrical communication with the electrochemical bath and with a
second terminal of the voltage supply, and a sonic energy source is
in communication with the substrate one of across the electrode,
across the substrate, and along a face of the substrate.
[0021] An embodiment of a method in accordance with the present
invention for processing a substrate, comprises the steps of,
providing an electrode in electronic communication with an
electrochemical bath, disposing a substrate within the
electrochemical bath, applying a potential difference between the
substrate and the electrode across the electrochemical bath, and
applying sonic energy across one of the electrode and the
substrate, to a surface of the substrate.
[0022] An embodiment of a liquid for processing a substrate with
sonic energy, comprises, a gas dissolved in a liquid component, the
gas exhibiting a solubility in the liquid component at least as
great as a solubility of the gas in deionized water under
equivalent temperature and pressure conditions.
[0023] An alternative embodiment of a liquid for processing a
substrate with sonic energy, comprises, a gas dissolved in a liquid
component, the gas exhibiting a solubility in the liquid component
at least as great as a solubility of air in deionized water under
equivalent temperature and pressure conditions.
[0024] An embodiment of a method in accordance with the present
invention for processing a substrate with sonic energy, comprises,
causing a gas to be dissolved in a component of a liquid, the
component exhibiting a solubility of the gas that is at least as
great as a solubility of the gas in deionized water under
equivalent temperature and pressure conditions. A substrate is
exposed to the liquid component, and sonic energy is applied to the
substrate within the liquid component.
[0025] An embodiment of a processing apparatus in accordance with
the present invention comprises a vessel configured to support a
substrate, a source configured to direct a jet comprising energy or
matter against a surface of the substrate, and a vibration energy
source in communication with the substrate.
[0026] An embodiment of a method in accordance with the present
invention for processing a substrate, comprises, disposing a
substrate within a processing vessel, directing a jet comprising
energy or matter against a surface of the substrate, and applying
sonic energy to the substrate at least one of before, during or
after application of the jet.
[0027] An embodiment of an apparatus in accordance with the present
invention for processing a substrate utilizing ultrasound energy,
comprises, a tank having walls configured to contain a liquid bath,
an energy source in sonic communication with the liquid bath, and a
substrate holder configured to support a substrate within the
liquid bath, at least one of a member of the substrate holder and a
feature of the tank lying between the energy source and a portion
of the substrate. A reflecting surface is in contact with the
liquid bath and configured to receive ultrasound energy incident
from the source at an angle of greater than 26.degree., and to
reflect the ultrasound energy to the substrate portion.
[0028] An embodiment of a method in accordance with the present
invention for processing a substrate utilizing ultrasound energy,
comprises, providing a liquid bath within a tank having walls,
supporting a substrate within the liquid bath on a holder, and
directing ultrasound energy to the substrate, such that at least
one of a portion of the tank and a portion of the substrate holder
lies between the energy source and a portion of the substrate. The
ultrasound energy incident at an angle of greater than 26.degree.
to a surface is reflected to contact the substrate portion.
[0029] An embodiment of a method in accordance with the present
invention for drying a substrate, comprises, positioning a
substrate within a processing chamber, pressurizing the processing
chamber by at least one of introducing a gas and a vapor and a
processing liquid into the chamber, and wetting at least part of
the substrate surface comprising one of submerging, spraying and
condensing liquid onto the substrate. A surface tension lowering
component is concentrated at a gas-liquid interface, one of the
substrate and the processing liquid level are moved relative to one
of the substrate and the gas-liquid interface such that a surface
tension gradient is created between the liquid comprising a
meniscus on the substrate surface and a remaining portion of the
processing liquid, wherein the surface tension gradient draws
liquid from the substrate surface into the bulk processing liquid.
Radiation is applied to at least one of a part of the substrate and
at least one of the processing liquid on the substrate surface. The
pressurizing step may occur before, after, or during the radiation
applying step, and the applying radiation step may occur before,
after, or during the moving step.
[0030] A further understanding of the nature and advantages of the
inventions disclosed herein may be realized by reference to the
remaining portions of the specification and the attached
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] FIG. 1A shows a cross-sectional view of an embodiment of an
apparatus in accordance with the present invention wherein
megasonic energy is transferred directly from the vibration member
through the substrate into the cleaning or processing liquid on the
other surface.
[0032] FIGS. 1B shows a simplified cross-sectional view of an
embodiment of an apparatus in accordance with the present invention
wherein the substrate may be held tightly against a vibration
member in contact with crystals by vacuum through a series of small
holes in the vibration member
[0033] FIG. 1BA shows an enlarged view of the embodiment of FIG.
1B.
[0034] FIG. 2A shows a simplified cross-sectional view of an
embodiment of an apparatus in accordance with the present invention
utilizing an air-liquid interface as a reflecting surface.
[0035] FIG. 2B shows a simplified cross-sectional view of an
embodiment of an apparatus in accordance with the present invention
utilizing a randomly pitted surface facing the substrate as a
reflecting surface.
[0036] FIG. 2C shows a simplified cross-sectional view of an
embodiment of an apparatus in accordance with the present invention
utilizing a vibrating plate as a reflecting surface.
[0037] FIG. 2D shows a simplified cross-sectional view of an
embodiment of an apparatus in accordance with the present invention
utilizing a rotating plate as a reflecting surface.
[0038] FIGS. 3A-B show simplified cross-sectional views of
embodiments of apparatuses in accordance with the present invention
utilizing fluid introduction coupled with reflection of megasonic
energy.
[0039] FIGS. 3C-D show simplified cross-sectional views of
embodiments of apparatuses in accordance with the present invention
utilizing fluid introduction and removal coupled with reflection of
megasonic energy.
[0040] FIG. 4A shows a simplified cross-sectional view of an
embodiment of an apparatus in accordance with the present invention
for performing brush scrubbing.
[0041] FIG. 4B shows a simplified cross-sectional view of an
embodiment of an apparatus in accordance with the present invention
for performing chemical mechanical polishing.
[0042] FIG. 4C shows a simplified cross-sectional view of an
embodiment of an apparatus in accordance with the present invention
for performing dual brush scrubbing combined with a megasonic
nozzle.
[0043] FIG. 4D shows a simplified cross-sectional view of an
embodiment of an apparatus in accordance with the present invention
similar to FIG. 4C, but having the capability to remove fluid via
suction during processing.
[0044] FIG. 4E shows a simplified cross-sectional view of an
embodiment of an apparatus in accordance with the present invention
showing a wide area megasonic nozzle based on a flat plate
vibration member design.
[0045] FIG. 4F shows a simplified cross-sectional view of an
embodiment of an apparatus in accordance with the present invention
showing a wide area megasonic nozzle based on a wedge element
design with multiple piezoelectric crystals or vibration
elements.
[0046] FIG. 4G shows a simplified cross-sectional view of an
embodiment of an apparatus in accordance with the present invention
showing a narrow area megasonic nozzle based on a small wedge
element with curved fluid contact face.
[0047] FIG. 5A shows a simplified cross-sectional view of a
vertical embodiment of an apparatus in accordance with the present
invention where processing fluid is introduced through spray
nozzles.
[0048] FIG. 5B shows a simplified cross-sectional view of a
horizontal embodiment of an apparatus in accordance with the
present invention where processing fluid is introduced through
spray nozzles.
[0049] FIG. 6A shows a simplified cross-sectional view of an
embodiment of an apparatus in accordance with the present invention
wherein applied megasonic energy is incident perpendicular to a
substrate bounded on both sides by liquid
[0050] FIG. 6B shows a simplified cross-sectional view of an
alternative embodiment of an apparatus in accordance with the
present invention, wherein substrates are positioned at a proper
angle to the incidence megasonic waves to maximize cross substrate
energy transfer.
[0051] FIG. 6C shows a simplified cross-sectional view of an
alternative embodiment of an apparatus in accordance with the
present invention, wherein one substrate has a surface in contact
with the vibration member and another substrate does not.
[0052] FIG. 6D shows a simplified cross-sectional view of an
alternative embodiment of an apparatus in accordance with the
present invention where the angle of the first substrate relative
to the vibration member is changed or "wobbled" relative to the
angle of the second substrate to the same vibration member.
[0053] FIG. 7A shows a simplified cross-sectional view of an
alternative embodiment of an apparatus in accordance with the
present invention where energy is transferred from piezoelectric
crystals through a solid wedge into a thin flat plate.
[0054] FIG. 7B shows a simplified cross-sectional view of an
alternative embodiment of an apparatus in accordance with the
present invention where the substrate can be rotated.
[0055] FIG. 7C shows a simplified cross-sectional view of an
alternative embodiment of an apparatus in accordance with the
present invention where the wedge is hollowed with a passage and
flushed with a cooling fluid.
[0056] FIG. 7D shows a simplified cross-sectional view of an
alternative embodiment of an apparatus in accordance with the
present invention, where energy is transferred from multiple
piezoelectric crystals present on more than one face, through a
solid wedge, into a thin flat plate.
[0057] FIG. 7E shows a simplified cross-sectional view of an
alternative embodiment of an apparatus in accordance with the
present invention, where energy is transferred from piezoelectric
crystals through multiple solid wedge devices into a thin flat
plate.
[0058] FIG. 7F shows a simplified cross-sectional view of an
alternative embodiment of an apparatus in accordance with the
present invention, where energy is transferred from piezoelectric
crystals through multiple solid wedge devices placed at opposite
ends of a thin flat plate.
[0059] FIG. 7G shows a simplified cross-sectional view of an
alternative embodiment of an apparatus in accordance with the
present invention, where energy is transferred from piezoelectric
crystals through a solid wedge device into a flat substrate across
a gap filled with liquid.
[0060] FIGS. 7HA-C shows a simplified perspective, end, and side
views of an alternative embodiment of an apparatus in accordance
with the present invention, where energy is transferred from
piezoelectric crystals through a solid wedge device into a transfer
member having a triangular cross section, into a substrate across a
gap filled with liquid.
[0061] FIG. 7I shows a simplified cross-sectional view of an
alternative embodiment of an apparatus in accordance with the
present invention, where energy is transferred from piezoelectric
crystals through multiple edges of a triangular solid wedge device,
across a fluid filled gap and into a substrate.
[0062] FIG. 7JA-D show simplified cross-sectional views of an
alternative embodiment of an apparatus in accordance with the
present invention, where energy is transferred from an elongated
transfer member having various shapes, across a fluid filled gap
and into a substrate.
[0063] FIG. 7KA shows a shows a simplified perspective view of one
embodiment of a wedge structure in accordance with the present
invention, featuring a hollow wedge filled with liquid, which exits
through multiple holes or nozzles.
[0064] FIG. 7KB shows a cross-sectional view of the megasonic
nozzle structure of FIG. 7KA, where energy is transferred from
piezoelectric crystals through the wedge to the substrate.
[0065] FIG. 8A shows a simplified cross-sectional view of an
alternative embodiment of an apparatus in accordance with the
present invention, where a wedge or device is used to convert
megasonic pressure waves into surface waves for processing with
substrate in direct contact with one face of the wedge and liquid
sprays.
[0066] FIG. 8B shows a simplified cross-sectional view of an
alternative embodiment of an apparatus in accordance with the
present invention, where a distinct cooling member is adjacent to
the wedge.
[0067] FIG. 8C shows a simplified cross-sectional view of an
alternative embodiment of an apparatus in accordance with the
present invention, where the wedge is designed to include hollow or
fluid-filled passages for cooling.
[0068] FIG. 8D shows a simplified cross-sectional view of an
alternative embodiment of an apparatus in accordance with the
present invention, where piezoelectric crystals are located on two
faces of a wedge structure, and a substrate contacts the third
face.
[0069] FIG. 8E shows a simplified cross-sectional view of an
alternative embodiment of an apparatus in accordance with the
present invention, where piezoelectric crystals are located on two
faces of a wedge structure, and the third face contacts the wall of
a processing vessel.
[0070] FIG. 8F shows a simplified cross-sectional view of an
alternative embodiment of an apparatus in accordance with the
present invention, where piezoelectric crystals are located on two
faces of a wedge structure, with a processing vessel in contact
with a top side of the wedge
[0071] FIG. 8G shows a simplified cross-sectional view of an
alternative embodiment of an apparatus in accordance with the
present invention where piezoelectric crystals are located on
multiple faces of a polygonal structure with sides forming a right
angle with the top side and a process vessel in contact with a
portion of that top side.
[0072] FIG. 8H shows a simplified cross-sectional view of an
alternative embodiment of an apparatus in accordance with the
present invention, where piezoelectric crystals are located on
multiple faces of a polygonal structure whose sides form an angle
with the top side, a portion of the top side in contact with a
process vessel.
[0073] FIG. 8I shows a simplified perspective view of wedge
structure with piezoelectric crystals on two faces at right angle
with the top and bottom sides and contact with a substrate is made
through a bottom side though a thin liquid layer or meniscus.
[0074] FIG. 8J shows a simplified perspective view of an
alternative embodiment of a processing apparatus in accordance with
the present invention similar to that shown in FIG. 8I, except the
sides of the wedge form an angle with the top and bottom sides of
the wedge.
[0075] FIGS. 8KA-C show simplified perspective views of an
embodiment of a wedge structure in accordance with the present
invention.
[0076] FIGS. 8LA-B show simplified perspective and edge views,
respectively, of another embodiment of a wedge structure in
accordance with the present invention.
[0077] FIGS. 9A-B show simplified cross-sectional views of
alternative embodiments of apparatuses in accordance with the
present invention which utilize direct cooling of transducers.
[0078] FIG. 10 shows a simplified cross-sectional view of one
embodiment of a ultrasonic processing apparatus in accordance with
the present invention utilizing reflection of sonic energy to
perform processing behind an obstruction.
[0079] FIG. 11 depicts a simplified cross-sectional view of another
embodiment of an ultrasonic processing apparatus in accordance with
the present invention wherein a curved tank wall provides the
reflecting surface for incident ultrasonic energy.
[0080] FIG. 12 shows a simplified cross-sectional view of an
alternative embodiment of an apparatus in accordance with the
present invention where reflection of incident sonic energy occurs
at the curved wall of a tank surrounded by gas.
[0081] FIG. 13 shows a simplified cross-sectional view of a member
comprising a hollow, gas-filled pipe or plate that is readily able
to reflect sonic energy.
[0082] FIG. 14 shows a simplified cross-sectional view of a member
exhibiting a semicircular cross-section, where sonic energy is
incident within the preferred range of angles for energy
transfer.
[0083] FIG. 15 shows a simplified cross-sectional view of a member
exhibiting an oval cross-section, where sonic energy is incident
outside of the preferred range of angles for energy transfer and is
reflected off of the member.
[0084] FIG. 16 shows a simplified perspective view of a
conventional cassette design.
[0085] FIG. 17 shows a simplified perspective view of another
conventional cassette design.
[0086] FIG. 18A shows a simplified end view of one embodiment of a
cassette in accordance with the present invention.
[0087] FIG. 18B shows a simplified isometric view of the cassette
shown in FIG. 18A.
[0088] FIG. 18C shows a simplified expanded side view of a
rectangular plate component support member having periodic slots to
contain edges of multiple substrates.
[0089] FIG. 19 shows a simplified end view of an alternative
embodiment of a cassette in accordance with the present
invention.
[0090] FIG. 20A shows a simplified cross-sectional view of a
three-support arrangement.
[0091] FIG. 20B shows a simplified enlarged cross sectional view of
the V shaped support of the cassette of FIG. 20A.
[0092] FIG. 21 shows a simplified expanded cross sectional view of
a non-rectangular support member.
[0093] FIG. 22 illustrates a simplified expanded cross sectional
view of a typical rectangular support member comprising a composite
of first inner material and second outer material.
[0094] FIGS. 23A and 23B show simplified perspective and cross
sectional views, respectively, of an inverted V-shaped support
member comprising plate components having slots for supporting the
edges of wafers and allowing fluid flow.
[0095] FIG. 24, which show a simplified cross-sectional view of a
wide rectangular support member for a flat bottom tank having a
bottom mounted transducer.
[0096] FIG. 25 illustrates an alternative embodiment in accordance
with the present invention, wherein a cassette comprises a
plurality of vertically-oriented support plate components.
[0097] FIG. 26 shows a simplified cross-sectional view of an
alternative embodiment of the present invention employing such a
hydrodynamic support member to move sound waves around the
member.
[0098] FIG. 27 shows a simplified cross-sectional view of an
alternative embodiment of an apparatus in accordance with the
present invention, wherein the flow of processing fluid is diverted
against the substrate that is to be processed, and the sonic energy
follows the fluid path.
[0099] FIG. 28 shows a simplified cross-sectional view of a member
to be processed, immersed in a surrounding liquid in a conventional
process tank.
[0100] FIGS. 29A-B show simplified cross-sectional and schematic
views, respectively, illustrating nonuniformity in the resulting
energy field of the near field.
[0101] FIG. 30 shows a simplified cross-sectional view of a
megasonic energy source comprising a transducer including multiple
individual piezoelectric crystals.
[0102] FIG. 31 shows a simplified cross-sectional view of a
megasonic generator featuring a modified transducer in
communication with a sequenced firing generator that sequentially
fires the individual piezoelectric crystals.
[0103] FIG. 32 shows a simplified cross-sectional view of a
conventional megasonic cleaning system configured to apply energy
to a member in a tank, with energy projected parallel to the
surface of a substrate.
[0104] FIG. 33 shows a simplified cross-sectional view of an
embodiment of an apparatus in accordance with the present invention
featuring second transducer elements included on a vertical wall at
or near a 90.degree. angle relative to the first transducer
element(s) located on the bottom of the tank.
[0105] FIG. 34 shows a simplified cross-sectional view of another
embodiment of an apparatus in accordance with the present invention
featuring first transducer elements located on one angled bottom
wall while the second transducer elements are located on the
other.
[0106] FIG. 35 shows a simplified cross-sectional view of another
embodiment of an apparatus in accordance with the present
invention.
[0107] FIG. 36 shows a simplified cross-sectional view of another
embodiment of an apparatus in accordance with the present invention
featuring additional elements added to the overhang above the
V-shape to allow energy to be projected across the face of the
sloped side walls of the tank.
[0108] FIG. 37 shows a simplified cross-sectional view of another
embodiment of an apparatus in accordance with the present invention
featuring the vibration member separated from the substrate by a
distance.
[0109] FIG. 38 shows a simplified cross-sectional view of another
embodiment n accordance with the present invention including two
vibration members aligned parallel to each other and spaced apart
with a substrate inserted between them.
[0110] FIG. 39 shows a simplified cross-sectional view of one
embodiment of an apparatus for employing vibrational energy to
enhance processing of a substrate.
[0111] FIG. 40 shows a simplified cross-sectional view of an
alternative embodiment of an apparatus in accordance with the
present invention for employing vibrational energy to enhance
processing of a substrate.
[0112] FIG. 41 shows a simplified cross-sectional view of another
alternative embodiment of an apparatus in accordance with the
present invention employing vibrational energy to enhance
processing of a substrate.
[0113] FIG. 42 shows a simplified cross-sectional view of yet
another alternative embodiment of an apparatus in accordance with
the present invention for the application of vibrational energy to
enhance processing of a substrate.
[0114] FIG. 43 shows a simplified cross-sectional view of still
another alternative embodiment of a processing apparatus in
accordance with the present invention, incorporating a mechanical
structure for generating vibrations.
[0115] FIG. 44 shows a simplified cross-sectional view of another
alternative embodiment of a processing apparatus in accordance with
the present invention.
[0116] FIG. 45 shows a simplified cross-sectional view of another
alternative embodiment of a processing apparatus in accordance with
the present invention, incorporating a sonic nozzle configured to
operate within a correct range of angles to generate
vibrations.
[0117] FIG. 46 shows a simplified cross-sectional view of one
embodiment of a processing apparatus for electrochemical processing
in accordance with the present invention.
[0118] FIG. 47 shows a simplified cross-sectional view of another
alternative embodiment of a processing apparatus in accordance with
the present invention.
[0119] FIG. 48 shows a simplified cross-sectional view of another
embodiment of an apparatus for performing electrochemical
processing in accordance with the present invention.
[0120] FIG. 49 shows a simplified cross-sectional view of an
alternative embodiment of an apparatus in accordance with the
present invention, wherein an electrode is disposed at an angle
lying between a range of critical angles relative to the direction
of sonic energy incident from a vibrating member.
[0121] FIG. 50 shows a simplified cross-sectional view of another
embodiment of an electrochemical processing apparatus in accordance
with the present invention.
[0122] FIG. 51 shows a simplified cross-sectional view of an
embodiment of an electrochemical processing apparatus in accordance
with the present invention, wherein the substrate holder supporting
the substrate within a chemical bath, is rotated.
[0123] FIG. 52 shows a simplified perspective view of an embodiment
of an electrochemical processing apparatus in accordance with the
present invention utilizing an open electrode structure.
[0124] FIG. 53 shows a simplified cross-sectional view of an
alternative embodiment of a processing apparatus similar to that
shown in FIG. 48, but featuring the substrate supported within the
bath over the electrode which is open for fluid flow.
[0125] FIG. 54 shows a simplified cross-sectional view of another
alternative embodiment of a processing apparatus in accordance with
the present invention, which is inverted relative to that shown in
FIG. 53, but further features a mechanical vibrator rather than
piezoelectric crystals.
[0126] FIG. 55 shows a simplified cross-sectional view of yet
another alternative embodiment of a processing apparatus in
accordance with the present invention.
[0127] FIG. 56 shows a simplified cross-sectional view of an
alternative embodiment of a structure in accordance with the
present invention, where a shaped electrode exhibits a tear-drop or
airfoil profile in order to promote circulation of bath fluid with
smooth or steady streamlines characteristic of laminar, rather than
turbulent, flow.
[0128] FIG. 57 shows a simplified cross-sectional view of one
embodiment of an apparatus in accordance with the present invention
utilizing induced pressure drops.
[0129] FIG. 58 shows a simplified cross-sectional view of yet
another alternative embodiment of a processing apparatus in
accordance with the present invention.
[0130] FIG. 59 depicts a simplified cross-sectional view of an
embodiment where the substrate and the electrode are separated by a
thin fluid layer, but are not fully submerged within a liquid
bath.
[0131] FIG. 60 shows a simplified cross-sectional view of another
embodiment of an apparatus in accordance with the present invention
utilizing microwave energy.
[0132] FIGS. 61A-B shows simplified cross-sectional views of
vertically- and horizontally-oriented embodiments, respectively, of
an apparatus in accordance with the present invention wherein
ultrasonic energy is transferred directly from the vibration member
through the substrate into residual liquid on another surface.
[0133] FIG. 62 shows a simplified cross-sectional view of an
embodiment of an apparatus in accordance with the present invention
using relatively incompressible fluids, where the gaps can become
quite large and still have significant energy transfer occur.
[0134] FIG. 63 shows a simplified cross-sectional view of yet
another embodiment of a processing apparatus in accordance with the
present invention, where energy is transferred across a substrate
bounded on both sides by liquid, such that the thickness of the
substrate should match an even multiple of one quarter
wavelength.
[0135] FIG. 64 shows a simplified cross-sectional view of one
embodiment of an apparatus for performing cleaning of a substrate
with a jet of carbon dioxide snow (dry ice), and/or cryogenic argon
aerosol.
[0136] FIG. 65 shows a simplified cross-sectional view of an
embodiment of an apparatus in accordance with the present invention
which may be utilized to perform laser shock cleaning.
[0137] FIG. 66 shows a simplified cross-sectional view of an
alternative embodiment of a processing apparatus in accordance with
the present invention, wherein the substrate is clamped on
substrate holder by vacuum suction.
[0138] FIG. 67 shows a simplified cross-sectional view of another
embodiment of an apparatus in accordance with the present
invention, wherein the substrate is positioned in substrate holder
utilizing mechanical vibration.
[0139] FIG. 68 shows a simplified cross-sectional view of yet
another embodiment of an apparatus in accordance with the present
invention, wherein a liquid layer is formed on the surface of a
substrate secured on combination substrate holder/vibration
member.
[0140] FIG. 69 shows a simplified cross-sectional view of another
embodiment of an apparatus in accordance with the present invention
featuring an ultrasonic nozzle for the gas/liquid/solid jet
utilized to form pressure pulses in the gas/liquid/solid jet prior
to the jet striking the surface of the substrate.
[0141] FIG. 70 shows a simplified cross-sectional view of another
embodiment of an apparatus in accordance with the present
invention, featuring a megasonic nozzle used in conjunction with a
vibration member.
[0142] FIG. 71 shows a simplified cross-sectional view of an
embodiment of a processing apparatus in accordance with the present
invention incorporating microwave energy.
[0143] FIG. 72 shows a simplified cross-sectional view of one
embodiment of a processing apparatus in accordance with the present
invention including a substrate holder cooler or heater.
[0144] FIG. 73 shows a simplified cross-sectional view of an
alternative embodiment of a processing apparatus in accordance with
the present invention, wherein the substrate is contained within
enclosure pressurized above atmospheric pressure during
processing.
[0145] FIG. 74 shows a simplified cross-sectional view of an
embodiment of a processing apparatus in accordance with the present
invention featuring piezoelectric crystals attached to the outside
of at least one wall of a processing chamber, with the thickness of
that wall chosen to approximately equal an odd multiple one-quarter
wavelength of the applied sonic energy to maximize energy
coupling.
[0146] FIG. 75 shows a simplified cross-sectional view of an
embodiment of a processing apparatus in accordance with the present
invention, featuring crystals bonded to a vibration plate that is
then rigidly attached to the chamber wall.
[0147] FIG. 76 shows a simplified cross-sectional view of one
design of a processing apparatus in accordance with the present
invention wherein reflections from a moving reflector impinge upon
a stationary substrate.
[0148] FIG. 77 shows a simplified cross-sectional view of one
design of a processing apparatus in accordance with the present
invention wherein sonic energy bounces off the substrate and off
walls of the vessel.
[0149] FIG. 78 shows a simplified cross-sectional view of a
processing apparatus in accordance with the present invention
wherein substrate is in motion and simultaneously receives sonic
energy from piezoelectric crystals outside the vessel.
[0150] FIG. 79 shows a simplified cross-sectional view of an
alternative embodiment of a processing apparatus in accordance with
the present invention, featuring jets which incorporates the
benefits of the unique physical and chemical properties of
supercritical fluids and sonic energy with the hydrodynamic
properties of a fluid jet.
[0151] FIG. 80 shows a simplified cross-sectional view of an
alternative embodiment of a processing apparatus in accordance with
the present invention, wherein a uniform low velocity of a
partially condensable gas is flooded over the face of the
substrate.
[0152] FIG. 81 shows a simplified cross-sectional view of an
alternative embodiment of a substrate processing apparatus in
accordance with the present invention, having vibration members
oriented for cross-substrate energy transfer.
[0153] FIG. 82 shows a simplified cross-sectional view of an
alternative embodiment of a substrate processing apparatus in
accordance with the present invention, having vibration members
oriented for cross-substrate energy transfer while providing
substrate rotation.
[0154] FIG. 83 shows a simplified cross-sectional view of an
alternative embodiment of a substrate processing apparatus in
accordance with the present invention, having multiple vibration
members in the form of transducers spaced about the height of the
tank.
[0155] FIG. 84 shows a simplified cross-sectional view of an
alternative embodiment of a substrate processing apparatus in
accordance with the present invention, featuring multiple vibration
members in the form of transducer pairs in physical contact with a
wedge having a triangle cross-section.
[0156] FIG. 85 shows a simplified cross-sectional view of an
alternative embodiment of a substrate processing apparatus in
accordance with the present invention, featuring multiple vibration
members in the form of a plurality of transducers in physical
contact with a wedge having a polygonal cross-section.
[0157] FIG. 86 shows a simplified cross-sectional view of an
alternative embodiment of a substrate processing apparatus in
accordance with the present invention, wherein multiple vibration
members are configured to perform processing of a substrate
supported horizontally.
[0158] FIG. 87 shows a simplified cross-sectional view of an
alternative embodiment of a substrate processing apparatus in
accordance with the present invention, utilizing a single
processing liquid jet spray and multiple sources of vibration from
a flat plate wide area sonic nozzle design with multiple sonic
nozzles elements.
[0159] FIG. 88A shows a simplified cross-section of an embodiment
of an apparatus in accordance with the present invention featuring
a diffraction grating between the sonic energy source and the
substrate.
[0160] FIG. 88B shows a plan view of one example of a diffraction
grating for use in accordance with embodiments of the present
invention.
[0161] FIG. 88C shows a plan view of another example of a
diffraction grating for use in accordance with embodiments of the
present invention.
[0162] FIG. 88D shows an enlarged view of a portion of FIG.
88C.
[0163] FIG. 88E shows a simplified cross-sectional view of an
alternative embodiment of an apparatus in accordance with the
present invention utilizing a diffraction grating.
DETAILED DESCRIPTION OF THE INVENTION
[0164] A pressurized processing system incorporating megasonic
energy is disclosed in U.S. nonprovisional patent application Ser.
No. 10/150,748, incorporated hereby for all purposes. A technique
for rapid, effective and efficient substrate heating in pressurized
systems is disclosed in U.S. nonprovisional patent application Ser.
No. 10/456,995, also incorporated by reference herein for all
purposes. Incorporated also by reference herein for all purposes is
published U.S. patent application Ser. No. 2001/0013355 A1.
[0165] Apparatuses in accordance with embodiments of the present
invention relate generally to the application of megasonic energy
to enhance processing of a substrate with a fluid. The substrate is
disposed within a processing region configured to receive a
processing fluid, and is exposed to megasonic energy.
[0166] The substrate need not be immersed in the processing fluid,
which can be present on a substrate surface in the form of droplets
or a thin film (meniscus). In addition, the processing region may,
but need not be, partially or completely enclosed by walls or a
tank in order to contain the fluid. Furthermore, the processing
region can, but need not be, enclosed within a processing vessel to
allow for processing at elevated or reduced pressures.
[0167] Conventionally, the term "megasonic" was used to describe
ultrasonic acoustic energy having a frequency of about 700-1800
kHz, while the term ultrasonic has been used to describe a lower
frequency range of about 20-200 kHz when applied to semiconductor
substrate processing. For purposes of the instant application, the
term "megasonic" is utilized to describe acoustic energy having a
frequency of about 10-10,000 kHz and higher that may be applied to
process a variety of substrates. Therefore, the term "megasonic" is
used interchangeably with "ultrasonic" in the instant patent
application.
[0168] In general, to achieve effective and efficient cleaning and
processing of substrates, it is important that the megasonic energy
be applied uniformly. Historically, piezoelectric crystals have
been attached to vibration plates, or directly onto tank walls. The
vibration plates or active tank walls have then been acoustically
or sonically coupled with process or cleaning solutions. These
vibration members were generally located in the bottom of process
tanks or more recently on sidewalls. When the piezoelectric
crystals were excited with a RF frequency, they caused the
vibration plates to vibrate, which in turn caused a series of
pressure or dilatational waves to propagate through the solution.
The substrate to be processed or cleaned was suspended within the
solution. With semiconductor substrates, the pressure waves flowed
up both sides, parallel to the wafer surface causing cleaning or
reactions to occur.
[0169] In order to efficiently transfer energy from a vibration
member to a substrate, effective sonic coupling between the
vibration member and the substrate should occur. This generally had
been accomplished by transferring energy from the vibration plate
to the solution, and then from the solution to the substrate. With
better coupling, more of the energy input into the piezoelectric
crystal is transferred to the process vessel and ultimately to the
substrate. With poor energy coupling, more energy is wasted as it
is reflected back to the generator or converted to heat,
necessitating cooling of crystals and more robust generator
designs. As a consequence of poor coupling, less actual energy
could be applied to a given substrate, which led to longer cleaning
and processing times.
