U.S. patent application number 11/497410 was filed with the patent office on 2006-11-23 for method and apparatus for wafer cleaning.
Invention is credited to Rick R. Endo, Alexander Ko, J. Kelly Truman, Steven Verhaverbeke.
Application Number | 20060260661 11/497410 |
Document ID | / |
Family ID | 27084514 |
Filed Date | 2006-11-23 |
United States Patent
Application |
20060260661 |
Kind Code |
A1 |
Verhaverbeke; Steven ; et
al. |
November 23, 2006 |
Method and apparatus for wafer cleaning
Abstract
An apparatus for wet processing individual wafers comprising; a
means for holding the wafer; a means for providing acoustic energy
to a non-device side of the wafer; and a means for flowing a fluid
onto a device side of the wafer.
Inventors: |
Verhaverbeke; Steven; (San
Francisco, CA) ; Truman; J. Kelly; (Morgan Hill,
CA) ; Ko; Alexander; (Sunnyvale, CA) ; Endo;
Rick R.; (San Carlos, CA) |
Correspondence
Address: |
BLAKELY SOKOLOFF TAYLOR & ZAFMAN
12400 WILSHIRE BOULEVARD
SEVENTH FLOOR
LOS ANGELES
CA
90025-1030
US
|
Family ID: |
27084514 |
Appl. No.: |
11/497410 |
Filed: |
July 31, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09891849 |
Jun 25, 2001 |
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11497410 |
Jul 31, 2006 |
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09603792 |
Jun 26, 2000 |
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09891849 |
Jun 25, 2001 |
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Current U.S.
Class: |
134/184 ;
134/198; 134/902; 134/94.1 |
Current CPC
Class: |
B08B 2203/0288 20130101;
H01L 21/02041 20130101; H01L 21/67051 20130101; B08B 3/02 20130101;
Y10S 134/902 20130101; B08B 3/12 20130101 |
Class at
Publication: |
134/184 ;
134/902; 134/198; 134/094.1 |
International
Class: |
B08B 3/00 20060101
B08B003/00; B08B 3/12 20060101 B08B003/12 |
Claims
1. An apparatus for processing a wafer, comprising: a bracket for
positioning and rotating the wafer about an axis; a platter aligned
beneath and parallel to the bracket, with the platter having a
through hole; a fluid source connected to the through hole for
flowing a first chemical within a gap between the wafer and the
platter; and a plurality of acoustic wave transducers positioned on
the platter that are capable of transmitting a plurality of
frequencies.
2. The apparatus of claim 1, further comprising one or more nozzles
positioned over the top of the wafer.
3. The apparatus of claim 2, wherein at least one of the one or
more nozzles is connected to a source of a gas.
4. The apparatus of claim 2, wherein at least one of the one or
more nozzles is connected to a source of a second chemical.
5. The apparatus of claim 1, wherein at least one of the plurality
of frequencies is a transparent frequency.
6. The apparatus of claim 2, wherein at least one nozzle is capable
of imparting acoustic energy to the second chemical flowing through
the nozzle.
7. The apparatus of claim 6, wherein the acoustic energy is at a
frequency greater than 400 kHz.
8. An apparatus for processing a wafer, comprising: a bracket for
positioning and rotating the wafer about an axis; a platter aligned
beneath and parallel to the bracket, with the platter having a
through hole; a fluid source connected to the through hole for
flowing a first chemical within a gap between the wafer and the
platter; a plurality of acoustic wave transducers positioned on the
platter that are capable of transmitting a plurality of megasonic
frequencies; and at least one of the plurality of megasonic
frequencies is a transparent frequency.
9. The apparatus of claim 8, wherein at least one of the plurality
of megasonic frequencies is a whole integer multiple of the lowest
frequency.
10. The apparatus of claim 8, wherein at least one of the plurality
of megasonic frequencies is approximately 5.4.+-.30% MHz.
Description
RELATED APPLICATIONS
[0001] The present divisional application is related to,
incorporates by reference and hereby claims the priority benefit of
the following U.S. Patent Applications, assigned to the assignee of
the present applications: U.S. patent application Ser. No.
09/891,849, filed Jun. 25, 2001 which is continuation-in-part of
U.S. patent application Ser. No. 09/603,792, filed Jun. 26,
2000.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to the field of cleaning of a
substrate surface and more particularly to the area of chemical and
megasonic cleaning of a semiconductor wafer.
[0004] 2. Discussion of Related Art
[0005] In semiconductor wafer substrate (wafer) cleaning, particle
removal is essential. Particles can be removed by chemical means or
by mechanical means. In current state of the art, particles are
usually removed by both a combination of mechanical means and
chemical means. The current state of the art is a batch process
that places a number of wafers into a bath filled with a liquid and
to apply high frequency (megasonic) irradiation to the liquid.
Megasonic cleaning uses a ceramic piezoelectric crystal excited by
a high-frequency AC voltage that causes the crystal to vibrate. The
vibration causes sonic waves to travel through the liquid and
provide the mechanical means to remove particles from the wafer
surface. At the same time, chemicals in the liquid provide a slight
surface etching and provide the right surface termination, such
that once particles are dislodged from the surface by the
combination of etch and mechanical action of the megasonics on the
particles, these particles are not redeposited on the surface. In
addition, chemicals are chosen such that an electrostatic repulsion
exists between the surface termination of the wafer and the
particles.
[0006] Until now, most megasonic irradiation has been applied to a
bath in which the wafers are immersed. When using a cleaning bath
filled with a liquid to immerse the wafer in, it is necessary to
immerse multiple wafers at the same time to be efficient. Single
wafer cleaning is possible in a bath, but then the chemicals have
to be reused, because of the volume of a single wafer bath.
[0007] So far, mechanical agitation in a single wafer cleaning
method has been achieved in several ways. At first, when wafers are
completely flat, brushes can be used to scrub the wafer surface.
However, this method is not possible when the wafers have any
topography (patterns) that can be damaged by the brushes. Moreover,
the brushes don't reach in between the wafer patterns. Megasonic
energy, which is the preferred mechanical agitation when patterns
are present, can be applied to a liquid in a nozzle and this liquid
can then be sprayed on the wafer. When spray methods are used in
this way, the sonic pressure waves are confined to the droplets of
the spray where they then lose a lot of their power. When the
droplets hit the wafer surface, most of the remaining sonic energy
is lost. Another method used is to apply megasonic pressure waves
with a quartz rod suspended over the wafer surface with the
cleaning solution building up between the rod and the wafer
surface.
[0008] None of these attempts to apply megasonics to a single wafer
surface is sufficiently efficient as they do not reduce the single
wafer cleaning time enough, which is of the utmost importance. A
single wafer cleaning approach should be much faster than a batch
cleaning process in order to be competitive. Moreover, none of the
current single wafer techniques are able to clean sufficiently both
the front and the backside of the wafer at the same time. The only
known technique to clean the front and backside at the same time is
to immerse a batch of wafers in a bath and apply the acoustic waves
from the sides of the wafers. In this manner, the acoustic waves
travel parallel to the wafer surfaces to be cleaned. In silicon
wafer cleaning, it is important to clean both sides of the wafer
even though only the device side (front side) contains active
devices. Contamination left on the device side can cause a
malfunctioning device. Contamination left on the non-device side
(backside) can cause a number of problems. Backside contamination
can cause the photolithography step on the front side to be out of
focus. Contamination on the backside can cause contamination of the
processing tools, which in turn can be transferred to the front
side of the wafer. Finally, metallic contamination on the backside,
when deposited before a high temperature operation, can diffuse
through the silicon wafer and end up on the device side of the
wafer causing a malfunctioning of the device.
