U.S. patent application number 16/559950 was filed with the patent office on 2020-05-14 for device and method for plasma treatment of electronic materials.
This patent application is currently assigned to Surfx Technologies LLC. The applicant listed for this patent is Surfx Technologies LLC. Invention is credited to Siu Fai Cheng, Robert F. Hicks, Thomas Scott Williams.
Application Number | 20200152430 16/559950 |
Document ID | / |
Family ID | 70550735 |
Filed Date | 2020-05-14 |
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United States Patent
Application |
20200152430 |
Kind Code |
A1 |
Williams; Thomas Scott ; et
al. |
May 14, 2020 |
DEVICE AND METHOD FOR PLASMA TREATMENT OF ELECTRONIC MATERIALS
Abstract
Plasma applications are disclosed that operate with argon and
other molecular gases at atmospheric pressure, and at low
temperatures, and with high concentrations of reactive species. The
plasma apparatus and the enclosure that contains the plasma
apparatus and the substrate are substantially free of particles, so
that the substrate does not become contaminated with particles
during processing. The plasma is developed through capacitive
discharge without streamers or micro-arcs. The techniques can be
employed to remove organic materials from a substrate, thereby
cleaning the substrate; to activate the surfaces of materials,
thereby enhancing bonding between the material and a second
material; to etch thin films of materials from a substrate; and to
deposit thin films and coatings onto a substrate; all of which
processes are carried out without contaminating the surface of the
substrate with substantial numbers of particles.
Inventors: |
Williams; Thomas Scott; (Los
Angeles, CA) ; Cheng; Siu Fai; (Culver City, CA)
; Hicks; Robert F.; (Los Angeles, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Surfx Technologies LLC |
Redondo Beach |
CA |
US |
|
|
Assignee: |
Surfx Technologies LLC
Redondo Beach
CA
|
Family ID: |
70550735 |
Appl. No.: |
16/559950 |
Filed: |
September 4, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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PCT/US19/49556 |
Sep 4, 2019 |
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16559950 |
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62726905 |
Sep 4, 2018 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01J 2237/3321 20130101;
H01L 21/02126 20130101; H01L 21/02274 20130101; H01J 37/32174
20130101; H01J 2237/335 20130101; H01L 21/02068 20130101; H01L
21/67069 20130101; H01J 37/32091 20130101; H01L 21/4835 20130101;
H01L 21/02216 20130101; H01L 21/4828 20130101; H01L 21/02057
20130101; H01J 37/32825 20130101; C23C 16/50 20130101; H01J 37/3244
20130101; H01J 2237/3341 20130101; H01L 21/02046 20130101; H01L
21/67028 20130101 |
International
Class: |
H01J 37/32 20060101
H01J037/32; H01L 21/67 20060101 H01L021/67; H01L 21/02 20060101
H01L021/02; H01L 21/48 20060101 H01L021/48; C23C 16/50 20060101
C23C016/50 |
Claims
1. An apparatus for producing an ionized gas plasma, comprising: a
housing having an inlet for gas flow comprising argon and one or
more molecular gases, an outlet for argon plasma comprising
reactive neutral species, and a flow path within the housing for
directing the gas flow; a power electrode disposed within the
housing having a powered electrode surface exposed to the gas flow;
a ground electrode disposed adjacent to the power electrode such
that a grounded electrode surface is closely spaced from the power
electrode surface and the gas flow is directed therebetween; a
power supply for delivering radio frequency power coupled to the
power electrode and the ground electrode to ionize the gas flow and
produce the argon plasma comprising the reactive neutral species;
an enclosure having the housing contained within and including an
enclosure gas flow wherein the enclosure gas flow has been filtered
to remove particles from the flow; and a material substrate
disposed within the enclosure near the outlet of the housing to
receive the reactive neutral species in the gas flow from the
ionized gas plasma.
2. The apparatus of claim 1, wherein the argon plasma is produced
by a capacitive discharge without substantially any streamers or
micro-arcs.