[0170] Embodiments in accordance with the present invention provide
for designs that introduce megasonic energy to substrate surfaces
in ways that address the concerns and limitations mentioned with
conventional designs. The multiple embodiments of this invention
provide great flexibility in delivering a very uniform energy field
to a variety of substrate surfaces, as well as providing
alternative energy wave patterns, especially on the surface of the
substrate. For example, most megasonic designs are based on
dilatational waves (also known as longitudinal or pressure waves)
being generated and transmitted through fluid boundaries to
substrate surfaces. Some of the embodiments of the new invention
disclosed herein are based on generating and using shear and
surface waves as well. Wave modes generated in substrates by these
different wave types can impact contamination particles on
substrate surfaces differently than previously seen with
traditional megasonic systems where only longitudinal waves
emanated from vibration members.
[0171] In one embodiment of the new invention, the vibration plate
is brought into intimate contact with one surface of the substrate,
while cleaning or processing fluid contacts the other. This
embodiment is shown in FIG. 1A, where the megasonic energy is
transferred directly from the vibration member 104 through the
substrate 102 into the cleaning or processing liquid 110 on the
other surface.
[0172] Many methods of ensuring adequate contact between the
substrate 102 and a vibration member to provide reasonable energy
transfer are possible. For example, with a smooth substrate surface
and a smooth vibration member surface, the substrate 102 may be
held tightly against a vibration member 115 in contact with
crystals 106 by vacuum through a series of small holes in the
vibration member and a vacuum tube 114 as is shown in the
embodiment of FIGS. 1B-1BA. Additionally, as shown in FIG. 1BA, a
recess 116 could be machined into the vibration member to match the
cross section of the substrate to be processed.
[0173] Good physical contact does not in and of itself ensure good
energy coupling between the substrate and the vibration member. One
effective way to obtain optimal energy coupling when the substrate
is in intimate contact with the vibration member, is to have the
sum of the thickness of the vibration member and of the substrate,
equal to an odd multiple one-quarter wavelength (n.lamda./4, where
n=1,3,5,7 . . . ) of the megasonic energy wave. The speed of sound
in each material must be taken into account to determine the
corresponding thickness for that material. The greater the
deviation from this optimal thickness, the poorer the coupling and
less energy that is transferred, even with substantial physical
contact. In many embodiments, transmission of megasonic energy
having a wavelength that is within +/-30% of this desired substrate
thickness may be sufficient to transfer adequate energy to the
substrate.
[0174] In theory, the thickness of a vibration member, or a
combination of a vibration member and contacting substrate, should
equal an odd multiple of one-quarter of the wavelength of incident
radiation, when the piezoelectric crystal is of the correct
thickness and operating at its natural resonant frequency. In
practice, however, the piezoelectric crystal can act partly like a
vibration-generating element and partly like a vibration
member.
[0175] This result changes the preferred thickness of the vibration
member slightly from the theoretical value, especially when the
crystal is not operated at its natural resonant frequency. For
purposes of the present invention, slight differences between
theoretical and actual thickness are not quantified for each
possible embodiment. These thickness differences, however, are
often relatively small and generally fall within the specification
ranges listed above.
[0176] For purposes of the instant patent application, a "vibration
member" is defined as a member such as a tank wall, flat plate, or
wedge face, to which a vibrating element such as a piezoelectric
crystal or mechanical actuator is attached, or the combination of
the member and the vibrating element. Thus the thickness of the
vibration member can refer only to the thickness of the member
bearing vibrating element, or may refer to the combined thickness
of that member plus the vibrating element.
[0177] The substrate may be wetted by the processing fluid by total
or partial immersion, or from having the fluid being sprayed
directly onto the substrate, or from the condensation of a volatile
gas or vapor as the temperature changes within the process vessel.
Especially for the immersion approach, when megasonic energy
emanates from a vibration member into a liquid, or from a
combination of substrate and vibration member in intimate contact
with each other into a liquid, at a distance into the liquid from
the surface of the substrate, the energy field may not be uniform.
Standing waves can be established within this distance due to
localized reinforcement and cancellation of the energy wave. These
standing waves can lead to spots of non-uniformity. This region of
non-uniform field intensity is often referred to as the near field,
and generally extends to a distance on the order of ten wavelengths
away from the vibrating surface, depending upon frequency and
intensity of the incident energy. For water, ten wavelengths at a
frequency of 800 KHz translates into a distance of approximately
1.85 cm. Many conventional batch processing systems are designed to
keep substrates separated from vibration members (or tank walls) by
more than this amount in order to avoid the nonuniform nature of
the near field.
[0178] In another embodiment where the substrate and the vibration
member are in intimate contact, a reflecting surface is provided to
cause emanated energy to be reflected back into the near field and
make it more uniform. Various examples of reflecting devices are
shown in FIGS. 2A, 2B, 2C and 2D. Some of the devices are static,
such as the air-liquid interface 202 of FIG. 2A, or the randomly
pitted reflecting surface 204 facing the substrate of FIG. 2B.
[0179] As used herein, the term "static" does not necessarily imply
stationary. It is meant to imply a situation where the member is
not subjected to active mechanical agitation or repetitive
motion.
[0180] In the case of the air-liquid interface 202 of FIG. 2A, the
energy is reflected back at this interface relatively randomly as
the surface of the liquid moves during operation. This interface
can occur at the top of the fluid level within the process vessel,
or could occur as bubbles are generated or introduced into the
processing fluid within the energy path.
[0181] In other cases, the reflecting devices are dynamic or
moving, such as the vibrating reflective plate 206 of FIG. 2C or
the rotating reflective plate 208 of FIG. 2D. The dynamic devices
reflect energy back to cancel out the spots of non-uniformity
within the field. While repetitive, these moving reflecting devices
tend to sweep points of constructive and destructive interference
of sonic energy across the surface of the substrates through the
near field. While not truly random reflections, over time they tend
to make the energy patterns in the near field and those contacting
the substrate more uniform.
[0182] Whether moving or stationary, the reflecting member should
be constructed to reflect, rather than transmit, the megasonic
waves. When separated from a vibration member, an effective
reflecting member may be constructed of solid material with a
thickness equal to approximately an odd multiple one-quarter
wavelength of the energy wave, the same combined thickness that
would afford strong energy coupling if the reflecting member were
in intimate contact with the vibration member. Regardless of the
thickness of the reflecting member, embodiments featuring a gas or
vacuum on the other side of the reflecting member will produce
effective reflection, much as with the case at the gas-liquid
interface.
[0183] FIGS. 3A and 3B show embodiments in accordance with the
present invention where fluid introduction is coupled with
reflection of megasonic energy. The combination of reflection with
fluid introduction not only tends to make the energy field more
uniform, but also may increase uniformity of the processing
chemistry near the surface of the substrate. Uniform concentrations
and temperatures at the substrate surface become more important
when it is desired that controlled chemical reactions take place at
a substrate surface.
[0184] FIGS. 3C and 3D show embodiments in accordance with the
present invention where fluid introduction is coupled not only with
reflection of megasonic energy, but also with fluid removal. Like
the embodiment of FIGS. 3A-B, fluid is introduced through conduits
302. In the embodiment of FIGS. 3C-D, however, fluid is
simultaneously or subsequently removed through local conduits 304.
Processing can thus be accomplished where fluids are both
introduced and removed locally.
[0185] Thus far, the above description has focused upon the use of
reflected sonic energy to enhance the quality of processing in near
field regions. However, embodiments in accordance with the present
invention may also utilize reflected sonic energy to achieve
processing of substrate regions that would otherwise be shadowed
from receiving that energy, for example by an intervening substrate
support member.
[0186] A discussion of such alternative embodiments utilizing
reflected sonic energy is provided below in Section A. A discussion
of other approaches for avoiding sonic shadowing of substrate
regions is provided below in Section B.
[0187] And while the above description has focused upon the use of
reflected sonic energy to enhance processing in near field regions,
alternative embodiments in accordance with the present invention
may utilize other techniques. One such alternative approach is to
vary the character of the sonic energy applied, and is discussed in
detail below in Section C.
[0188] Two applications of particular interest in accordance with
embodiments of the present invention are brush scrubbing and
chemical mechanical polishing (CMP) processing. FIGS. 4A and 4B
show embodiments for brush scrubbing and CMP respectively, wherein
megasonic energy is transmitted across the substrate and directly
into the brush 402 or pad 404. In this manner the mechanical energy
and megasonic energy are focused simultaneously at a single spot or
region of the substrate.
[0189] One benefit of the approach shown in FIGS. 4A and 4B may be
that as the brush 402 is removing macroscopic particles from a
smooth substrate surface portion, the megasonic energy is
simultaneously lifting particles out of recesses such as trenches
and vias. By contrast, conventional systems are limited to applying
megasonic energy to the side or periphery of mechanical members
such as brushes or CMP pads, as those mechanical members are
generally not conducive to the transfer of energy. Thus those
conventional systems can only apply the mechanical and megasonic
energies to a substrate sequentially rather than
simultaneously.
[0190] A further embodiment in accordance with the present
invention could incorporate a localized liquid feed, with liquid
removal via suction. Such an embodiment could aid in the removal of
unwanted materials from recessed regions of a substrate such as
trenches and vias. Further discussion of simultaneous
addition/removal of processing liquid is set forth below.
[0191] Embodiments in accordance with the present invention do not
require the substrate to be completely submerged in a liquid. In
another embodiment shown in FIG. 5A, where the substrate 500 and
the vibration member 502 are in intimate contact as a spray of
liquid 504 processing fluid may wet the exposed surface.
[0192] While FIG. 5A depicts the substrate and vibration member
positioned vertically, they could alternatively be positioned
horizontally as shown in FIG. 5B. The continuing spray 504
replenishes the film of processing fluid on the surface of the
substrate 500, so that energy can interact with the chemistry at
the substrate surface. This embodiment could be utilized in
designing single wafer processing equipment.
[0193] In an alternative embodiment energy may be transferred
across a substrate 600 bounded on both sides by liquid 602 with
incidence of megasonic energy 604 normal to the substrate surface.
FIG. 6A shows such an embodiment, wherein the substrate thickness
should match an even multiple of one-quarter wavelengths
(n.lamda./4, n=2,4,6,8 . . . ) of the applied megasonic energy.
Conventional megasonic cleaning systems have not been very
successful in transferring energy across or through silicon wafers
because they use megasonic frequencies/wavelengths that would
require wafers to be much thicker than is practicable.
[0194] In another embodiment, energy may effectively be transferred
across relatively thin substrates submerged in a liquid, even if
the substrate thickness does not approximate the even multiple of
one-quarter wavelengths just described. As shown in FIG. 6B, this
may be accomplished when the substrates 600 are positioned at a
proper angle to the incidence megasonic waves. This incidence angle
should lie between first and second critical angles for the
fluid/substrate/fluid arrangement.
[0195] In sonic theory for non-destructive testing, the terms first
and second critical angles may have well defined meanings. For
example, when an incident dilatational wave traveling in one medium
strikes the surface of a second material having a higher impedance,
the incident wave can form both a dilatational wave and a shear
wave in the second material.
[0196] As the angle of the incident wave to the surface of the
second material is increased, so do the angles of the refracted
dilatational and shear waves created. The angle when the
dilatational wave is refracted to 90.degree. (parallel with the
surface of the second material) is termed the first critical angle.
The second critical angle occurs when the refracted shear wave lies
90.degree. to the surface.
[0197] For purposes of the instant patent application, however, the
first critical angle is defined as the incident angle at which a
significant fraction of incident energy is transferred across a
substrate when that substrate is submerged in a liquid and not
positioned perpendicular to energy incidence. The second critical
angle is defined as the angle of incidence which marks the halt of
significant energy transfer.
[0198] The optimal range of critical incidence angles may differ
under different conditions. Where sonic energy is applied to a
member surrounded by a liquid, the critical angle will range
between about 18-58.degree., more preferably between about
25-50.degree., and most preferably between about 30-45.degree.. At
such incident angles, a significant amount of energy is transferred
across the substrates, while less energy would be expected to be
transferred if the megasonic waves impinged upon the substrate at
angles outside these ranges.
[0199] For example as the angle becomes larger or smaller, less
energy would be transferred and more reflected, especially for
angles approaching 90.degree. (0.degree. deviation from the normal
to the substrate surface), where little coupling occurs with
substrates of thickness not conforming to the rule set forth above
of an even multiple one-quarter wavelength.
[0200] A further advantage with across-wafer transfer of energy in
accordance with the present invention is that multiple substrates
can be processed simultaneously. This is true for the embodiment of
FIG. 6B where the liquid contacts both surfaces of the substrate.
This is also true for the embodiment shown in FIG. 6C, where one
substrate 600a has one surface in contact with the vibration member
and another substrate 600b does not.
[0201] The number of substrates that a single wave can penetrate is
not limitless. Some attenuation of signal occurs due to the
thickness of each of the substrates, and fluid conditions between
substrates.
[0202] Another potential advantage in accordance with the present
invention is that the wave mechanics and interaction at the surface
are different than occur with traditional designs. These new and
different interactions may result in different forces at the
substrate surface on contaminant particles.
[0203] Without wanting to be bound by any particular theory,
across-substrate energy transfer may be visualized as follows. As
the propagating dilatational megasonic wave in the liquid strikes
the substrate surface at the proper angle, it is converted into a
shear wave in/on the solid substrate. The shear wave in the
substrate is then converted back into a dilatational or pressure
wave as it emanates from the opposite side of the substrate. Where
a gas or vacuum rather than liquid is present on the other side of
the substrate, the shear wave could continue along the substrate as
a shear or surface wave, or be partially or totally reflected.
[0204] The concepts of wave mechanics, including incidence,
reflection, refraction and mode conversion are well known in
classical acoustic and elasticity theory. These concepts describe
that dilatational waves involve particle motion in the same
direction as wave propagation. With shear waves, which occur only
in solids, particle motion is at right angles to the direction of
propagation. For a free surface on a solid half-space, particle
motion is complicated and decays rapidly away from the surface.
Surface waves can also form in thin substrates, often in substrates
comprised of multiple thin layers.
[0205] In another embodiment showing energy transfer across a
substrate whose thickness is not approximately an even multiple
one-quarter wavelength of the applied megasonic energy, the
megasonic energy may be directed perpendicular to the surface of
the substrate. Simultaneously, a bulk fluid movement may create a
fluid velocity parallel to the substrate surface. The fluid
velocity is adjusted so that the resultant angle at which the
megasonic energy hits the substrate surface is within the range of
angles conducive to across substrate-energy transfer. For example,
where megasonic energy is at a frequency of approximately one
megahertz, it is estimated that a water fluid velocity of
approximately six meters per second could be chosen to produce an
acceptable resultant angle. Because a range of resultant angles is
acceptable for across substrate energy transfer, a range of fluid
velocities at each frequency can be utilized. As is discussed in
more detail below, a steady flow of processing fluid without
significant mixing is preferred to maintain integrity of transport
of sonic energy within the liquid.
[0206] The bulk fluid movement could be produced in various ways,
including but not limited to pumping, gravity feed, and the
introduction of a second megasonic wave at an appropriate angle. It
is desirable to have the resultant angle of the energy pattern
created by the intersection of two separate waves fall within the
range of acceptable angles for acceptable cross substrate energy
transfer. When the frequency of the first megasonic wave and second
megasonic wave are similar, the intersection of the two waves could
be at near right angles. Since acoustic streaming velocity is a
function of frequency and intensity, dissimilar frequencies and
intensities could result-in other angles.
[0207] Alternatively, relative motion between the substrates and
the vibration members could be established to produce a relative
velocity on the order of the fluid velocity mentioned above. The
relative motion could be linear, or angular as in the case of
rotation. Either of these approaches could lead to enhanced energy
transfer across the substrate, where the thickness of the substrate
does not conform to the approximate even multiple of 1/4 of the
applied megasonic energy previously described.
[0208] In another embodiment, a first substrate positioned at an
angle to the incidence of the megasonic energy coming from a
vibration member may be moved or vibrated. This movement can occur
along both the x- and the y-axes. This movement could be
represented in FIG. 6D where the angle of the first substrate 600a
relative to the vibration member is changed or "wobbled" relative
to the angle of the second substrate 600b to the same vibration
member 606. As a consequence of this "wobbling" the resulting
energy field transferring across this first substrate and
contacting the second substrate varies.
[0209] This variation in field can include variation in intensity
as well as orientation. When the first substrate has a thickness of
approximately an even multiple of one-quarter wavelength of the
megasonic energy, the resulting change in field can be particularly
evident. The varying field essentially causes a continual sweeping
of energy across the surface of the second substrate. The second
substrate can be any thickness or even be positioned parallel to
the vibration member. Energy field non-uniformity can thus be moved
around over time, essentially making the resulting time-averaged
field more uniform, which may in turn lead to more uniform
processing under certain conditions.
[0210] While the vibrating substrate is shown as a flat plate-like
object of uniform thickness in FIG. 6D, the vibrating substrate may
have a varying thickness. It may be shaped as either a concave or a
convex lens-like structure. Additionally, while shown as being held
parallel to the vibration member, the first substrate could be
positioned at some selected angle, or allowed oscillate over some
range of angles.
[0211] Another benefit of direct contact between the substrate and
the vibration member requires only that the sum of the thickness of
the substrate and the vibration member be approximately equal to an
odd multiple one-quarter wavelength of the megasonic energy. Thus
as the substrate thickness changes, the thickness of the vibration
member can easily be adjusted to compensate. This avoids requiring
new generation frequencies, which may be difficult or impossible to
do cost effectively today.
[0212] In another embodiment shown in FIG. 7A, energy is
transferred from piezoelectric crystals 700 through a solid wedge
702 into a thin flat plate 704. At the proper angle, the
dilatational or longitudinal pressure wave created by the
piezoelectric crystals mounted on the vibration member is
transferred through the solid wedge and converted into a shear or
surface wave in the thin solid plate. When the thickness of the
contacting plate is on the order of one wavelength or less, the
surface/shear wave is sometimes known as a Lamb wave. For thicker
plates, these surface waves may be called Rayleigh waves. For
multiple layer plates with one layer thinner than another, the
resulting surface waves may be called Love waves. The wave
mechanics may be different in each case.
[0213] Additionally there are torsion and flexural waves that
transfer down the length of the plate. Each type of wave has its
own distinctive surface movement, which can translate into
different forces being applied to a surface particle. While other
lengths and thicknesses can be effective for processing, in order
to maximize the transfer of energy along the flat plate for
different applications, its thickness and length generally should
be controlled to certain multiple one-quarter wavelength
values.
[0214] With the wedge/flat plate embodiment depicted in FIG. 7A,
the substrate 708 to be cleaned or processed is kept parallel to
the vibrating plate member and maintained at a separation distance
forming a gap. The gap between the substrate and the vibration
member is filled with processing liquid. Energy is transferred from
the vibration member through the liquid 706 into the substrate.
Ideally the gap should be on the order of an even multiple of
1/4.lamda. of the applied megasonic energy. In practice, since the
gap is filled with a relatively incompressible liquid, its
thickness can vary widely. Alternatively, the substrate could be in
direct contact with the plate vibration member.
[0215] A further advantage of such a wedge arrangement occurs when
the wedge is hollowed with a passage 710 and flushed with a cooling
fluid, as is shown in the embodiment of FIG. 7C. Piezoelectric
crystals tend to heat up during operation. The less efficient the
energy coupling, the higher the operating temperatures. Also,
higher applied power density tends to develop higher crystal
operating temperatures. Higher operating temperatures make it more
difficult to keep piezoelectric crystals attached to a vibrating
member because of differential expansion between various glue
materials and the piezoelectric crystals and the vibration member.
At excessively high temperatures, the crystals can even loose their
ability to function. Therefore an efficient way to cool them during
operation is important.
[0216] As an alternative to cooling channels within the wedge,
another type of cooling structure could be attached to one of the
surfaces. Such an alternative cooling structure could include a
Peltier (thermo-electric) cooler or some other non-liquid
device.
[0217] A further advantage of a wedge arrangement with an extended
vibration plate is that it allows the entire substrate surface, or
a major portion of it be covered and processed with megasonic
energy at the same time.
[0218] A further embodiment in accordance with the present
invention is shown in the simplified cross-sectional view of FIG.
7D, where piezoelectric crystals 700 are placed on multiple sides
702a-b of a wedge design 702. The crystals may operate at the same
or at different frequencies. Variation of the operating frequencies
of these crystals can lead to enhance energy field uniformity over
time.
[0219] While the embodiments of FIGS. 7A through 7D show a single
wedge device, this is not required by the present invention. A
second device 703 could be present, as illustrated in the
simplified cross-sectional views of FIGS. 7E-F. In FIG. 7E, a
second wedge device 703 is added directly under a first wedge
device 702 and transfer plate 704. In FIG. 7F, a second wedge
device 703 could be placed on the opposite end of the transfer
plate 704 with the substrate situated between them.
[0220] Alternatively, one could combine a wedge device with a flat
plate vibration member. Such a combination of wedge device and flat
plate vibration member may produce unconventional sonic energy
patterns when used for substrate processing. These patterns may
include both surface and pressure waves simultaneously leading to
complex waveforms, and complex interactions between the waveforms
and the contaminant particles.
[0221] While the cross-sectional shape of the wedge device of the
embodiments of FIGS. 7A-F is shown as a right triangle, this is
also not required by the present invention. For example, in
accordance with one alternative embodiment, the cross-sectional
profile of the wedge device could exhibit two angles of 30.degree.,
and another of 120.degree..
[0222] Moreover, while the cross-sectional profile of the wedge
device of the embodiments of FIGS. 7A-F is shown as a triangle,
that is also not required by the present invention. Other shapes
may be favored with energy waves exiting a crystal which act
through a surface at some non-zero angle. Not only can angles of
less than 30.degree. or greater than 60.degree. be used, but as
shown in FIGS. 8F-H, even an angle of 90.degree. can be used under
certain conditions.
[0223] Such wedge-like devices may exhibit several useful
characteristics not realized by conventional flat plate vibration
members. First, the energy intensity of individual crystals can be
combined into a single larger energy output on the face utilized
for energy transfer.
[0224] Second, the energy from the individual crystals may be
spread out relatively uniformly over the target face. While
generally larger than the area of an individual crystal, it could
also be smaller. Thus the energy intensity (energy density) on the
active face can be greatly different from the energy density on an
individual crystal. This is in contrast with a conventional flat
plate vibration member where the sonic energy at megasonic
frequencies is well collimated, and falls off rapidly outside the
edged of the energy projection (crystal outline) through the
plate.
[0225] Third, it is possible to operate individual crystals at
either similar or different frequencies, resulting in a combined
waveform for substrate processing. Conventional piezoelectric
crystal act like a large number of separated and distinct vibration
point sources when excited, leading to various wave interference
interactions in the near and far fields. When the output wave from
individual or multiple crystals goes through mode conversion across
the appropriate angles, such as in a wedge-like structure, it is
speculated that perhaps the resulting activated surface from which
the sonic energy is emitted, may actually act a little bit more
like a uniform set of point sources acting at the conditions
dictated by the combined waveform.
[0226] Also, the rod offers only a small cross-sectional area for
contact with a vibration element in order to transfer energy down
its length. Such a small energy-transfer cross section requires
that the vibration element operate at high energy density or
intensity in order to transfer sufficient energy down the rod to
accomplish the processing.
[0227] The wedge design of the instant invention stands in contrast
with that of U.S. Pat. No. 6,463,938 ("the '938 patent"), where
dilatational waves from a piezoelectric crystal attached to the end
of an elongated rod are transferred down the length of the rod.
Being only a rod and not a plate, just a small interaction area
between rod and substrate can occur at any one time. In order to
process or cover the entire substrate surface in the '938 patent,
the substrate must be rotated.
[0228] With embodiments of the current invention, however,
substrate rotation is not required. FIG. 7B shows a simplified
cross-sectional view an embodiment employing such optional
rotation.
[0229] With the wedge design producing a shear or surface wave, not
only is a different waveform used to transfer energy, but also a
large area of contact between substrate and vibration member can
exist. While rotation or movement of the substrate, or rotation or
movement of the vibration member is also possible to enhance
processing, it is not required.
[0230] Alternative embodiments in accordance with the present
invention may utilize a relatively large surface area for contact
with a vibration element, a much larger vibration element, or even
several vibration elements operating at relatively low energy
intensity or density, or a combination of these aspects. Lower
energy density operation of the vibration elements generally
translates into lower operating temperatures requiring less
cooling.
[0231] The '938 patent describes transferring energy from a
piezoelectric crystal to a substrate surface utilizing rods of
various shapes. In accordance with embodiments of the present
invention, however, a wide variety of other shapes can be used,
including various elongated transfer members.
[0232] For example, in FIGS. 7G, 7H, 7I, and 7K, the cross section
of the transfer member is triangular. In FIG. 7G, each of the two
angled sides (angled with respect to the substrate surface) are
positioned at an angle that may actually enhance transfer of
applied sonic energy across the thickness of the substrate 708. The
angled incidence of energy may result in either better cleaning or
reduced damage, under certain processing conditions.
[0233] In the embodiment of FIG. 7H, rather than attaching the
crystals directly to the top of the triangle as in the embodiment
of FIG. 7G, a wedge device 702 is brought into contact with the top
surface of the triangular member 720. The transfer member 722
illustrated in FIG. 7I has similar cross sectional shape to that
shown in FIG. 7G, except crystals 700 are attached to one or both
of the slanted sides, and the third side is rotated 180.degree. and
kept parallel with the substrate surface 708. The flat side that
was parallel to the substrate surface but spaced away from it in
the embodiment of FIG. 7G; is instead positioned near the substrate
surface.
[0234] In the embodiment of FIG. 7J, a transfer member 724
exhibiting a "w" shape is used. In accordance with this embodiment,
the crystals 700 can either be attached directly to the member 724,
or attached to a wedge that is attached to the member.
[0235] While the embodiments of FIGS. 7G through 7J have shown
energy transfer members exhibiting triangular cross-sections, this
is not required by the present invention. Other shapes may also be
useful. Such other shapes could include triangular shapes having
one corner rounded or flattened (FIG. 7JC), or could include
cross-sections having either concave and/or convex portions as
illustrated in FIGS. 7JB and 7JD.
[0236] Most conventional batch type megasonic systems utilize a
flat plate vibration member, having piezoelectric crystals attached
to a flat plate of the appropriate thickness, or attached to a tank
wall of the appropriate thickness. Traditionally the surfaces of
these vibrating members that contact processing fluids have been
smooth and flat. Non-flat plate designs may be desirable for some
applications when cross substrate energy transfer is desired. In
such cases, the surface of the vibration member contacting the
processing fluid (flat plate or inside tank wall), can have a
contoured cross section.
[0237] For example, the cross sections shown in 7JB and 7JD could
be extended to much wider structures. Many other contours are
acceptable.
[0238] In such cases, the resulting vibration plate surface might
appear to have a sawtooth or washboard (corrugated) cross section.
The repetitive "undulating" surface can cause fluid to be pushed
away from the vibration plate surface at various angles. Such
contoured cross sections can also be applied to wedge type devices,
not just flat plate vibration members.
[0239] At first glance, constructing vibration members that
intentionally create energy and fluid flow field interference
patterns may seem counterintuitive. Specifically, the prior art has
generally emphasized designs minimizing energy interference. In
certain embodiments in accordance with the present invention,
designs purposely creating interference patterns are acceptable and
even preferred. As such various fluid and sonic interference
patterns are established, however, variation in the frequency and
intensity of applied sonic energy can be utilized to render the
"effective" energy fields more uniform for substrate processing.
Use of such variation in applied sonic energy is discussed below in
Section C.
[0240] In accordance with additional embodiments of the present
invention, the transfer member could be constructed with a hole or
channel to allow expulsion of processing fluid simultaneous with
application of sonic energy from either a single or multiple
nozzles or jets. Such an embodiment is illustrated in simplified
cross-section as structure 754 of FIG. 7K. The fluid exiting holes
750 as these jets 752 would be ultrasonically activated. Designs
for sonic nozzles are discussed in detail below, including sonic
nozzles utilizing wedge structures.
[0241] Various other shapes for energy transfer members are
possible, as long as energy can readily be transferred to the
members without forcing the energy to be transferred solely along
the length of the rod through only the rod's cross section, as with
the '938 patent.
[0242] In another embodiment, processing fluid is applied to a
substrate through a megasonic nozzle that is set to deliver
megasonic energy to the substrate surface at a selected angle. When
selected from a proper range of angles, significant energy transfer
across the substrate can occur when the substrate is at least
partially submerged in a processing fluid. Alternatively, when the
substrate is not submerged, or is in contact with a support member,
the application of megasonic energy from a megasonic nozzle at a
proper angle can result in the wave mode conversion. The
dilatational or pressure waves exiting the megasonic nozzle can be
converted to surface waves in the substrate. Depending on frequency
and substrate and/or support thickness, these surface waves can
take the form of Rayleigh, Lamb, or Love waves, the last waveform
type being described below in connection with FIG. 8A.
[0243] One application that can benefit from this mode conversion
of megasonic energy is dual brush scrubbing, where two brushes 450
and 452 are directly opposed to each other across a substrate as
shown in the simplified cross-sectional view of FIG. 4C. In the
earlier mentioned application with a single brush embodiment shown
in FIG. 4A, megasonic energy was transferred directly across the
substrate into the brush/substrate contact area as well as on both
sides of that area simultaneously. With the dual brush example,
across-substrate energy transfer in the same way may not occur.
[0244] Therefore to get megasonic energy delivered to the
brush/substrate/brush contact areas simultaneously, a megasonic
nozzle 456 is utilized. The nozzle 456 is directed to the surface
of the substrate 400 at an angle between a first and second
critical angle, where the pressure waves in the liquid stream
exiting the nozzle are converted to shear or surface waves in the
substrate. These shear or surface waves then travel into the
brush/substrate/brush contact areas. This allows the simultaneous
localized introduction of sonic energy and the scrubbing action of
dual brushes onto a single substrate or substrate area.
[0245] To cover the entire contact area, the nozzle could be moved
back and forth adjacent to one of the brushes, or multiple nozzles
could be used. The nozzle could face either the top or bottom
surfaces of the substrate, or multiple nozzles could be directed at
both surfaces at the same time. FIG. 4D shows a simplified
cross-sectional view of an alternative embodiment of a processing
apparatus, wherein fluid applied by nozzle 456 is removed by
suction through outlet 458.
[0246] An alternative embodiment of an ultrasonic nozzle is shown
in FIG. 4E, which depicts a wide active area with liquid 401
exiting a porous or liquid permeable transducer plate 460. In this
embodiment, liquid is pumped into hollowed section 462 of vibration
member plate 460 and forced out through multiple small holes or
openings 464 simultaneously. The thickness of the outer surface of
the plate 460 is selected to be an even multiple of one-quarter
wavelength of the sonic energy, to ensure substantial transfer of
energy from the surface of the vibration member submerged in a
liquid. When not fully submerged, or when high velocity jets of
fluid exit the surface of the vibration member, the fluid is
sonically energized locally as it exits the cavity. The fluid can
exit perpendicular to the vibration member surface or at some angle
to enhance cross substrate energy transfer or to accomplish more
effective processing, as shown in FIG. 87.
[0247] In accordance with embodiments of the present invention, it
is not necessary to select the liquid gap thickness within the
vibration member to be an even multiple one-quarter wavelength.
However, such a design in conjunction with the use of flat plate
vibration members may enhance transfer of energy across the
internal liquid layer when the gap is relatively narrow (i.e. few
wavelengths thickness).
[0248] Holes in the vibration member can be of any shape, with
shapes leading to a maximized steady flow of exiting fluid often
preferred. Shaping the hole to aerodynamically enhance fluid flow
and minimize mixing and turbulence or the generation of eddy
currents within the fluid flow, may also be preferred.