[0009] Polysilicon or amorphous silicon is deposited on a silicon
wafer for different purposes. It can be the gate material of the
transistor, or it can be used for local interconnects or it can be
used as one of the capacitor plates in a capacitor structure. Most
commonly, polysilicon or amorphous silicon is deposited on an
insulating material, such as silicon dioxide. Polysilicon or
amorphous silicon is usually deposited by a CVD (chemical vapor
deposition) technique. The deposition of polysilicon or amorphous
silicon usually occurs unselectively, that is, the entire wafer is
covered with a layer of polysilicon or amorphous silicon. After
such a blanket deposition, the wafers are covered with photoresist,
the photoresist is exposed with UV light according to a certain
designed pattern, and developed. Then the polysilicon or amorphous
silicon is etched in a plasma reactor. The exposure of the
photoresist determines the pattern in which the polysilicon or
amorphous silicon will be etched. Usually, the polysilicon is used
to conduct current from one place to another place or to collect
charge as in a capacitor. In both cases, the dimensions are scaled
down with every new generation of technology.
[0010] Until recently, dimensions not smaller than 0.3 .mu.m
(micron) were being used. However, technologies using poly-line
dimensions smaller than 0.3 .mu.m, such as 0.14 .mu.m and even down
to 0.1 .mu.m are now being used. These poly-line dimensions and
capacitor plate dimensions are so fragile a construction that they
are prone to breakage. These constructs are so fragile that
agitation may break them and cause a defective chip. After etching
and photoresist removal, such as with an oxygen plasma (i.e. the
ashing of the photoresist), the silicon wafers are usually riddled
with particles. These particles have to be removed before going to
the next device fabrication operation.
[0011] These particles are usually removed in a cleaning tool such
as a wet bench. The particles are removed by immersing the wafers
into a cleaning liquid and agitating the cleaning liquid with
megasonic sound waves. This has worked well with poly-lines of 0.3
.mu.m and above, however, when using poly-lines with dimensions
smaller than 0.3 .mu.m, megasonic sound agitation cannot be used as
the megasonic sound agitation damages these fragile structures.
Therefore, only chemicals can be used to clean particles when these
fragile structures are exposed to the cleaning liquid. Although,
even simple immersion into a cleaning liquid without agitation does
remove some of the particles, it cannot remove all of the particles
or even enough of the particles. Nevertheless, no alternative has
existed and therefore, this is the only cleaning technique used on
these fine structures.
SUMMARY OF THE INVENTION
[0012] A method and apparatus is disclosed for single wafer
processing that applies a cleaning or rinse solution to one or both
sides of a wafer positioned above a platter. The wafer can be
positioned in a bracket, the bracket rotated, and the platter can
apply megasonic energy in the form of one or more frequencies to a
side of the wafer. The bracket can hold the wafer at three or more
points where wafer position is maintained by gravity. At least one
frequency applied to a 300 mm wafer can be at 5.4 MHz. The wafer
side facing the platter may be the non-device side, and the platter
can generate the megasonic energy at one or more frequencies with
one or more acoustic wave transducers positioned on the platter
backside.
[0013] The frequencies selected may be un-reflected by the platter
and the wafer such that a large percentage of the megasonic energy
will reach the wafer side not facing the platter. While a
cleaning/rinse solution is applied to the wafer non-device side, a
second cleaning/rinse solution may be applied to the wafer device
side. The megasonic energy may be pulsed and/or applied at varying
power.
[0014] According to the present invention, chemicals area applied
requiring low volumes and no-reuse of the cleaning and rinse
chemicals. Applying chemicals between the platter, having a dished
out center, and the wafer, to be held in position by natural forces
and then spinning the wafer to remove the chemicals is also
disclosed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1A is an illustration of one embodiment of a wafer
cleaning chamber.
[0016] FIG. 1B is an illustration of an alternate embodiment of the
wafer cleaning chamber.
[0017] FIG. 2A is an illustration of one embodiment of a megasonic
single wafer cleaning chamber.
[0018] FIG. 2B is an illustration of one embodiment of the
center-section of the platter and the wafer having a flow of
chemicals therein.
[0019] FIG. 3 is an illustration of an embodiment of a venturi
nozzle design.
[0020] FIG. 4A illustrates in a top view, one embodiment of the
rotatable wafer holding bracket (bracket).
[0021] FIG. 4B illustrates the bracket in a 3D perspective
view.
[0022] FIG. 4C illustrates the effects of airflow above and below
the wafer in the bracket rotating over a platter.
[0023] FIG. 5A is an illustration of a cross-section of one
embodiment of the platter.
[0024] FIG. 5B is an illustration of a bottom view of one
embodiment of the platter assembly showing a single acoustic wave
transducer attached to the platter.
[0025] FIG. 5C is an illustration of one embodiment having acoustic
wave transducers positioned in a strip fashion on the platter.
[0026] FIG. 6A illustrates one embodiment where a half circle of
the platter surface is coated with a first acoustic wave transducer
that vibrates in the 925 kHz range and the remaining platter half
is covered with a second acoustic wave transducer vibrating in the
1.8 MHz range.
[0027] FIG. 6B illustrates an alternate embodiment of the platter
having two groups of acoustic wave transducers in diagonal
quadrants.
[0028] FIG. 6C illustrates an alternate embodiment where the
platter has two groups of transducers positioned on the platter in
linear strips that each runs substantially the diameter of the
platter surface.
[0029] FIG. 7 is an illustration of wafer removal for one
embodiment of the cleaning chamber.
[0030] FIG. 8 is an illustration of one embodiment where a
plurality of megasonic frequencies is applied to quartz rods.
[0031] FIG. 9 is an illustration of one embodiment where a
plurality of megasonic spray nozzles is used to transfer acoustic
energy.
[0032] FIG. 10 is an illustration of one embodiment of an apparatus
for batch processing a plurality of wafers using two or more
megasonic frequencies.
[0033] FIG. 11 is an illustration of a cluster of four single wafer
cleaning apparatus that are positioned about a robot arm
assembly.
[0034] FIG. 12 is an illustration of a single wafer cleaning
apparatus.
[0035] FIG. 13 is an illustration of an alternate embodiment of a
top chamber.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
[0036] An apparatus and method of use to provide single wafer
cleaning is disclosed. A process chamber (chamber) can process
either or both a top and a bottom side of a single wafer in chip
processing. The chamber can offer high wafer throughput along with
good process control while providing low use of cleaning
solutions.
[0037] In one embodiment, a single wafer is positioned in a wafer
holding bracket (bracket) above a platter. Chemicals such as
cleaning and rinse solutions are transferred through the platter
from below to contact the bottom side of the wafer. Sufficient
chemical flow is provided to fill a gap between the wafer and the
platter. Once the gap is filled, little additional chemicals may be
required, with the solution within the gap maintained in position
by natural forces such as surface tension and capillary forces.
[0038] In another embodiment, a first group of chemicals (first
chemical) are transferred to the bottom side of the wafer while
chemicals from a different source (second chemical) are transferred
to a top surface of the wafer. In either embodiment mentioned
above, megasonic sound waves can be emitted from the platter to
transfer through the first chemicals flowing from below and strike
the wafer bottom surface. In yet another embodiment, which can
include elements of the above embodiments, megasonic sound waves
are placed within chemicals that are applied to the topside of the
wafer where the solutions may be in the form of a spray or a thin
film.