3. The apparatus of claim 1, wherein the reactive neutral species
from the ionized gas plasma are used for cleaning organic
contamination from the material substrate, activating the material
substrate surface for adhesion, etching a thin film off of the
material substrate, or depositing a thin film onto the material
substrate, all substantially without the deposition of
particles.
4. The apparatus of claim 1, wherein the gas inside the enclosure
is at atmospheric pressure.
5. The apparatus of claim 1, wherein the gas flow through the
housing is laminar.
6. The apparatus of claim 1, wherein the gas flow from the outlet
of the housing for the argon plasma is between 25 and 200.degree.
C.
7. The apparatus of claim 1, wherein the outlet of the housing for
argon plasma comprises a linear opening.
8. The apparatus of claim 7, wherein the linear opening is at least
as wide as the material substrate and the material substrate is
passed at a constant speed relative to and contacting the reactive
gas beam.
9. The apparatus of claim 1, further comprising a means of
translating the housing with the outlet for the ionized gas plasma
relative to the surface of the material substrate such that the
entire surface of the material substrate is uniformly treated with
the reactive species from the ionized gas plasma.
10. The apparatus of claim 1, wherein the power supply operates at
a radio frequency of 13.56 or 27.12 MHz and includes an auto-tuning
network that impedance matches the radio frequency power supply to
the argon plasma to minimize reflected power.
11. The apparatus of claim 1, wherein the one or more molecular
gases are added to the argon gas flow at a concentration between
0.5 to 5.0 volume % and a fraction of the one or more molecular
gases dissociates into atoms inside the argon plasma, and then
flows out of the outlet, wherein the atoms are selected from the
group consisting of O, N, H, F, C and S atoms.
12. The apparatus of claim 1, wherein the enclosure gas flow is
laminar.
13. The apparatus of claim 1, wherein the enclosure includes no
more than 100,000 particles larger than 0.1 micron per cubic meter
of air.
14. The apparatus of claim 13, wherein the enclosure comprises a
cleanroom.
15. A method of producing an ionized gas plasma comprising:
directing gas flow comprising argon and one or more molecular gases
from an inlet through a flow path within a housing to an outlet for
argon plasma comprising reactive neutral species; directing the gas
flow within the housing between a powered electrode surface of a
power electrode and a grounded electrode surface of a ground
electrode, the grounded electrode surface closely spaced from the
powered electrode surface; delivering radio frequency power coupled
to the power electrode and the ground electrode from a power supply
to ionize the gas flow and produce the argon plasma comprising the
reactive neutral species; disposing the housing with the argon
plasma within an enclosure including an enclosure gas flow wherein
the enclosure gas flow has been filtered to remove particles from
the flow; and disposing a material substrate within the enclosure
near the outlet of the housing to receive the reactive neutral
species.
16. The method of claim 15, wherein the argon plasma comprising the
reactive neutral species is produced by capacitive discharge
substantially without any streamers or micro-arcs.
17. The method of claim 15, wherein the reactive neutral species
from the ionized gas plasma are used for cleaning organic
contamination from the material substrate, activating the material
substrate surface for adhesion, etching a thin film off of the
material substrate, or depositing a thin film onto the material
substrate, all substantially without the deposition of
particles.
18. The method of claim 15, wherein the gas inside the enclosure is
at atmospheric pressure.
19. The method of claim 15, wherein the gas flow inside the housing
is laminar.
20. The method of claim 15, wherein the gas flow from the outlet of
the housing for the argon plasma is between 25 and 200.degree.
C.
21. The method of claim 15, further comprising a means of
translating the housing with the outlet for the ionized gas plasma
relative to the surface of the material substrate such that the
entire surface of the material substrate is uniformly treated with
the reactive species from the ionized gas plasma.
22. The method of claim 15, wherein the radio frequency power is
delivered at 13.56 or 27.12 MHz.
23. The method of claim 15, wherein the one or more molecular gases
are added to the argon gas flow at a concentration between 0.5 to
5.0 volume % and a fraction of the one or more molecular gases
dissociates into atoms inside the argon plasma, and then flows out
of the outlet, wherein the atoms are selected from the group
consisting of O, N, H, F, C and S atoms.