Additionally, various ports for sucking liquid off the substrate
surface during processing could also be employed.
[0249] While the vibration member has been illustrated so far as a
flat plate structure, this is not required by the present
invention. A wedge having a triangular or multi-edged polygonal
cross-sectional profile could also be utilized, as illustrated in
the alternative embodiment of FIG. 4F.
[0250] And while the cavity within the vibration member has
generally been depicted as being rectangular in shape, this is also
not required by the present invention. Other shapes of various
sizes can be utilized.
[0251] Moreover, with non-flat plate-like vibration member
structures such as a wedge device where energy waves intersect a
surface at an angle other than perpendicular, the thickness of the
top of the fluid cavity need not be of an even multiple one-quarter
wavelength thickness as suggested for a flat plate vibration
member.
[0252] FIG. 4G illustrates the simplified cross-section of a narrow
area ultrasonic nozzle 480 based on a wedge-like device. The fluid
is maintained in a steady flow condition as its direction is
changed in the nozzle, leading to sonic activation of the exiting
spray. A wide area nozzle arrangement could be made with an
analogous design as well.
[0253] And while the face or side from which energy applied by the
megasonic sources exits the wedge device ("the activated face") has
generally been portrayed as flat, this is not required. Various
contours are possible. For example, in either a large or small area
sonic nozzle, the third face of a wedge device could be curved
instead of flat (straight). Such a curved face may help to promote
steady flow. Thus the activated face could be totally or partially
flat, concave or convex with constant or variable radius of
curvature.
[0254] Another embodiment utilizing the concept of a wedge or
device to adjust the incident angle of megasonic energy onto a
plate is illustrated in FIG. 8A. The substrate 802 and vibration
plate 804 is in intimate contact and at the proper angle to gain
mode conversion from longitudinal to shear wave. When a shear wave
is formed in a multi-layer vibration member comprised of a thin and
a thick section, it is concentrated in the thin layer and is termed
a Love wave. Processing liquid is sprayed onto the substrate
surface allowing unique surface energy patterns to be formed and
interact with contaminant particles. Alternative embodiments
include a distinct cooling member 806 adjacent to the wedge (FIG.
8B), and designing the wedge to include hollow or fluid-filled
passages 810 for cooling (FIG. 8C). Additionally, a vacuum chuck
arrangement (complex of vacuum holes) could be constructed to keep
the substrate in good mechanical contact with the vibration
member.
[0255] Still another embodiment utilizing the concept of a wedge
device is illustrated in FIG. 8D, where piezoelectric crystals 812
are attached to more than one side of a wedge 8.times..times.
simultaneously. The substrate 802 is brought into close contact
with the third face (hypotenuse) 804a of the triangular wedge
device 804. The dilatational waves produced by the crystals are
likely converted to surface waves that perform the substrate
processing. Complex waveforms can be generated from multiple
crystals operating at different frequencies on each of the adjacent
sides. Processing fluid can be applied to the processing face by at
least a spray, mist or immersion.
[0256] A further embodiment utilizing a wedge device is illustrated
in FIG. 8E, with crystals on two surfaces 804b-c and the third face
(hypotenuse) 804 in direct contact with a tank 814. The sonic
energy is readily transferred from the wedge device into the
processing tank. As shown in FIG. 8E, any number of crystals may be
used by making the wedge "thicker" or deeper. Crystals having
almost any aspect ratio can be oriented in any direction on any of
the wedge faces.
[0257] One of the advantages of having the tank wall or even the
substrate in direct contact with the vibration as stated
previously, is that the thickness of the substrate can be easily
compensated for to produce maximum energy transfer either across
the substrate or into the processing vessel or tank.
[0258] In accordance with another embodiment of the present
invention shown in simplified cross-section in FIG. 8F, a tank or
vessel may contact either (top or bottom) sides of the wedge
device, rather than the third face as in the embodiment of FIG. 8E.
In such an embodiment, crystals 812 may be arranged on multiple
faces or sides of a wedge 804. In the embodiment of FIG. 8F, energy
unexpectedly flows perpendicular to the plane of the dilatational
or pressure waves emanating from the crystals into the wedge and
then into the overlying tank 814. Spray, immersion or contact with
a tank may be utilized for processing.
[0259] While the embodiment of FIG. 8F shows crystals on two sides,
this is not required by the present invention. In alternative
embodiments in accordance with the present invention, crystals
could be present on only one side, or on three or more sides as
desired.
[0260] In accordance with another embodiment of the present
invention, the bottom surface 804d of a modified wedge device 804
illustrated in FIG. 81 is placed parallel to the top or bottom
surface of a rotating substrate 802. The sides of the wedge where
the crystals are mounted are at right angles (90.degree.) with the
bottom surface. Bottom surface 804d and substrate 802 are brought
into relatively close proximity to each other with the gap being
filled with a thin film 880 of processing liquid.
[0261] In accordance with yet another embodiment of the present
invention shown in FIG. 8J, the bottom surface of a wedge structure
that has crystals attached to sidewalls angled at other than the
90.degree. shown in the embodiment of FIG. 8I, is positioned above
a substrate. The sonic energy emanating from the crystals impinges
the bottom of the wedge at an angle. The angled incidence likely
converts the dilatational waves from the crystals into surface
waves on the bottom of the wedge. The surface wave then interacts
with the substrate through a thin liquid layer or meniscus.
[0262] In accordance with still other embodiments of the present
invention, shapes other than a wedge of triangular cross section
can be used in the transfer device design. FIG. 8G illustrates a
polygonal shape 882 fitted with multiple crystals 812. Energy may
be transferred as shown, from the side of the device directly into
a tank.
[0263] While the embodiment of FIG. 8G shows the crystals mounted
on edges at right angles to the surfaces to which sonic energy is
applied, this is not required by the present invention. Instead,
the edges could be constructed at an angle with the side, as shown
in the embodiment of FIG. 8H, thereby possibly allowing the
transfer of even more energy to the top side 882b of the device
882.
[0264] FIGS. 8LA-B show perspective and edge views, respectively,
of an alternative embodiment of a vibration member in accordance
with the present invention. FIGS. 8LA-B show that the
energy-transferring surface can extend out past the sides of the
polygonal structure on which the vibrating elements are attached.
Further, the polygonal structures need not be regular. Each side
could have a different length.
[0265] In a further refinement of this and other embodiments
disclosed where a thin liquid film or meniscus is formed on the
surface of the substrate, the gas atmosphere can be charged with
various species to enhance processing. For example, to remove
organic particles, ozone gas could be introduced into the gas
space. The ozone molecules could then easily diffuse across the
fluid layer on the surface of the substrate and react with the
surface or contaminates on the surface.
[0266] To enhance the rate of ozone transfer, the gas phase could
be pressurized. Such an approach would contrast with conventional
industry processes, which operate at atmospheric pressure.
[0267] Instead of ozone, hydrogen peroxide could be added to the
processing liquid or sprayed onto the thin liquid film on the
substrate surface. Other processing chemicals could also be chosen,
such as organic acids, inorganic acids, bases, oxidants, reducing
agents. They could be in the form of solids, liquids or gases.
[0268] When combined with the introduction of megasonic energy, the
liquid film layer can be broken up and mixed rapidly, increasing
transfer of a gas species from the atmosphere to the surface of the
substrate, whether operated at either elevated or even atmospheric
pressure. The megasonic energy could also aid in removing loosened
material from the substrate surface. Such a processing mechanism
could be helpful when applied to a number of processing steps, but
could be especially advantageous when applied to processes such as
photoresist stripping. Additionally, the application of megasonic
energy can lead to the formation of desirable chemical species in
the processing fluid or on the surface of the substrate.
[0269] The thickness of the vibration member and the substrate may
be dictated or set by other constraints. If the combination of
their individual thickness do not add up to the desired thickness
for good energy coupling at a given frequency, it may be desirable
to adjust the frequency. Conventionally, this can only be
accomplished over a relatively narrow range around various harmonic
frequencies of the piezoelectric crystals to produce the desired
odd multiple one-quarter wavelength of the megasonic energy. With
megasonic generators manufactured by PCT Systems Inc. of Fremont,
Calif., in accordance with embodiments of the present invention,
however, some degree of frequency adjustment of individual crystals
to accommodate minor thickness differences or changes is possible.
With many conventional fixed frequency systems using generators
with RF outputs that cannot be changed for individual crystals,
this type of adjustment may not possible.
[0270] In some applications, less than optimal energy transfer may
be acceptable, and only partial energy transfer may be adequate to
perform the processing. For example, in the case of optimal energy
transfer, no gap(s) or voids are present between the vibration
member and the substrate. Practically, however, some gaps or voids
may exist due to machine tolerances, or machine designs requiring
separation. In such applications, it may be especially important
that the gaps or voids not be filled with compressible fluids, such
as gases.
[0271] When unavoidable gaps or voids between the vibration member
and the substrate are filled with relatively incompressible fluids
such as water, acceptable energy transfer can often occur. In
applications using relatively incompressible fluids, the gaps can
become quite large (>1 m) and still demonstrate significant
energy transfer, especially with fluids containing relatively low
levels of dissolved gases. FIG. 6A is a representation of such an
embodiment.
[0272] In practice, the gap between the vibration member and the
substrate may be filled with a solution containing a significant
amount of dissolved gas. Some of that gas is liberated in the form
of bubbles as the megasonic energy is applied because of the
reduced pressure region on the trailing edge of the pressure
wave.
[0273] These liberated bubbles may interfere with optimal energy
transfer through the solution. However, with the high intensity
energy surges occurring with multiplexed crystal operation, bubbles
may effectively be "pushed" out of the sonic pathway, so that
little problem is encountered with such solutions.
[0274] Historically, high fluid velocities have been used in an
attempt to sweep the bubbles out of the active energy field in
order to enhance energy transfer. However, such designs based on
forced high fluid velocities have only met with limited success. A
very large fluid velocity may be utilized to dislodge some bubbles
from energized/vibrating surfaces. If the fluid velocities become
too large, or if large turbulence or eddy currents are generated in
the fluid, the transfer of energy through the fluid may be
adversely affected.
[0275] Thus in another embodiment, the system is operated at
increased pressure to overcome these limitations. The increased
operating pressure acts to both reduce the volume of any bubbles
formed, and to increase the concentration of gas that may be
dissolved in the liquid at a given temperature. Therefore, any
bubbles formed by the megasonic energy tend to be smaller and are
forced back into solution more quickly. This leads to better energy
transfer, and can lead to more uniform processing as well,
especially when active gas species are dissolved in the liquid.
[0276] As a consequence, solutions with high dissolved gas content
may now be utilized effectively. These high gas concentrations can
enhance chemical reactivity. Additionally, they can promote
increased microcavitation that may aid in substrate cleaning.
Further, increased pressure can force solutions into smaller
crevices than surface tension of the fluids alone would allow at
atmospheric pressure.
[0277] In another embodiment, where the substrate is in intimate
contact with the vibration member, an electrochemical reaction may
be carried out simultaneously or intermittently with the
application of megasonic energy. And where the substrate is
separated from the vibration member and submerged in a fluid with
the megasoric incident angle allowing across-substrate or
across-electrode transfer of energy, an electrochemical reaction
may be carried out simultaneously or intermittently during the
application of megasonic energy. In either case, transfer of energy
across the substrate and/or across the electrode can be important
to maximize fluid exchange in small diameter holes, vias, and
narrow trenches, to produce more uniform processing.
[0278] Section D below presents a detailed discussion of the
application of ultrasonic energy to perform electrochemical
processing of a substrate, in accordance with embodiments of the
present invention.
[0279] In certain of the various embodiments described above, it
may be preferable that the substrate front side, rather than back
side, contact the vibration member. Moreover, either the substrate
front side or the back side may face away from the vibration member
when the substrate is fully submerged. These configurations could
be important where the cross-substrate energy transfer aids in
cleaning out surface voids such as vias and trenches. The
cross-substrate energy transfer may help push contaminants out the
front side of the substrate when energy enters from the backside of
the substrate. By contrast, for conventional processing where
energy is parallel to the substrate surface, little or no cleaning
action within recesses or voids results, as these features are
positioned at right angles to the flow of energy. Moreover, with
conventional processing where the energy is directed perpendicular
to the substrate surface, much of the applied energy may be
reflected back toward the vibration member, possibly pushing
contaminants even deeper into voids and crevices.
[0280] As mentioned above, suction can be applied sequentially or
simultaneously with megasonic substrate processing. When liquid is
both directed towards the substrate surface (multiple feed nozzles)
and removed via suction (single or multiple suction nozzles), there
is a greater likelihood that liquid within high aspect ratio
trenches and vias can be exchanged more easily or more frequently.
Both effects could result in enhanced processing.
[0281] In accordance with an alternative embodiment in accordance
with the present invention, megasonic energy may first be
transferred across the substrate in one direction, and then
transferred across the substrate in another direction, thus
treating both sides of the substrate with energy and momentum. For
example, U.S. Pat. No. 6,098,643 ("the '643 patent") is hereby
incorporated by reference for all purposes.
[0282] The '643 patent describes a tank that may be modified to
produce cross-substrate energy transfer. Substrates positioned
within the tank may have their orientation rotated 90.degree. in a
horizontal plane from the orientation shown in the '643 patent.
Megasonic energy would thus impinge the surface of the wafers at a
proper angle for cross wafer energy transfer from the end of the
substrate carrier, instead of the energy being directed parallel to
the substrate surface along the length of the substrate
carrier.
[0283] As the piezoelectric crystals are fired sequentially first
from one side of the side of the V-bottom tank and then from the
other, megasonic energy goes through the wafers first from one end
of the carrier and then from the other. Thus both sides of the
substrates may be treated equally by megasonic energy bursts. This
can be utilized for both single wafer as well as batch processing
of multiple wafers simultaneously.
[0284] As described above, there can be some attenuation of energy
as it crosses each substrate and the space separating the
substrates. Therefore the energy density is not necessarily
constant from one end of a cassette to the other.
[0285] In an arrangement wherein both sides of the substrates are
treated with bursts of megasonic energy, substrates at each end of
the cassette would receive a high energy density burst from one
direction and a low energy density burst from the other. This would
tend to even out the total applied sonic energy. For most effective
processing, it may be desirable to use a carrier holding fewer that
50 or even 25 substrates.
[0286] Embodiments in accordance with the present invention are not
limited to using any one type of megasonic generator or mode of
operation. For example, the megasonic generators could be of the
type in which the piezoelectric crystals are fired singly, or
several are fired simultaneously or in repeating sequence. The
megasonic generators could generate either at a fixed frequency, a
programmed variable frequency, or a randomly fluctuating frequency.
Thus while the above embodiment describing operation in a V-bottom
tank specifically refers to sequentially firing one transducer on
one side of a tank and then a transducer from the other side of the
tank, other modes of operation are possible as well.
[0287] In certain applications, process fluids may be maintained at
high temperatures that could damage either the megasonic
transducers or their method of attachment, or could lead to poor
megasonic performance. In such applications, direct cooling of the
piezoelectric crystal(s) can be employed. Such direct cooling of
the piezoelectric crystal(s) may also be useful in cases when high
megasonic energy density is required which would cause high
operating transducer temperatures, again leading to possible damage
or poor performance.
[0288] FIGS. 9A and 9B illustrate two embodiments of direct cooling
of transducers, where a fluid filled cooling member 904 is attached
to the backside of the piezoelectric crystal(s) 906 with adhesive
or other bonding means, or clamping and other holding means. In
order to minimize the amount of megasonic energy transferred into
the cooling member (and perhaps lost in the cooling fluid),
thickness of the sheath of the cooling member is chosen to be
approximately an even multiple one-quarter wavelength of the
ultrasound energy.
[0289] In practice, potentially some loss could occur should the
thickness of the sheath be closer to an odd multiple one-quarter
wavelength, promoting transfer of energy into the cooling member,.
Even so, some fraction of the energy transferred into the cooling
member would likely be reflected back into the vibration member
from the air-sheath interface on the backside of the cooling
member, minimizing actual loss of megasonic energy.
[0290] The sheath material can be comprised of a number of
materials, including but not limited to metals, plastics and
composites and combinations thereof. Generally a metal may be
preferred when good electrical insulation can be ensured between
the cooling member and the transducer body because of the generally
higher thermal conductivity of many metals. Non-electrically
conductive sheath materials can often be placed into direct
physical contact with the transducers without any additional
electrically insulating layers or films separating them.
[0291] The cooling or heat transfer fluids in the cooling members
can also be comprised of a number of materials including, but not
limited to both electrically conductive and non-conductive fluids.
These fluids could be introduced as gases, liquids or solids. They
may remain in their initial state or they may go through a phase
change as they absorb heat from the transducers. Further, they
could also include processing and rinsing fluids commercially used
in semiconductor processing and other industries. Perfluorinated
fluids such as Galden manufactured by 3M can be used either as
fluids circulating through the cooling member, or could be sprayed
directly onto the back of the exposed transducer body causing rapid
cooling of the transducers. When dealing with structures exhibiting
an angle relative to incident energy (i.e. other than flat plate
vibration members), limitations in thickness are changed when
conditions promoting significant cross substrate energy transfer
are used.
[0292] In the embodiment of FIGS. 9A-B, as well as with many others
already disclosed, a more rigorous discussion of this subject
matter could include impedance terms rather than mere thickness
criteria, taking into account physical parameters like Yong's
modulus, density and speed of sound in differing materials.
[0293] Processing in accordance with embodiments of the present
invention can occur in a single chamber with a single processing
step, or could comprise multiple processing steps with various
chemistries. Alternatively, processing may be carried out in
multiple chambers sequentially. Processing could include wet
processing, dry processing, or a combination of both wet and dry
processing.
[0294] Coupled with megasonic energy, a variety of radiation types
may be useful for different applications. Types of applied
radiation include, but are not limited to: microwave, ultraviolet,
infrared, and electromagnetic induction. In another embodiment of
the invention, the radiation may heat the substrate or processing
liquid on the substrate surface, thus promoting more rapid reaction
without necessitating heating of the entire process chamber.
Microwave, infrared, and electromagnetic induction could be useful
in heating a processing liquid or substrate.
[0295] In another embodiment, the radiation could promote a
particular reaction at a surface of the substrate. In such an
approach, the application of ultraviolet radiation can offer
particular advantages. An oxidant could be utilized in combination
with the radiation to enhance degradation of the residual
contaminants or to modify surfaces. Examples of useful oxidants
include but are not limited to ozone, hydrogen peroxide, and oxides
of nitrogen.
[0296] In another embodiment, residual processing liquid on the
surface of a substrate can be vaporized off the surface by the
addition of megasonic energy. As with many of the other
embodiments, the residual processing liquid could be in the form of
droplets or even a film covering the substrate surface. In some
cases, very small cavities within the substrate may be filled with
processing liquid as well. It has been discovered that thin films
or droplets of liquid can be quickly vaporized, leaving substrate
surfaces dry by applying megasonic energy directly to, or through,
a substrate. A detailed description of the application of sonic
energy in accordance with embodiments of the present invention to
accomplish substrate drying is provided below in Section E.
[0297] A detailed description of the application of sonic energy in
accordance with embodiments of the present invention to accomplish
other processing applications is provided below in Section F. A
detailed description of the use of different types of fluids in
megasonic processing is provided below in Section G.
[0298] For purposes of this patent application, the term "wedge
device" can be defined to include devices with various cross
sections. For example, a wedge device could be comprised of a
triangular cross section with three faces and two sides
(top/bottom), or could be comprised of a polygonal structure with a
larger number of faces.
[0299] In the case of wedge devices in particular, the face or side
from which energy emanates may demonstrate a more uniform vibration
pattern resembling a row of point sources firing sequentially,
rather than a number of randomly firing point sources as is more
typical in a flat plate vibration member. This characteristic could
translate into more uniform processing for some substrates within
various vessel and holder configurations.
[0300] The energy in a wedge device tends to be spread over the
entire surface. Therefore, sequential firing of multiple crystals
on one or more sides of the device would result in energy waves
covering essentially the entire wedge/face of interest. Such an
arrangement would result in a multiplexing generator design in
which the crystals are fired sequentially, appearing as if a
generator of the same total wattage output were used having all
crystals continuously energized at a reduced energy density.
Instead of producing a continuous energy output, such an
arrangement would result in continuous pulses of energy having a
duration equal to the length of time each crystal is fired, before
being switched to the next crystal.
[0301] With megasonic energy transmitted through a fluid, when the
crystal is first energized, the initial magnitude of the pressure
wave increases to a maximum and then decreases over the remainder
of the energy pulse dependent upon pulse duration. The pulsating,
"always on" characteristic described above for a crystal
multiplexing of the wedge device could thus be beneficial for
substrate processing, with both continuous power and peak surges
during each on duration for each crystal.
[0302] To prevent the spreading of the energy from a single crystal
over the entire bottom of a tank or process vessel, the wedge
device could be constructed of multiple narrower wedge device
elements 804 rather than a single larger device, as illustrated in
the side and perspective views of FIG. 8KA-C. With a minute air gap
between neighboring wedge devices 804, or less than optimal
coupling between them, transfer of energy from one device element
to the next could be minimized, thus maintaining a higher energy
density on each individual section with multiplexing systems. With
some appropriate limited transfer between adjacent sections (energy
overlap), there would not be totally dead areas (areas of
non-uniformity) between the elements, which could adversely impact
localized substrate processing.
[0303] Generally, sonic nozzles are used to direct a small,
energized liquid stream at a non-submerged surface. This approach
is successful where the nozzle is relatively close to the surface,
and a continuous liquid column or stream between the nozzle and
surface can be achieved. Separation distances larger than several
centimeters, however, generally results in inefficient energy
transfer.
[0304] In accordance with an alternative embodiment of the present
invention, the nozzle may be incorporated in a fully submerged
processor. Thus, not only is megasonic energy transferred from the
vibration member to the substrate, but significant high fluid
velocity profiles are developed between the two. In some cases, the
megasonic energy follows the fluid pulses from the jet as with a
megasonic nozzle. This combination of megasonic energy and high
localized fluid velocities, may great enhance processing under some
conditions, especially those in which mass transfer is an important
issue. Examples of the enhanced processing as a consequence of high
fluid velocities, absent even the addition of megasonic energy, are
evident from the enhanced rates of photoresist stripping of
co-pending U.S. patent application Ser. No. 10/150,748.
[0305] The speed of sound in a material may differ according to
wave type. For example, dilatational or pressure waves often
exhibit nearly twice the speed of sound in many materials relative
to a shear or surface wave in the same material. Thus, with mode
conversion or diffraction of an incident dilatational wave, the
speed of the resulting waveform could be significantly different,
perhaps leading to different processing performance.
[0306] With wedge devices, the dilatational or pressure waves
generated by the piezoelectric and other crystal types can be
converted into various waveforms depending not only upon the angles
the wave encounters, but also upon the environments through which
the wave(s) must travel. In the embodiment of FIG. 7C, for example,
even with a fluid-filled region the dilatational waves are
converted to shear or surface waves on the activated face or in the
attached plate.
[0307] By contrast, in a somewhat similar wedge device shape of the
embodiment of FIG. 8E, the energy waves leave the activated surface
at an orientation normal to that surface and enter the process
tank. This mode conversion can occur over a broad range of
angles.
[0308] Such mode conversion over a broad range of angles, may be
contrasted with the ability of dilatational or pressure waves to
transfer across a submerged substrate over a much narrower range of
incident angles. For example, if the fluid filled wedge of the
embodiment of FIG. 7C were separated from the extending plate, and
liquid were placed on the face of the wedge that had been in direct
contact with the plate, it could be difficult to predict the
resulting transfer of energy types and patterns across the
activated face if the shell of the wedge were thin.
[0309] Depending on the angle of incidence of the dilatational
waves transmitted through the fluid inside the wedge, upon striking
the shell of the activated face, both mode conversion and transfer
could occur. At one range of incidence angles, energy transfer
across the thin shell could predominate. At another range of
incidence angles, mode conversion to surface waves may occur. At
still other incidence angle ranges, varying degrees of internal
reflection could occur. Testing may reveal with greater precision
the results of applying sonic energy at various angles with such a
complex structure design.
[0310] FIG. 81 illustrates a simplified cross-sectional view of
another embodiment in accordance with the present invention
comprising a single wafer processor having vibration members
oriented for cross-substrate energy transfer. Vibration members
8100a and 8100b positioned on opposing sides are energized
sequentially, not simultaneously. Substrate 8105 is supported
within tank 8107 by supports 8109. Energy from each vibration
member travels through the liquid 8102 and impinges the surface of
the substrate 8104 at an angle proper for reasonable transfer of
energy there across.
[0311] Energy that transfers across the substrate continues on in
the liquid on the other side in a relatively straight line until it
impinges one of the two reflecting sidewalls 8106. The transferred
energy is then reflected back towards the substrate. The energy
again impinges the surface of the substrate at a correct angle for
significant cross substrate energy transfer, and passes again
through the substrate at a higher point. The energy continues on to
reflect off the other sidewall and back through the substrate yet
again at a point higher in the tank.
[0312] Some attenuation occurs each time the energy crosses the
substrate, the further it travels in the liquid and each time it
reflects off a sidewall. The character of substrate processing
could thus change as the energy travels from the bottom to the top
of the tank.
[0313] To enhance overall uniformity of processing, a substrate
could be rotated during processing so that all areas of the
substrate are exposed to similar energy impingement. Such an
apparatus is illustrated in simplified cross-section in FIG.
82.
[0314] While the vibration members of the embodiment shown in FIG.
81 are described as being fired sequentially, this is not required
by the present invention. Alternatively, both vibration members
could be fired simultaneously. Any resulting interference patterns
generated from such simultaneous firing could be moved around with
proper frequency and power variation, thus allowing uniform
processing of a substrate.
[0315] While the above embodiment is shown with only two vibration
members, this is not required by the present invention. Multiple
vibration members could be spaced about the height of the tank. A
section of such a design is illustrated in simplified cross-section
in FIG. 83. Use of such multiple levels 8300 of vibration members
8302 reduces the need to rotate the wafer 8304 in order to effect
more uniform processing.
[0316] FIGS. 84 and 85 illustrate simplified and enlarged
cross-sectional views of a portion of an additional embodiment of
processing apparatuses in accordance with the present invention.
Apparatus 8400 of FIG. 84 is similar to that shown in FIG. 83,
except that the simple flat plate transducer has been replaced with
a pair of transducers 8402 in contact with wedge structure 8404
having a triangular cross-section. Apparatus 8500 of FIG. 85 is
similar to that shown in FIG. 83, except that the simple transducer
has been replaced with multiple transducers 8502 in contact with a
wedge structure 8504 having a cross-sectional profile. The
embodiment of FIG. 85 produces two paths of incident sonic energy
8506 and 8508, respectively. While the sonic energy paths 8508 may
occur at any depth in the process vessel, it can also occur at the
gas/liquid interface at the top of the tank. Thus, the substrate
could be receiving sonic energy while it is being withdrawn from
the processing liquid.
[0317] Another embodiment of an apparatus in accordance with the
present invention for performing horizontal processing is
illustrated in FIG. 86. In the embodiment of FIG. 86, substrate
8600 is positioned in relatively close proximity to the multiple
vibration members 8602, which are oriented at appropriate angles to
obtain efficient cross-substrate energy transfer. Gap 8604 between
vibration members 8602 and substrate 8600 is filled with liquid
8608. Top surface 8600a of substrate 8600 can bear a thin liquid
layer, or be exposed to the ambient within the processor, or could
be wetted with a liquid spray from nozzle 8610.
[0318] The substrate 8600 can optionally be rotated. Either the
frontside 8600a or the backside 8600b of the substrate 8600 can
face the vibration members 8602. Both the frontside 8600a and the
backside 8600b of the substrate 8600 may be processed
simultaneously. While nozzle 8610 is shown as producing a liquid
spray of processing liquid, the nozzle could also comprise a
megasonic nozzle configured to sonically energize the exiting
liquid. The nozzle could even be oriented to operate within a range
of angles promoting transfer of energy across a substrate.
[0319] Another embodiment of and apparatus for horizontal single
substrate processing is illustrated in simplified cross-section in
the embodiment of FIG. 87. Specifically, the vibration members of
FIG. 86 are replaced with a single wide area sonic nozzle 8700
having liquid inlet 8701 such that jets 8702 directed at an
appropriate angle for some transfer of energy across substrate
8704. Optionally, the substrate could be rotated, submerged,
exposed to the atmosphere or wet with a liquid spray. Processing of
both the frontside and the backside of the substrate can occur
simultaneously.
[0320] When the gap between the wide area sonic nozzle and the
substrate is not completely filled with liquid, energy may be
transferred through the various fluid jets. The energy travels
through the liquid in the jet stream as long as the jet stream is
comprised of a continuous liquid stream between the vibration
member and the substrate. When the gap is completely filled with
liquid, energy transfer can occur by a combination of both energy
transfer with the jet and direct transfer from the surface of the
vibration member through the filling liquid to the substrate.
[0321] Physical fluid flow collimating devices added to the near
field may modify the processing performance of megasonic systems.
One type of such a collimating device could resemble a grating
having relatively small openings for the transmission of fluid and
energy. In some embodiments openings on the order of one wavelength
or less may preferred. In other embodiments, openings larger than
one wavelength may be preferred. The thickness of the grating could
vary from less than one wavelength, to tens of wavelengths or
more.
[0322] Depending upon the characteristics of the fluid, including
but not limited to the degree of gas saturation, smaller openings
of a grating may undesirably become "blinded" by gas bubbles
generated in the megasonic energy field during operation. In
certain embodiments, this could be avoided by utilizing gases
having different surface tension characteristics, or utilizing
higher pressure differentials to force fluid flow though the
opening. As frequency or intensity of ultrasonic energy applied is
varied, bubbles that form could be redissolved.
[0323] The diffraction of energy waves through narrow slots or
openings in a grating may seem counterintuitive when attempting to
form a uniform energy field. When employed, however, in conjunction
with proper frequency or power variation, points of non uniformity
can be moved around through the confined liquid space effectively
making the sonic processing better or more uniform. Generally, it
would be desirable to consider both fluid flow/velocity profile
with energy uniformity.
[0324] FIG. 88A shows a simplified cross-sectional view of an
embodiment of an apparatus 8800 in accordance with the present
invention featuring a diffraction grating 8802 between the
substrate 8804 and a vibration member 8806 in contact with multiple
transducers 8808. FIG. 88B shows a plan view of one example of a
diffraction grating for use in accordance with embodiments of the
present invention, comprising a screen 8810 defining openings 8812
much larger than one wavelength of the incident sonic energy.
[0325] Alternatively, the grating could comprise thin-plates having
relatively large aspect ratios arranged parallel to each other are
separated by a distance on the order of less than about 6 mm. As
the sonic energy moves up between the plate elements, the energy
field in this near field region changes and becomes more
collimated. As frequencies or power levels are varied, points of
field non uniformity are moved around may make the effective
substrate processing more desirable. Substrates to be processed
could be positioned on the top edge of these plates in a generally
parallel or some other arrangement.
[0326] FIG. 88C shows a plan view of another example of a
diffraction grating 8814 for use in accordance with embodiments of
the present invention, comprising openings defining various shapes
and sizes. FIG. 88D shows an enlarged view of a portion of the
diffraction grating of FIG. 88C.
[0327] While the embodiment of FIG. 88A shows the diffraction
grating as separated from the vibration member by a distance, this
is not required by the present invention. FIG. 88E shows a
simplified cross-sectional view of an alternative embodiment of an
apparatus in accordance with the present invention utilizing a
diffraction grating 8816 in direct physical contact with the
vibration member.