[0039] The use of acoustic wave transducers generating frequencies
in the megasonic range has recently become common in wafer
cleaning. The difference between ultrasonic cleaning and megasonic
cleaning lies in the frequency that is used to generate the
acoustic waves. Ultrasonic cleaning uses frequencies from
approximately between 20-400 kHz and produces random cavitation.
Megasonic cleaning uses higher frequencies beginning at between
350-400 kHz and may use frequencies well into the MHz range. An
important distinction between the two methods is that the higher
megasonic frequencies do not cause the violent cavitation effects
found with ultrasonic frequencies. Megasonic significantly reduces
or eliminates cavitation erosion and the likelihood of surface
damage to the wafer. In general, the higher the frequency, the
lower the damage to the wafer.
[0040] Megasonic cleaning produces more controlled cavitation.
Cavitation, the formation and activity of bubbles, is believed to
be an important mechanism in the actual particle removal process
because cavitation has sufficient energy to overcome particle
adhesion forces and cause particles to be removed. Controlled
cavitation becomes acoustic streaming which can push the particles
away so they do not reattach to the wafer. Megasonic cleaning may
be improved by varying and/or pulsing the input power to the
megasonic transducers, which can provide better control over
cavitation than applying power continuously at a constant level.
Megasonic cleaning may be improved through the use of a plurality
of frequencies to be simultaneously generated, or by changing one
or more frequencies during the clean and rinse the cycles, or a
combination thereof. Megasonic cleaning may also be improved
through a selection of the frequency or frequencies used.
[0041] In semiconductor processing, there are a number of occasions
requiring processing of the wafer backside (non-device side)
without processing the front side (device side), such as to remove
backside particles before exposing the wafer to UV light from a
lithography tool. Particles on the backside can cause
depth-of-focus problems. In other occasions, deposition tools
deposit materials on the front side on the wafers to form a film,
but inadvertently, some deposits end up on the backside of the
wafer. In other tools, such as copper electroplating tools, copper
contamination can end up on the backside of the wafer. In all these
cases, the backside has to be cleaned of particles and/or dissolved
metals or certain layers have to be stripped.
[0042] FIG. 1A is an illustration of one embodiment of a single
wafer cleaning chamber 100. Disclosed is an apparatus and method of
use for exposing the bottom side of the wafer 106 to cleaning,
rinsing and drying chemicals 112 without exposing the topside of
the wafer 106 to any chemicals. In one embodiment, the wafer
non-device side 114 is facing down to be exposed to chemicals 112,
while the wafer device side 116 is facing up and is not exposed to
chemicals 112.
[0043] In one embodiment, to initiate a wafer process cycle, a
rotatable wafer holding bracket (bracket) 148 translates along an
axis 145 a distance upward. A robot arm (not shown) holding the
wafer 106 enters the interior of the chamber 160 through an access
door 158 and the wafer 106 is placed in the bracket 148. The
bracket 148 is then lowered so as to align the wafer 106
horizontally a distance from a circular platter 108. The wafer 106,
resting in the bracket 148, is parallel to the platter 108 and
located a distance from the platter 108, i.e. the gap. The platter
108 is flat where it faces the wafer 106 and therefore, the
distance separating the platter 108 and the wafer 106 is uniform.
The gap between the wafer 106 and the platter 108 may be in the
range of approximately 1-5 millimeters (mm) and preferably
approximately 3 mm.
[0044] In one embodiment, the wafer 106 when positioned in the
bracket 148 can rest on three or more vertical support posts
(posts) 110 of the bracket 148. The vertical support posts 110 can
contain an elastomer pad (shown in FIG. 4A later) to contact the
wafer 106 directly. The wafer 106 is rotated while chemicals 112
are dispensed from below to contact the wafer backside 114. A tube
128 connects to a through hole (feed port) 142 in the platter 108.
As a result of wafer 106 rotation (spin), chemicals 112 applied to
the wafer backside 114 are restricted from reaching devices 121 on
the wafer front side 116. In addition, a nozzle 117 may move in
over the wafer 106 o be positioned within approximately 5 mm of the
wafer surface and in the outer half of the wafer radius. The nozzle
117 can apply a stream of inert gas 113 such as N.sub.2 to the
wafer device side 116 to further limit chemicals 112 applied to the
wafer backside 114 from migrating onto the wafer front side 116.
Gravity and the downward flow of air 123 from a filter 111 such as
a High Efficiency Particulate Arresting (HEPA) filter or an Ultra
Low Penetration Air (ULPA) filter can act to maintain the wafer 106
positioned on the posts 110. Chemicals 112 placed between the wafer
106 and the platter 108 can be maintained in position by natural
forces such as capillary action and surface tension. As a result, a
chemical flow rate required to maintain the chemicals 112 against
the wafer backside 114 can be reduced during processing, which can
allow for a small chemical use in each cycle and can also allow for
an efficient "no reuse" of chemicals 112. During the cleaning
portion of the process, the wafer rotation may be stopped allowing
the wafer 106 to remain still while the cleaning chemicals 112
contact the wafer bottom surface 114. The wafer 106 can be rotated,
however, to wet out the wafer bottom surface 114 initially with the
cleaning chemicals as well as for the rinse and dry cycles.
[0045] FIG. 1B is an illustration of an alternate embodiment of a
single wafer cleaning chamber 101. In this embodiment, the platter
108' has a dished-out center area 119 on the platter side facing
the wafer 106. For processing, chemicals 112 can be placed in the
dished-out area 119 and the wafer 106 can be positioned within the
dished-out area 119 such that the wafer backside 114 is contacting
the chemicals 112. This dished-out area 119 of the platter 108' can
function to contain the chemicals 112 and further reduce the amount
of chemicals 112 needed during a process cycle. The dished out
center 119 can be deep enough to submerge the bottom surface 114 of
the wafer 106 while the top surface 116 of the wafer 106 remains
outside of the chemicals 112. In one embodiment, approximately one
half of the total surface area of the wafer 106 is submerged within
the chemicals 112. A nozzle 117' may be placed in the top area of
the chamber 160 to flow a gas such as nitrogen onto the wafer
topside. The nozzle 117'' may have to move or pivot to avoid
contact with the wafer 106 during wafer placement and removal as
well as for the rinse and spin cycles. The gas flow from the nozzle
117' along with centrifugal forces if the wafer is spinning, can
shift the chemicals 112 toward the wafer edge 115, further limiting
migration of any chemicals 112 onto the wafer top surface 116.
[0046] FIG. 2A is an illustration of one embodiment of a megasonic
single wafer cleaning chamber. FIG. 2B is an illustration of one
embodiment of the center section of the platter and the wafer
having a flow of chemicals therein. The megasonic single wafer
cleaning chamber 200 can incorporate the methods, features and
benefits of the single wafer cleaning chambers 100 and 101
illustrated in FIGS. 1A & 1B. Within the cleaning chamber 200,
megasonic energy is generated by one or more acoustic wave
transducers (transducers) 202 attached to the platter 208 and the
megasonic energy can pass into the wafer 206 through chemicals 212
in contact with both the wafer 206 and the platter 208. As a
result, the wafer 206 can be cleaned with a variety of combinations
that include wafer rotation, megasonic energy, and chemical action,
all under temperature control. Between and after the cleaning and
rinsing cycles, the single wafer cleaning chamber 200 can dry the
wafer 206.
[0047] The platter 208 has a topside 217 and a bottom side 219,
with the set of transducers 202 attached to the bottom side 219.