24. The method of claim 23, wherein the molecular gas added to the
argon gas flow is dissociated into atoms selected from the group
consisting of oxygen (O), nitrogen (N), and hydrogen (H).
25. The method of claim 15, wherein the enclosure gas flow is
laminar.
26. The method of claim 15, wherein the enclosure includes no more
than 100,000 particles larger than 0.1 micron per cubic meter of
air.
27. The method of claim 15, wherein the enclosure comprises a
cleanroom.
28. The method of claim 15, wherein the molecular gas is selected
from the group comprising oxygen and nitrogen and the material
substrate is a semiconductor wafer and the surface of the
semiconductor wafer is cleaned of organic contamination with the
plasma.
29. The method of claim 15, wherein the molecular gas is hydrogen
and the material substrate is a copper substrate and copper oxide
on the copper substrate is etched away with the plasma.
30. The method of claim 29, wherein the copper substrate is a lead
frame strip.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit under 35 U.S.C. .sctn.
119(e) of the following U.S. provisional patent application, which
is incorporated by reference herein:
[0002] U.S. Provisional Patent Application No. 62/726,905, filed
Sep. 4, 2018, and entitled "DEVICE AND METHOD FOR PLASMA TREATMENT
OF ELECTRONIC MATERIALS," by Williams et al. (Attorney Docket
SRFXP010.P1).
BACKGROUND OF THE INVENTION
1. Field of the Invention
[0003] The invention is related to a plasma apparatus and methods
of using the plasma apparatus for cleaning, surface activation,
etching and deposition on electronic materials.
2. Description of Related Art
[0004] Ionized gas plasmas have found wide application in materials
processing. Plasmas that are used in materials processing are
generally weakly ionized, meaning that a small fraction of the
molecules in the gas are charged. In addition to the ions, these
plasmas contain reactive species that can clean, activate, etch and
deposit thin films onto surfaces. The temperature in these wealdy
ionized gases is usually below 250.degree. C., so that most
thermally sensitive substrates are not damaged. The physics and
chemistry of weakly ionized plasmas are described in several
textbooks. See for example, Lieberman and Lichtenberg, "Principles
of Plasma Discharges and Materials Processing," (John Wiley &
Sons, Inc., New York, 1994), and Raizer, Y. P., "Gas Discharge
Physics", (Springer-Verlag, Berlin (1991).
[0005] According to the literature, weakly ionized plasmas are
generated in vacuum at pressures between 0.001 and 1.0 Torr (see
Lieberman and Lichtenberg (1994)). Electrical power is applied
across two electrodes to break the gas down and ionize it. The
electricity may be provided as a direct current (DC), alternating
current (AC), radio frequency (RF), or microwave (MW) source. The
electrode may be constructed to provide either capacitive or
inductive coupling to strike and maintain the plasma. In the former
case, two conducting electrodes are placed inside the vacuum
chamber filled with a small amount of gas. One of the electrodes is
powered, or biased, by the RF generator, while the other one is
grounded. In the latter case, the RF power is supplied through an
antenna that is wrapped in a coil around the insulating walls of
the chamber. The oscillating electric field from the coil
penetrates into the gas inducing ionization.
[0006] Electronic materials, including silicon, gallium arsenide,
silicon carbide, sapphire and glass wafers, are inserted into the
vacuum chamber and processed by striking the plasma and running it
for a period of time to modify the surface of the wafer. Standard
semiconductor wafer sizes are 100, 150, 200 and 300 mm in diameter.
The vacuum chamber is designed specifically to fit one of the wafer
sizes. One way to clean the silicon surface is to feed oxygen and
argon to the chamber. Energetic free electrons in the plasma
convert a portion of the oxygen molecules to O atoms and other
reactive species that attack the organic contaminants on the
substrate surface and convert them into gaseous carbon dioxide. In
addition, the surface may be physically sputtered of contaminants
by bombardment of the substrate with positively charge argon ions
(Ar.sup.+). Following oxygen plasma treatment for several minutes
in the vacuum chamber, the silicon wafer surface is clean and
activated for other semiconductor processing steps, including, but
not limited to, wafer bonding.