[0328] A. Reflected Energy
[0329] In many megasonic cleaning and processing applications,
non-uniform distribution of sonic energy within a processing tank
may result from the presence of various obstructions therein, for
example built-in supports for substrate carriers, or the structural
members of the substrate carriers themselves. Portions of a
processed substrate behind such obstructions may be "sonically
shadowed" from receiving the full impact of ultrasound energy
transmitted into the tank.
[0330] Over the years, a variety of designs of processing tanks and
substrate carriers have been proposed to lessen such nonuniform
energy distribution attributable to sonic shadowing. One
conventional approach taught elimination of shadowing by imparting
relative motion between the substrates and the sonic energy
sources, for example by moving the energy sources, moving the
substrates, or both. Such processing equipment designs however, can
be mechanically complex, offering the disadvantage of higher
maintenance costs. Other disadvantages of designs involving
numerous moving parts include additional space requirements, the
unwanted generation of particles, and the non-uniform flow of fluid
within the bath.
[0331] Another conventional approach to reducing ultrasound
shadowing involves designing the tank to promote reflection of
ultrasonic energy to process areas otherwise shadowed behind
various obstructions, including carrier supports. One such approach
is shown and described in U.S. Pat. No. 6,523,557, incorporated by
reference herein for all purposes.
[0332] That patent illustrates a design where sidewalls of the
processing tank include a convex portion, with sonic energy
reflected off the convex portion into regions that would otherwise
be sonically shadowed. This specific design required that the
incidence angle of the sonic energy impinging the curved wall be
less than a critical angle for that material. For quartz, this
critical angle was 26.degree..
[0333] One limitation of such a design, is that the curved areas
must be relatively large, owing to the limitation of maintaining a
small angle of incidence (e.g. less than .about.26.degree. for
quartz). This forces the processing tanks to become larger, and
hence more difficult and costly to fabricate. For example with
quartz, fabricating large curves into a tank wall are much more
difficult and expensive than fabricating tanks from flat
plates.
[0334] The relatively small angle of incidence described in this
patent also forces reflecting surfaces within a bath to be
relatively large, which may interfere with fluid flow and efficient
utilization of space within the tank volume.
[0335] A further limitation with conventional reflecting-type
ultrasonic processing systems becomes apparent when applied
ultrasound beams are relatively wide, or multiple piezoelectric
crystals are fired simultaneously. In such cases, the reflected
portion of the beam can interfere with the unreflected portion of
the beam. This can result in localized constructive and destructive
interference, generating points of high- and low-energy intensity.
Such non-uniformity in the energy field can correspondingly result
in processing of substrates in a non-uniform manner. As described
below, embodiments in accordance with the present invention may
resolve such non-uniform processing by altering the position of
such high- and low-energy intensity points over time.
[0336] One approach to overcoming this constructive and destructive
interference, is to design the tank such that energy reflected off
of the carrier and tank walls is directed towards the liquid
surface within in the tank. One such a design is disclosed in U.S.
Pat. No. 6,098,643, incorporated by reference herein for all
purposes.
[0337] Another conventional approach to reducing sonic shadowing of
conventional ultrasonic/megasonic processing systems, places the
piezoelectric crystals on more than one wall or location of a
processing tank or vessel, thereby ensuring areas shadowed from
receiving energy from one transducer would not also be shadowed
from receiving energy from another transducer. Tank designs
providing such features are disclosed in U.S. Pat. Nos. 5,279,316,
6,098,643 and 6,595,224, each of which is incorporated by reference
herein for all purposes.
[0338] Still another conventional approach to eliminating sonic
shadowing in ultrasonic/megasonic processing systems reduces the
size, shape, and/or number of supporting members of the substrate
carrier. For example, U.S. Pat. No. 6,209,555 teaches that
substrate carrier structural members be constructed out of flat
plate material having a thickness equal to a precise multiple of a
fractional wavelength of the sonic energy traveling through the
bath. This design promotes the transfer of energy across the
carrier structural supports of the carrier, rather than absorption
or reflection of incident ultrasound energy by the supports.
[0339] However, different systems utilize different frequencies of
applied ultrasound energy. Thus, one disadvantage of this
conventional approach is that the thickness of the plate components
would need to be matched with the particular frequency of the
applied ultrasonic energy, or vice-versa, reducing flexibility in
operation of the device.
[0340] Moreover, the speed of sound through a particular material
depends upon its composition. Thus another disadvantage of this
conventional approach is that the thickness of the plate components
would also need to be matched to the composition of the plate
components, or vice-versa, also reducing flexibility in
construction and operation of the device.
[0341] Therefore, a need exists for both tank designs and cassette
designs that overcome limitations inherent with current designs to
ensure uniform substrate processing with various frequencies of
ultrasonic energy.
[0342] A tank and a substrate carrier in accordance with an
embodiment of the present invention, for supporting either single
or multiple substrates in a megasonic processing bath, allow
ultrasonic energy introduced into the bath to be reflected from a
surface in order to process a substrate portion located behind a
sonically obstructing member. In accordance with one embodiment of
the present invention, the other side of the reflecting surface may
be in contact with a gas such as air, thereby ensuring reflection
of sonic energy incident at a wide range of angles. In accordance
with another embodiment of the present invention, where the other
side of the reflecting surface is in contact with a liquid,
reflection of the sonic energy will be achieved where the angle of
incidence is either less than the first critical angle or greater
than the second critical angle. The reflecting surface can be flat,
or partially or entirely curved (convex or concave) with a constant
or variable radius of curvature. Direction of reflected ultrasonic
energy to shadowed regions may be enhanced by controlling the
direction of flow of fluid within the tank.
[0343] A tool for processing substrates with ultrasonic energy in a
liquid bath in accordance with an embodiment of the present
invention, allows some amount of ultrasonic energy introduced into
the bath to be reflected off a reflecting surface to process a part
of the substrate located behind a sonically obstructing member. In
accordance with certain embodiments, the angle of incidence of the
sonic energy relative to the reflecting surface is less than a
first critical angle or greater than a second critical angle. In
other embodiments, a gas is present on the side of the reflecting
surface opposite to the incident ultrasound energy, such that
reflection takes place regardless of the angle of incidence.
[0344] FIG. 10 shows a simplified cross-sectional view of one
embodiment of a ultrasonic processing apparatus in accordance with
the present invention. Tank 1000 comprises walls 1020 in contact
with sidewalls 1030 that are angled with respect to one another.
Ultrasonic transducers 1040 are in contact with sidewalls 1030.
[0345] Ultrasound energy 1060 is emitted from transducers 1040
toward substrate 1080 supported by members 111. Energy 1060 is
reflected from walls 100 toward formerly shadowed portions S of the
substrate 1080 that would otherwise be shadowed by the presence of
the intervening support member 1101.
[0346] While the specific embodiment illustrated in FIG. 10 depicts
the reflecting surface of the tank as flat, this is not required by
the present invention. FIG. 11 depicts a simplified cross-sectional
view of another embodiment in accordance with the present
invention, wherein tank wall 1100, providing the reflecting surface
for incident ultrasonic energy 1102, is curved.
[0347] And while the embodiment of FIG. 11 shows the application of
ultrasonic energy to a reflecting surface that is concave and
continuous, this is also not required by the present invention.
Alternative embodiments could employ a reflecting surface that this
only partially curved, and which may exhibit a convex or concave
shape with a constant or variable radius of curvature.
[0348] Embodiments in accordance with the present invention
overcome many limitations of known designs for substrate processing
apparatuses. Specifically, embodiments in accordance with the
present invention may advantageously utilize angles larger than the
first critical angle and less than the second critical angle under
certain conditions. For example, when one side of the reflecting
surface is in contact with a gas (i.e. air surrounding the tank),
while the side of the reflecting surface receiving the incident
ultrasound energy is in contact with a liquid (i.e. inside the
processing bath), essentially all of the incident sonic energy is
reflected back into the liquid.
[0349] Such an embodiment is shown in simplified cross-section in
FIG. 12, where reflection of incident sonic energy occurs at the
curved wall 1200 of tank 1202 that is surrounded by gas. Unlike
conventional approaches, for example where the curved tank wall
comprises quartz and the incidence angle of sonic energy impinging
the wall is less than .about.26.degree., if a gas is present on the
opposite side of the reflecting surface, the ultrasound energy need
not be applied incident at any particular angle or range of
angles.
[0350] In accordance with still other embodiments of the present
invention, incident sonic energy could be reflected by walls of a
pipe-like structure that is filled with a gas, such that internal
reflectance is exhibited. In such embodiments, surfaces receiving
ultrasonic energy incident at angles larger than the first critical
angle and less than the second critical angle, could readily
reflect significant amounts of energy back into the liquid.
[0351] As described previously, depending on the material
composition of the solid reflecting surface, various amounts of
incident ultrasound energy may be directly absorbed by that
material and hence unavailable for reflection. Under almost all
conditions, polymer amorphous materials such as PTFE absorb energy
without significant reflection or transmission. By contrast, solid
crystalline materials such as quartz, or metals such as stainless
steel, either reflect or transmit sonic energy depending upon
incidence angles and whether only one side or both sides of the
reflecting/transmitting surface are in contact with a liquid. Many
construction materials, such as crystalline polymeric materials,
exhibit partial absorbency and partial reflection or
transmission.
[0352] Thus one factor for selecting the correct range of incidence
angles for applied ultrasonic energy is whether the reflecting
surface is in contact with liquid on one or both sides. If the
surface of the solid member has liquid on both sides, sonic energy
directed toward one of those sides could actually be transferred
across the member when the incident angle is larger than a first
critical angle and smaller than a second critical angle. Where the
applied ultrasound energy is incident at an angle of less than the
first critical angle or larger than the second critical angle,
reflection of the sonic energy predominates.
[0353] Conventional approaches for ultrasonic/megasonic processing
teach the presence of large, gently sloping convex surfaces
receiving energy within a narrow range of angles, or the presence
of flat planar surfaces designed to reflect energy to the surface
of the liquid to prevent interference. Embodiments in accordance
with the present invention, however, utilize surfaces of various
shapes. These shapes can include convex, concave, and planar or
even be comprised of composite designs. The radius of curvature can
remain constant or may vary.
[0354] Further, embodiments in accordance with the present
invention describe embodiments where energy may impinge surfaces at
different than the "critical angle or range of angles" described by
conventional approaches, and still readily reflect back into the
liquid. In accordance with alternative embodiments of the present
invention, a structure having a predetermined thickness may be
oriented to receive incident sonic energy outside of range of
critical angles. For quartz, those critical angles range between
about 26.degree. and 60.degree.. At angles less than about
26.degree. or greater than about 60.degree., most sonic energy
impinging the surface will be reflected from quartz surfaces. At
angles between about 26-60.degree. with liquid on both sides of the
reflecting member, significant energy can be transferred directly
across the member. When liquid only contacts one surface and gas
contacts the other, most sonic energy is reflected, regardless of
incident angle.
[0355] And while some prior art designs feature a water boundary
layer between the transducer and the bottom of the tank, this is
not required by embodiments in accordance with the present
invention, but may be employed when useful. For example, transducer
plates or individual piezoelectric crystals can be either directly
bonded to the wall of the tank or processing vessel, or fabricated
with a water boundary layer, without deviating from the spirit and
teachings in accordance with embodiments of the present
invention.
[0356] Further, different embodiments in accordance with the
present invention also envision utilizing of megasonic systems that
energize individual piezoelectric crystals within a transducer or
transducer array either simultaneously, or sequentially.
[0357] In accordance with another embodiment of the present
invention, supports for the wafer carrier may be constructed
integral to the tank wall from materials and designs promoting
transfer of significant amounts of sonic energy across those
structures.
[0358] For example, rather than using round solid rod stock to
construct the wafer carrier support, or relying upon a separate
reflecting surface within a tank, support stock exhibiting an oval
cross section may instead be used. Such an embodiment is shown in
simplified cross-section in FIG. 15.
[0359] A substrate holder member or substrate holder support 1500
having such an oval cross section may be oriented to ensure that a
significant amount of the surface area of the rod lies within a
range of angles between a first and a second critical angle,
thereby maximizing transfer of sonic energy directly across the
rod. Conversely, if the member or support is oriented such that a
significant amount of surface area of the rod lies at less than the
first critical angle or at greater than the second critical angle,
energy reflection is maximized.
[0360] Still other embodiments in accordance with the present
invention may utilize designs that reflect or re-direct fluid flow
smoothly in another direction. The reflector structures can be
planar, curved either convex or concave, oval, or otherwise
aerodynamic in shape to promote smooth fluid flow either past, or
off of, the surface of the structure.
[0361] By maintaining steady flow with smooth streamlines and
pathlines where the re-direction of fluid flow does not cause
turbulence or mixing, sonic energy follows along those lines. The
fluid flow can result from acoustic streaming, or from forced fluid
flow such as results from pumping.
[0362] Alternatively, if instead of being solid, the rod is hollow
and gas filled, the orientation of the oval could be such that the
incident sonic energy strikes the surface at greater than the first
but less than the second critical angle and still reflect the
majority of the energy. Other embodiments in accordance with the
present invention may thus utilize a hollow, gas-filled pipe or
other structure readily able to reflect sonic energy.
[0363] Such an embodiment is shown in simplified cross-section in
FIG. 13. Such a reflecting structure 1300 containing a gas 1302 in
accordance with embodiments of the present invention, allows for
significant reflection, rather than absorption or transmission, of
incident sonic energy 1304 impinging the surface of the structure,
regardless of whether the incident sonic energy lies within the
correct range of angles.
[0364] FIG. 13 shows the contrast in behavior between the hollow
reflecting structure 1300 and a solid reflecting surface 1306 that
is surrounded by liquid. Specifically, solid reflecting surface
1306 would tend to transmit ultrasonic energy incident at angles
within a critical range (for example 26-60.degree.), reflecting
ultrasonic energy incident at angles outside that critical
range.
[0365] In accordance with further embodiments of the present
invention, composite shapes may be employed to construct the wafer
carrier. For example, a rod with a semicircular cross section may
be used. Such an embodiment is shown in simplified cross-section in
FIG. 14.
[0366] In this embodiment, semi-circular rod 1400 may be oriented
such that the sonic energy 1402 impinges the flat planar surface
1400a at an angle between the first and second critical angles.
Energy will be transferred across the cross-section of the rod,
emanating from the curved rod surface 1400b. Other cross-sectional
profiles could exhibit surfaces having concave or convex
shapes.
[0367] B. Transfer of Sonic Energy Across Member
[0368] In a number of industries, ultrasonic energy may be applied
to a liquid bath in order to enhance substrate processing. The use
of the ultrasound energy has become especially common in the
manufacture of electronic components. As structures on the
substrates have become smaller and more delicate and easily
damaged, higher ultrasonic frequencies have been employed to remove
smaller contaminant particles without harming the underlying
structures.
[0369] Historically, multiple substrates (often as many as 25 to
50) have been held in a partially closed single cassette or carrier
in a generally parallel orientation, with uniform separation
between adjacent substrates. FIG. 16 shows a perspective view of
one such conventional cassette design.
[0370] During processing, this loaded cassette was lowered into a
liquid bath fitted with a means to introduce ultrasonic energy.
Many early tank designs featured the ultrasonic transducers located
generally in the bottom or sides of a rectangular tank. Since at
relatively low ultrasonic frequencies the energy pattern was almost
omni-directional within the tank, energy to interact with the
surfaces of the substrates was readily obtained, even when the
substrates were held in a partially closed cassette structure.
[0371] However, as the frequency of applied ultrasonic energy has
increased to near the megahertz range to remove smaller particles
and cause less substrate damage, the resulting energy waves have
become more collimated and travel through the bath in a generally
straight path. This allows the megasonic waves to rise up through
the bath between the substrates within the cassette at an
orientation that is generally parallel to the substrate
surfaces.
[0372] As the ultrasonic waves encounter obstructions such as
structural members of the cassette, the waves may typically either
be absorbed or reflected, resulting in an area behind the
obstructions to be shadowed from the ultrasonic energy. This
shadowing can in turn practice non-uniform processing across the
substrate surfaces, as some surface regions receive different
amounts of energy from other surfaces.
[0373] Previously, several approaches have been adopted to enhance
uniformity of the ultrasonic energy reaching the surface of the
substrates. One approach was to physically rotate the wafers.
However, this resulted in larger and much more complicated cassette
structures which were often expensive and cumbersome. The cassettes
were also potential sources of particle generation, as the
substrate edge rubbed against the structural members of the
cassette.
[0374] Another conventional approach to ensuring uniform
application of ultrasonic energy opens up the cassette by using
just three or four spaced-apart rods to support the substrates.
Such designs omit the conventional sidewall, plate and finger
members of earlier cassette designs. FIG. 17 shows a conventional
low profile cassette having four support rods.
[0375] While such cassette designs allowed more uniform energy
coverage, there remained shadowed areas located directly behind the
support rods. Additionally, reduced contact between the substrates
and the cassette resulted in the substrates being more easily
knocked out of proper orientation by bursts of fluid or bubbles.
Other conventional cassette designs open and close the holder
slightly (thereby increasing and decreasing the separation between
adjacent supports), so that substrates were constrained from
popping out of their desired resting slots within the carriers
during processing, yet were easily removable when processing was
completed. Examples of such a compliant cassette design are
presented in U.S. Pat. Nos. 6,041,938 and 6,153,533, incorporated
herein by reference for all purposes.
[0376] Still another conventional approach to cassette design was
to construct supports from materials of critical thickness to match
a multiple partial wavelength of the applied ultrasonic energy.
More specifically, physical theory teaches that thickness of the
cassette member should be equal to an even multiple of one-quarter
wavelength (n1/4.lamda., n=even integer.gtoreq.2) of the applied
megasonic energy. It was hoped that a significant amount of energy
would be transmitted across support members having this particular
thickness, thereby partially eliminating sonic shadowing behind
them. These cassette designs and the underlying theory required
applied ultrasonic energy to encounter the support members at
approximately a right angle (90.degree.) to the surface of the
cassette support member.
[0377] However, because frequencies varied between ultrasonic
systems, different thickness of supports were required. This
resulted in specific, unique holders for each ultrasonic processing
system. Thus a cassette constructed for one system operating at a
particular frequency could not be used in another system operating
at a different frequency. Additionally, flexibility in choosing
materials for cassette construction was reduced by the precise
material thickness as required. This translated into a more costly
and less flexible cassette.
[0378] In yet another conventional approach, a tank with slanted
sidewalls was specially designed and constructed, so that
ultrasonic energy encountered the substrate from two directions
nearly orthogonal to one another to eliminate any shadowed areas
behind obstructions. Such a design is disclosed in U.S. Pat. No.
6,098,643, incorporated by reference herein for all purposes.
[0379] However, a need still exists for a cassette that overcomes
limitations inherent with current designs to ensure uniform
substrate processing with various frequencies of ultrasonic
energy.
[0380] In accordance with one embodiment of the present invention,
a cassette or wafer carrier for supporting single or multiple
substrates in a megasonic processing bath allows a significant
amount of incident ultrasonic energy directed towards the supported
substrates to be transmitted across cassette structural supports in
the path of the sonic energy, thereby reducing sonic shadowing on
substrate surfaces lying directly behind the supports. An
embodiment of a carrier in accordance with the present invention
includes a side or bottom support connecting an end panel forming a
structure to maintain substrates in a desired orientation during
processing. At least one of the side and/or bottom support(s) of
the carrier is formed from plate components. A surface of the plate
components is maintained between first and second critical angles
relative to the surface of the megasonic transducer. The angle is
chosen to allow the maximum amount of projected megasonic energy to
be transmitted across the plate components. By not absorbing or
reflecting all of the incident ultrasonic energy, the plate
components do not create a total shadowing of supported
substrates.
[0381] The side and bottom supports of the cassette are formed of
relatively narrow plates of any convenient thickness, and may be
constructed of a single material or of a composite of multiple
materials. When positioned at the proper critical angle, a
significant portion of incident megasonic energy at any frequency
can be transmitted across the supports. The narrow plates can also
feature holes and cutouts to minimize disruption to uniform fluid
flow within the cassette and around the substrates held
therein.
[0382] Embodiments in accordance with the present invention
comprise substrate cassette or carrier for supporting either single
or multiple substrates in an ultrasonic or megasonic processing
bath. A cassette design in accordance with embodiments of the
present invention allows a significant amount of incident applied
ultrasonic energy to be transmitted across those structural
supports of the cassette in the path of the sonic energy, thus
reducing sonic shadowing on substrate surfaces directly behind such
supports is reduced over designs utilized by the prior art.
[0383] In accordance with embodiments of the present invention,
sonic energy applied incident to a planar surface of a supporting
member over a range of critical angles can result in significant
transfer of energy across the supporting member. Such supporting
members featuring an incident planar surface facing the
ultrasonic/megasonic transducer are referred to herein as "plate
components". Transfer of energy across plate components in
accordance with the present invention was observed even when the
thickness of the support components varied over a wide range.
[0384] The ability to transfer applied sonic energy across a member
is highly dependent upon the material comprising the member. For
example, members comprising the fluorpolymers PFA (perfluoroalkoxy)
and PTFE (polytetrafluoroethylene) likely would not allow transfer
of as much incident angled energy as a member comprising quartz.
With some materials, energy incident perpendicular to the surface
of a member having a thickness of an even multiple one-quarter
wavelength, could result in the transfer of greater energy than if
incident within the range of critical angles. This is particularly
possible if conversion between pressure and surface waves occurs
with energy applied at critical angles.
[0385] By failing to absorb or reflect all of the incident
ultrasonic energy, the plate components avoid total sound shadowing
of portions of substrates supported within the carrier. This result
is unexpected and in stark contrast with prior art designs
requiring the ultrasonic energy to impinge the surface of a plate
support member at approximately right angles or normal to the
support surface, and also requiring the cassette support members to
be of a very precise thickness specific to each applied ultrasonic
frequency.
[0386] FIG. 18A shows an end view of one embodiment of a cassette
1800 with the present invention having four support members 1802
for a flat bottom tank 1804 with megasonic transducer elements 1806
located on the tank bottom 1804a. FIG. 18B shows an isometric view
of the cassette shown in FIG. 18A.
[0387] Cassette or carrier 1800 includes at least a single side or
bottom support 1802 connecting to at least a single end panel 1808
forming a structure to maintain substrates 1810 in a desired
orientation during processing. Generally, two, three, or in some
cases more side/bottom supports will be used to provide adequate
structural strength for the cassette.
[0388] At least one of the side/bottom supports 1802 of the carrier
1800 is formed of plate components. A plate component comprises a
member having a planar surface incident to the applied sonic
energy. FIG. 18C shows an expanded side view of a rectangular plate
component support member 1802 having periodic slots 1812 to contain
the edges of multiple substrates 1810.
[0389] In order to effectively transmit sonic energy across plate
components of moderate thickness, the surface of the plate
components are maintained within a correct range of angles with
respect to the surface of the megasonic transducer. In accordance
with embodiments of the present invention, the normal to the
incident planar surface of the plate component is maintained at an
angle between a first and second critical angles relative to the
normal to the surface of the megasonic transducer.
[0390] With structures such as wafers comprising silicon, these
first and second critical angles are typically between about
18.degree. and 58.degree., respectively. Other materials may
exhibit different ranges of critical angles.
[0391] This orientation is shown in FIG. 18A, wherein the incident
surfaces of supporting plate components 1800 are oriented at an
angle of 38.degree. with respect to the megasonic transducer
elements 1802 present at the bottom of processing tank 1804. The
normal of the incident surface of the plate components is thus
orientated at an angle of 52.degree. with respect to the megasonic
transducer elements.
[0392] FIG. 19 shows an end view of an alternative embodiment of a
cassette in accordance with the present invention. The cassette of
FIG. 19 is similar to that of FIG. 18a except that the relative
orientations of two of the supporting plate component members 1900
and 1902 is reversed. However, the incident surfaces of all the
plate components remain inclined within the critical angle range of
about a relative to the plane of megasonic transducers located in
the bottom of the processing tank.
[0393] While FIGS. 18A-19 show cassette structures formed from
plate components comprising single flat surfaces, this is not
required by embodiments in accordance with the present invention.
FIG. 20 shows a cross-sectional view of a three-support
arrangement, with bottom support 2000 comprising an inverted
V-shaped member for a flat bottom tank with megasonic transducer
2002 positioned on the bottom.
[0394] As shown in FIG. 20A, two plate components 2004 having
rectangular cross sections are bonded together along one lateral
surface, forming a V shaped cross section allowing the use of thin
plate components not otherwise exhibiting adequate strength. In
order to function as a cassette support, the normal to one of the
surfaces of each individual piece would form an appropriate angle
with the normal to the surface of the megasonic transducer.
[0395] FIG. 20B shows an enlarged cross sectional view of the V
shaped support of the cassette of FIG. 20A illustrating a correct
angle with respect to the normal of both the support and transducer
surfaces. Apex 2000a of V shaped support structure 2000 could point
either toward or away from the plane defined by the surfaces of the
ultrasonic/megasonic transducers.
[0396] And while FIGS. 18A-B and 19 show the support plates as
having a rectangular cross section that leads to the incident
ultrasonic energy exiting the back surface of the plate at the same
angle, this is not required by the present invention. In accordance
with other embodiments, one surface of the plate could be flat
while the opposite surface could be curved, either concave or
convex or in another more complex shape.
[0397] FIG. 21 shows an expanded cross sectional view of a
non-rectangular support member 2100. In the case of FIG. 21, the
angle at which the energy leaves the plate could be different from
the angle of incidence.
[0398] Many types of materials could be used to form the plate
component supports in accordance with embodiments of the present
invention. Selection of material type may be based upon its desired
physical characteristics such as strength and flexibility. In
general, more sonic energy can be transmitted across thicker
members exhibiting lower energy attenuation, than members
exhibiting higher energy attenuation.
[0399] For example, greater sonic energy may be transmitted across
metallic or highly crystalline materials of the same thickness
exhibiting relatively low energy attenuation, than can be
transmitted across amorphous polymer materials of the same
thickness exhibiting relatively large energy attenuation.
Relatively stiff materials such as quartz allow greater transfer of
energy as compared with more flexible materials such as
polytetrafluoroethylene (PTFE). A discussion of the attenuation of
sonic energy by various materials is presented by Buckin and
O'Driscoll, "Ultrasonic Waves and Material Analysis: Recent
Advances and Future Trends", LabPlus International (June 2002), and
also by McClements, "Ultrasonic Measurement in Particle Size
Analysis", from Encyclopedia of Anayltical Chemistry, Robert A.
Meyers, Ed. (John Wiley & Sons), both of which are incorporated
by reference herein for all purposes.
[0400] Moreover, support plate components in accordance with
embodiments of the present invention need not be created from a
single material. Composite structures fabricated from multiple
materials can be used, keeping in mind considerations regarding
physical properties and energy attenuation just discussed. Thus in
accordance with certain embodiments of the present invention, one
material exhibiting desired structural properties with low energy
attenuation could be combined with another material exhibiting
excellent chemical resistance and relatively high energy
attenuation, to produce a support element exhibiting acceptable
structural strength and energy transfer properties.
[0401] Such an alternative embodiment is shown in FIG. 22. FIG. 22
illustrates an expanded cross sectional view of a typical
rectangular support member 2200 comprising a composite of first
inner material 2202 and second outer material 2204. While the
composite support structure of FIG. 22 is depicted as being
comprised of solid materials, this is not required by the present
invention. In accordance with alternative embodiments, the support
could comprise a solid outer shell filled with fluid. In this
manner, hollow tubes exhibiting various cross-sectional profiles
could be utilized.
[0402] In accordance with other alternative embodiments in
accordance with the present invention, the plate components can
contain holes and cutouts to minimize disruption of uniform fluid
flow within the cassette and about the substrates being treated.
With substrate processing, non uniform localized fluid velocities
and energy densities can lead to nonuniform substrate processing,
whether that processing be cleaning, etching or other substrate
surface modification. Thus for certain applications it will be
preferred that the substrate support feature holes or cutouts.
[0403] The sizes of such openings or cutouts can vary. In some
circumstances, the openings can be relatively large, or
substantially greater than one wavelength of the applied energy.
For processing with gas saturated solutions, depending upon the
surface tension of the processing fluid, gas bubbles generated
during processing may lodge in small-sized openings, interfering
with the effective transmission of sonic energy through the
apparatus to the substrate.
[0404] Accordingly, FIGS. 23A and 23B show perspective and cross
sectional views, respectively, of an inverted V-shaped support
member 2300 comprising plate components 2302 having slots 2304 for
supporting the edges of wafers. Support member 2300 further defines
cut-outs 2306 between plate components 2302. Cut-outs 2306 promote
circulation of processing fluid between the supporting plate
components during use.
[0405] In other embodiments, it may be desirable for the cutouts or
openings to be sized approximately equal to or less than the
wavelength of the applied sonic energy. In such a configurations,
incident ultrasound energy encountering the cassette may be
diffracted. Such diffraction can potentially have beneficial
results, enhancing the uniformity or effectiveness of ultrasonic
processing.
[0406] Certain of the embodiments previously shown and described
depict a wafer support structure comprising side and bottom
supports are formed of relatively narrow plate components, this is
not required by the present invention. While in certain embodiments
relatively narrow plate components may be preferred to minimize
localized disruption in fluid flow, in other applications much
wider plates could alternatively be used when different fluid flow
was desired. Such an alternative embodiment is depicted in FIG. 24,
which shows a cross sectional view of a wide rectangular support
member 2400 for a flat bottom tank having a bottom mounted
transducer 2402.
[0407] And while the embodiments of FIGS. 18A-19 illustrate a wafer
support structure featuring plate components oriented relative to
sonic transducers positioned in a common plane at the bottom of a
processing tank, this configuration is not required by the present
invention. FIG. 25 illustrates an alternative embodiment in
accordance with the present invention, wherein cassette 25000
comprises a plurality of vertically-oriented support plate
components 25002. Processing tank 25004 features V-shaped bottom,
with transducers 25006 mounted on sides 25004a of tank bottom. The
sides of the processing tank are inclined at the critical angle
relative to the vertically-oriented support plate components
comprising the cassette.
[0408] Embodiments in accordance with the present invention offer a
number of possible advantages over conventional approaches. For
example, unlike conventional substrate support designs, the
thickness of the plate component support member in accordance with
the present invention need not be any specific multiple of applied
ultrasonic wavelength. Thus as frequency changes, the thickness of
supports for the new invention need not be changed.
[0409] In accordance with alternative embodiments of the present
invention, support members may be designed to produce steady flow
with smooth streamlines (pathlines) in fluid flowing past them.
Such smooth fluid streamlines around support structures allow
applied sonic energy to flow around the structures, instead of
being transferred directly across or through them. The
cross-sections of support members may be hydrodynamically designed
to offer minimum drag, allowing fluid streamlines to be smoothly
split at the leading edge, and then smoothly rejoined at the
trailing edge, without the formation of eddies or turbulence. In
this manner, members in accordance with embodiments of the present
invention can be shaped to allow significant amounts of sonic
energy to essentially "bend" around them.
[0410] The flow of sonic energy in a fluid causes movement of fluid
referred to herein as "acoustic streaming". This acoustic streaming
is comprised of macroscale streaming involving bulk fluid flow and
movement, as well as microscale streaming (localized streaming of
several types). When fluid streamlines (pathlines) containing
acoustic streaming of sonic energy are smoothly split and then
smoothly recombined, much of the energy remains with the fluid
flow, effectively allowing transfer of sonic energy around
appropriately-shaped structures.