The platter topside 217 can be facing the wafer 206. The platter
208 is fixed in this embodiment, but alternate embodiments can have
the platter 208 able to translate along the bracket rotation axis
245 to open the gap during wafer rinse or dry cycles. The robot arm
(not shown) can place the wafer 206 in the rotatable wafer holding
bracket (bracket) 248 such that the wafer device side 216 is facing
up and away from the platter 208. When placed in the bracket 248,
the wafer 206 can be centered over and held substantially parallel
to the platter 208 to create the gap. The gap distance is
approximately 3 mm but can fall within the range of approximately
1-5 mm. Positioned beneath the platter 208 can be an electric motor
222 for rotating the bracket 248. A through hole 225 can exist in
the electric motor through which is passed the wiring 246 from the
platter 208 as well as a tube 228 that can transfer the chemicals
212 to the feed port 242.
[0048] Referring still to FIG. 2A, the platter 208 can have an
approximate 0.190'' diameter through-hole 242 that acts as a feed
port for the chemicals 212 dispensed from below. This feed port 242
can be located at the center of the platter 208 or the feed port
242 can be placed off-center by up to a few millimeters (not
shown). Attached to each of the acoustic wave transducers 202 can
be a copper spring 244. The spring 244 could be of a variety of
shapes to maintain electrical contact such as a wire coiled shape
(shown) or a flexed foil constructed from sheet metal (not shown).
Soldered to the spring 244 free ends are the wiring leads 246 to
form the electrical connections. The platter 208 can be connected
to the cleaning chamber 200 so as to act as ground for the
electrical connections 244 and 246 to the acoustic wave transducers
202.
[0049] In one embodiment, located above the platter 208 and the
wafer 206, may be positioned a nozzle 251. Through the nozzle 251
can pass a second set of chemicals 223, 224, 225, and 227 (second
chemicals) during processing. The nozzle 251 can direct a fluid
flow 250 onto the wafer device side 216 with each of the chemicals
223, 224, 225, and 227 in the cleaning process. The nozzle 251 can
apply the chemicals 223, 224, 225, and 227 to the wafer 206 while
the wafer 206 is not moving or while the wafer 206 is spinning. The
nozzle 251 can apply the chemicals 223, 224, 225, and 227 at a flow
rate to maintain a coating of the chemicals 223, 224, 225, and 227
on the wafer device side 216 surface with minimal excess.
[0050] The nozzle 251 can apply a continuous chemical flow to
maintain a film thickness on the wafer 206 of at least 100 microns.
To keep the chemical film at the 100 microns thickness, the
chemicals 223, 224, 225, and 227 may be converted at the nozzle 251
into a mist having a particular mean diameter droplet size. All
nozzle designs are limited as to how small a droplet size they can
create. To meet the requirements of minimal fluid usage, a further
reduction in droplet size may be required. One method of reducing
the droplet size beyond a theoretical limit is to entrain a gas
into the chemicals. The nozzle 251 can entrain or dissolve enough
H.sub.2 gas 205 or any other gas from the group of O.sub.2,
N.sub.2, Ar, or He into the chemicals 223, 224, 225, and 227 to
further reduce the mean droplet size. And in addition, entraining
the gas 205 can have the added benefit of optimizing cavitation
within the chemicals 223, 224, 225, and 227 when the megasonics are
applied.
[0051] FIG. 3 is an illustration of an alternate embodiment of a
venturi nozzle design. The nozzle, in the shape of a "showerhead",
is provided as an illustration of the use of a venturi to draw
gases into the flow of cleaning chemicals. The venturi shape can
inject a gas source 305 such as H.sub.2 into the fluid steam 352
before the fluid stream 352 passes out holes 360 in a plate 358 in
the nozzle 351 as a spray 350. Using this approach, the chemicals
flow past a throat 354, which increases the flow rate thereby
reducing the fluid pressure. A small hole (injector port) 356 is
placed in the throat 354 and is attached to a gas source 305 such
as H.sub.2. As the fluid stream 352 passes by the injector port
356, the gas 305 is drawn into the lower pressure of the fluid
stream 352. Alternatively, the gas 305 may simply be injected into
the fluid stream 352 under sufficient pressure thereby avoiding the
need for a venturi design (not shown). Other approaches (not shown)
for entraining gas into the chemicals can be to bubble the gas into
each cleaning fluid or to mist the cleaning fluids through a volume
or stream of gas. The gas-entrained chemicals then exit the nozzle
351 through a perforated surfaced 358 where the perforations 360
are sized to generate a particular mean droplet diameter.
[0052] FIG. 4A illustrates in a top view, one embodiment of the
rotatable wafer holding bracket (bracket). FIG. 4B illustrates the
bracket in a 3D perspective view. The wafer 406 (shown in dashed
line) can be held in place by the bracket 448 to position the wafer
406 parallel to and near the platter (not shown for clarity).
Initially, the bracket 448 can hold the wafer 406 by gravity at
four points 409 and 409' along the wafer edge 415 such that the
wafer front side 416 and the wafer backside 414 are clear of the
bracket 448 structure and fully exposed to both cleaning/rinsing
liquids and thus to megasonic energy. The number of points of
contact 409 and 409' for the bracket 448 with the wafer 406 can be
three or more and can be made with an elastomeric material such as
a plastic or rubber to friction grip the wafer 406 during the start
and stop phases of rotation. In one embodiment, the contact points
are O-rings that are positioned at the ends of bracket support
posts (posts) 411 where the posts 411 have been given an airfoil
shape to minimize vibrations during high-speed rotations.
[0053] FIG. 4C illustrates the effects of airflow above and below
the wafer 406 in the bracket 448 rotating over the platter 408.
When there are no chemicals between the wafer 406 and the platter
408 (portions of the rinse cycle and the dry cycle), air can flow
in circular swirls or patterns 460 and 462 during wafer 406
rotation. The gap (not to scale) between the platter 408 and the
wafer 406 limits the area of airflow and as a result, air flow
circulating above the wafer 460 is at a different flow rate than
air flowing between the platter and the wafer 462. At the higher
rinse and dry wafer rotation speeds, the difference in flow rate
provides different pressures above and below the wafer 406
(Bernoulli forces), which can operate to provide a downward force
acting on the wafer 406 that maintains the wafer 406 onto the
bracket 448.
[0054] Referring again to FIGS. 2A and 2B, one embodiment of a
method of use rotates the bracket 248 and the wafer 206 while the
first cleaning solution 212 is applied from below to be in
simultaneous contact with the platter 208 and the non-device side
of the wafer 214. The second cleaning solution 223, 224, 225, and
227 is wetted out onto the device side 216 of the wafer 206. The
acoustic wave transducers 202 generate megasonic waves through the
platter 208 into the first cleaning solution 212, captured by the
wafer 206 and the platter 208. The megasonic waves may be incident
to the wafer non-device side 214 at an angle substantially normal
(perpendicular) to the wafer surface 214. A percentage of the
megasonic waves, depending on the frequency or frequencies used can
pass through the wafer 206 to exit the wafer device side 216 and
enter the second cleaning solution 223, 224, 225, and 227 that is a
film on the wafer device side 216. The megasonic waves acting
within the second cleaning solution 223, 224, 225, and 227 can
produce cleaning on the wafer device side 216. For optimal
throughput speed, the total area of the acoustic wave transducers
202 can be sufficient to provide approximately between 80-100% area
coverage of the platter surface 219. The platter 208 diameter may
be approximately the same size or larger than the wafer 206
diameter. The invention is scalable to operate on a wafer 206 that
is 200 mm (diameter), 300 mm (diameter), or larger in size. If the
wafer diameter is larger than the platter diameter, the vibrations
from the megasonic energy striking the wafer 206 can still travel
to the wafer 206 outer diameter (OD) providing full coverage for
the cleaning action.