[0007] Another way to clean the surface of an electronic material
is to insert the substrate into the vacuum chamber and to strike
the plasma with a feed gas containing a mixture of hydrogen and
argon. In this case, the energetic free electrons will produce H
atoms and Ar.sup.+ ions that will strike the surface of the
substrate and remove metal oxide contaminates, including, but not
limited to, copper oxide, silver oxide, tin oxide, or indium oxide.
To prevent a safety hazard by introducing hydrogen into a chamber,
the feed gas to the plasma must contain less than 4.0% H.sub.2 in
argon. At the operating pressure of the vacuum chamber, 0.05 to
0.50 Torr, the resulting concentration of hydrogen is only 2 to 20
milliTorr. The rate of hydrogen atom etching of the metal oxide
will be extremely low, so that most of the oxide contaminant is
removed by sputtering.
[0008] Semiconductor substrates are etched in vacuum plasmas by
generating reactive species in the gas that convert elements on the
substrate surface into stable gas molecules. The stable gas
molecules are then pumped out of the chamber. For example, silicon
is removed from Si wafers by the following reaction:
Si.sub.(s)+4F.sub.(g).dbd.SiF.sub.4(g), where the subscripts s and
g refer to solid and gas, respectively. Fluorine atoms are
generated in the plasma by the electron impact dissociation of
carbon tetrafluoride (CF.sub.4) into carbon and fluorine atoms. If
a polymer mask is deposited on the surface that prevents etching of
the underlying silicon substrate in selected areas, then a pattern
of silicon transistors can be etched into the Si wafer (see
Lieberman and Lichtenberg (1994)). Ion bombardment of the wafer
surface adds vertical directionality to the silicon etching
process. The etching of other materials, including certain metals,
can be accomplished with chlorine plasmas, and are obvious to those
skilled in the art of plasma processing.
[0009] Etching copper oxide films off of copper substrates is an
important process in semiconductor manufacturing. The dies
containing the integrated circuits are attached to copper lead
frame strips, where wires are attached from output pads on the dies
to the leads, and then the entire package is encapsulated in a mold
compound. It has been found that if copper oxide films are present
on the lead frame strips delamination will occur at the die
pad--mold interface (see for example, L. C. Yung, L. C. Ying, C. C.
Fei, A. T. Ann and S. Norbert, "Oxidation on copper lead frame
surface which leads to package delamination," IEEE Proceedings of
the International Conference on Software Engineering, 2010, Kuala
Lumpur, Malaysia, p. 654; and C. T. Chong, A. Leslie, L. T. Beng,
and C. Lee, "Investigation on the effect of copper leadframe
oxidation on package delamination," IEEE Proceedings of the 45th
Electronic Components and Technology Conference, Las Vegas, Nev.,
USA, p. 463) Failure occurs between the copper oxide and copper
metal base rather than between the mold compound and copper oxide
surface. As the oxide layer grows on the base metal, voids develop
in the interface, weakening the bond between the layers. Therefore,
removing the oxide layer and eliminating the interfacial voids is
necessary to prevent delamination of the protective mold. Vacuum
plasmas are poorly suited to this application due to the very low
concentration of hydrogen that may be fed into the chamber.