[0411] An alternative embodiment of the present invention employing
such a hydrodynamic support member is shown in simplified
cross-section in FIG. 26. Support member 2602 is positioned within
the tank between vibration member 2606 and substrate 1104 that is
to be processed. Support member 2602 exhibits a teardrop shape that
promotes hydrodynamic fluid flow such that illustrated streamlines
2600 are split and recombined around support member 2602 without
turbulence and mixing. Shaped support member 2602 need not be
oriented at some critical angle, but is shaped so that it smoothly
separates and recombines streamlines/pathlines.
[0412] The particular support member 2602 shown in FIG. 26
represents a portion of a substrate support structure that is not
in direct physical contact with the supported wafer 2604. In
accordance with alternative embodiments, however, the
hydrodynamically-shaped support member could be in direct contact
with the wafer, for example supporting the wafer on the tip of the
teardrop-shaped member 2602.
[0413] And while support 2602 is shown as being solid, this is not
required. The support could optionally be comprised of a hollow
tube. The tube could then be either empty or fluid filled. The
fluid could either be a gas or a liquid. As long as the
cross-sectional profile of the member did not lead to disruption of
streamlines resulting in mixing, significant transfer of sonic
energy around the support member could occur.
[0414] Hydrodynamic structures may be oriented relative to the
direction of acoustic streaming flow, so that turbulence is not
created at their trailing edge. Orientation of the hydrodynamic
structures should also take into account any substantial fluid
velocity components not caused by the sonic energy, for example a
flow of fluid caused by pumping action. Care should accordingly be
employed in designing the processing tank to minimize the adverse
effects of the hydrodynamic flow on the acoustic streaming, and
vice-a-versa.
[0415] Acoustic streaming in accordance with embodiments of the
present invention may not be limited to transmission of ultrasound
energy in a direction across a supporting substrate. FIG. 27 shows
a simplified cross-sectional view of an alternative embodiment in
accordance with the present invention, wherein the flow 2700 of
processing fluid 2702 encountering structure 2704 is diverted
against substrate 2706 that is to be processed. This deflection of
fluid flow 2700 by structure 2704 may be accompanied by a similar
deflection of sonic energy 2708 emitted by vibration member 2710
against substrate 2706, according to an acoustic streaming
mechanism.
[0416] To summarize, sonic energy applied perpendicular to a
surface of a member having a thickness of an even multiple of
one-quarter the wavelength of applied energy, results in energy
being transferred across the member. If the sonic energy is applied
between first and second critical angles relative to the surface of
the member, energy can be transferred across a member if there is
liquid on both sides. If there is gas present on both sides of the
member, mode conversion can arise. If there is liquid on one side
of a member and liquid on the other side of the member, energy will
be reflected from the member regardless of the angle of incidence
on the liquid side.
[0417] C. Improved Near Field Uniformity
[0418] In a number of industries, ultrasonic energy may be applied
to a liquid bath in order to enhance processing of substrates
positioned therein. The use of the sonic energy has become
especially common in the manufacture of electronic components. FIG.
28 shows a simplified cross-sectional view of member 2800 to be
processed, immersed in a surrounding liquid 2802 in a conventional
process tank 2805. Beam(s) 2804 of ultrasonic energy is applied
from vibration source 2806 to liquid 2802, which in turn transmits
the energy to member 2800. The interface between liquid 2802 and
the surface of member 2800 in turn vibrates in response to the
ultrasonic energy received. Cavitation, microcavitation, acoustic
streaming, and microstreaming of various types can all occur at or
near this interface and can be useful in promoting cleaning and
processing of member 2800.
[0419] The near field 2807 is a region extending a distance X from
the vibration source 2806 into the surrounding bath. The near field
2807 is characterized by nonuniformity of the resulting energy
field, as shown in FIGS. 29A and 29B. The distance that the energy
field extends into the liquid and degree of non-uniformity that it
exhibits is a function of frequency, intensity, and vibration
member design. Specifically, as standing waves and various
interference patterns between interacting waves develop in the near
field region 2807, locations of high and low energy intensity
develop and are dispersed throughout the near field region
2807.
[0420] While not wishing to be bound by any particular theory of
operation, the following examples and explanation are discussed for
reasons of understanding only. It is thought that these points of
high and low energy intensity are generally created by standing
wave and interference patterns leading to cancellation or
reinforcement of energy waves. These interference patterns can
arise because there can be slight localized material variation or
non-uniformity from point to point within a single vibration member
piezoelectric crystal. This localized nonuniformity can lead to not
all points on the face of the vibration member resonating at
identical frequency. Instead of the entire piezoelectric crystal
vibrating as a single uniform vibrating point source, it can
vibrate as a collection of many individual point sources. Thus, at
a given generator frequency, not all of the points will vibrate
with the same intensity, as some will be excited at other than
their resonance frequency. There can even be slight phase shifts of
the applied or driving voltage between points on an individual
crystal introduced by the crystal's non-uniformity.
[0421] Even if all of the point sources on a vibration member
crystal did vibrate at the same resonance frequency, with the same
amplitude, and were energized completely in phase with each other,
the distance from each point source to a selected point on the
surface of member 2800 within the bulk liquid 2802 can be
different. The waves emanating from each point source into the
liquid tend to spread out as they travel away from the vibrating
member to varying degrees based on a number of factors, including
frequency, intensity and shape of the vibration member.
[0422] At a given frequency, a vibrating member will generate a
pressure wave in the liquid with a well-defined wavelength. If
distances from two closely-spaced point sources on a single crystal
to a given position within the bath are different, even if the
waves are generated in phase with each other, they would reach the
given point at slightly different times. Therefore the waves from
the individual point sources can arrive at selected points near the
surface of member 2800 in varying degrees of phase alignment or
agreement.
[0423] At a certain distance from the vibrating element (the
dividing line between the near field and the far field shown in
FIG. 28, the intensity of the resulting energy field becomes
relatively uniform. The region outside of the near field 2806 is
known as the far field 2810. The far field 2810 is characterized by
field uniformity and field attenuation with distance from the
vibrating member 2806. Specifically, the intensity of the energy
field drops off at a fairly uniform rate, depending upon localized
fluid properties as distances from the surface of the vibration
element increase. The length of the near field, or distance that it
extends into the liquid from the vibration member, increases with
increasing frequency and increasing applied intensity.
[0424] The quality of interaction between a substrate and a
surrounding processing bath is partly dependent upon the energy
field at the substrate/bath interface. Thus where ultrasonic energy
is applied to a substrate within a processing bath within the near
field region, non-uniform energy in the near field may result in
non-uniform cleaning and processing of substrates or parts of
substrates.
[0425] Accordingly, there is a need in the art for methods and
apparatuses for applying ultrasound energy to substrates which
ensures uniformity of substrate processing.
[0426] The frequency, power, phase between simultaneously fired
crystal elements, and/or pulse width of ultrasonic energy applied
to a substrate within a processing bath, may be varied over time in
an attempt to make the near field more uniform. This variation over
time causes movement of the -location of points of constructive and
destructive interference, or localized points of high and low
energy intensity in the near field. As a result of the changing
character of the applied ultrasonic energy, the overall or
effective intensity of energy in the near field, and hence
processing of the substrate in near field regions, is rendered more
uniform.
[0427] As shown in FIG. 30, megasonic energy source 3000 comprises
transducer 3002 including multiple individual piezoelectric
crystals 3004. In accordance with embodiments of the present
invention, megasonic energy source 3000 is operated to sequentially
fire these individual piezoelectric crystals 3004. Specifically,
voltage at a selected or resonance frequency and magnitude is
applied to individual piezoelectric crystals 3004 is varied in a
uniform or predetermined manner about this resonance frequency and
selected magnitude. Typically, a different frequency is chosen for
each piezoelectric crystal in an array in an attempt to match that
crystal's resonance frequency that will maximize that crystal's
performance. The magnitude of the applied voltage determines the
vibration intensity generated by each crystal.
[0428] As a result of varying the frequency and magnitude of the
voltage applied to the piezoelectric crystals, the frequency and
intensity of oscillation produced within the bath by of the
piezoelectric crystals 3004 also varies.
[0429] Like phased array radar with its multiple transmitter
points, each piezoelectric crystal is comprised of multiple
vibration point sources. When the frequency/phase between the
various emitters in the phased array is varied, the direction of
the energy beam sent out from each emitter changes slightly.
[0430] If the frequency were cycled, the emanating beams would tend
to move or "flicker" analogous to the flame of a burning candle.
Where this "flickering" occurs with the ultrasonic waves in a
process bath, it would have the effect of moving the various nodes
(points of high energy intensity) and nulls (points of low energy
intensity) from position to position within the near field region.
Historically, megasonic processing has emanated energy waves out
parallel to the surface of a substrate. For embodiments with
significant energy transfer directly across the thickness of a
substrate member, varying the vibrational frequency also modifies
the ultrasonic/megasonic near field emanating from the exiting side
or surface of the substrate that is to be processed.
[0431] Absent such frequency variation, the non-uniform energy
field known as the near field would be established off of the front
side of the vibration member, or exiting side of a substrate,
extending for up to several inches into the bulk liquid in the
tank. This non-uniform field would result in non-uniform cleaning
and processing of substrates or parts of a substrate situated
within the non-uniform field area.
[0432] When a standing wave is generated, points of maximum
reinforcement (nodes), can have intensity twice as large as the
intensity seen in a single free wave (amplitude of first wave adds
to amplitude of second wave). At points of maximum cancellation
(nulls), the wave intensity goes to zero (first wave cancels the
second wave). At constant frequency, these points of cancellation
and reinforcement do not move within the bath. It is only when the
frequency/phase/power etc. are varied, that the location of these
points move. Various methods and apparatus for measuring field
intensity and even localized field intensity are commonly know in
industry and academia and are readily available. For measuring
gross values of intensity, hydrophones are often used.
[0433] As the frequency varies slightly, the intensity, direction,
and wavelength of the wave of ultrasonic energy also varies
slightly. This slight variation in direction, intensity, and
wavelength can result in changes in the position of points of
positive and negative reinforcement in the near field, but there
may also even be conditions that prevent the formation of standing
waves altogether. As these locations of high and low energy
intensity change, or the magnitude of differences between them are
minimized, the entire surface of a substrate is more likely to
experience similar processing. By exposure to both high and low
intensity points, as these points move about the substrate surface,
the resulting processing is likely to be more uniform than if those
points were stationary within the bath.
[0434] In a sense, this movement of high- and low-energy points
generates a near field region exhibiting more energy uniformity or
smoothness over a period of time. This time period is a function of
the rate at which the frequency and magnitude of the applied
voltage is varied. For example, some piezoelectric crystals may be
excited adequately with a constant applied voltage of 500 VAC
operating at a frequency of 750 kHz. Points of high and low energy
intensity will be generated in the near field region and perhaps
even in the far field region depending upon tank
characteristics.
[0435] For example, instead of being held constant, the frequency
of the applied voltage may be cycled between values. If cycled
between 748 and 752 kHz once per second, the points of high and low
intensity established in the liquid when the frequency is 748 kHz,
will switch every second to the new positions associated with 752
kHz. If the rate of change of the frequency of the applied voltage
is now raised to one hundred thousand cycles per second, rapid
switching back and forth, or movement between node positions
associated with 748 kHz to node positions associated with 752 kHz,
will occur.
[0436] Alternatively switching could occur between five discrete
frequencies, for example of 748, 749, 750, 751 and 752 kHz. The
location of nodes and nulls formed at each of these five discrete
frequencies will differ. As the frequency is changed from one value
to another, the nodes and nulls will move accordingly. The more
rapidly the switching of frequency from one value to another, the
faster the nodes and nulls will change position.
[0437] Further alternatively, frequencies could be altered
continuously and be swept over a range, with nodes and nulls in the
near field continuously moving from one position to another. In
such a continuously varying approach, standing waves would not be
established in the far field even for very poorly designed tanks,
since standing waves in the far field could not even form under
such varying conditions.
[0438] The position of nodes and nulls within the near field is
also a function of field intensity, which is determined by the
magnitude of the applied voltage to each piezoelectric crystal.
Therefore, varying the magnitude of the applied voltage can also
result in movement of the nodes and nulls generated. If such a
change in intensity occurred continuously, the nodes and nulls may
appear to effectively disappear.
[0439] In accordance with one embodiment of the present invention,
the backside of the wafer being treated may be placed in direct
contact with a modified transducer, thereby forming a single
vibration element. The megasonic generator is a sequenced firing
generator that sequentially fires the individual piezoelectric
crystals comprising the modified transducer. Such an embodiment of
a modified transducer is shown in FIG. 31.
[0440] In the embodiment of FIG. 31, the voltage frequency applied
to each crystal 3104 by the generator 3100 is varied, either
randomly or in predetermined stepwise or other manner. This voltage
frequency variation by a predetermined amount not only increases
the uniformity of the near field emanating from the front side of
the wafer 3106, but also increases the likelihood that each portion
of each crystal is operating about its resonance frequency for at
least part of the time. The frequency of this variation in
frequency of the applied voltage could range from just a few cycles
per second to the megahertz range. It could vary both above and
below a set point frequency by an equal amount in a smooth
sinusoidal or other manner, or could vary randomly within some
preset range.
[0441] Alternatively, it could vary stepwise in a cyclic manner.
For example, the frequency of the applied voltage could be bumped
up above a set point frequency by a certain number of kilohertz in
a single step. The frequency could then be dropped back to the
original set point frequency in smaller increments over 10 or more
steps. These steps could occur at a rate of just a few per second
to a rate of over a megahertz.
[0442] Piezoelectric crystals often can be excited at more than a
single frequency. These various frequencies at which the crystals
can be excited are often referred to as resonance harmonic
frequencies. As such it is possible to have the frequency variation
of the applied voltage occur between different harmonic frequencies
and not merely cycle about a single resonance frequency.
[0443] In addition to varying the frequency of vibration of the
piezoelectric crystals of the transducer by varying the frequency
of the applied voltage supplied by the generator, the pulse width
of the energy beam can be varied. This can be accomplished by
rapidly switching the generator output voltage on and off in either
a random or in a predetermined stepwise or other manner. Pulse
widths can vary from being continuously on to switching on and off
at a frequency in the megahertz range. This switching frequency
need not be constant and can also vary in a stepwise or other
manner.
[0444] It is possible to vary the intensity and the uniformity of
the ultrasonic energy in the near field by varying the magnitude of
the applied voltage. As with the variation in frequency of the
applied voltage, this variation in the magnitude of the applied
voltage can take any number of the forms of continuous or discreet
variation as described above.
[0445] It is further possible to vary simultaneously the frequency
and power or magnitude of the applied energy, or even the
frequency, intensity and pulse width. Varying both the frequency
and the magnitude of the applied voltage either randomly or in
predetermined stepwise or other manner, or additionally varying the
pulse width as described above accomplishes this.
[0446] Frequency, power and pulse width variation is not limited to
sequenced firing of individual piezoelectric crystals. For systems
designed for continuous firing of multiple crystals simultaneously
at a single frequency, frequency variation could also be
advantageously applied. In such an embodiment, at a single
frequency not all crystals are firing at their individual resonance
frequency. As frequency varies, the likelihood of all of the
crystals and perhaps all points of all crystals being fired at
their individual resonance frequency for at least part of the time,
is increased. And, similar to embodiments employing sequenced
firing of piezoelectric crystals, the near field will also
experience an averaging or smoothing out as points of high and low
energy intensity change location.
[0447] Further, if multiple crystals are fired simultaneously, it
is possible to vary the phase between the waves generated by each
of the crystals. Varying the phase angle between these multiple
crystals fired simultaneously in a random fashion, or varying
according to some predetermined stepwise or other manner, can
result in enhanced uniformity as nodes and nulls move about or are
perhaps even prevented from forming.
[0448] Use of frequency, power, and pulse width variation to smooth
non-uniformity of the near field is not limited to having the
substrate contact, or become part of the transducer vibration
member. Traditional megasonic cleaning systems apply the energy to
a member in a tank, with energy projected parallel to the surface
of a substrate, as shown in FIG. 32. In such systems, the
substrates 3200 are not positioned too closely to the vibration
surfaces 3206 because of the non-uniform cleaning experienced in
this region. This distancing from vibration surfaces increases the
size of the processing tanks, consuming more chemicals than
architectures allowing close proximity of the substrate to the
vibration surface. By varying the frequency, intensity, and pulse
width of the megasonic beam, "effective" field uniformity is
improved in this region.
[0449] In accordance with another alternative embodiment of the
present invention, using a sequenced firing of the vibration
elements, a second element could be positioned at a distance from
the near field region of the first element. This second element
would send megasonic energy into the near field region of the first
element when the first element is inactive. The second element
would thus provide megasonic energy in its far field, to clean and
process substrates present in the original near-field of the first
element.
[0450] Such a combination of first and second vibrating elements
could be arranged in either a flat-bottomed or V-bottomed tank. In
the case of a flat-bottomed tank of FIG. 33, the second elements
3304b could be included on a vertical wall at or near a 90.degree.
angle relative to the first transducer element(s) 3304a located on
the bottom of the tank 3300. Such an embodiment is shown in FIG.
34. An acceptable tank design for this embodiment is disclosed in
U.S. Pat. No. 6,595,224, incorporated by reference herein for all
purposes.
[0451] In the case of a V-bottom tank, the first transducer
elements 3404a can be located on one angled bottom wall while the
second transducer elements 3404b can be located on the other as
shown in FIG. 34. Such a tank design is disclosed in U.S. Pat. No.
6,098,643 and is herein incorporated by way of reference for all
purposes. Additionally, the transducer plates shown in FIG. 34
could be widened, or additional elements could be added to the
overhang above the V-shape to allow energy to be projected across
the face of the sloped side walls of the tank. Such alternative
embodiments are shown in FIGS. 35 and 36, respectively.
[0452] The design approach shown in FIGS. 33-36 could also be
applied to a megasonic unit firing all vibration elements
simultaneously. Historically, when beams were directed from
multiple angles simultaneously, some beam interference and
resulting cancellation and reinforcement was expected. However, by
varying the frequency, power, phase angle, or pulse width of the
applied ultrasonic energy, points of high and low intensity could
be moved within the bath, effectively rendering the overall bath
more uniform for purposes of processing or cleaning.
[0453] In accordance with still another embodiment of the present
invention, a single substrate is oriented such that its surface is
held parallel to the surface of a vibration member. While either
partially or fully submerged in the processing fluid, the substrate
may be brought into close proximity to, but not in direct contact
with, the vibration member. The distance separating the vibration
member and the substrate could range from between about 1 .mu.m to
about several inches or more. Such an arrangement is shown in FIG.
37.
[0454] In FIG. 6A, a similar concept is shown, except that
substrate thickness should approximate an even multiple of
one-quarter the wavelength of the applied megasonic energy in order
to accomplish significant transfer of energy across the substrate.
In general, the thickness of semiconductor wafers would not promote
transfer of energy at frequencies conventionally employed in single
wafer megasonic processors. With primarily reflection of megasonic
energy occurring off the wafer surface back towards the vibration
member, a very strong and pronounced standing wave can be
established. Because the wafer is held parallel to the vibration
member, this standing wave is comprised of a fixed array of points
of constructive and destructive interference, leading to less than
optimal wafer processing.
[0455] To overcome the disadvantages of such non uniform energy
exposure during processing, embodiments of the present invention
disclose techniques for eliminating points of constructive and
destructive interference, minimizing the energy difference between
them, or changing over time the physical location of their
location. One approach to enhance uniformity in such a situation is
to vary frequency or intensity of applied megasonic energy in an
appropriate manner. Another approach to enhancing uniformity is to
change a position of the wafer relative to an energy source, for
example by wobbling or rocking. Both approaches are described in
detail in earlier sections of this application.
[0456] FIG. 38 shows an embodiment in accordance with the present
invention including two vibration members 3802a and 3802b, aligned
parallel to each other and spaced apart with a substrate 3800
inserted between them. The gap between the vibration members is
completely filled with processing fluid 3802. The processing fluid
can either be flowed from one end of the vibration members to the
other past the substrate, or may enter into the gap between the
vibration members through slots in one or both vibration members
positioned between adjacent piezoelectric crystals 3804.
[0457] Varying the frequency and/or power level can act to even out
the negative effects of the non-uniform near field region, which
results in non-uniform processing of the substrate. And
serendipitously, the variation of some combination of frequency,
power, phase angle, and pulse width can also reduce damage to
sensitive structures on the surface of the substrate both in the
near and far fields.
[0458] In yet another embodiment in accordance with the present
invention, the substrate may be positioned non-parallel to the
vibration member, and separated from a vibration member by a
distance. When the angle between a substrate surface and the
surface of a vibration element lies within a certain range, a
significant portion of impinging ultrasonic energy is transferred
across the thickness of the substrate. However, at angles outside
this critical range, the majority of the energy is reflected off
the surface of the substrate. In either case, embodiments in
accordance with the present invention may help to minimize adverse
affects of near field non-uniformity where frequency, power, phase
angle, and pulse width are varied appropriately.
[0459] In any of the embodiments in accordance with the present
invention, the substrates could exhibit motion relative to the
surface of the vibration element. This relative motion could take
the form of rotation, lateral movement, moving closer to or farther
away, or a combination of rotation, lateral, and other movement.
Motion of the substrate relative to the vibration element can be
uniform or variable, depending upon the stage of processing.
[0460] For example rapid rotation of a single wafer not submerged
in a tank, but instead wetted by spray nozzles, may be desired for
a resist strip processing step. The rapid rotation tends to reduce
the thickness of the liquid layer on the substrate surface. A
thinner liquid layer could allow faster diffusion of a gas
component, such as ozone gas, from the surrounding atmosphere to
the surface of the wafer where it may participate in a desired
reaction.
[0461] Conversely, in a later step it may be desirable to clean a
wafer in an SC1 solution and rinse the wafer in DI water by fully
submerging the wafer in a liquid in a tank. A much slower rotation
or even back and forth motion may be preferable for these two
steps. In a processing step wherein the substrate is submerged, any
components desired for reaction at the surface of the wafer would
need to be dissolved in the liquid. Rotation of the submerged wafer
would not appreciably reduce the hydrodynamic boundary layer on the
surface of the wafer over the thickness of the acoustic boundary
layer formed by the use of ultrasonic energy, either alone or in
combination with the rotation, unless the rotation speed was very
high. Therefore, rotation would not appreciably increase the
diffusion rate of a dissolved species in the bulk fluid to the
substrate surface.
[0462] Additionally, various chemicals could contact the substrate
surface before, following, or during the application of ultrasonic
energy of varying frequency, power, or pulse width. For example, a
fine mist or spray of a liquid comprised essentially of DI water
could be applied to the substrate surface simultaneously with the
application of ultrasonic energy, while an oxidizing gas is
introduced into the atmosphere surrounding the substrate.
Alternatively, the liquid could be comprised of an organic acid or
an inorganic base dissolved in DI water.
[0463] While the preceding embodiments and examples have dealt with
the application of ultrasound at frequencies commonly used in the
manufacture of electronic component substrates, the invention is
not limited to these frequencies or components. For example,
frequencies outside the traditional ultrasonic and megasonic
processing ranges may also be used advantageously. Historically,
frequencies in the range from a few kilohertz to several hundred
kilohertz have often been referred to as ultrasonic processing
frequencies and those from a few hundred kilohertz to several
thousand kilohertz have been referred to as megasonic processing
frequencies. For purposes of the present invention, the terms
ultrasonic and megasonic frequency can refer to any frequency
range. Frequencies much higher than these, even in the conventional
microwave processing range, can also be used without departing from
the spirit and intent of this invention.
[0464] An example of microwave processing of various substrates is
disclosed in U.S. patent application Ser. No. 10/150,748 and is
hereby incorporated by way of reference for all purposes. Various
embodiments of this incorporated application disclose the
processing of substrates even when they are not fully or even
partially submerged in a liquid. With microwave processing, various
standing wave patterns can be established in the processing
chamber. By varying the frequency, power, phase angle of multiple
sources, and/or the pulse width of the applied energy, the negative
impact of these standing waves with their attendant points of high
energy and low energy can be minimized or even eliminated.
[0465] Additionally, processing can occur at pressures above
atmospheric pressure and include the application of megasonic
energy. Such processing is disclosed in U.S. patent application
Ser. No. 10/456,995, herein incorporated by way of reference for
all purposes. Also, various processing chemistries and processing
sequences are disclosed which can be advantageously used with the
instant invention of this current application.
[0466] Further, substrates other than electronic components can
also be advantageously treated with sonic energy in accordance with
embodiments of the present invention. Such substrates could come a
wide variety of industries, such as from the disk drive, optics,
flat panel, medical, microbiological and pharmaceutical industries
just to name a few.
[0467] In addition, embodiments in accordance with the present
invention are not limited to changing the character of sonic energy
in the manner described to achieve more uniform processing in the
near field. In accordance with alternative embodiments of the
present invention, the character of other forms of applied
radiation, such as microwave radiation, may also be varied in the
manner described to achieve more uniform processing of a substrate
receiving the applied radiation. Moreover, multiple forms of
radiation could be applied simultaneously or in sequence to
accomplish the desired processing.
[0468] While the most common vibration elements used in electronic
component substrate processing are comprised of piezoelectric
crystals bonded to a plate-like structure, other types of crystals,
materials, and structures, may also be used without departing from
the spirit and teachings of this instant invention. Many of these
alternative transducer element materials are well known in
industry.
[0469] Moreover, other methods of applying pressure pulses to the
processing fluid and processing fluid/substrate interface may also
be used to advantage in accordance with embodiments of the present
invention, including but not limited to pulsed high pressure jets
and vibrating surfaces moved by mechanical agitation.
[0470] And while the above description has focused upon improving
uniformity of processing in near field regions proximate to a
vibration member, embodiments in accordance with the present
invention are not limited to this application. For example, in
alternative embodiments of the present invention, megasonic energy
could be applied from a vibration member to a substrate located in
far field regions, thereby generating regions of constructive and
destructive interference. Subsequently, utilizing techniques such
as at least 1) energy reflection, 2) the use of multiple energy
sources, or 3) variation in the character of energy emitted from
the megasonic energy sources, the location of points of
constructive and destructive interference could be altered over
time, thereby enhancing the uniformity of processing in far field
regions outside of the vibration member near field.
[0471] D. Mechanical and Electrochemical Processing
[0472] With most conventional chemical mechanical polishing or
planarization (CMP)-type processing, substrates are gripped in a
device and then set in rotational motion adjacent to a large
rotating abrasive pad. Both the substrate and the abrasive pad may
be in motion with respect to the machine as well as with respect to
each other).
[0473] As the substrate is brought into contact with the abrasive
pad, various processing slurries are introduced onto the surface of
the pad to aid in uniform grinding and removal of unwanted material
from the substrate surface. Various pressure profiles are
established across the diameter (or length, not thickness) of the
substrate due to the relative velocity between the substrate and
the abrasive pad.
[0474] Typically, the established pressure profiles are not uniform
across the entire substrate. Sometimes the first part of the
substrate to contact the pad dips towards the abrasive pad, while
the last part of the substrate to contact the pad lifts further
away from the pad. In other applications, the leading surface of
the substrate may lift or hydroplane while the lagging surface
dips.
[0475] The pressure profiles are often nonlinear across the entire
diameter or length of the substrate, taking the form of a curved
surface containing minimums and/or maximums. Localized pressure can
increase or build leaving the leading edge of the surface of the
substrate, and then fall away when approaching the other end or
trailing edge. Alternatively, the localized pressure may first
decrease, and then increase, from one side of the substrate to the
other.
[0476] Conventional approaches have attempted to apply ultrasonic
energy to clean accumulated grit from the surface of abrasive pads.
Since these pads are generally constructed of flexible polymeric
materials that readily absorb vibrational energy, it is generally
difficult to transfer vibrational or sonic energy across them.
[0477] This transfer of sonic energy during an abrasive polishing
or planarization process is further complicated when the pads
and/or substrates are moving or spinning. One conventional design
for a polishing unit utilizes a belt/loop design for the abrasive
pad. The belt continually loops around drive pulleys, in a manner
analogous to a belt sander. The substrate is held stationary and
allowed to come into contact with the abrasive belt.
[0478] Ultrasonic energy may be applied to the top side of the belt
during this operation, with the intent of having some small portion
of the energy transfer across the polymeric belt to the interface
between the belt and substrate, aiding in processing or cleaning
accumulated grit from the belt. Because of the poor energy transfer
across flexible polymeric materials, such conventional designs have
only met with limited success.
[0479] Accordingly, a need exists in the art for methods and
apparatuses for applying sonic or vibrational energy to enhance a
semiconductor fabrication process.
[0480] In accordance with embodiments of the present invention
vibrational energy is applied to a substrate or workpiece being
subjected to chemical mechanical planarization or some other form
of processing. In accordance with one embodiment, the vibrational
energy may be imparted to the workpiece from an energy source
through contact with a stationary energy-transfer member. In
accordance with an alternative embodiment of the present invention,
the vibrational energy may be imparted to the workpiece through an
energy-transfer member featuring a rotating element such as a
bearing. Application of vibrational energy in accordance with the
present invention may alter a pressure profile experienced by a
substrate in contact with an abrading member, thereby affecting a
quality of the resulting processing.
[0481] Embodiments in accordance with the present invention
generally relate to modification of surfaces of workpieces by
processing in conjunction with the application of vibrational
energy. The applied vibrational energy may take various forms, and
may be produced from various sources.
[0482] In accordance with one embodiment, of the present invention,
vibrational energy in the form of mechanical vibrations,
sonic/ultrasonic waves, or microwaves, may be applied to a
substrate prior to, during, or after processing of the substrate to
improve the uniformity of local or global substrate processing. One
specific application in accordance with the present invention
involves planarization of a semiconductor workpiece, either
generally over an entire wafer, or over individual features present
on that surface.
[0483] Other more common forms of planarization and surface
modification include mechanical polishing, lapping, grinding
(removal of material by abrasion), and electrochemical machining
processes involving removal of material/planarization due to
electrochemical reactions on a surface. Alternatively, embodiments
in accordance with the present invention can apply vibrational
energy to a substrate in the course of adding, as well as removing,
material to a workpiece. Such additive sonic-assisted processes
include but are not limited to electroplating and various forms of
coating.
[0484] In the specific field of CMP processing, introducing
vibrational energy into and/or through a substrate during
planarization/processing may render more uniform the localized
pressure profile developed between the substrate and the contacting
abrasive pad. This more uniform pressure profile may in turn lead
to more uniform removal of material from the substrate surface.
[0485] In the field of electrochemical machining, introduction of
vibrational energy may lead to more uniform concentration and
temperature profiles near the surface of the substrate. More
uniform profiles may lead to more uniform and rapid removal of
material from substrate surfaces or more uniform and rapid
reactions.
[0486] The applied vibrational energy may also reduce the thickness
of fluid boundary layers, referred to as the acoustic boundary
layers in the case of applied ultrasonic energy, or as the
hydrodynamic boundary layers in the case of forced fluid flow. Such
thinned fluid/hydrodynamic/acoustic boundary layers may lead to
faster mass transfer across the layers to the substrate and
correspondingly accelerated processing when mass transfer limiting
steps are involved. This effect can occur generally over an entire
surface, or be limited to specific local areas only, or act
differently in different local areas.
[0487] For example, during processing the concentration of a
specific ionic species of interest can be depleted in the fluid
boundary layer next to an active surface. The individual ions
cannot diffuse or move as rapidly into and through the fluid
boundary layer under the applied potential, as they can in the bulk
fluid by means of the bulk fluid flow. The resulting depletion of
specific ions in the fluid boundary layer is often referred to as
concentration polarization, and results in slower and non-uniform
processing.
[0488] By reducing the thickness of the fluid boundary layer
through the application of vibrational or sonic energy in
accordance with embodiments of the present invention, the rate at
which the desired ionic species becomes available at the reacting
surface, is increased.