[0055] During the cleaning, rinse and dry cycles, the wafer 206 is
rotated at a selected revolution per minute (rpm) about an axis 245
that runs through the bracket 248 pivot point. Additionally, to
optimize any particular cycle, the wafer spin rate may be stopped
or varied and the sonic energy varied by changing any combination
of the power setting, the frequency or frequencies, and by pulsing.
In one embodiment, the bracket 248, powered by the motor 222, can
rotate the wafer 206 during cleaning operations at an rpm of
approximately between 10-1000 and during the dry and rinse cycles
at an rpm of greater than 250 rpm where a range of approximately
between 250-6000 rpm is preferable. Therefore, when the bracket 248
is in operation, the wafer 206 is seeing a first cleaning solution
212 on the non-device side 214, a second cleaning solution 224 on
the device side 216, while the wafer 206 is being rotated and
radiated with megasonic energy.
[0056] Continuing with FIG. 2A, acoustic waves can first strike the
wafer non-device side 214 where no devices 221 exist that could be
damaged by the full force of the acoustic energy. Depending on the
frequency or frequencies used, the megasonic energy may be dampened
to a degree when passing through the platter 208 and wafer 206 to
exit into the cleaning or rinse chemicals 223, 224, 225, and 227 at
the wafer device side 216. As a result, the megasonic energy
striking the wafer non-device side 214 may be powerful enough that
only de-ionized (DI) water is used as the first cleaning solution
212.
[0057] A thin film (not shown) of the second cleaning solution 223,
224, 225, and 227 may be applied to wet the wafer device side 216
surface. If not DI water 225, the second cleaning solution 224 may
be a stronger chemistry such as used in an RCA (Radio Corporation
of America) cleaning process. The action of the megasonic energy on
the device structures 221 is confined to a small volume (thin film)
that contacts the device structures 221, absorbs the sonic waves,
and maintains useful cavitation.
[0058] In an embodiment, megasonic energy is applied to the
rotating wafer 206 throughout the cleaning process. The megasonic
energy is in a frequency range of 400 kHz-8 Mz but may be higher.
The RCA type cleaning process, along with the prior use of an
etchant such as hydrofluoric acid (HF) 223 having a concentration
of 0.5% by weight of HF, may be used on the wafer device side 216.
The RCA cleaning process is commonly used and is well known to
those skilled in the art. The RCA process or a similar cleaning
process may include a first standard clean (SC-1) cycle
(NH.sub.4OH+H.sub.2O.sub.2) 224, a rinse (DI water 225 ending with
IPA vapor in N.sub.2), an SC-2 clean (HCl+H.sub.2O.sub.2) 224, a
rinse (DI water 225 ending with IPA vapor in N.sub.2), and a dry
cycle (blowing N.sub.2 on the rotating wafer 206). The application
of IPA vapor in N.sub.2 can be accomplished while DI water still
exists on the wafer. As a result, some of the previous cleaning
chemicals still remain on the wafer, immersed in the DI water. The
use of start of IPA vapor in N.sub.2 blowing on the wafer can
reduce the rinse time since it begins prior to complete rinse, i.e.
complete removal of the cleaning chemicals by the DI water. The
effect of the IPA vapor in N.sub.2 is to assist the rinse cycle and
shorten the rinse cycle duration. The IPA vapor in N.sub.2 256 can
be applied through a second nozzle 253 to support a rinse cycle on
the top side 216 of the wafer. The second nozzle 253 can be placed
off-center to the wafer axis of rotation 245. In yet another
embodiment (not shown), more than two nozzles can be used which can
be positioned in a variety of other patterns, such as equally
distant from the axis 245, so as to provide chemical and gas
coverage onto the topside 216 of the wafer.
[0059] The wafer non-device side 214 may have the same cycles of
clean, rinse, and dry or could use only DI water 212 in the clean
and rinse cycles. The temperature of the cleaning chemicals, as
well as the rinsing chemicals, etchants, and gasses can be between
15-85.degree. C. during use. A drain 262 may be provided within the
cleaning chamber housing 260 to collect the cleaning fluids. A
cleaning chamber floor 263 may be angled toward the drain 262 to
improve flow of the chemicals 212, 223, 224, 225, and 227 to the
drain 262.
[0060] Cleaning of the wafer backside (non-device side) surface 214
may be accomplished in a different manner. Because the acoustic
energy is higher on the backside of the wafer, no RCA type cleaning
solutions 224 may be necessary. The vibrations alone in water may
be sufficient to separate the particles from the wafer 206 and move
them away. DI water 212 may be selected as the medium to transfer
the acoustic energy in the area around the wafer backside 214 for
both the cleaning and rinse cycles. In one embodiment, non-gas
entrained DI water or even de-gassed DI water is preferred for use
on the wafer backside 214. The DI water 212 is fed through the tube
228, the feed port 242, and onto the wafer backside surface 214 at
a sufficient rate to continually fill the area between the platter
208 and the wafer 206 which will guarantee constant fluid contact
with the wafer surface 214. The DI water 212 can be vacuum degassed
before directing it to the fluid inlet port 242, by passing the DI
water 212 through a membrane degassifier (not shown) such as with
Liqui-Cel membrane contactors such as supplied by Celgard
(Charlotte, N.C.). Alternatively, if a vacuum is placed on the gas
side of the membrane, most of the dissolved gases can be removed
from the incoming DI water 212. Alternatives to the rinse and dry
cycles can include the rinse cycle using IPA along with or instead
of H.sub.2O, and the dry cycle may use wafer spinning and an inert
gas such as N.sub.2.
[0061] In one embodiment there is little use and no reuse of
cleaning solutions. This is a result of the small volumes of
chemicals used in the process such that it is efficient to use the
chemicals once and then discard them. With such a small volume of
chemicals used, the single pass concept is economical and does not
increase the burden to the environment. With the present invention,
spraying a thin film may use 1/10 or less the water volume as
compared to existing wafer megasonic batch processes using
immersion. To reduce chemical use, the bracket 248 may be rotated
initially at a first speed to dispense the first chemical 212 onto
the non-device side 216 of the wafer 206 and to dispense the second
chemical 223, 224, 225, and 227 onto the device side 216 of the
wafer 206. Once dispensed, the bracket rotation speed can be slower
than the first speed while megasonics are applied to the wafer
non-device side 214. The bracket 248 can then be rotated at a speed
higher than the first speed to rinse the wafer 206 and the bracket
248 rotated at a speed higher than the first speed to dry the wafer
206.
[0062] After the chemicals are dispensed, the wafer rotation is
slowed so that the first chemicals 212 can remain trapped between
the wafer and the platter as well as keeping the second chemicals
223, 224, 225, and 227 wetted out on the wafer opposite side. In
one embodiment, the initial wafer spin rate can in the range of
approximately 50-300, where an rpm of 150 is preferable, while the
cleaning solutions 212, 224, and 225 are applied. In one
embodiment, once the device side 216 of the wafer 206 is wetted
with the chemicals 224 or 225, the wafer rotation speed may be
reduced to a range of approximately 10-50, where an rpm of
approximately 15 is preferable, and/or the cleaning solutions 224
or 225 applied at a lower rate, which in either case can reduce the
cycle time and result in conserving chemical use. Finally, in one
embodiment, after the cleaning process, during a rinse and/or dry
cycle, the rpm can be increased to over 1000 to remove the
chemicals remaining on the wafer 206.