[0010] The plasma-enhanced chemical vapor deposition of thin films
(PECVD) is carried out in vacuum plasma chambers as well. Here, a
volatile precursor molecule is introduced into the chamber that
contains the elements in the desired film. Reactive species
produced in the plasma breakdown the precursor molecules,
liberating the elements that are subsequently incorporated into the
growing film on the wafer surface. For example, silicon dioxide
(SiO.sub.2) can be deposited in trenches etched onto silicon wafers
by the decomposition of tetraethoxysilane
(Si(OC.sub.2H.sub.4).sub.4) in an oxygen plasma. The overall
reaction may be represented as follows:
Si(OC.sub.2H.sub.4).sub.4(g)+22O.dbd.SiO.sub.2(s)+8CO.sub.2(g)+8H.sub.2O.-
sub.(g). The plasma process conditions are selected to achieve the
desired film properties. For example, if a stable insulator with
high dielectric strength is desired to isolate adjacent
semiconductor transistors, then a pure glass film is deposited in
trenches on the Si wafer by heating the substrate to a high
temperature, for example, 500.degree. C., and limiting the amount
of precursor in the vacuum chamber to slow down the deposition rate
and to achieve a large excess of oxygen to convert all the carbon
and hydrogen from the precursor into carbon dioxide and water. Many
other thin films on electronic materials are produced by
plasma-enhanced chemical vapor deposition, and are obvious to those
skilled in the art of PECVD.
[0011] One of the drawbacks of vacuum plasma processing of
electronic materials is that the chambers become dirty upon
repeated processing of semiconductor wafers. The chambers slowly
fill up with particles (i.e. contamination) ranging in size from
0.01 to 10.0 microns in diameter. This problem has been documented
in many publications (see for example, G. S. Selwyn, et al., J.
Vac. Sci. Technol. A 7, 2758 (1989); ibid., 8, 1726 (1990); M. J.
McCaughey and M. J. Kushner, Appl. Phys. Left. 55, 951 (1989); R.
N. Nowlin and R. N Carlile, J. Vac. Sci. Technol. A 9, 2824 (1990);
G. S. Selwyn, Jpn. J. Appl. Phys. 32, 3068 (1993); and S. J. Choi,
et al., Plasma Sources Sci. Technol. 4, 418 (1994); and R. L.
Merlino and J. A. Goree, Physics Today 1 (July 2004)). When the
plasma is turned on, the particles become negatively charged and as
a result of the electric field in the chamber, float over the wafer
surface. When the plasma is turned off, the particles drop down on
the wafer, forming a thin layer of contamination. It is well known
to those skilled in the art that particles present during wafer
processing can kill solid-state devices. The semiconductor industry
is obsessed with eliminating them, and spends billions of dollars
constructing cleanrooms that are free of particles above 0.01
microns in diameter. Dirty plasma chambers must be routinely
cleaned to reduce particle contamination. In addition, wafers have
to be wet cleaned after plasma immersion to eliminate any particles
that may have stuck to them. All this drives up the cost of
manufacturing electronic devices. In conclusion, there is a need
for improved plasma processing devices and methods that do not
generate particles.
[0012] Atmospheric pressure plasmas have been developed as an
alternative to vacuum plasmas. The different types of atmospheric
pressure plasma devices have been described in multiple
publications (Schutze, et. al., IEEE Trans. Plasma Sci. 26, 1685
(1998); Goldman and Sigmond, IEEE Trans. Electrical Insulation
EI-17, no. 2, 90 (1982); Eliasson and Kogelschatz, IEEE Trans.
Plasma Sci. 19, 1063 (1991); Fauchais and Vardelle, IEEE Trans.
Plasma Sci. 25, 1258 (1997); Moravej, et al., J. Appl. Phys. 96,
7011 (2004); and Babayan and Hicks, U.S. Pat. No. 7,329,608 (Feb.
12, 2008) and U.S. Pat. No. 8,328,982 (Dec. 11, 2012)). These
plasmas have not been adopted for semiconductor manufacturing for a
number of reasons. They generate non-uniform beams of reactive gas
containing sparks or streamers that can damage the solid-state
devices on the semiconductor wafer. They can generate too much UV
light, or cause electrostatic discharge onto the substrate. In many
cases, atmospheric pressure plasmas do not have a protective sheath
at the electrode surfaces, so that energetic ions collide with said
surfaces and etch off particulate matter. It is known to those
skilled in the art that these plasmas are dirtier than the vacuum
plasmas used in semiconductor processing.
[0013] In view of the foregoing, there is a need for a plasma
device and method that is suitable for electronic materials
processing, and that does not generate particles which can be
harmful to manufacturing operations. These and other needs are met
by embodiments of the present invention as described hereafter.