[0489] For substrate processing involving planarization, the
inverse effect can be observed. Specifically, a boundary layer
having thickness reduced by the application of vibrational energy
in accordance with the present invention, allows species removed
from the substrate surface to more rapidly enter the bulk fluid.
This results in both faster and more uniform processing.
[0490] Vibrational energy applied to enhance processing in
accordance with embodiments of the present invention can be
produced by any convenient mechanical/electrical source. Examples
range from a piston/eccentric arrangement exhibiting relatively low
frequency, to electronic excitation of ultrasonic piezoelectric
crystals exhibiting moderate frequency, to generation and
application of higher frequency microwave energy. These generated
vibrations may be communicated to the interface between the
substrate and the processing entity (i.e. the abrasive pad and/or
the processing chemistry) in various ways.
[0491] As previously described, polymeric pads which may be
employed in abrasive processing may absorb vibrational energy.
Therefore, instead of attempting to transfer significant amounts of
energy across such a flexible energy absorbing element, embodiments
in accordance with the present invention transfer significant
amounts of vibrational energy directly across a relatively rigid
substrate and/or substrate holder, where little of the energy is
absorbed and lost.
[0492] One specific design for accomplishing this application of
vibrational energy is described above. Specifically, by
transferring large amounts of energy directly across the substrate
instead of only transferring small amounts across the abrasive pad,
much more energy is available at the interface between the pad and
the substrate to modify the localized pressure profile initially
developed.
[0493] While not wishing to be bound by any particular theory, the
applied vibrational energy may disrupt or modify localized pressure
profiles arising during substrate processing. For application of
moderate frequency vibrational energy, the full pressure profile
across the entire substrate resulting from a conventional
processing apparatus machine/unit may be prevented from fully
forming, because the time between vibration pulses is too short to
allow full development of a non uniform pressure profile.
[0494] An analogous situation may be described for conventional
megasonic processing of substrates in a liquid bath. As fluid flows
past a stationary substrate, a hydrodynamic boundary layer is
formed above the substrate where the fluid velocity ranges from
zero at the substrate surface to the bulk fluid velocity at some
distance removed from the substrate surface. This profile grows in
thickness or develops the further the fluid flows along the plate.
Therefore, the profile of "reduced" fluid velocity in the resulting
fluid boundary layer is thinner near the nose of the substrate and
thicker near the tail.
[0495] By applying megasonic energy during processing, however, the
full localized profile is never allowed to fully develop. As a
result, the acoustic-determined boundary layer may be less than
1/1000 the thickness of the hydrodynamic-determined boundary layer.
In this manner, application of megasonic energy can promote
cleaning or processing operations in which mass must be transferred
across the fluid boundary layer.
[0496] One issue addressed by embodiments in accordance with the
present invention is the difficulty in transferring vibrational
energy across the substrate while the substrate is rotating or
moving. Several embodiments in accordance with the present
invention accomplish this goal without also requiring the source of
the vibrational energy to also move. Such movement of the
vibrational energy source would add complexity to the apparatus by
requiring moveable power connections.
[0497] FIG. 39 shows a cross-sectional view of one embodiment of an
apparatus for employing vibrational energy to enhance processing of
a substrate. Processing apparatus 3900 of FIG. 1 shows solid member
3902 having one end 3902a fitted with source of vibrational energy,
for example an ultrasonic transducer 3904 including piezoelectric
crystals 3905 in electrical communication with a power source 3907.
Second end 3002b of solid member 3902 is in contact with the
substrate 3906 through substrate support plate 3908. Energy is
transmitted through the solid energy transfer member, the
substrate, and into interface 3910 between substrate 3906 and
abrasive pad 3910 in the presence of slurry 3912. Solid member 3902
is stationary and does not rotate or move while abrasive pad 3910
rotates in contact with the substrate.
[0498] FIG. 40 shows a cross-sectional view of an alternative
embodiment of an apparatus in accordance with the present invention
for employing vibrational energy to enhance processing of a
substrate. Apparatus 4000 of FIG. 40 is similar to that of the
first embodiment of FIG. 39, except that transmission member 4002
includes rotatable bearing 4004. Bearing 4004 allows top portion
4002a of solid member 4002 in contact with vibration source 4008 to
remain stationary, while bottom portion 4002b in contact with
substrate support 4006 is free to turn.
[0499] Various bearing designs are known that could allow transfer
of vibrational energy simultaneous with rotation of the substrate
holder. Examples of such bearing designs include but are not
limited to mechanical bearings and low friction slip surfaces.
[0500] While FIG. 40 does not explicitly illustrate how the
substrate holder can be rotated, many methods are well known in the
industry that could be employed to accomplish this movement.
Examples of such mechanisms for inducing rotation include but are
not limited to belts/pulleys, gears, and friction with other moving
members.
[0501] FIG. 41 shows a cross-sectional view of another alternative
embodiment of an apparatus in accordance with the present invention
for employing vibrational energy to enhance processing of a
substrate. Apparatus 4100 shown in FIG. 41 is similar to that shown
in FIG. 40, except that a mechanical vibrator 4002 is used as the
vibration element.
[0502] While the substrate holder is shown as static in FIG. 41,
this is not required by the present invention. In accordance with
other embodiments, the vibration transfer element could be rotated
as the mechanical vibrator operates.
[0503] Liquid is well known as a medium for the transmission of
vibrational and sonic energy. Accordingly, FIG. 42 shows a
cross-sectional view of yet another alternative embodiment of an
apparatus in accordance with the present invention for application
of vibrational energy. Apparatus 4200 utilizes a hollow vibrational
energy transmission member 4202 filled with liquid 404 in
conjunction with a bearing/seal structure 4206 to facilitate
transfer of vibrational energy to substrate support 4208 during
processing. One example of such a bearing/seal structure for use in
accordance with the present invention is the ferrofluidic seal
structure.
[0504] In the specific embodiment shown in FIG. 42, it may be
preferable to employ a degassed liquid to limit bubble formation,
which would reduce the transfer of sonic energy from source 4210 to
the substrate 4212.
[0505] FIG. 43 shows a cross-sectional view of still another
alternative embodiment of a processing apparatus in accordance with
the present invention. Apparatus 4300 of FIG. 43 employs
vibrational energy from mechanical vibrator 4302 actuated at a
relatively low frequency (e.g..about.1-1000 Hz or .about.1-10 kHz),
in combination with vibrational energy from a megasonic unit 4304
having crystals stimulated at a moderate frequency
(e.g..about.0.7-1 MHz). In accordance with still other embodiments,
microwave or other higher frequency energy pulses may also be
applied, either in conjunction with the low and moderate frequency
energy, or in place of these other energy frequencies.
[0506] FIG. 44 shows a cross-sectional view of another alternative
embodiment of a processing apparatus in accordance with the present
invention. Apparatus 4400 of FIG. 44 utilizes slip ring structure
4402 to allow transfer of electrical energy from a stationary
source of power (not shown) to a rotating moving vibrational
element 4404 (e.g. a piezoelectric crystal). Slip ring 4402 may
feature contacts comprised of an electrically conducting liquid
such as mercury, or formed from a solid conductor such as copper
metal. Such an arrangement as shown in FIG. 44 allows the rotation
of both the abrasive pad 4406 as well as substrate 4408 itself,
without requiring a bearing or bearing surface through which
vibrational energy must be transferred.
[0507] FIG. 45 shows a cross-sectional view of another alternative
embodiment of a processing apparatus in accordance with the present
invention. Apparatus 4500 of FIG. 45 includes megasonic nozzle 4502
positioned within a specific range of angles
(.theta..sub.1-.theta..sub.2) with respect to the front or back
surface of substrate 4504, or the surface of the holding or support
plate 4506. Within the specific range of angles
(.theta..sub.1-.theta..sub.2), mode conversion occurs where
dilatational pressure waves 4505 exiting megasonic nozzle 4502 are
converted into surface waves in the substrate, substrate holder, or
substrate support.
[0508] FIG. 46 shows a cross-sectional view of yet another
embodiment of a processing apparatus in accordance with the present
invention. Apparatus 4600 is configured to perform electrochemical
polishing or machining. Rather than utilizing an abrasive pad to
perform substrate planarization, in apparatus 4600 electrolyte
liquid 4602 acts as a medium to transfer various ionic species to
and from the surface of substrate 4604. Electrochemical processes
utilizing either constant current or constant voltage operational
modes are particularly suited to take advantage of benefits offered
by embodiments in accordance with the present invention.
[0509] While the embodiment of FIG. 46 shows vibrational energy
being transferred across the thickness of the substrate, this is
not required by the present invention. In accordance with
alternative embodiments, vibrational energy can be directed
parallel to the substrate surface in order to reduce the thickness
of any associated fluid boundary layers.
[0510] And while the embodiment of FIG. 46 shows a large electrode
positioned parallel to the substrate surface, this is not required
by the present invention. Because of the electrical conductivity of
the electrolyte solution, in accordance with alternative
embodiments the electrode may be positioned some distance from the
substrate surface, and/or in a non-parallel orientation relative
thereto.
[0511] While electrochemical processing is generally conducted in a
tank with the substrate either partially or completely submerged,
this is also not required by the present invention. In accordance
with still other alternative embodiments of the present invention,
it may be preferable to employ only a thin fluid layer between an
electrode closely spaced from the substrate. Such an arrangement
could be particularly useful in applying microwave energy when the
electrode is relatively transparent to microwave energy.
[0512] For example, many semiconductor substrate materials are
relatively transparent to microwave energy. Such substrate
materials may also possess sufficient electrical conductivity, or
may contain conducting traces or other sufficiently conductive
features, to allow the substrate to act as an electrode. Thus in
accordance with alternative embodiments of the present invention,
high frequency microwaves may be applied in order to enhance
electrochemical processing.
[0513] FIG. 47 shows a cross-sectional view of another alternative
embodiment of a processing apparatus in accordance with the present
invention. Apparatus 4700 of FIG. 47 shows delivery of microwave
energy 4702 from microwave source 4710 to interface 4708 between
abrasive pad 4704 and substrate 4706, utilizing wave-guide 4712. A
wave-guide delivers the microwave energy to the desired point of
application to maximize energy transfer directly to the
substrate/pad interface. In the particular embodiment shown in FIG.
47, a thin liquid layer at interface 908 and/or on the surface of
the substrate holder could be advantageously used to absorb the
microwave energy applied from the source.
[0514] While FIG. 47 shows the microwave radiation as being
directed by the waveguide parallel to the substrate surface and the
substrate/pad interface defined thereby, this is not required by
the present invention. In accordance with alternative embodiments,
a waveguide could be disposed at an angle relative to the substrate
surface to maximize efficient transfer of energy to the
substrate/pad interface. Mode conversion of the waveform could
occur under the proper conditions.
[0515] While the previously-described embodiments have depicted the
processed substrate as being positioned above a processing member,
this is not required by the present invention. In accordance with
alternative embodiments, the substrate could be disposed beneath or
beside a processing member.
[0516] Before, during, or after performance of a sonic-assisted
processing step in accordance with the present invention, a second
processing step may be commenced. This second processing step may
include electrochemical processing including electroplating,
electropolishing and electromachining. The incorporation of
vibrational energy of any frequency, but especially ultrasonic and
microwave frequency energy into the interface between the substrate
and any contacting liquid, can be of great benefit in reducing the
thickness of any associated fluid boundary layers formed on the
surface of the substrate.
[0517] Reduction of such fluid boundary layers in accordance with
the present invention can lead to more rapid and/or uniform
processing, especially for processing involving a
mass-transfer-limiting step. Additionally, reduction of the
thickness of fluid boundary layers renders more uniform the fluid
concentration and temperature contacting the substrate surface.
This in turn leads to more uniform processing, and can prove
especially effective for processing uneven surfaces of
substrates.
[0518] It is further noted that the steps of processing described
in the various above embodiments can be performed in any order, and
still remain within the scope of the present invention. For
example, vibrational energy may be applied before, during, or after
processing to enhance results.
[0519] Electrochemical processing is finding increasing use in the
fabrication of semiconductor devices. One prevalent use is in the
formation of copper metallization of interconnect structures. In
such damascene applications, copper is deposited by electroplating
within recesses formed in dielectric material.
[0520] In such electrochemical processing, it may be difficult to
obtain/maintain uniform addition or removal of materials from a
substrate surface exhibiting complex topography. One difficulty
encountered with conventional electrochemical processing is that
the concentration of various ionic and other active chemical
species, and fluid flow velocity profiles, change across the
hydrodynamic boundary layer at the surface of the substrate or
within recessed features present thereon. Thus, with the
electroplating of metal layers over substrates, it may be difficult
to form layers of a uniform thickness over lower portions of trench
features exhibiting high aspect ratios, or to insure that such
features are filled uniformly without the inclusion or creation of
voids.
[0521] In particular, as electrolyte fluids flow over the recessed
features, the fluids form a hydrodynamic boundary layer on the
substrate surface and the fluid-filled trenches may become
stagnant. Liquid in the trenches may thus fail to rapidly transfer
via fluid convection or be regularly replaced with fresh bulk fluid
overlying the boundary layer. In some cases, a vortex within the
trench may be formed, causing localized fluid circulation within
the trench but otherwise preventing fluid contained therein from
mixing with the surrounding bulk fluid. In such circumstances, the
mechanism of transporting various ionic and other chemical species
across stagnant regions may be limited to diffusion, which is
generally quite slow.
[0522] Alternatively or in conjunction with diffusive transport,
chemical species may move across stagnant regions under the
influence of an applied potential. Under such conditions,
concentration polarization arising in these stagnant regions can be
amplified. Such concentration polarization may lead to slow and
non-uniform processing, especially around sharp edges or in corners
within structures.
[0523] In an attempt to render electrochemical processing more
uniform, various additives are conventionally introduced in the
electrolyte baths. Some additives may enhance or speed up
deposition or removal of material in selected areas, such as in
fluid stagnant areas where ionic concentrations have dropped
significantly. Other additives may retard or slow down deposition
or removal in areas where fluid velocities and ionic concentrations
are elevated. Combinations of additives may be used to make the
final deposited material layers or surfaces more level, especially
across various non-planar substrates with high aspect ratio
features.
[0524] While processing uniformity often improves in the presence
of additives, processing may still exhibit significant
non-uniformity. Such nonuniformity can necessitate additional
processing steps to remove or add material in some locations, and
to compensate for voids formed in other locations. Such additional
steps decrease throughput and increase expense.
[0525] Introduction of additives such as enhancers, retarders and
levelers, can also serve as another source of contamination that
must be removed from the substrate before further processing steps
can be implemented. Such contamination removal steps can also
decrease throughput and increase expense.
[0526] It is known that the introduction of sonic energy may reduce
the thickness of the hydrodynamic boundary layer. In certain cases,
the acoustic boundary layer resulting from introduction of sonic
energy at near 1 MHz, may only be .about.1/1000 the thickness of
the original hydrodynamic boundary layer. This reduction in the
effective thickness of fluid boundary layers can be especially
important in processing of trenches, vias, or other structures on
the substrate surface that exhibit high aspect ratios.
[0527] Introduction of sonic energy of the correct frequency can
diminish the effective thickness of fluid boundary layers, thereby
increasing the rate of fluid flow/chemical transport in these
areas. Where fluid flow is essentially nonexistent, or circulates
only locally, fluid transfer out of the feature to the surrounding
bulk fluid is prevented. Sonic energy can be utilized to disrupt
such localized vortices, and thereby enhance fluid transfer between
stagnant areas and the bulk fluid.
[0528] However, conventional attempts to use sonic energy to reduce
the thickness of the fluid boundary layers during electrochemical
processing have typically met with only limited success. In certain
cases, these conventional approaches proved unable to effectively
transfer energy across an electrode positioned parallel to the
surface of the substrate. For designs where the electrode was not
positioned parallel to the surface of the substrate, and not
positioned between the substrate and the vibration member,
non-uniform processing often resulted.
[0529] Accordingly, a need exists in the art for methods and
apparatuses for improved electrochemical processing of
semiconductor substrates.
[0530] Electrochemical processing of a semiconductor workpiece in
accordance with embodiments of the present invention may be
enhanced by application of various forms of energy, including sonic
energy, across the thickness of the electrode, especially an
electrode positioned between the substrate and a vibrating sonic
energy source and pulsed fluid flow. In accordance with embodiments
of the present invention, sonic energy may be transferred across
electrodes having solid, composite, open (i.e., mesh), or porous
structures. Electrochemical processing in accordance with
embodiments of the present invention may also be accomplished by or
be accompanied by pulsed fluid flow to further reduce the thickness
and impact of hydrodynamic boundary layers on processing
effectiveness and uniformity.
[0531] Embodiments in accordance with the present invention
disclose methods and apparatuses for transferring energy to the
substrate surface across solid, as well as composite, open, or
porous electrode structures, alone or in combination with pulsed
fluid flow. Embodiments in accordance with the present invention
reduce the thickness of fluid boundary layers, and enhance the
uniformity of those layers, and prevent fluid stagnation in high
aspect ratio structures. Embodiments in accordance with the present
invention thus result in faster and more uniform processing, while
eliminating or reducing the need for additives.
[0532] FIG. 48 shows a simplified cross-sectional view of one
embodiment of an apparatus for performing electrochemical
processing in accordance with the present invention. Vibration
member 48 in direct physical contact with piezoelectric crystals
4812 is partly or fully immersed in electrochemical processing bath
4814 contained within walls 4816 of processing vessel 4818.
Electrode 4820 is positioned in bath 4814 spaced apart from
vibration member 4810. Substrate 4822 is positioned in bath 4814
spaced apart from, and on the opposite side of, electrode 4820.
[0533] Piezoelectric crystals 4812 are in electrical communication
with RF power source 4824. Electrode 4820 and substrate 4822 are in
electrical communication with opposite poles 4826a and 4826b of
voltage source 4826, respectively.
[0534] During processing, a potential difference is applied across
substrate 482 and electrode 4820 by power source 4826. This
potential difference creates an electric field that activates the
chemical species of bath 4814 to process the substrate surface.
This potential difference can be held constant, allowed to change
magnitude, or even alternate between positive and negative values,
depending upon the specific processing desired to be accomplished.
Electrochemical processing modes utilizing either constant or
variable current operation are possible in accordance with
embodiments of the present invention.
[0535] Prior to or during application of the potential difference,
piezoelectric crystals 4812 receive voltage from RF source 4824,
and commence vibrating. The vibrational energy of piezoelectric
crystals 4812 is in turn imparted to the adjoining vibrating member
4810. Vibrating member 10 in turn communicates the vibrational
energy to electrochemical bath 4814.
[0536] As a result of the application of vibrational energy to bath
4814, the hydrodynamic boundary layer on both the surface of the
substrate and the electrode are substantially lessened. This
promotes transfer of chemical species to and from the surface of
both the substrate and the electrode.
[0537] Processing apparatuses in accordance with embodiments of the
present invention may feature electrodes specifically designed to
promote their conduction of ultrasonic energy. For example, a solid
electrode may be designed to have a thickness of an even multiple
of one-quarter of the wavelength of the applied sonic energy (i.e.
thickness=n.lamda./4, n=2,4,6 . . . ).
[0538] While FIG. 48 shows the electrode disposed orthogonal to
sonic energy incident from the vibrating member, this is not
required by the present invention. FIG. 49 shows an alternative
embodiment, wherein electrode 4900 is disposed at an angle .theta.
lying between a range of critical angles
.theta..sub.1-.theta..sub.2 relative to the direction of sonic
energy incident from vibrating member 4902. Positioning the
electrode at an angle in the manner shown in FIG. 49 allows
vibrational energy to be transmitted across the electrode without
requiring that the thickness of the electrode be approximately
equal to an even multiple one-quarter wavelength, as is the case
shown in FIG. 48.
[0539] In the embodiment of FIG. 49, there is no limitation on the
electrode thickness, and cross electrode transfer of energy will
occur as long as the surface of the electrode is at a proper angle
with respect to the surface of the vibration member. While FIG. 49
depicts an angle between the electrode and the substrate, as well
as between the electrode and the vibration member, this is not
required. The substrate may be parallel to the electrode, or
oriented at any angle to it. Further, an angle between the
electrode and the substrate may be held constant or may change
during processing. An angle between the electrode and the vibration
member may also be held constant or may change during
processing.
[0540] FIG. 50 shows a simplified cross-sectional view of another
embodiment of an electrochemical processing apparatus in accordance
with the present invention. Specifically, the apparatus of FIG. 50
is similar to that shown in FIGS. 48 and 49, except that electrode
5000 is in direct physical contact with vibrating member 5002
including piezoelectric crystals 5004. The combined
electrode/vibration member structure 5000/5002 can be designed in a
number of ways to optimize the transfer of vibrational energy from
member 5002 across electrode 5000.
[0541] For example, while electrode 5000 need not exhibit any
particular thickness, the thickness of the resulting electrode
vibration member combination should be approximately an odd
multiple of one-quarter the wavelength of the applied vibrational
energy (thickness=n.lamda./4, n=1,3,5 . . . ). This odd multiple
one-quarter wavelength thickness is desirable for effective
coupling of sonic energy from a vibration member into a fluid.
[0542] In certain electrochemical processes, portions of the
electrode material may be consumed, thereby altering the overall
thickness of the electrode over time. In such applications, where
the thickness of the electrode element does not comprise a
significant % of the overall thickness of the combination of the
electrode/vibrating member, changes in the electrode thickness will
not greatly effect package operation. For example, if the thickness
of the vibration member is four times the thickness of the
electrode, a reduction of 50% in the electrode thickness will
reduce the overall thickness of the electrode/vibration member by
only 10%. While overall thickness variation of the combination of
as much as +/-50% may be acceptable under some conditions,
variation of less than +/-30% is generally preferred.
[0543] Moreover, certain generators of sonic energy allow
adjustment in the firing frequency of the piezoelectric crystals.
Over time, the firing frequency of the piezoelectric crystals could
be adjusted, thereby compensating for changes in electrode material
thickness.
[0544] In accordance with still other embodiments of the present
invention, the vibrating member may comprise the electrode itself.
Specifically, the vibrating member could be fabricated from the
appropriate electrically conducting material, such that application
of a potential difference produces the desired electric field and
resulting electrochemical processing. In embodiments where
vibration results from application of current to a piezoelectric
crystal in contact with the vibration member, the piezoelectric
crystal could be electrically, but not mechanically, insulated from
the vibration member.
[0545] Sonic-assisted electrochemical processing in accordance with
embodiments of the present invention may be enhanced utilizing
relative motion between one or more elements and the surrounding
electrochemical bath. FIG. 51 shows a simplified cross-sectional
view of one such embodiment, wherein substrate holder 5100
supporting substrate 5102 within chemical bath 5104, is rotated.
This relative motion serves to further reduce the thickness of the
hydrodynamic boundary layer between the substrate and the
surrounding bulk fluid, thereby enhancing transport of ions and
other chemical species to and from the substrate surface during
processing.
[0546] Perhaps more importantly, relative motion can also increase
the uniformity of processing, especially when the substrate and
electrode are positioned relatively close to one another. This
increase in processing uniformity can result from the relative
motion of the various equipment elements through non-uniform energy
fields. In this manner, nodes of high and low energy intensity
created in the non-uniform fields are not concentrated onto fixed
points on the substrate during the entire processing sequence.
[0547] For example, if the applied potential field contains
localized non-uniformity, movement of the
substrate/electrode/vibration member can minimize the negative
impact on processing. Without relative motion, points on the
substrate surface located in a high energy intensity node could
experience an increased rate of material addition, while a point in
a low energy intensity node could experience a reduced rate of
material addition. Relative motion would tend to smooth out the
impact of these high and low intensity nodes on processing
uniformity.
[0548] Similarly, relative motion may also tend to minimize
negative impact of a non-uniform sonic field, which may be created
as sonic energy departs a vibrating surface. The region near a
vibrating surface is often termed the near field and is
characterized by field non-uniformity. This non-uniformity may
extend up to several inches from the vibrating surface depending
upon constraints imposed by frequency, intensity, and geometry.
[0549] While FIG. 51 shows imparting relative movement between the
substrate and the surrounding bath, this is not required by the
present invention. In accordance with other embodiments, one or
both of the electrode or vibrating member could experience motion
within the bath, alone or in combination with movement of the
substrate. Such relative motion would tend to minimize adverse
effects on processing of non-uniformity either in the applied
potential field, or in the generated sonic field emanating from a
vibrating surface.
[0550] And while FIG. 51 shows imparting relative rotational
movement between the substrate and the surrounding bath, this is
also not required. Forms of relative motion other than rotational
motion may be employed, for example, lateral movement or
vibrational movement.
[0551] The specific embodiments described above have focused upon
the use of a solid electrode structure. In accordance with
alternative embodiments, however, other than solid electrodes may
be employed to enhance electrochemical processing in accordance
with the present invention.
[0552] For example, FIG. 52 shows a simplified perspective view of
an embodiment of a processing apparatus in accordance with the
present invention utilizing an open electrode structure.
Electrochemical processing apparatus 5200 is similar to that shown
in FIG. 48, except that electrode 5202 features a pattern of
openings 5204 and 5206. Openings 5204 and 5206 allow and direct the
flow of fluid of the electrochemical processing bath through the
electrode, thereby promoting uniform distribution of chemical and
ionic species in the bulk fluid. The overall thickness or
orientation of the electrode plate 5202 remains within the
desirable ranges described above, so that vibrational energy is
transferred across the electrode 5202.
[0553] The size and number of openings 5204 and 5206 in electrode
5202 are designed to minimize any nonuniformity in sonic energy
received at the substrate surface. In certain embodiments, the
width of the openings could be greater than or equal to one
wavelength (or multiple wavelengths) of the incident sonic energy,
in order to allow passage of the energy wave.
[0554] In other embodiments, the width of the openings in the
electrode can be designed to be less than one full wavelength. Such
an electrode structure could result in diffraction of the incident
wave of sonic or other energy types, creating various diffraction
interference patterns. By varying frequency, power level, and phase
angle of the incident sonic energy, such interference patterns can
be caused to change over time, producing different processing
effects.
[0555] FIG. 53 shows a simplified cross-sectional view of an
alternative embodiment of a processing apparatus similar to that
shown in FIG. 52, but featuring substrate 5300 supported within
bath 5302 over electrode 5304. The cross-sectional view illustrated
in FIG. 53 also shows the circulation of electrochemical processing
fluid through openings 5304a in electrode 5304.
[0556] Unlike FIG. 52, which suggests the flow of fluid straight
through the electrode, FIG. 53 depicts an electrode that imposes a
change in the direction of fluid flow during operation. This change
of the direction of the flowed fluid could range from a single
shift of only a few degrees, to repeated changes as might result
from operation of a static mixer.
[0557] FIG. 54 shows a simplified cross-sectional view of another
alternative embodiment of a processing apparatus in accordance with
the present invention, which is similar to that shown in FIG. 52
but features a mechanical vibrator 5400, rather than piezoelectric
crystals, in contact with vibrating member 5400. In this
embodiment, bulk electrochemical processing fluid 5402 flows to
substrate 5404 through openings in electrode 5406. Such a
mechanical vibrator design could be particularly effective when a
relatively low frequency of vibration is desired, especially if
such vibration is to be accompanied by a large amplitude or
displacement of the vibration member.
[0558] FIG. 55 shows a simplified cross-sectional view of yet
another alternative embodiment of a processing apparatus in
accordance with the present invention. Apparatus 5500 FIG. 55 is
similar to that shown in FIG. 53, but features electrode 5502
direct contact with 5504a in manifold 5504, such that jets of fluid
flowing out of the electrode are created.
[0559] In a further refinement of the embodiment shown in FIG. 55,
the flow of fluid can be pulsed on and off. A pump (not shown) can
be used to generate the pulsed fluid flow. The generated fluid
pulsation may roughly simulate the pressure pulses present in the
previous examples, which were caused by either the movement of
piezoelectric crystals, or by a mechanical vibrator coupled to the
vibration member.
[0560] The pulsed fluid flow can be effective not only in reducing
the thickness of a hydrodynamic boundary layer present on the
surface of one or more elements of the system, but may also be
particularly effective in disrupting vortices that have arisen in
stagnant fluids present within recessed features on the substrate
surface.
[0561] Embodiments in accordance with the present invention may
match the frequency of fluid pulsation with dimensions of a
recessed feature, in order to maximize mixing and fluid transfer
between the recess and the bulk fluid. For example, if the
frequency of pulsation is either too high or too low, sub-optimal
fluid transfer may occur between a recessed feature and the
surrounding fluid.
[0562] FIG. 58 shows a simplified cross-sectional view of yet
another alternative embodiment of a processing apparatus in
accordance with the present invention.
[0563] Apparatus 5800 of FIG. 58 is similar to that shown in FIG.
55, but features vibration member 5802 in direct physical contact
with manifold 5804. Thus, fluid pulses generated from a pump source
(not shown) can be augmented with pressure pulses from either
piezoelectric crystals (not shown) or a mechanical vibrator (not
shown) coupled to vibration member 5802. Such an arrangement could
result in multi-mode vibration frequency. Thus it would be possible
to simultaneously produce vibrations at a high, medium and low
frequency, each vibrational frequency having a different
intensity.
[0564] Embodiments of apparatuses in accordance with the present
invention described above have focused upon utilization of an
electrode member that is solid or has openings. In accordance with
alternative embodiments, however, an electrode made out of other
than solid material may be utilized. For example, porous,
electrically conducting materials are used as electrodes in fuel
cells. Such electrodes are often combined with various ion exchange
membranes and separators. Embodiments in accordance with the
present invention could also utilize electrodes, membranes, and
separators formed from such materials. The porosity exhibited by
such an electrode structure would enhance the flow of bulk
electrochemical processing fluid through the electrode, in a manner
similar to that described above in connection with the open
electrode structure.
[0565] Further, incorporation of ion exchange membranes and
separators could actually prevent or inhibit transfer of
undesirable chemical species contained in the bulk fluid, to the
surface of the substrate, while promoting transfer of desirable
species. Thus different reactions could occur simultaneously within
the electrochemical processing cell.
[0566] Still further alternatively, the electrode structure
utilized in connection with embodiments of the present invention
may be formed from a composite of materials. For example, the
electrode may be hollow or fluid-filled to enhance the transmission
of sonic energy. Alternatively, the electrode may be filled or
coated, partially or completely, with a polymer material, also to
enhance its ability to transfer applied sonic energy, while
producing desirable electrochemical reactions.
[0567] While the previous figures have depicted the electrode as
planar member, this is not required by the present invention. In
accordance with alternative embodiments, the electrode could
exhibit different shapes. For example, where the surface of the
processed substrate exhibits a curved shape, the electrode could be
similarly shaped to conform thereto.
[0568] In accordance with still other embodiments, the electrode
may be shaped to promote the smooth flow of fluid. Accordingly,
FIG. 56 shows an alternative embodiment of a structure in
accordance with the present invention, where shaped electrode 5600
exhibits and tear-drop or airfoil profile in order to promote
circulation of bath fluid with smooth streamlines characteristic of
laminar, rather than turbulent, flow. The shape of the electrode
may further promote transmission of sonic energy along a path
conforming to these fluid streamlines. Such an electrode shape
would promote the propagation of sonic energy around obstructions
in the electrochemical bath such as the electrode or supporting
members, thereby preventing sonic shadowing resulting in nonuniform
processing. While shown as a single large electrode in FIG. 56, the
active electrode surface could actually comprise multiple smaller
hydrodynamically-shaped elements.
[0569] As described more fully in U.S. nonprovisional patent
application Ser. No. 10/150,748, performing electrochemical
processing performed at elevated pressures may involve the pulsed
flow of fluid. The pressure pulsed operation could also include
pressure/release operation, where dissolved gas at higher pressure
is released as bubbles when the system or localized pressure is
reduced. This could include the inclusion of a pressure drop as a
result of high velocity fluid flow within the tank. FIG. 57 shows
one embodiment utilizing such induced pressure drops.