[0063] The use of chemicals can be further decreased by wetting the
wafer surface 216 with a finer spray of chemicals as opposed to a
more coarse spray or even a solid stream of liquid. The finer spray
can be achieved through an effective design of one or more nozzles
251 to apply the cleaning solution, by adjusting the temperature of
the cleaning solution applied, by adjusting the chamber pressure
acting on the spray, the fluid pressure in the nozzle 251, the
chemical makeup of the cleaning solutions 223, 224, 225 or 227, and
the amount and type of entrained gases 205 within the cleaning
solution 223, 224, 225, and 227.
[0064] When the chemicals are not reused, the use of the platter
208 has the benefit of containing the various liquids 223, 225,
224, and 227 that would otherwise fall by gravity from the wafer
non-device side surface 214. Containing the cleaning liquids 223,
224, 225, and 227 against the wafer 106 can reduce cleaning liquid
use, optimize the acoustic energy transmitted from the platter 208
to the wafer 206 and can allow the cleaning liquids 223, 224, 225,
and 227 to act longer on the wafer surface 214. Finally, cleaning
solutions 223, 224, 225, and 227 applied to the wafer non-device
side, can be more dilute, i.e. made of a higher concentration of
water, which will further reduce cleaning chemical consumption.
[0065] After the last rinse cycle is complete there can be a dry
cycle to dry the wafer. During the dry cycle, a few milliliters of
isopropyl alcohol (IPA) vapor, mixed with nitrogen gas (N.sub.2),
can be injected through the fluid feed port 242 to contact the
wafer device side 216 and non-device side 214. The IPA, having a
lower surface tension than water, will wet out the surface better
and form a smaller boundary layer. The combination of high wafer
rpm, IPA vapor as a wetting agent, and N.sub.2 gas pressure
striking the wafer 206 reduces the drying time for the wafer
206.
[0066] FIG. 5A is an illustration of a cross-section of one
embodiment of the platter 500. The platter 508 can be made of
aluminum that is polished and may have a surface finish of 16 or
smoother and having an approximate 300 mm diameter. Alternatively,
it should be noted that the platter 508 can be made from a variety
of materials such as sapphire, stainless steel, tantalum, or
titanium. The platter 508 is approximately 3.43 mm thick (530) and
the platter front side 517 can be coated with a protective
fluoropolymer 534 such as Halar.RTM. (Ausimont USA, Thorofare,
N.J.), having a coating thickness (536) of between 0.015-0.045''.
The platter backside 514 can have one or more acoustic wave
transducers 502 bonded directly to the aluminum with an
electrically conductive epoxy adhesive or a solder having an
adhesive/solder thickness 540 of approximately 0.001-0.010''. The
opposite side of each of the one or more acoustic wave transducer
502 can be flexibly attached 544 to electrical wiring 520 to
provide power at a frequency while the platter 508 can be connected
to ground.
[0067] FIG. 5B is an illustration of a bottom view of one
embodiment of the platter 500 showing a single acoustic wave
transducer attached to the platter. The shape shown is circular;
however, any number of individual acoustic wave transducers 502,
made into any shape such as square, round, or rectangular, can be
used to meet area coverage and manufacturing requirements. If more
than one acoustic wave transducer 502 is used, the acoustic wave
transducers 502 can be positioned close together so as to provide
the 80% or greater coverage of the platter backside 514 surface
area. The wafer 506 (dashed), upon receiving megasonic energy to a
portion of the wafer backside surface 507, can transmit that
megasonic energy to the entire wafer backside surface 507. This
complete coverage of the wafer backside surface 507 can occur if
the megasonic energy from the platter 508 is incident to between
50-100% of the wafer surface backside surface 507, however, optimal
throughput can require the 80-100% coverage, with 90-100% coverage
preferred. In one embodiment, 80% or greater acoustic wave
transducer coverage on the platter 508 is provided and as a result,
megasonic energy will be applied to the entire wafer backside
surface 507 dramatically reducing the cycle time and hence
increasing the throughput of wafers. In another embodiment (not
shown) the bracket can translate the wafer in linear travel,
without rotation, to pick up acoustic energy over the entire wafer
surface.
[0068] Acoustic wave transducer thickness t (FIG. 5A) can be sized
to generate sound at a particular frequency. When a signal,
generated at the frequency for which the transducer has been
designed to respond, arrives at the transducer, the transducer will
vibrate at that frequency. A typical acoustic wave transducer is
made from a piezoelectric material having a thickness of 0.098'',
which is designed to respond to a frequency of 920 kHz. For a 300
mm wafer 506 (dashed to show a position on the opposite side of the
platter 508 in FIG. 5B), the frequency of 5.4 MHz has a special
utility in that the 300 mm wafer 506 is transparent for those sound
waves. At 5.4 MHz.+-.30%, the sound waves can travel substantially
through the wafer 506 to exit the opposite wafer surface. To obtain
a frequency of 5.4 MHz, the thickness of the acoustic wave
transducer 502, as well as each thickness of all the other layers
(platter 508 and adhesive/solder 540, FIG. 5A), are multiplied by a
factor 920/5400=0.17 or alternatively the layer thicknesses of the
acoustic wave transducer piezoelectric material, adhesive, and
aluminum platter are to be divided by a factor of 5.87. This will
provide for a transducer to respond to a frequency of 5.4 MHz and
for a reduced bounce back from the other layers of materials 508
and 540, that the sound must pass through on its way to the wafer
506. An exception may be the thickness 536 of the fluoropolymer
coating 534 (not to scale) which can be kept similar in all
embodiments. In one embodiment, the piezoelectric material is a
ceramic of lead zirconate titanate with the transducer 502
manufactured by Channel Industries, Inc of Santa Barbara, Calif. In
one embodiment, an efficiency of at least 30% of the energy applied
to the transducers 502 can reach the wafer 506.
[0069] FIG. 5C is an illustration of one embodiment having acoustic
wave transducers positioned in a strip fashion on the platter. The
acoustic wave transducers 502 and 503 linearly placed on the
platter backside 514 can run a distance on the platter surface 514.
The acoustic wave transducers 502 and 503 on the platter backside
514, could be positioned as a strip that runs at least
substantially the diameter (referring here to the outer diameter)
of the platter 508 covering approximately 40% of the platter
backside 514 area. The acoustic wave transducers 502 may transmit
at a frequency that is different from the other acoustic wave
transducers 503. In one embodiment the acoustic wave transducers
502 can form one strip while the acoustic wave transducers 503 form
a second parallel strip. In an alternate embodiment (not shown) the
acoustic wave transducers 502 and 503 can be uniformly mixed. In
another embodiment (not shown), the acoustic wave transducers could
be a strip that runs substantially a radius (R), the distance from
the platter inner diameter to the platter outer diameter. For this
embodiment, the acoustic wave transducers 502 and 503 could cover
approximately 20% of the platter backside 514 surface area. As a
result of less than 80% acoustic wave transducer coverage of the
platter, the wafer throughput may be reduced if the power is not
increased to compensate, but complete coverage of each wafer with
megasonics can still be maintained.
[0070] The effectiveness of cleaning by sound, in particular
removing particles, can be related to frequency, and different
sized particles can be more effectively removed with different
megasonic frequencies. Currently, a large percentage of the
particles to be removed from a wafer (not shown) exist in the 0.3
.mu.m (micron) and 0.1 .mu.m sizes. It has been determined that in
cleaning wafers, the megasonic removal of particles in the 0.3
.mu.m size range is efficient in the 900 kHz range while the
megasonic removal of particles in the 0.1 .mu.m range is efficient
in the 1.8 MHz range. In one embodiment, to provide two different
frequencies to a wafer for megasonic cleaning, a single signal is
sent to all of the transducers that contains a combination of
frequencies superimposed. The different transducers that exist on
the platter will each only respond to the corresponding frequency
they are sized for. In this manner, within the single signal,
individual frequencies can be added and subtracted or power varied,
for each frequency throughout the wafer processing cycles.