SUMMARY OF THE INVENTION
[0014] One embodiment of the invention comprises a plasma apparatus
and method that utilizes argon and other molecular gases to clean,
activate, etch and deposit thin films onto electronic materials.
The plasma apparatus and substrate is placed inside an enclosure
that contains a means of removing the particles from the air so
that no particles come in contact with the substrate that is being
processed by the plasma. The enclosure may be a cabinet encasing
the apparatus and substrate, or it may be a cleanroom. The plasma
is generated in a self-contained housing that contains two
electrodes driven with radio frequency (RF) power. A high density
of reactive species is generated within the device by collisions
between molecules and the energetic free electrons in the
discharge. The reactive species flow out of the housing and onto
the substrate that is placed a short distance downstream. The
housing is supplied with RF power and with a controlled flow of
argon and other molecular gases in suitable proportion to generate
the stable plasma that cleans, activates, etches, or deposits the
thin film onto the substrate. The gas flow system is cleaned and
made free of any contaminants that can be a source of particle
generation in the plasma. The plasma apparatus and method further
contains a means of uniformly contacting the substrate by scanning
the self-contained housing over said substrate, or by spinning the
substrate underneath the housing.
[0015] In one embodiment of the invention, contaminated surfaces
are cleaned through exposure to the reactive gas species generated
by the plasma. To remove organic, contamination, oxygen molecules
are added to the argon gas entering the housing. The concentration
of oxygen fed to the plasma is in the range from 0.1% to 5.0%,
preferably from 0.5% to 1.5%. The plasma then converts the oxygen
molecules into 0 atoms and other reactive species. This gas flow is
directed onto the substrate surface to be cleaned. An example of
this process is the cleaning and activation of silicon wafers. The
native oxide layer on the silicon is initially contaminated with a
layer of adsorbed organic compounds. The reactive oxygen species
generated in the argon/oxygen plasma react with the organic
compounds, converting them into water vapor and carbon dioxide,
leaving behind a silicon dioxide surface free of contamination. The
plasma apparatus in this embodiment of the invention is in an
enclosure which is free of particles so there is no deposition of
particles onto the silicon wafer surface either from the gas flow
emanating from the plasma, or the air inside the enclosure. In one
embodiment, the enclosure is a cleanroom.
[0016] In another embodiment of the invention, silicon dies are
cleaned of organic contamination with the plasma apparatus and
method immediately prior to attaching the dies to the lead frame
strip. Die attach is the first step in the semiconductor packaging
operation. Organic contamination on the dies can prevent adhesion
of the die to the lead frame strip, and result in a defective
package. This embodiment of the invention ensures that strong
adhesion of the die to the lead frame is achieved.
[0017] In another embodiment of the invention, the plasma gas is
used to etch layers of material, including metals, metal oxides,
polymers, or semiconductors, from a substrate surface. In the case
of etching metals, metal oxides, semiconductors or ceramics, gas
molecules containing hydrogen, fluorine, or chlorine are added to
the argon flow into the self-contained housing. When RF power is
applied to the electrodes inside the housing, the gas flow becomes
ionized, and the energetic free electrons produced therefrom
collide with the gas molecules, causing them to dissociate into
fragments and liberate hydrogen, fluorine or chlorine atoms. These
atoms flow out of the housing and impinge on the substrate placed a
short distance downstream. Etching occurs by the reaction of the H,
F or Cl atoms with the metals, metal oxides, semiconductors, or
ceramics exposed on the wafer surface.