Opening/closure of the two valves 5700 and 5702 can be sequenced to
develop various pressure and flow pulsed operation within the tank.
The generation of bubbles can also contribute to fluid uniformity
within the tank.
[0570] Embodiments in accordance with the present invention are not
limited to the transmission of sonic energy across an electrode
submerged in an electrochemical processing bath. FIG. 59 depicts an
embodiment where substrate 5900 and electrode 5902 are separated by
a thin fluid layer 5904, but are not fully submerged within a
liquid bath. This thin fluid layer 5904 allows the electrochemical
reaction to take place. The specific apparatus for introducing
fresh fluid to the interface between electrode 5902 and substrate
5900 could include forced fluid flow from either a narrow or wide
area nozzle or jet, gravity flow from vertical operation or the
introduction from a porous electrode and/or vibration member.
Electrode 5902 could be of solid, open or porous design. Vibration
member 5906 could be fitted with piezoelectric crystals or with a
mechanical vibrator.
[0571] FIG. 60 shows another embodiment of an apparatus in
accordance with the present invention. Specifically, apparatus 6000
of FIG. 60 includes electrode 6002 oriented proximate and parallel
to surface 6004a of substrate 6004. Rather than being totally
submerged in electrolyte liquid, a thin layer 6006 of liquid is
provided between electrode 6002 and substrate 6004.
[0572] During processing, a potential difference is applied between
electrode 6002 and substrate 6004 to promote electrochemical
processing. Also during this process, microwave energy 6008 from a
source (not shown) is transmitted through microwave conduit 6010 to
backside 6002a of electrode 1302. This microwave energy 6008 is
transmitted across electrode 6002 and impinges the surface of
substrate 6004. Application of microwave energy across an electrode
to a substrate surface during electrochemical processing can be
effective to enhance the speed or uniformity of the resulting
electrochemical processing. Such application of microwave energy
may or may not be accompanied by the application of sonic
energy.
[0573] While FIG. 60 shows introduction of microwave energy solely
through the electrode, this is not required. In accordance with
alternative embodiments of the present invention, microwave energy
can be introduced through the substrate or at some angle relative
to the electrode or substrate. These angles could include the
introduction of the energy parallel to, perpendicular to, or
orientated at some angle relative to either the electrode or the
substrate.
[0574] While much of the electrochemical processing discussion has
dealt with transfer of energy across an electrode, it should be
understood that this is not required. Energy could also be
reflected off of electrode structures and still fall within the
spirit of this invention. And, rather than utilizing energy
transferred across electrodes in the manner shown, these
embodiments could be modified to illustrate the transfer of energy
across substrates or substrate holders.
[0575] E. Substrate Drying
[0576] Drying of semiconductor substrates after various
wet-processing steps has become even more important as feature
sizes shrink. Incomplete or ineffective drying can leave
watermarks, which lead to device defects and reduced manufacturing
yields. Some early dryers were based on the use of hot gases to
evaporate water and other processing liquids directly from
substrate surfaces. Others were based on the use of organic
solvents to displace residual water or processing liquids. The
residual solvent films were then removed by evaporation with heated
gas flow.
[0577] This solvent-based drying technology was eventually
supplanted by surface tension gradient (STG) dryer technology,
sometimes referred to as marangoni drying. While more effective
than earlier dryer designs, current STG designs suffer from slow
processing speed, fugitive alcohol and solvent emission, and
potential fire hazards.
[0578] Accordingly, there is a need in the art for methods and
apparatuses for rapidly and effectively drying processed
substrates.
[0579] In accordance with embodiments of the present invention,
drying of substrates exposed to liquid processing solutions may be
enhanced utilizing a number of techniques, alone or in combination.
In accordance with one embodiment, the substrate drying process
occurs at elevated pressure to increase concentration of a
surface-tension reducing component. In accordance with another
embodiment, radiation applied during the drying process, and in
particular applied at the meniscus, may reduce surface tension. In
accordance with still another embodiment, ultrasonic energy may be
applied during the drying process to vaporize residual liquid from
the substrate surface.
[0580] The present invention addresses the limitations mentioned
for these various earlier dryer designs. The multiple embodiments
of this invention provide flexibility in overcoming those
limitations. An embodiment of a pressurized STG dryer is disclosed
in copending U.S. nonprovisional patent application Ser. No.
10/150,748 ("the '748 application"). The use of rapid, effective
and efficient heating of substrates during processing is disclosed
in U.S. non-provisional patent application Ser. No 10/456,995, also
incorporated by reference herein for all purposes.
[0581] In accordance with one embodiment of the instant invention,
a pressure-based STG dryer may incorporate radiation heating and
modification of substrate surfaces. By operating under pressure, a
surface tension lowering component may be forced into solution at
the gas-liquid interface at a faster rate. Increasing the rate of
dissolution can lead to an increased amount of component being
dissolved in the liquid at the gas-liquid interface in a limited
time. Increasing the amount of the component dissolved in the
processing fluid at this interface can result in a lower localized
surface tension. A lower localized surface tension in the fluid,
especially in the meniscus area, can lead to a larger surface
tension gradient or differential between that surface tension and
the surface tension of the bulk processing fluid. This larger
surface tension gradient can lead to faster and more effective and
efficient drying.
[0582] One problem sometimes encountered in conventional STG
drying, is condensation of gases and vapors on the substrate
surface as the substrate is raised out of the processing liquid.
Thus in accordance with another embodiment in accordance with the
present invention, the STG gas may be heated while the processing
liquid is not, or while the processing liquid is actually
cooled.
[0583] In such an alternative embodiment, the heated STG gas heats
the substrate as the substrate exits the processing liquid, which
prevents condensation of water vapor or STG gas components on the
exposed substrate surface. Since the heat capacity of the STG gas
is small compared to the heat capacity of the processing liquid,
even though the gas is heated, it does not appreciably raise the
temperature of the processing liquid. Keeping the temperature of
the processing liquid low insures that the solubility of the STG
gas in the processing liquid remains relatively unchanged, or
increased in the case of cooling of the processing liquid.
[0584] Embodiments in accordance with the present invention may
operate at elevated pressures of between about 1 and 10 ATM,
although operation at pressures higher than 10 ATM and as high as
100 ATM or higher are possible. Operation at elevated pressures may
be accomplished by flowing a gas, a processing fluid, and/or a
rinsing fluid into a closed or substantially closed processing
vessel, as described generally in the '748 application.
[0585] Suitable surface tension lowering agents can exist as
solids, liquids, vapors, and gases, and in combinations thereof.
The surface tension lowering gases can be either locally applied at
the gas-liquid interface, or they can be generally applied within a
vapor space. In some cases the agents might be applied to the bulk
liquid phase itself to reduce the surface tension of all of the
processing fluid. Some of these agents exhibit the lower vapor
pressures common with some liquids and solids, while other agents
can exhibit high vapor pressures associated with extremely volatile
gases. By processing in a closed chamber or system, little volatile
surface tension lowering agent can escape into the atmosphere. It
is also easy to recover and reuse these agents if desirable. When
it is necessary to treat or destroy used or excess STG agents, they
can be treated at a point of maximum concentration, avoiding the
need for installation and use of expensive treatment systems
required to treat an entire air-handling system.
[0586] In another embodiment, surface tension lowering components
with limited solubility in the processing liquid can be utilized.
For example, carbon dioxide and ozone gas show limited solubility
in most common aqueous based solutions. As the operating pressure
is increased, not only the rate, but also the amount of these
partially soluble surface tension lowering components entering the
processing solution is increased. Higher solubility resulting from
the increased pressure can lead both to reduced localized surface
tensions, and increased surface tension differentials or gradients
between liquids at the gas-liquid interface and those of the bulk
fluid.
[0587] In accordance with still another embodiment of the present
invention, STG gas can be dissolved in a carrier liquid. The
carrier liquid can then be added to the surface of the processing
liquid without mixing to maintain a separate and distinct layer of
the STG rich carrier liquid on top of the processing liquid. The
substrate is then raised through the carrier liquid layer.
[0588] In various embodiments, the carrier liquid and the
processing liquid may be the same or different materials. An STG
gas is not required. A carrier liquid with a surface tension
different than the processing liquid can be used without the
addition of a separate STG gas component. The carrier liquid can be
of the same or a different temperature than the processing liquid.
While either the processing liquid or the carrier liquid may be
water based or miscible in aqueous solution, this is not required.
Further, one of the carrier liquid and the processing liquid may be
water miscible, while the other liquid is not miscible. Differences
in physical properties such as density may also be used to help
keep the layers separated.
[0589] In accordance with still other embodiments, radiation may be
applied locally to the substrate and/or processing liquid,
particularly in the area of the meniscus, in order to enhance the
surface tension gradient. This applied radiation can result in the
localized heating of the substrate or of the processing liquid in
this meniscus region. With many processing fluids, an increase in
temperature lowers surface tension. For example, microwave
radiation can heat both aqueous solutions and silicon
substrates.
[0590] As a silicon substrate emerges from an aqueous solution,
applied microwave energy heats both the substrate and the solution.
The portion of the substrate not immersed in the aqueous solution
heats up much more rapidly than the bulk solution, or the portion
of the substrate still immersed in the bulk solution. This heating
differential is due to the smaller heat capacity of the exposed
substrate as compared to the heat capacity of the bulk solution.
Therefore, even with poorer coupling of microwave energy with the
silicon substrate than with the solution, a given radiation flux
can cause much faster heating of the exposed substrate.
[0591] In addition, the liquid in the meniscus region also heats
faster than the bulk solution. Not only is the solution volume in
the meniscus areas very small (surface area/volume larger in
meniscus than bulk solution), the solution in this area also
receives energy from the hot exposed substrate. Therefore the
solution in the meniscus heats more rapidly and its surface tension
is reduced. In another embodiment, the bulk solution itself may be
heated prior to removal of an immersed substrate.
[0592] Temperature changes can affect the solubility of a number of
suitable agents. In accordance with still another embodiment, with
some partially soluble surface tension lowering gases, as the
liquid temperatures increase, their solubility in those liquids
decrease. By increasing the process pressure, more of the surface
tension lowering gas is forced into solution. This increased
solubility resulting from an increased pressure can act to offset
or compensate for the reduced solubility at higher temperatures.
This increased solubility also allows for the generation of higher
surface tension gradients, leading to enhanced drying. This
increased solubility further also allows the use of some previously
unavailable surface tension lowering components.
[0593] Additionally, the chemical reactivity of many processing
fluids is a function of process temperature. As such, it may be
desirable to change the temperature or pressure during the
processing to take advantage of the characteristics of different
components when several are used in combination. In some
embodiments, it may be desirable to perform processing with one
agent, followed by a second. In another embodiment, it might be
desirable to perform processing in a single vessel. In still
another embodiment, processing may take place in multiple vessels
in a sequential process.
[0594] A variety of radiation types may be useful for different
applications. These could include, but are not limited to
microwave, ultraviolet, infrared, and electromagnetic induction. In
another embodiment of the invention, the radiation may heat the
substrate or processing liquid on the substrate surface, thus
promoting faster evaporation. For example microwave, infrared and
electromagnetic induction could be useful as various types of
energy useful in heating.
[0595] In another embodiment, the radiation could promote
decomposition of an organic drying liquid present on the surface of
the substrate. The residual drying liquid could be oxidized, either
partially or fully, thus increasing the rate of clearing of the
liquids from the substrate. In such an embodiment, application of
ultraviolet radiation can offer certain advantages. Further, an
oxidant could also be utilized in combination with the radiation to
enhance degradation of the residual liquids. Examples of useful
oxidants include, but are not limited to, ozone, hydrogen peroxide,
and oxides of nitrogen.
[0596] In still other embodiments, the residual processing liquid
on the surface of a substrate can be vaporized off the surface by
the application of ultrasonic energy. As with many of the other
embodiments, the residual processing liquid could be in the form of
droplets or even a film covering the substrate surface. In some
cases very small cavities within the substrate may contain
processing liquid as well. It has been discovered that thin films
or droplets of liquid can be quickly vaporized by applying
ultrasonic energy directly to a substrate, thereby leaving
substrate surfaces dry.
[0597] In order to effectively and efficiently transfer energy from
a vibration member to a substrate, an effective sonic coupling
between the vibration member and the substrate should occur. The
energy should then be coupled between the substrate and the
residual liquid. The energy then causes the liquid to vaporize or
form a mist, which comes off the substrate surface.
[0598] One effective way to ensure optimal coupling is to cause the
substrate to be in intimate contact with the vibration member, and
have the sum of their individual thickness' equal to an odd
multiple one-quarter wavelength (substrate plus vibration member
thickness=n.lamda./4, n=1,3,5,7 . . . ) of the ultrasonic energy
wave. The speed of sound in each material must be taken into
account. The more deviation from this thickness, the poorer the
coupling and less energy is transferred. In the horizontally- and
vertically-oriented embodiments shown in FIGS. 61B and 61A,
respectively, the ultrasonic energy is transferred directly from
the vibration member 6100 through the substrate 6108 into residual
liquid on another surface.
[0599] In some applications, it may not be essential to have
optimal energy transfer, and only partial energy transfer may be
adequate to perform the drying acceptably. For example, in the
optimal case there would be no gap(s) between the vibration member
and the substrate. In many practical applications, because of
machine tolerances or machine designs requiring separation, some
gaps occur. In those cases, it may generally be important that the
gaps not be filled with compressible fluids such as gases. When
those gaps are filled with relatively incompressible fluids such as
water, acceptable energy transfer can occur. FIG. 62 represents an
embodiment using relatively incompressible fluids 6206, where the
gaps can become quite large (>1 m) and still have significant
energy transfer occur.
[0600] In still another alternative embodiment, when transferring
energy across a substrate bounded on both sides by liquid, the
substrates thickness should match an even multiple of one quarter
wavelengths (substrate thickness=n.lamda./4, n=2,4,6,8 . . . ).
FIG. 63 illustrates such an embodiment. Because wafers 6302 would
be required to be much thicker for the megasonic frequencies
generally employed today, this is likely one reason that
conventional megasonic cleaning systems have had limited success
transferring energy across or through silicon wafers. When in
direct contact with the vibration member, it is only required that
the sum of the thickness of the substrate and the vibration member
approximately equal an odd multiple one quarter wavelength.
[0601] Moreover, the thickness of the vibration member and the
substrate may be dictated by other constraints. If the combination
of their individual thickness do not add up to the desired
thickness for favorable energy coupling at a given frequency, it
may be desirable to adjust the frequency to produce an odd multiple
one-quarter wavelength of the ultrasonic energy equal to the sum of
their combined thicknesses. While preferred megasonic systems in
accordance with embodiments of the present invention, some degree
of frequency adjustment of individual crystals to accommodate
thickness differences or changes is desirable, it is not required.
Such frequency adjustment is generally not available for many
conventional fixed frequency systems, yet they can be made to
operate satisfactorily.
[0602] While these embodiments are useful for processing and drying
substrates, energy transfer through the substrate resulting from
contact with a vibration member of the correct thickness can also
be useful in substrate cleaning and processing applications. Single
wafer processing equipment is especially well suited to
advantageously employ embodiments of this invention. These and
other embodiments could be particularly useful in chemical
mechanical polishing (CMP), wafer cleaning by brush scrubbing, and
electrochemical processing as well.
[0603] Other concepts of acoustics can be used to advantage with
new embodiments of the invention by applying the principles of
reflection, transmission, refraction and mode conversion to the
generated ultrasonic wave. Of particular interest are designs that
utilize mode conversion to transform longitudinal or dilatational
waves into surface waves. These surface waves can include Rayleigh,
Lamb and Love waves. Each of these waves produces a different wave
pattern on the surface of the vibrating article. These differing
wave patterns can have unique interactions with substrates and
contaminants on substrates. As device feature sizes shrink, it is
becoming even more important to find "gentler" ways of applying
ultrasonic energy without causing device damage. Additionally, mode
conversion can play a big role in effectively transferring energy
across silicon wafers immersed in an aqueous bath at commercially
available frequencies.
[0604] While embodiments in accordance with the present invention
may relate to chemical processing of substrates utilized during the
manufacture of semiconductor devices, for example substrates
comprising silicon, SiGe, GaAs, Si, GaAs, GaInP, and GaN to name a
few. However, the present invention is not limited to processing of
semiconductor substrates, and other materials may be subjected to
microwave heating during processing. Examples of other candidates
for chemical processing utilizing the present invention include,
but are not limited to, hard disks and hard disk substrates,
optical devices such as mirrors, lenses, or waveguides, and
substrates utilized in the fabrication of micro-electrical
mechanical systems (MEMS), liquid crystal display devices,
bio-medical slides, optical devices, mirrors, lenses, waveguides,
substrates for DNA or genetic markers, liquid crystal displays, and
other media.
[0605] F. Other Megasonic Processing Applications
[0606] Processing a substrate utilizing a directed force (i.e., a
flow of energy or matter), or utilizing a supercritical fluid, may
be enhanced by the simultaneous application of sonic energy, which
causes vibration displacement of the substrate or contacting fluid.
Other forms of energy including mechanical vibration and microwave,
can also be applied, either in conjunction with or in place of the
sonic energy. In accordance with one embodiment of the present
invention, coupling sonic or vibration energy to the substrate with
greater effectiveness and uniformity may enhance the processing of
a substrate with supercritical carbon dioxide.
[0607] Different types of substrate processing that might be
accomplished in accordance with the present invention, include but
are not limited to, cleaning or removing particles and
contaminants, coating or depositing materials, etching or removing
materials and chemically or electrically modifying various
substrate surfaces, structures and layers.
[0608] Embodiments in accordance with the present invention may
also be useful to enhance the effect of processing substrates
utilizing a number of different techniques. One such processing
technique is the application of a jet of gas, liquid, solid, or
mixtures thereof, to a substrate surface.
[0609] In accordance with one embodiment, the present invention is
useful to enhance cleaning of a substrate with a jet of carbon
dioxide snow (dry ice), and/or cryogenic argon aerosol. FIG. 64
shows a simplified cross-sectional view of one embodiment of an
apparatus 6400 for performing such processing. Vibration member
6402 is in mechanical communication with piezoelectric crystals
6404. Substrate 6406 bearing contaminant particles 6408 on its
surface, rests on vibration member 6402. Nozzle 6410 ejects jet
6412 of a cryogenic aerosol at the substrate surface, thereby
physically displacing particles 6408 therefrom.
[0610] For cleaning a substrate with dry ice or snow, a gaseous or
liquid carbon dioxide is allowed to expand (e.g. at constant
enthalpy ) in nozzle 6410, thereby cooling the gas and liquid to
the point that solid carbon dioxide particles are formed in the gas
stream/jet 6412. These particles of frozen carbon dioxide are
directed towards a substrate surface and knock off contaminant
particles from the substrate surface.
[0611] It has been suggested that some of the solid carbon dioxide
particles melt due to deformation resulting from collision with
contaminant particles or the substrate surface. The resulting
"liquid" carbon dioxide provides a good solvent for dissolving or
dislodging contaminants from a substrate surface. The melted dry
ice particle then may re-solidify as it rebounds back off the
substrate, carrying the contaminant with it.
[0612] Impingement of the cold stream/jet also can lead to
substantial cooling of the substrate surface as processing
proceeds. Energy could be introduced to the substrate to prevent
the usual cooling, and minimize condensation of moisture or other
vapors. Such processing is generally carried out at atmospheric
pressure, although this is not required.
[0613] For cryogenic aerosol cleaning with argon, the gaseous argon
is generally allowed to expand in a nozzle (e.g. adiabatic
expansion/cooling) into a chamber maintained at substantially less
than atmospheric pressure, thereby causing some of the argon gas to
be converted into small frozen argon particles. These frozen
particles propelled in the gaseous argon stream/jet are similarly
directed towards a substrate surface to knock off particulate and
other contaminants. Again, substantial substrate cooling can occur
during processing.
[0614] While not wishing to be bound by any particular theory, it
is possible that processing in accordance with embodiments of the
present invention may be enhanced by the following mechanism. As
the surface of the substrate vibrates, and its position in space is
repeatedly changed, the dynamics of various processing events
occurring with different technologies are modified.
[0615] For example, where a cryogenic aerosol jet comprised of cold
argon gas and frozen argon particles is applied against a substrate
surface, the nature of the interaction between those frozen argon
particles and the contaminant particle on the substrate surface is
changed. Where the substrate surface is stationary, a particular
collision dynamic is present between the frozen argon particle and
the contaminant particle, resulting in a particular momentum
exchange and particle deformation and rebound.
[0616] When vibration or rapid movement of the substrate
surface/particle occurs, the dynamics of the collision may be
modified. Thus depending upon the frequency of vibration, during
one portion of the collision event (i.e. the duration of
particle-to-particle interaction) the surface and contaminant borne
thereon may be accelerated towards the incoming frozen argon
particle. However, during another portion of the collision event,
the surface/contaminant may be accelerated away from the incoming
frozen argon particle. The interaction energy/force is increased
with acceleration toward the incoming particle, and decreased
during acceleration away from the incoming particle.
[0617] Moreover, with high frequency vibration, several changes in
the direction of acceleration of the surface/contaminant may occur
during a single collision event. Thus instead of involving a single
collision and corresponding momentum transfer, high frequency
vibration may actually provide for repeated increase and decrease
in the force applied to the particle during the collision time
period. The resulting momentum transfer may exhibit a pulsed
nature, in a manner analogous to a jackhammer.
[0618] Several of the processing techniques and technologies
discussed herein may cause substrates/surfaces to vibrate during
operation. Historically, there have not been convenient ways to
change the resulting frequency, intensity, and uniformity of such
processing-induced vibration to more desirable frequencies,
intensity or uniformities that can be used to enhance processing.
Therefore, convenient ways to adjust or modify frequency, intensity
and uniformity of vibration of various substrates are needed.
[0619] One approach to adjusting the character of substrate
vibration during processing is to effectively couple sonic and
other vibration-causing energy with the substrate. Several methods
and apparatuses demonstrating increased coupling of sonic energy
with various substrates have been disclosed above. Enhanced sonic
coupling with the substrates can occur with the front side, back
side, or edge of the substrate. In the latter case, the application
of sonic energy is parallel to the substrate surface.
[0620] Another approach to adjusting the character of the substrate
vibration during processing is to enhance the uniformity of energy
emanating from vibrating surfaces. Methods and apparatuses
demonstrating increased uniformity of the energy field emanating
from a vibrating surface are also disclosed above. Such increased
field uniformity can correspondingly lead to increased uniformity
of processing of a substrate surface.
[0621] Other types of applied energy, exhibiting a wide range of
frequencies, intensities, and uniformity, may enhance processing.
Such alternative forms of energy include, but are not limited to,
thermal, microwave, mechanical vibration, IR, UV, hydrodynamic or
fluid flow.
[0622] The application of thermal energy can cause a change in
substrate temperature. Energy can be added, resulting in substrate
heating. Alternatively, energy can be removed, resulting in
substrate cooling. The characteristics of adhesion of various
particles and contaminants to the substrate surface can be modified
as the temperature changes.
[0623] With some substrates, microwaves and infrared radiation can
be used to increase substrate temperature, while other substrates
may require direct contact between the substrate and a heated
element. Cooling can be accomplished effectively by bringing the
substrate into intimate contact with a cooling member containing
channels able to accept liquid nitrogen.
[0624] Processing at elevated or reduced pressure may be
beneficial, especially at conditions above the critical point for
various processing fluids including carbon dioxide. The substrates
may be caused to vibrate at various frequencies utilizing different
techniques. These approaches include, but are not limited to: sonic
coupling through direct contact with a vibrating member or
indirectly through an intervening fluid layer; contact with a
flowing or pulsating fluid and presence within an oscillating or
pulsed electrical, RF or magnetic field. The energy can be coupled
to, or impinged upon, any side or surface of the substrate, or at
any angle with respect thereto.
[0625] In accordance with some embodiments, at some angles, the
applied energy may reflect from a substrate surface, while at other
angles the energy might be transferred across the substrate. At
still other applied angles, one form of energy wave, (such as a
dilatational pressure wave) can be transformed into another (such
as a surface or shear wave) depending upon the environment around
the substrate.
[0626] By way of further example, the substrate surface/contaminant
can be made to vibrate by coupling sonic energy to the backside of
the substrate as also shown in FIG. 1. This is accomplished by
bringing the substrate into direct contact with one lateral surface
of a vibration member. This vibration member can be comprised of a
plate with piezoelectric crystals attached to the opposite lateral
surface. The plate can be constructed of various materials
including but not limited to such widely differing materials as
aluminum, stainless steel, quartz and ceramics. The piezoelectric
crystals can be energized with an applied potential alternating at
a frequency of near 1 MHz. Once vibrating, a jet of carbon dioxide
snow can be directed towards the substrate surface bearing
contaminant particles.
[0627] There is no upper or lower limit to the range of acceptable
frequency of vibration. In practice, however, usable frequencies
will likely lie within the range of from 1 Hz to 1 GHz. Other types
of energy having even higher operating frequencies, such as
conventional microwaves, could also be used under certain
conditions.
[0628] Alternatively, instead of having direct contact between the
substrate and the vibration member, a coupling fluid may be used
when the vibration member is separated from the substrate. Various
methods of coupling and energy transfer include the use of sonic
nozzles. Some methods allow coupling with the front surface of the
substrate, while others allow coupling with the back surface of the
substrate. Other methods allow energy to be transferred across the
thickness of the substrate, and still other methods lead to
reflection of energy. Some methods completely immerse the substrate
in a processing fluid, while others call for the application of
only a thin liquid film.
[0629] In another embodiment shown in FIG. 69, an ultrasonic nozzle
6900 for the gas/liquid/solid jet 6902 is utilized to form pressure
pulses in the gas/liquid/solid jet prior to the jet striking the
surface of the substrate 6904. In this embodiment, piezoelectric
crystals are incorporated into the expansion nozzle 6900 used in
cryogenic aerosol cleaning with argon. While this application is
generally carried out at below atmospheric pressure, this is not
required by other types of jet cleaning. In some cases, pressures
above atmospheric may be desirable.
[0630] In an alternative embodiment, shown in FIG. 70, a megasonic
nozzle 7002 can be used in conjunction with a vibration member
7004. In another alternative embodiment, shown in FIG. 71, a
megasonic nozzle 7112 can be used in conjunction with the
application of microwave energy. In either of the embodiments shown
in FIGS. 70-71, relative movement between the substrate and the
nozzle is possible.
[0631] In accordance with another embodiment, the substrate may be
cooled prior to, during, and/or after introduction of the jet of
energy or matter. While the jet can be comprised of the typical
mixtures of gas and frozen particles described above, in certain
embodiments the jet may be comprised of a gas, a liquid, or a
combination of the two. Sonic energy or substrate surface vibration
may be used, but is not required.
[0632] This cooling can be accomplished in a number of ways and
performed at various rates and to different temperatures depending
upon the individual characteristics of the substrate and the
contaminant particle. FIG. 72 shows a simplified cross-sectional
view of one embodiment of a processing apparatus 7200 in accordance
with the present invention. Substrate holder 7202 features maze
7204 of cooling channels. Liquid or gaseous nitrogen could be
introduced into channels 7204 to rapidly reduce the temperature of
substrate 7206 to a cryogenic range. Rapid cooling may lead to a
reduction in the force of adhesion exhibited between the substrate
surface and a contaminant particle, facilitating particle
removal.
[0633] In another embodiment as shown in FIG. 80, a uniform low
velocity of a partially condensable gas 8000 is flooded over the
face of the substrate 8010 with contaminant particles 8020 thereon.
Cold chuck 8030 is rapidly cooled. As some of the gas 8000
condenses and freezes between the substrate surface 8010 and
contaminant particles 8020, the bond between substrate 8010 and
particle 8020 is broken. An optional high velocity fluid jet (not
shown) blows the dislodged contaminant particles 8020 off substrate
surface 8010 while piezoelectric crystals 8050 provide high
frequency vibration to substrate 8010.
[0634] Less energy is typically required to break the bond between
ice and a substrate than is required to melt a covering ice layer.
Thus in accordance with still another embodiment of the present
invention, after the condensable gas has formed an ice layer over
the particles and surface of the substrate, sufficient energy may
be applied to heat the substrate and break the bond between the ice
and substrate, but not sufficient to completely melt the ice layer.
Once the substrate-ice bond is broken, ice and contaminants can be
blown off or removed more easily, hopefully as a sheet or chunks
containing the contaminant particles.
[0635] In accordance with yet another embodiment, the substrate may
be heated prior to introduction of a jet of energy or matter
comprising at least one of a gas, a liquid, a solid, or
combinations thereof. Microwave energy can be an effective means of
heating a substrate rapidly for those substrates that are able to
accept microwave energy.
[0636] Additionally, microwave energy will heat some substrates
while not heating certain gases and solids. This can allow the
substrate to be maintained at a desired temperature without heating
the incoming jet of material prior to contact with the
substrate.
[0637] FIG. 71 shows a simplified cross-sectional view of an
embodiment of a processing apparatus 7100 in accordance with the
present invention. Substrate 7102 is supported within processing
chamber 7104. Backside 7102b of substrate 7102 is exposed to
microwave energy from source 7106, and is heated thereby. Front
side 7102a of substrate 7102 bearing contaminant particles 7108, is
exposed to jet 7110 of cryogenic aerosol from nozzle 7112.
[0638] The addition of vibration or other forms of energy is
optional. Additionally, a liquid jet could also be included along
with, or in place of a cryogenic aerosol cleaning jet to further
extend the range of possible processing.
[0639] In order to maximize displacement of the substrate surface
during vibration, it may be beneficial to physically hold the
substrate into contact with the vibration element. This could be
accomplished in a number of ways including use of a vacuum chuck or
other holding fixture. Various holding and clamping designs not
relying upon vacuum could also be employed.
[0640] FIG. 66 shows a simplified cross-sectional view of an
alternative embodiment of a processing apparatus 6600 in accordance
with the present invention, wherein substrate 6602 is clamped on
substrate holder 6604 by vacuum suction. Piezoelectric crystals
6606 are attached to the back of vibration member 6612 that is in
turn in contact with the back of holder 6604. Crystals 6606 are
energized with a high frequency 6602 while holder 6604 is rotated
to introduce relative motion between the aerosol jet and the
substrate surface. Alternatively, the jet could be moved while the
substrate and holder remains stationary.
[0641] Mechanical vibration can be utilized either in place of or
in combination with, sonic energy. In another embodiment of the
present invention shown in the simplified cross-sectional view of
FIG. 67, substrate 6700 is positioned in substrate holder 6702.
Mechanical vibrator 6706 is coupled to the back of the substrate
holder that also serves as vibration member 6704 to allow transfer
of vibrational energy from vibrator 6706 to substrate 6700 to cause
displacement of the substrate surface and contaminants 6708 present
thereon. Cryogenic aerosol 6710 is directed toward the substrate
surface from nozzle 6712.
[0642] The substrate surface can face any direction, or be in any
orientation and still operate in accordance with embodiments of the
present invention. In certain embodiments the substrate surface may
preferably face upward, and in other embodiments may face downward
or at some angle with respect to vertical.
[0643] In a similar fashion, the jet can be directed towards a
substrate surface at any angle, ranging from perpendicular to
parallel. During processing this angle need not remain constant.
Positioning of the jet at a proper angle with respect to the
substrate surface may lead to mode conversion of pulsating pressure
waves exiting the jet nozzle into vibrating surface waves in the
substrate.