[0071] FIGS. 6A, 6B, & 6C illustrate one embodiment of acoustic
wave transducers 650 and 652 that output more than one frequency.
It has been determined that there is a relationship between the
size of the particle to be removed and the effectiveness of the
megasonic frequency to remove that particle. When cleaning a wafer,
particle sizes to be removed are often in the 0.3 micron (.mu.m)
and 0.1 micron sizes. Megasonic frequencies in the 925 kHz range
have been found to be effective at removing particles having a
diameter of approximately 0.3 .mu.m, and megasonic frequencies in
the 1.8 MHz range have been found to be effective at removing
particles having a diameter of approximately 0.1 .mu.m. The
acoustic wave transducers 650 and 652 are attached to the platter
608 where some of the acoustic wave transducers 650 output a
frequency that is different from the remaining acoustic wave
transducers 652. FIG. 6A illustrates one embodiment where a half
circle of the platter surface 614 is coated with a first transducer
650 that vibrates in the 925 kHz range and the remaining platter
half is covered with a second transducer 652 vibrating in the 1.8
MHz range. As the wafer (not shown) rotates, the entire wafer is
radiated with both frequency ranges. Even though these transducers
650 and 652 are not vibrating at the 5.4 MHz frequency to be
transparent, sufficient energy can still reach the wafer to be
effective in cleaning.
[0072] A variety of transducer placement arrangements are possible
to transfer multiple frequency acoustic energy to the wafer. A few
additional transducer arrangements are described below but the
invention is not limited to them. FIG. 6B illustrates an alternate
embodiment of the platter 608 having two groups of transducers 650
and 652 in diagonal quadrants. FIG. 6C illustrates an alternate
embodiment where the platter 608 has two groups of transducers 650
and 652 positioned on the platter in linear strips that each runs
substantially the diameter 654 of the platter surface 614. In an
embodiment, each transducer group 650 and 652 covers approximately
20% of the platter surface area 614. In the embodiments using the
half circle transducer placement (FIG. 6A), the quadrant transducer
placement (FIG. 6B), and the linear strip placement (FIG. 6C),
rotation of the wafer (not shown) will allow both frequencies to
strike at least 80% of the wafer surface. As a result of less than
80% acoustic wave transducer coverage, the through put may
[0073] If the transducers 650 and 652 are not generating at the 5.4
MHz frequency, i.e. transparent for the conditions that drove the
5.4 MHz selection, the various thicknesses making up the
transducers 650, and 652, adhesives 540 (FIG. 5B), and platter 608
can still be sized to minimize acoustic reflection and improve
efficiency of the sound waves reaching the wafer. With an
embodiment having a first group of transducers vibrating at a
frequency approximately twice that of the second group of
transducers, a platter thickness 530 (FIG. 5A) selected to minimize
reflection for one transducer group 650 frequency will be equally
efficient at reducing reflection for the other transducer group 652
frequency. The use of two frequencies has been given in the above
embodiments for purposes of example, however, it should be
appreciated that any number of different frequencies could be
provided and that the percent of coverage from each transducer type
producing each of the frequencies could be varied. When a platter
thickness has been selected that minimizes reflection from one
frequency, all of the other frequencies that will be applied can
also have minimized reflection if the ratio of each frequency used
is an integer multiple of the lowest frequency.
[0074] FIG. 7 is an illustration of wafer removal for one
embodiment of the cleaning chamber 700. During wafer 706 removal,
an alternate bracket 748, and the nozzle 751 can translate along an
axis 745, moving upward approximately 1'' to allow for wafer 706
engagement with the external robot arm (not shown). Next, a
cleaning chamber door 758 moves to provide access to the cleaning
chamber housing 760. With this opening, the robot arm can enter the
cleaning chamber housing 760, engage and remove the wafer 706, and
replace it with the next wafer (not shown) to be cleaned. In this
manner, the wafer 706 can be installed, cleaned, and removed
without requiring the system 700 to move complex components of the
cleaning apparatus such as the platter 708, the electric motor 722,
the fluid tubing 728 and the electrical wiring 746.
[0075] FIG. 8 is an illustration of one embodiment where a
plurality of megasonic frequencies are applied to quartz rods. In
this embodiment, a chemical 806 is applied to the wafer 814 through
a nozzle 816. A first quartz rod 802 and one or more additional
rods 804 may be placed close to the wafer 814 so as to collect the
liquid 806 between the quartz rods 802 and 804 and the wafer 814.
The quartz rods 802 and 804 can each transfer a different frequency
to the liquid couplant 806 from transducers attached at the ends of
each rod (not shown). The quartz rods 802 and 804 may be placed
with their axes 808 and 810 running parallel to the rotating wafer
814 to transfer sound pressure waves to the wafer top surface 812
which may be the wafer non-device side or the wafer device
side.
[0076] FIG. 9 is an illustration of one embodiment where a
plurality of megasonic spray nozzles 902 and 904 are used to
transfer acoustic energy. Each nozzle 902 and 904 imparts sonic
energy to a water spray 908 and 909 that strikes a wafer 906
rotating in a platter 907. The acoustic energy is placed in water
droplets 908 and 909, as imparted by the nozzles 902 and 904, and
the megasonic energized water can be sprayed onto the rotating
wafer non-device side surface 910. The platter 907 may have a
dished out center 912 to contain cleaning chemicals 911 and in
which the wafer 906 may "float". The cleaning chemicals 911 can be
pumped into an area between the wafer device side 913 and the
platter 907. With this embodiment, more than one megasonic spray
nozzle 908 and 909 may be used in which a different frequency is
imparted to one nozzle 902 than is imparted by the other nozzle
904. As a result of wafer rotation, the wafer 906 will receive both
megasonic frequencies during the process. Alternatively, one or
more megasonic frequencies can also by emitted from the platter 907
such that both sides of the wafer are receiving acoustic energy
directly, i.e. not just the acoustic energy transmitted through the
wafer to the opposite side.
[0077] FIG. 10 is an illustration of one embodiment of an apparatus
for batch processing a plurality of wafers using two or more
megasonic frequencies. A number of transducers 1004 and 1008 are
positioned on a chamber 1001 of the cleaning apparatus 1000.
Transducers of a first type 1004 generate at a first frequency
while transducers of a second type 1008 generate at a second
frequency. The transducers of the first type 1004 are positioned on
a first chamber surface 1002 while the transducers of the second
type 1008 are positioned on a second chamber surface 1006 that can
be approximately perpendicular to the first surface 1002. In this
manner, sound waves generated by transducers of the first type 1004
and the second type 1008 both travel parallel to a stack of wafers
1010 (only the top wafer is visible). To minimize wave interference
in the process chamber 1000 from the two frequencies, neither of
the transducer sets are positioned 180 degrees from the other set.
In addition, one or both of the two frequencies can be pulsed. In
alternate embodiments, the transducers may be at angles other than
perpendicular. In one embodiment, a number of transducers,
transmitting a number of frequencies, can each be positioned at
angles less than 90 degrees, i.e. acute angles, to meet constraints
of the megasonic cleaner housing 1012 shape and the number of
frequencies to be generated. In an alternate embodiment (not
shown), the transducers 1004 and 1006 can be positioned so as to be
mixed on any surface.