[0018] In one embodiment, copper oxide is etched with hydrogen
atoms by the following reaction:
CuO.sub.(s)+2H.sub.(g).dbd.Cu.sub.(s)+H.sub.2O.sub.(g). In this
case, hydrogen molecules are added to the argon gas entering the
housing and dissociated into H atoms by the plasma. The H atoms
flow out of the housing and impinge on a substrate comprising
copper and other materials, and remove the copper oxide layer. The
concentration of hydrogen fed to the plasma is in the range from
0.1% to 5.0%, preferably from 0.5% to 1.5%. One example of a
substrate is copper lead frame strips. Copper oxide can form on the
surface of copper lead frame strips preventing strong adhesion of
wires to the copper bond pads, and of the mold compound to the
copper leads. Removing the copper oxide from the copper lead frame
surface ensures that strong adhesion is obtained with the wire
bonds and the mold compound to the lead frame. Additionally, no
particles are deposited onto the lead frame strips by this process
that could prevent adhesion between the mold compound and the
copper surface.
[0019] In another embodiment, silicon dioxide is etched with
fluorine atoms by the following reaction:
SiO.sub.2(s)+4F.sub.(g).dbd.SiF.sub.4(g)+O.sub.2(g). An example of
this process is the etching of thin glass films on the surface of
silicon wafers. Many other etching reactions are possible and would
be obvious to those skilled in the art. In this embodiment of the
invention, the reactive gas flow emanating from the plasma inside
the housing does not contain any particles and the apparatus is
contained within an enclosure free of particles, so that there is
no deposition of particles onto the substrate during the etching
reaction.
[0020] NOM Another embodiment of the invention comprises an
apparatus and method for depositing thin films onto a substrate
without the co-deposition of particles. A suitable precursor
molecule is selected so that its reaction products will generate
the desired coating. In this case, oxygen, nitrogen, hydrogen or
another gas can be mixed with the argon gas fed into the plasma
source. These molecules get dissociated inside the plasma and
produce a reactive gas stream that includes for example, O, N, or H
atoms. Precursor molecules are mixed with the reactive gas stream
at the exit of the plasma source. The resulting mixture then
impinges on a substrate where deposition of the thin film occurs.
In one embodiment of the invention, O.sub.2 and argon are fed to
the plasma, generating a flow of reactive species at the exit of
the plasma source that contains O atoms. Then a volatile
organosilane precursor, including tetraethoxysilane, or
tetramethyl-cyclotetrasiloxane, is mixed with the reactive gas
flow. This reactive mixture impinges onto a substrate, including a
silicon, gallium arsenide, silicon carbide, sapphire, or glass
wafer or sheet, placed in the flow path, resulting in the
deposition of a silicon dioxide film without co-deposition of a
substantial number of particles.
[0021] In one notable example embodiment, an apparatus for
producing a low-temperature, atmospheric pressure plasma, comprises
a housing having an inlet for gas flow comprising argon and one or
more molecular gases, an outlet for plasma comprising reactive
neutral species, and a flow path within the housing for directing
the gas flow to become laminar, a power electrode disposed within
the housing having a powered electrode surface exposed to the
laminar gas flow, a grounded electrode disposed adjacent to the
powered electrode such that the grounded electrode surface is
closely spaced from the powered electrode surface and the laminar
gas flow is directed there between, a power supply for delivering
radio frequency power coupled to the powered electrode and the
grounded electrode to ionize the laminar gas flow and produce the
plasma comprising the reactive neutral species. Typically, the
housing can comprise the grounded electrode, and/or the outlet can
comprise a linear opening. In this embodiment, the housing is
placed inside an enclosure with flowing gas, wherein said gas has
been filtered to prevent the introduction of particles into the
space containing the housing. In addition, a substrate, including a
silicon, gallium arsenide, silicon carbide, sapphire, or glass
wafer or sheet, is placed a short distance downstream of the outlet
of the housing so that the substrate is exposed to the reactive gas
emanating therefrom.
[0022] In further embodiments, the molecular gas can be added to
the argon gas flow at a concentration between 0.1 to 5.0 volume %
and the molecular gas dissociates into atoms inside the plasma, and
then flows out of the outlet, wherein the atoms are selected from
the group consisting of O, N, H, F, Cl, C and S atoms.