[0644] In another embodiment of the present invention shown in the
simplified cross-sectional view of FIG. 68, liquid layer 6800 is
formed on surface 6802a of substrate 6802 secured on combination
substrate holder/vibration member 6804. Sonic energy is coupled to
the backside of substrate 6802 by attaching piezoelectric crystals
6806 to the backside of combination substrate holder/vibration
member 6804. Aerosol jet 6808 is directed towards front surface of
substrate 6802, which is covered with thin fluid layer 6800.
[0645] While jet 6808 may be of the cryogenic aerosol type, that is
not required. Jet 6808 can comprise a gas only, without inclusion
of frozen solid particles, or may comprise only a liquid, or may
comprise a combination of both phases. Processing can be
accomplished at any pressure or temperature.
[0646] Embodiments in accordance with the present invention are not
limited to the application of a jet of solid, liquid, or gaseous
particles. Another processing technique which may be enhanced in
accordance with embodiments of the present invention is treatment
of the surface of a substrate with energy from a laser. Examples of
such laser processing include shock cleaning, dry cleaning, and
explosive evaporation.
[0647] FIG. 65 shows a cross-sectional view of one embodiment of an
apparatus 6500 in accordance with the present invention, which may
be utilized to perform laser shock cleaning (LSC). LSC occurs when
laser 6502 is focused just above surface 6504a of substrate 6504
supported on vibration member 6506 in mechanical communication with
piezoelectric crystals 6508. Focused beam 6510 from laser 6502
creates a thermal shockwave 6512 by rapidly heating a localized
section of the atmosphere above substrate surface 6504a. This
shockwave propagates over surface 6504a of substrate 6504,
displacing contaminant particles 6514 therefrom.
[0648] Again, without wishing to be bound by any particular theory,
processing of a substrate with laser energy may be enhanced in
accordance with the mechanism described above for processing with a
jet of material. Thus for LSC, vibration of the proper frequency
and intensity of the surface/contaminant could modify the dynamics
of the interaction between the shockwave and contaminant particles
on the substrate surface. Rapid, repetitive movement of the
substrate surface/contaminant during the time period that the
shockwave interacts locally with individual particles or
contaminants could alter the effectiveness and efficiency of
particle and contaminant removal, allowing a change in the
intensity of applied energy.
[0649] FIG. 73 shows a simplified cross-sectional view of an
alternative embodiment of a processing apparatus in accordance with
the present invention, wherein during LSC processing, substrate
7300 is contained within enclosure 7302 pressurized above
atmospheric pressure. This elevated operating pressure changes the
velocity of propagation and intensity of shockwave 7304, thereby
resulting in a different dynamic interaction between shockwave 7304
and surface 7300a of substrate 7300. In still other embodiments,
the enclosure could be depressurized, thereby lessening the
intensity of the laser-induced shockwave.
[0650] The apparatus of FIG. 65 may be utilized to perform laser
processing other than LSC. For example, laser dry cleaning, which
is characterized by use of the laser to directly ablate/destroy
particles directly off a substrate surface, or to modify that
surface could be employed. Alternatively, explosive evaporation,
which occurs when a laser is focused into a thin layer of an
alcohol/water mixture on the substrate surface causing violent
evaporation/explosion of the mixture, could be practiced. The
resulting "explosion" wavefront propagates over the substrate
surface, dislodging contaminant particles therefrom.
[0651] In accordance with another embodiment, the substrate is
cooled and vibration introduced prior to the application of laser
shock. While generally performed at atmospheric pressure, the laser
shock can also be accomplished at either above or below atmospheric
pressure depending on surface and particle characteristics and the
collision dynamics desired. There is no theoretical upper or lower
limit for operating pressure, other than the limits imposed by
materials of construction and equipment designs. Equipment able to
operate at pressures exceeding several thousand pounds per square
inch are know in industry.
[0652] Still another processing technique enhanced in accordance
with embodiments of the present invention is treatment of a
substrate with a supercritical fluid, for example carbon dioxide
(CO.sub.2). At temperatures and pressures above a critical point of
a particular fluid, a material becomes a supercritical fluid.
[0653] Under such conditions, the material is technically neither a
gas, a liquid, nor a solid. Carbon dioxide in the supercritical
state exhibits a low surface tension characteristic of the gaseous
state, but also exhibits an elevated density characteristic of the
liquid state. While a high velocity jet can enhance processing,
even in the absence of a high velocity jet, useful processing can
result from unique characteristics of a fluid present above
supercritical conditions.
[0654] In certain conventional designs of processing equipment,
supercritical carbon dioxide is nearly stagnant within a processing
chamber. Substrates are present within the supercritical fluid,
which acts as a solvent to remove contamination from the substrate
surface.
[0655] In other conventional equipment designs, carbon dioxide is
continuously pumped through a processing chamber in an attempt to
create minimal hydrodynamic forces aiding in the removal
contamination. Rather than holding the operating pressure of the
system constant, the operating pressure of the system may be
allowed to fluctuate or pulse during processing. These pressure
pulses can either all be above the supercritical point, or can
cycle between supercritical and sub supercritical levels. At sub
supercritical conditions, a fluid such as carbon dioxide for
example can become a gas, liquid, or solid, and then transform back
into a supercritical fluid when supercritical conditions are
reestablished.
[0656] It has been suggested that introduction of sonic energy into
supercritical carbon dioxide may enhance substrate processing.
However, because the supercritical fluid is not in a liquid or
solid state, transfer of sonic energy through the supercritical
fluid may be substantially reduced or attenuated. In general,
attenuation within a medium varies as the square of the frequency
of the sonic energy. Thus, as the semiconductor industry migrates
toward application of higher frequency sonic energy to remove
smaller particles, it becomes increasingly difficult to transfer
significant amounts of sonic energy through a medium.
[0657] The difficulty in applying vibrational energy in the context
of supercritical processing becomes even more troublesome as the
size of substrates is increased. This effect is particularly
evident when the sonic energy is applied in a direction parallel to
the substrate surface, as has been traditionally done in batch
processing. Historical attempts to transfer significant amounts of
sonic energy directly across substrates has been hampered by the
requirement of matching substrate thickness to an even multiple
one-quarter wavelength of the pressure wave for maximum transfer.
For example, because of their small thickness of semiconductor
wafers, frequencies three or four time those currently used for
processing would be required.
[0658] Sonic energy has been transferred over long distances
parallel to substrate surfaces, in conventional batch processing
and cleaning utilizing aqueous solutions at atmospheric pressure.
However, such approaches are not likely to succeed in the context
of supercritical processing, because of the sonic characteristics
of the materials involved. Therefore it becomes even more important
to maximize energy coupling between the substrate and the vibration
member and to minimize distances over which sonic energy must be
transferred in the fluid. Efficient transfer of energy directly
across the thickness of a substrate is especially useful, allowing
one surface of the substrate to be in direct contact with a
vibration member or closely spaced therefrom. Such an arrangement
minimizes the distance that sonic energy must travel through highly
attenuating fluid.
[0659] A poor impedance match between the sonic impedance of the
supercritical fluid and of the material of the vibration surface
can prevent transfer of a sufficient amount of sonic energy. Thus,
the selection of materials having the proper thickness, becomes
important to minimize the impedance mismatch typically observed in
conventional processing.
[0660] Supercritical processing enhanced in accordance with
embodiments of the present invention can include the use of various
gases and liquid mixtures with the supercritical fluid which may be
referred to as co-solvents. Alternatively, substances other than
carbon dioxide can be utilized for processing at conditions above
their critical point. And while it is generally desirable for the
supercritical processing fluid to comprise multiple components
forming a homogeneous phase, this is not required. In accordance
with alternative embodiments, the processing chemistry could
comprise two or more phases. Two phase or even multiple phase
chemistries could be exploited.
[0661] Sonic energy may be transferred through a medium as a series
of pressure waves. Thus, whether of relatively low frequency from a
mechanical vibrator, or of relatively high frequency from a
piezoelectric crystal, sonic energy may be viewed as an extension
of slower pressure pulses historically utilized in supercritical
processing. In accordance with embodiments of the present
invention, it is now possible to have pulses at multiple
frequencies introduced simultaneously to even further enhance
processing.
[0662] In one embodiment, sonic energy may be introduced to a
processing chamber operating at supercritical conditions. A shown
in the simplified cross-sectional view of FIG. 74, this may be
accomplished by directly attaching piezoelectric crystals 7400 to
the outside of at least one wall 7402 of the chamber 7404, with the
thickness of that wall chosen to approximately equal an odd
multiple one-quarter wavelength of the applied sonic energy to
maximize energy coupling. As the energy in FIG. 74 is being
transferred across the vessel wall instead of being generated
within the vessel itself, issues associated with sealing electrical
leads exiting a high-pressure environment, and with compatibility
between the piezoelectric crystals 7400 and the processing
chemistry are avoided.
[0663] While the embodiment of FIG. 74 depicts a chamber with fluid
inlet 7406 and outlet 1008 oriented such as to suggest fluid flow
parallel to the surface of the substrate 7410, high velocity jets
may be utilized and directed at any angle relative to the surface
of the substrate.
[0664] FIG. 79 shows a simplified cross-sectional view of such an
alternative embodiment which incorporates the benefits of the
unique physical and chemical properties of supercritical fluids and
sonic energy with the hydrodynamic properties of a fluid jet. As in
several of the previous embodiments, jets 7900a-c provide a
convenient way to transfer momentum to contaminant particles.
Further, the multiple jets could be sequenced to sweep across
substrate 7902 if desired. This can be combined with a megasonic
system comprised of multiple piezoelectric crystals fired
sequentially or continuously.
[0665] In accordance with another alternative embodiment, where the
thickness of the vessel wall is determined by other constraints,
the frequency of excitation of the piezoelectric crystals can be
adjusted to make the given wall thickness equal the desired odd
multiple one-quarter wavelength as disclosed in the above
referenced application.
[0666] Further optionally, instead of bonding the crystals directly
to the outside of a wall of the chamber, the crystals could be
bonded to a vibration plate that is then rigidly attached to the
chamber wall as shown in the simplified cross-sectional view of
FIG. 75. The resulting sum of the thickness of the vibration plate
7500 in intimate contact with the tank wall 7502, and ideally
including the substrate thickness as well, should equal an odd
multiple one-quarter of the wavelength of the applied energy. While
vibration member 7500 is shown in FIG. 75 as being bonded to the
outside of chamber wall 7502, the vibration member could optionally
be bonded to the inside of the chamber wall. However, such an
alternative embodiment would make establishing reliable electrical
connection with the vibrating element slightly more difficult, and
could raise potential issues of compatibility between the material
comprising the vibrating member and the surrounding processing
chemistry to which it is exposed.
[0667] In such an alternative embodiment, the substrate may either
be suspended in supercritical fluid within the chamber, or may be
brought into direct contact with the chamber wall as was shown in
FIGS. 74 and 75. When brought into direct contact or close
proximity with the chamber wall, variation in the frequency or
intensity of vibration can be utilized to disrupt near-field
effects and enhance the uniformity of processing.
[0668] Where the substrate is in direct contact with the chamber
wall (or vibration member) transmitting the sonic energy, the
thickness of the substrate as well as the thickness of the chamber
wall (or vibration member) may need to be taken into account in
maximizing energy transfer. In such applications, the combined
thickness of the substrate and the chamber wall (or vibration
member), taking into account the material composition of each
element, should conform to approximately an odd multiple
one-quarter wavelength guideline at the operating frequency. While
an exact match of thickness is not required, it is generally
preferable to be within +/-50% of the guideline values.
[0669] The frequency of sonic energy or the thickness of the
chamber wall (and/or vibration member) may be selected or adjusted
to maximize the transfer of sonic energy across the substrate.
Megasonic generators manufactured by PCT Systems Inc. of Fremont,
Calif., allow for such adjustment.
[0670] Since the speed of sound can vary greatly from one material
to another, each material or combination of materials will require
a unique total thickness to match a selected frequency, or a unique
optimal frequency to match a resulting thickness sum. For example,
while the speed of sound in aluminum is approximately 6380 m/s, in
stainless steel it is only 5780 m/s. Thus to ensure optimal sonic
energy coupling, an aluminum plate of greater thickness than a
stainless steel plate would be required.
[0671] When the substrate is not in direct physical contact with a
chamber wall, the substrate may be oriented at any angle with
respect to any of the walls. When oriented within a proper range of
angles with respect to the incident sonic energy, transfer of sonic
energy across the substrate can also occur. Outside this range of
angles, the sonic energy may be reflected from the substrate
surface. FIG. 77 shows a design for a processing apparatus wherein
sonic energy 7700 bounces off substrate 7702 and off walls 7706 of
vessel 7704.
[0672] While the substrate is shown in FIG. 77 as in direct contact
with the vessel wall, this is not required to obtain desired
reflection off the substrate surface. When the incident angle of
the pressure wave is between the first and second critical angles
as discussed above, energy can either be transferred across the
substrate, or the incident pressure wave can be converted into a
surface or shear wave, with little or no reflection.
[0673] As discussed previously, the range of angles over which
significant mode conversion from dilatational to shear or surface
waves occurs, perhaps including some dilatational wave diffraction,
is likely to be larger than the range of angles over which cross
substrate transfer of energy occurs. As with any wave interactions
with multiple materials, some reflection may occur even within part
of the range of angles over which significant cross substrate
transfer or even mode conversion occurs.
[0674] When energy is transferred across a substrate, for example
when the substrate is in direct contact with a surface of a
vibrating member, is of a proper thickness or is oriented at an
appropriate angle, the chamber may be advantageously designed to
reflect transferred sonic waves back toward the substrate surface.
Such reflections can be from stationary or moving surfaces. FIG. 76
shows a simplified cross-sectional view of one such design 7600,
with reflections 7602 from a moving reflector 7604 impinging upon
stationary substrate 7606.
[0675] In accordance with an alternative embodiment, the substrate
can be in motion. As shown in the simplified cross-sectional view
of FIG. 78, substrate 7800 is in motion and simultaneously receives
sonic energy from piezoelectric crystals 7804 outside the vessel
7802. While the specific embodiment of FIG. 78 shows the
piezoelectric crystals generating the sonic energy, in accordance
with other embodiments a mechanical vibrator could alternatively be
utilized for such purpose. Relative motion between the substrate
and the chamber or the incident sonic energy, can thus be
incorporated into processing.
[0676] In another embodiment where sonic energy is added to a
process chamber operating at supercritical conditions, a vibration
member may be present within the process chamber. In such an
embodiment, the sonic energy is not transferred across a chamber
wall, and the thickness of the chamber wall need not equal an odd
multiple one-quarter wavelength of the applied sonic energy.
Rather, the thickness of the chamber wall need only provide
adequate strength to contain the elevated pressures implicated in
supercritical operation.
[0677] When sonic energy is transmitted across very thick walls or
substrates, power attenuation may occur. By not requiring the sonic
energy to be transferred across a thick vessel wall, more energy
may be available for transfer into or across a substrate residing
within the vessel.
[0678] Although the invention has been described in terms of
preferred methods and structures, it will be understood to those
skilled in the art that many modifications and alterations may be
made to the disclosed embodiments without departing from the
invention. For example, while the above description and figures
have focused upon methods and apparatuses wherein the vibrating
member is in direct physical contact with the substrate, this is
not required by the present invention.
[0679] The substrate can either be placed into direct contact with
the vibration member or be separated from it. When in direct
contact, and the chamber is small, it may be desirable to have
energy reflect off the chamber walls back towards the substrate
surface. To enhance reflection and minimize energy loss through the
chamber wall, the reflection should be conducted at the appropriate
angles. This can be accomplished in either a static mode or a
dynamic mode where there is relative motion between the substrate
and some surface. That surface being a chamber wall or some
additional energy-reflecting surface.
[0680] While the above description has emphasized enhancement in
processing through the application of sonic energy, other types of
energy could also be utilized. Such other types or forms of applied
energy can include hydrodynamic, microwave, infrared, ultraviolet
and thermal. In the case of thermal energy, you can have the
addition of energy (heating) or the removal of energy
(cooling).
[0681] F. Soft Megasonic Fluids
[0682] As semiconductor processing of smaller feature sizes becomes
necessary, damage not posing a problem at larger feature sizes has
become more of a concern.
[0683] The cavitation threshold pressure corresponds to the power
density required to produce cavitation events, or to produce
cavitation events that result in visible damage from those events.
According to classical ultrasonic theory, most batch megasonic
systems operate at a power density well below that traditionally
considered the cavitation threshold pressure at these given
frequencies.
[0684] The cavitation threshold pressure increases with increasing
frequency of applied sonic energy. With megasonic processing, the
typical power densities used in processing are far below where
cavitation is expected. Even so, damage to fragile semiconductor
structures is being reported at these higher megasonic
frequencies.
[0685] While not wishing to be bound by any particular theory,
recent ultrasonic theory addressing higher operating frequencies
utilized in megasonic processing, discusses several additional
parameters or possible sub-processes including microcavitation and
microstreaming in addition to traditional acoustic streaming. Under
some conditions, these sub-processes may lead to enhanced cleaning
and processing. Under other conditions, damage may result.
[0686] Examples of megasonic processing damage may be seen with
fragile polysilicon lines. Inspection after traditional batch or
single wafer megasonic processing may reveal portions of a line or
lines to be either broken free from the substrate, or missing
entirely. This damage may be caused by cavitation, microcavitation,
microstreaming, or even just the pressure waves traveling through
fluid impinging on the surface of the substrate, or on the
polysilicon lines directly, depending on conditions and method of
introduction of sonic energy to the processing vessel.
[0687] Specifically, when cavitation events occur near a line,
imperfections in a line, or the area of the line's bond with the
substrate, energy released by collapse of the cavities and
microcavities, may be manifested in the form of high velocity fluid
jets. These high velocity jets can cause the line to be pushed or
even dislodged from the substrate.
[0688] Alternatively, as gas bubbles are formed and grow between
the substrate and the line, a line or other feature may be lifted
or pried away from the substrate.
[0689] As a sonic energy pressure wave travels through the fluid,
gas bubbles may often form from gas dissolved in the liquid, at the
low pressure trailing edge of the wave. As multiple sequential
bubble forming events occur, each of the newly formed and growing
bubbles between the substrate and the overhanging line may cause
more of the line to be lifted or dislodged from the substrate.
[0690] Generally, the higher the content of the gas in the liquid,
the lower the cavitation threshold pressure and the less damage
that occurs. The higher gas content of the liquid may provide more
cavitation and microcavitation nucleation sites to form a larger
number of smaller and more stable cavities and bubbles, each
producing less energy upon collapse. For fluids containing little
or no dissolved gas, a higher power density is required to form
voids in the fluid. At high cavitation threshold pressures and
relatively low frequencies, the voids can be filled with vapor
instead of gas.
[0691] The vapor-filled cavities collapse more violently than gas
filled cavities (bubbles) and can cause more severe damage. Vapor
filled cavities may also likely be larger than the gas filled
cavities, and can store more energy that is released during
collapse. This may explain why macro-scale damage is observed at
relatively low frequencies, while micro-scale damage of the type of
emerging concern is observed at higher frequencies.
[0692] Ultrasonic processing in DI water can produce more damage to
a thin metal foil than equivalent ultrasonic processing in acetone
or an alcohol. It is theorized that water creates more damage due
to high internal cohesion between adjacent packets of water
molecules, as separated fluid packets are drawn back together
harder under higher attraction energy. Therefore, fluids with less
internal cohesive strength should produce less damage, even with
the smaller microcavitation events.
[0693] Recently, semiconductor processing has utilized more dilute
solutions of various processing chemicals, with the ultimate goal
of processing semiconductor substrates in DI water at near room
temperature. This trend is driven by the expense of preparing
chemicals of the necessary purity, and of disposing of the
chemicals in an environmentally-friendly way once used.
[0694] Simultaneously, the direction of the industry has been
towards the manufacture of smaller and smaller device features and
structures. The convergence of the trend to use ever more dilute
aqueous solutions to process ever-smaller features, has enhanced
the level of scrutiny regarding potential damage inflicted on
features by megasonic processing.
[0695] It has also been suggested that the use of water results in
greater damage than the use of a number of other fluids, because
water exhibits relatively low solubility for many gases, while
simultaneously possessing high thermal conductivity. Higher gas
solubility provides more cavitation nucleation points for less
energy intense cavitation events. And under the high temperatures
generated with sonoluminesence events, fluids possessing high
thermal conductivity may transfer that localized high energy to
substrate surfaces more easily, leading to increased damage.
[0696] Therefore, a need exists for fluids for use in megasonic
processing which impart less damage to small and fragile features
on the surface of a processed substrate.
[0697] In accordance with various embodiments of the present
invention, damage to small features inflicted by wet megasonic
processing, may be reduced utilizing a variety of techniques, alone
or in combination. According to one technique, megasonic energy may
be applied to liquids that are less prone to producing cavitation
and microcavitation damage, for example liquids comprising
substantial components other than water, such as methanol or
acetone. According to another technique, damage arising from
cavitation and microcavitation events may be reduced by the
presence of microbubbles within the processing fluid. According to
still another technique, megasonic processing may be conducted
under temperature or pressure conditions that control the void
fraction and function of entrained microbubbles, controlling the
degree of attenuation, the cavitation threshold pressure, and
cleaning and processing performance.
[0698] Embodiments of methods, apparatuses, and compositions in
accordance with the present invention utilize (1) alternative
megasonic fluid types, (2) introduction of microbubbles, and (3)
processing at elevated/reduced pressure or temperature conditions,
alone or in combination, to reduce the damage imparted to substrate
features during megasonic processing.
[0699] In ultrasonic processing, less cavitation damage is observed
with some fluids than with others. For example, in fluids like
methanol and acetone, applied sonic energy can produce less damage
to an aluminum metal foil than water, or to a lesser extent,
kerosene.
[0700] In an attempt to minimize damage inflicted during megasonic
processing, it has been proposed to consider alternative fluids
which do not cause as much cavitation and microcavitation damage,
or which are better able to contain small or micro-sized
bubbles.
[0701] Accordingly, a first technique in accordance with the
present invention utilizes -fluids exhibiting solubility for gases
or mixtures of gases, which exceeds their solubility in traditional
processing fluids such as deionized (DI) water. Components of such
alternative megasonic processing fluids can include, but are not
limited to, alcohols such as methanol, ethanol, and isopropyl
alcohol (IPA); ketones such as acetone and MEK; other types of
organic solvents such as hydrocarbons; and more exotic materials
such as perfluorinated solvents, including various combinations of
these various components. Many such liquid solutions may be
considered, based for example on their chemical properties
(reactivity with substrates), or on their physical properties (gas
solubility and thermal conductivity).
[0702] The goal underlying this first approach is to obtain
relatively high solubility of gas in the liquid, as compared with
the solubility of the gas in DI water. Such increased gas content
should facilitate cavitation and microcavitation, drop the
cavitation threshold pressure, and result in less damage during
processing.
[0703] Additionally, the identity of particular gases or gas
mixtures dissolved in the fluid may also comprise an important
processing parameter. This is so not only because alternative gases
may exhibit higher solubility in the processing liquid, but because
the alternative gases may lead to the formation of different ionic
species in a sonic energy field, especially when sonolumunessence
occurs.
[0704] A second technique in accordance with embodiments of the
present invention teaches the introduction of small bubbles,
preferably micro bubbles into the processing fluid near the surface
of the substrate. Small or micro-bubbles introduced into the
processing fluid can buffer or cushion cavitation or
microcavitation events which do occur. Other theories teach that
microbubbles actually facilitate the removal of small particles, or
cleaning and various sonic processing.
[0705] Larger, macro-scale bubbles introduced into and dispersed
within the processing fluid may attenuate sonic energy transfer,
simultaneously reducing damage from cavitation shockwaves.
Introduction of smaller micro-scale bubbles dispersed in the
processing fluid should not attenuate sonic energy as
significantly. Additionally, the small bubbles may still be able to
absorb damaging shockwaves generated by microcavitation events
within the fluid, particularly microcavitation events on or near
the surface of the substrate.
[0706] Introduction of small bubbles into a processing fluid in
accordance with embodiments of the present invention may also serve
as an extension of the first approach previously described, as
bubbles within the fluid may provide a dispersed gas phase
reservoir allowing the concentration of dissolved gas in the fluid
to be maximized.
[0707] In certain instances, the enhanced concentration of gas may
also assist in cleaning. According to some theories, pressure waves
of sonic energy flowing past small or microbubbles tend to cause
the microbubbles to oscillate rapidly, which in turn may result in
small, localized changes in fluid velocity. The resulting localized
fluid velocity gradients may remove particles from surface of
substrates near these oscillating bubbles. Rapid oscillation of
small bubbles caused by sonic pressure waves is often referred to
as microstreaming within the fluid.
[0708] There are a number of ways of generating and introducing
small bubbles or microbubbles within processing fluids. Approaches
for introducing small bubbles can range from vigorous high shear
mixing of fluids in a vessel with a liquid/fluid interface, to
introduction of gas into a rapidly accelerating fluid using a
venturi mixer.
[0709] For both of the approaches discussed above, chemical
characteristics of the liquid and the gases dissolved therein, may
impact not only the cavitation and microcavitation threshold
pressure (sonic watt density), but also the ability of the
megasonic fluid to form and retain small or micro-sized bubbles for
sufficient lengths of time to facilitate useful processing. Such
chemical characteristics include, but are not limited to, internal
cohesive force between molecules or groups of molecules, dissolved
solids, dissolved gases, surface tension, surface tension lowering
agents, viscosity, and density. As one or more of these parameters
are varied, alone or in combination with others, the nature of the
resulting processing may change.
[0710] A third technique for softening the action of megasonic
fluids relates to controlling the conditions under which processing
takes place. Elevated or reduced operating pressures or pressures
can be used to control not only the void fraction of the entrained
micro bubbles, but rates of chemical reaction as well. This will
allow control to be exercised over the degree of attenuation, as
well as the over the influence of cavitation and microcavitation
threshold pressure upon alternative processing fluids.
[0711] The following outlines general guidelines regarding the
impact of changed processing conditions on the substrate damage.
First, an increase in the cavitation threshold pressure may result
in the collapse of bubbles with greater violence and energy
intensity. This can in turn can lead to greater damage of small,
fragile device structures.
[0712] Solubility of a gas in liquid is based on the concentration
and pressure (partial pressure) of that gas above the liquid.
Processing above or below atmospheric pressure may also control the
dissolved gas content of partially soluble gases.
[0713] This higher gas solubility may not be absolute with every
conceivable gas in every liquid. Some gases are only partially
soluble, and soluble to different extents (e.g.
nitrogen<oxygen<ozone) in DI water. Other gases are only
partially soluble, but establish a chemical equilibrium as some of
the gas is converted to another species, for example as carbon
dioxide is converted to carbonic acid. Still other gases are
completely miscible (HCL or HF) in DI water, but may exhibit a much
different solubility response in liquids other than DI water, such
as benzene.
[0714] A special atmosphere may need to be maintained in a closed
vessel to maintain a high concentration of a particular gas above
the liquid, in order to maintain a relatively high concentration of
that gas dissolved in the liquid. For example, carbon dioxide
dissolved in DI water at an elevated concentration due to elevated
pressure, will bubble out when the pressure is reduced, or will
diffuse out if left in an open container, with air replacing some
of the carbon dioxide. As with the case of dissolved ozone allowing
secondary reactions with oxidizable materials, dissolved carbon
dioxide may form carbonic acid which can change process fluid pH
and impact processing.
[0715] Processing at pressures above or below atmospheric pressure
may be used to control the size and volume fraction of small
bubbles in the fluid, controlling energy attenuation and modifying
cavitation and microcavitation threshold pressure and the impact of
changes in surface tension of the processing fluid.
[0716] Thus, the use of elevated/reduced pressure can modify the
performance of various fluids by changing their ease of cavitation,
any volume fraction occupied by bubbles, and the degree of gas
saturation etc. as desired.
[0717] In addition to pressure, temperature is another parameter in
determining gas solubility and general fluid properties that may be
controlled to soften megasonic processing in accordance with
embodiments of the present invention. Thus according to embodiments
of the present invention, megasonic processing can operate at
higher/lower temperatures, (i.e. closer to a boil temperature or
farther away) to minimize megasonic damage.
[0718] As evidenced by the damage vs. temperature curves for
various fluids (particularly in the case of DI water) the trends
need not be totally linear. There can be maximums that occur over
the traditional temperature ranges used for semiconductor wet
processing. Therefore, purely from a damage perspective, operating
at temperatures approaching the boiling point of a fluid may make
it easier to form cavitation and microcavitation events producing
less damage. This consideration should be balanced against the
tendency of higher temperatures to lead to higher rates of various
chemical reactions, including undesirable reactions.
[0719] Controlling temperature can be important not only to
influence gas solubility, but also because other solution
characteristics such as surface tension and corrosion behavior can
be a function of temperature. Therefore, in accordance with still
other embodiments of the present invention, solution
characteristics such as surface tension of the processing fluid may
be modified with the use of additives, alone or in conjunction with
the three approaches outlined above.
[0720] While most megasonic processing takes place in liquid baths,
this is not required by the present invention. Alternative
embodiments can be utilized in single wafer processing equipment,
where a thin liquid layer is formed on the substrate surface and
megasonic energy is applied through a liquid meniscus. Yet other
alternative embodiments in accordance with the present invention
may use spray processing as another process variable to influence
the "hardness" of megasonic fluids and the resulting damage to
substrate features.
[0721] Still other alternative embodiments in accordance with the
present invention may use parameters of the applied sonic energy to
influence the softness of a processing fluid, in combination with
one of the above-described approaches. Examples of such sonic
energy parameters which may be controlled include, but are not
limited to, the frequency, phase, power density, and duration of
the applied energy.
[0722] In accordance with further alternative embodiments of the
present invention, marangoni or surface tension gradient drying may
be accomplished by producing a surface layer enriched in a material
having a lower surface tension (often a dissolved gas), over a bulk
fluid through which the substrate is moved. This movement causes
residual water droplets to be drawn off the surface of the
substrate into the bulk fluid. In accordance with certain
embodiments, alcohol/water mixtures could be used to not only
reduce damage, but also aid in the drying of substrates without
leaving watermarks, or leaving fewer watermarks.
[0723] For example, mixtures of alcohol and water, especially if on
top or where the surface layer is enriched in the alcohol, will
exhibit a lower surface tension than the bulk fluid. When the bulk
fluid is comprised of an alcohol/water mixture, that bulk solution
will exhibit a lower surface tension than pure water.
[0724] While removing a substrate from such a mixture is not
exactly like moving it through an enriched layer. By virtue of the
reduced surface tension alone, it is easier to pull out the
substrate in a more nearly dry state than just out of water alone.
The lower the surface tension, the less the tendency to leave large
liquid drops behind on the substrate surface or the formation of
watermarks.
[0725] The large liquid drops may take a long time to evaporate
because of their relatively small surface area to volume ratio.
When liquid drops stay on a surface longer, they have longer to
etch surfaces. The material etched (dissolved) then is deposited
back onto the substrate surface as the watermark when the drop
finally evaporates. If the processing fluid tended to form more of
a very thin sheet of liquid on the substrate surface, rather than
large drops, the liquid could evaporate relatively quickly owing to
the large ratio of surface area to liquid volume, and watermarks
would be much less likely to form.
[0726] Moreover, once processing in an alcohol/water mixture in
accordance with an embodiment of the present invention has occurred
with less damage to the substrate, the concentration of the surface
layer of the bath could be elevated when the substrate is removed.
Serendipitously, better drying may thus be achieved in combination
with processing with less damage.
[0727] Although the invention has been described in terms of
preferred methods and structures, it will be understood to those
skilled in the art that many modifications and alterations may be
made to the disclosed embodiments without departing from the
invention. Hence, these modifications and alterations are intended
to be considered as within the spirit and scope of the invention as
defined by the appended claims.
* * * * *