[0078] FIG. 11 is an illustration of a cluster 1100 of four single
wafer cleaning apparatus 1101 that are positioned about a robot arm
assembly 1102. Attached at a side of the machine 1100 are a number
of wafer cartridges 1104, each holding a plurality of wafers 1106
to be cleaned or that have been cleaned. The cleaning processes of
the cleaning chambers 1101 proceed in a sequence timed to optimize
the use of available space and the robot arm assembly 1102. One
possible sequence has the robot arm assembly 1102 take an unclean
wafer 1106 from a wafer cartridge 1104, install the wafer into a
cleaning chamber 1101, remove a clean wafer 1106 from another
process chamber 1101 and place the clean wafer 1106 into another
wafer cartridge 1104. This movement from process chamber 1101 to
wafer cartridge 1104 to process chamber 1101 and so on will
optimize wafer 1106 cleaning times, however other sequence
variations may be used to select an optimal wafer cleaning cycle
time.
[0079] FIG. 12 is an illustration of a single wafer cleaning
apparatus. The wafer cleaning apparatus 1200 is a stack of
machinery. The top of the stack can be a filter 1210 where air
flows through the filter 1210 using a fan or a turbine. The filter
1210 can be placed on a top chamber 1220 that positions the filter
1210 a distance from the cleaning chamber 1230 to reduce the
likelihood of chemical spray reaching the filter 1210. The cleaning
chamber 1230 can house the wafer holding bracket (not shown) along
with the other equipment needed to processes the wafer. Beneath the
cleaning chamber 1230 can be located various electronics 1240 used
to control the cleaning process and at the bottom can be placed the
cleaning and rinsing chemicals 1250 that feed up to the cleaning
chamber.
[0080] FIG. 13 is an illustration of an alternate embodiment of a
top chamber. In one embodiment, the air-flow from the filter above
(not shown) is partially re-directed 1320. A portion of the air
1310 flows down onto the wafer 1325 (platter removed for clarity),
however the remaining portion 1320 flows down a by-pass chamber
1330 of the top chamber 1350. A series of holes 1340 are spaced
annularly and in line with the spinning wafer 1325. Chemicals 1345
that are spun off the wafer 1325 during processing are drawn into
the annular holes 1340 to flow down the by-pass chamber 1330. In
this manner, the overall flow through the cleaning chamber 1300 is
more balanced and chemicals 1345 can be collected with less
contamination. Such chemicals 1345, collected with less impurities,
may be considered for reuse.
[0081] It is well known in the art that sonic energy may bounce
back or reflect when changing (material) boundaries. Therefore, it
is to be expected that a particular acoustic frequency generated by
a transducer through the transducer adhesive, the platter body, and
the platter fluoropolymer coating will have many opportunities to
reflect back and interfere with later transmitted sonic energies.
One approach is to design the various thickness of materials to
minimize or even eliminate this reflection. Another approach is to
allow bounceback, perhaps even up to an 80% reflection and then
pulse the transmitted sonic energy at a rate such that the new
outgoing sonic energy does not run into the reflected sonic energy.
As previously mentioned, pulsing the sonic energy has the
additional advantage of improving cavitation and therefore acoustic
streaming.
[0082] A thickness of a 300 mm wafer is nominally 0.775 mm. The
elimination or reduction in reflection can be done by choosing the
thickness of the layers to be a multiple of .lamda./2, where
.lamda. is the wavelength of the megasonic energy applied to the
wafer. Alternatively, for pulsing, the interference by reflection
can be eliminated by reducing the length of the signal pulse to
less than 2 L/c with c the velocity of the acoustic signal in the
layer and L the thickness of the layer. The velocity of an acoustic
wave in silicon is roughly 8430 meters/second (m/s). Therefore the
length of the pulse or burst should be less than (0.775
mm).sup.2/(8430 m/s)=0.18 .mu.s. Since this burst is very short, it
is a better practice to choose a frequency so that .lamda./2=0.775
mm and pulsing is not necessary. Since .lamda.=8430 m/s/f with f
the frequency, this gives a frequency of approximately 5.4 MHz.
[0083] After experimenting with 300 mm wafers, it was confirmed
that the optimum resonance frequency for transmission through the
wafer with minimum reflection is 5.4 MHz. Therefore, in one
embodiment this 5.4 MHz frequency is used to transmit megasonic
waves to the non-device side of the wafer. These frequency waves
transmit almost without any reflection through the platter and the
wafer to the wafer side not facing the platter, i.e. transparent
frequency. For a different wafer thickness than the present 300 mm
wafer thickness of 0.775 mm, 5.4 MHz would not be the correct
frequency. To generate a transparent wave through the wafer (and
the layers of preceding materials), a formula based on the
following factors; the .lamda./2 thickness of layers ratio and the
speed of sound in silicon, coupled with the wafer thickness, may be
used. The general formula for calculating the frequency that will
be transparent (i.e. not bounce back) is: 4215.+-.30% m/d, where m
is meters and d=the thickness of the wafer in meters. In another
embodiment, however, the 4215 m/d formula for calculating frequency
for the transparent wave may be used to apply the frequency to the
device side of the wafer. In this manner, for a given wafer
thickness, a sonic frequency having a wafer transparent to the wave
could be applied directly to the wafer device side and/or the wafer
non-device side. If more than one frequency is used that is
transparent to the stack of materials the sound waves must pass
through to arrive at the wafer surface, it could be desirable to
make as many of the frequencies multiples of the lowest frequency
as possible. This would allow for the transparency of such
frequencies passing through the stack of materials. If one of such
frequencies was transparent to the wafer, then additionally all
would have such advantage. This approach for generating transparent
frequencies could be used in other wafer cleaning apparatus such as
apparatus that totally immerse more than one wafer or apparatus
that use one or more quartz rods or apparatus that uses one or more
nozzles to place sonic energy in the spray.
[0084] Particulate removal without poly-line, i.e. poly-silicon or
amorphous silicon, damage to fine structures, i.e. having
dimensions less than 0.3 .mu.m, can be greatly reduced or
eliminated through the use of a cleaning solution used in
conjunction with megasonic energy that is applied normal to and
striking the wafer backside surface. Megasonic energy in the
frequency ranges of 900 kHz or higher can completely suppress
damage to the fragile poly-lines even when high acoustic power is
applied. 700 kHz or greater frequencies may be applied to the wafer
backside that can provide a megasonic power density of between 0.01
W/cm.sup.2 (Watt per centimeter squared) and 10 W/cm.sup.2 and
preferably between 0.1-5.0 W/cm.sup.2. Effective megasonic
frequencies may be in the range of 700 kHz-2.0 MHz but frequencies
are preferably higher than 900 MHz and most preferably
approximately 1.5 MHz.+-.30%.
[0085] In an embodiment, the cleaning solution used (with megasonic
energy), to reduce or eliminate poly-line damage, may be de-ionized
water or the cleaning solution may be a mixture from the SC-1
cleaning process (mentioned above) and applied at approximately
60.degree. C. The SC-1 cleaning process includes the cleaning
mixture of NH.sub.4OH+H.sub.2O.sub.2 added to water, and for this
embodiment, the cleaning mixture could consist of an
ammonia-to-hydrogen peroxide-to-water mixing ratio of approximately
1:2:80 by volume. The ammonia supplied could be an approximate 28%
solution by volume with water and the hydrogen peroxide supplied in
an approximate 31% solution by volume with water.
[0086] In the foregoing specification, the invention has been
described with reference to specific exemplary embodiments thereof.
It will, however, be evident that various modifications and changes
may be made thereto without departing from the broader spirit and
scope of the invention as set forth in the appended claims. The
specification and drawings are, accordingly, to be regarded in an
illustrative rather than a restrictive sense.
* * * * *