[0023] In a similar manner, a method embodiment of the invention
comprises the steps of directing gas flow comprising argon and one
or more molecular gases from an inlet through a laminar flow path
within a housing to an outlet for plasma comprising reactive
neutral species, directing the laminar gas flow within the housing
between the surface of a powered electrode and the surface of a
grounded electrode, the grounded electrode surface closely spaced
from the powered electrode surface, delivering radio frequency
power coupled to the powered electrode and the grounded electrode
from a power supply to ionize species in the laminar gas flow, and
directing the reactive neutral species generated by the plasma from
a head onto a substrate surface, wherein the housing and the
substrate are contained inside an enclosure that has a
particle-free gas flow. In one embodiment, the gas flow in the
enclosure is laminar.
[0024] The material surface can be cleaned or etched by the
reactive neutral species directed from the head. Alternately (or in
addition), the material surface can obtain increased surface energy
by the reactive neutral species directed from the head. This can
improve adhesion properties of the material surface. The material
surface can also have a thin film deposited thereon by the reactive
neutral species directed from the head. A chemical precursor can be
mixed with the reactive species directed from the head causing at
least one element from the chemical precursor to be incorporated
into the thin film deposited on the material surface. In each of
these processes, the plasma gas flow is substantially free of
particles and the enclosure containing the plasma source and the
substrate is substantially free of particles so that few if any
particles are deposited onto the substrate. This method embodiment
of the invention can be further modified consistent with any of the
apparatus or method embodiments described herein.
[0025] Another embodiment of the invention can also comprise an
apparatus for producing an ionized gas plasma, including a housing
having an inlet for gas flow comprising argon and one or more
molecular gases, an outlet for argon plasma comprising reactive
neutral species, and a flow path within the housing for directing
the gas flow, a power electrode disposed within the housing having
a powered electrode surface exposed to the gas flow, a ground
electrode disposed adjacent to the power electrode such that a
grounded electrode surface is closely spaced from the power
electrode surface and the gas flow is directed therebetween, a
power supply for delivering radio frequency power coupled to the
power electrode and the ground electrode to ionize the gas flow and
produce the argon plasma comprising the reactive neutral species,
an enclosure having the housing contained within and including an
enclosure gas flow wherein the enclosure gas flow has been filtered
to remove particles from the flow, and a material substrate
disposed within the enclosure near the outlet of the housing to
receive the reactive neutral species in the gas flow from the
ionized gas plasma. In this embodiment as well, the apparatus can
be placed inside an enclosure, including a cabinet or a cleanroom,
which is free of substantial numbers of particles. This apparatus
embodiment of the invention can be further modified consistent with
any of the apparatus or method embodiments described herein.
[0026] Typically, the argon plasma is produced by a capacitive
discharge without substantially any streamers or micro-arcs and the
gas inside the enclosure is at atmospheric pressure. The gas flow
through the housing can be laminar, the enclosure gas flow can be
laminar, and the gas flow from the outlet of the housing for the
argon plasma can be between 25 and 200.degree. C. In addition, the
reactive neutral species from the ionized gas plasma can be used
for cleaning organic contamination from the material substrate,
activating the material substrate surface for adhesion, etching a
thin film off of the material substrate, or depositing a thin film
onto the material substrate, all substantially without the
deposition of particles. The power supply can operate at a radio
frequency of 13.56 or 27.12 MHz and includes an auto-tuning network
that impedance matches the radio frequency power supply to the
argon plasma to minimize reflected power. The one or more molecular
gases can be added to the argon gas flow at a concentration between
0.5 to 5.0 volume % and a fraction of the one or more molecular
gases dissociates into atoms inside the argon plasma, and then
flows out of the outlet, wherein the atoms are selected from the
group consisting of O, N, H, F, C and S atoms. The enclosure can
include no more than 100,000 particles larger than 0.1 micron per
cubic meter of air and the enclosure can comprise a cleanroom.
[0027] In some embodiments, the outlet of the housing for argon
plasma comprises a linear opening. The linear opening can be at
least as wide as the material substrate and the material substrate
is passed at a constant speed relative to and contacting the
reactive gas beam.
[0028] In further embodiments, the apparatus can include a means of
translating the housing with the outlet for the ionized gas plasma
relative to the surface of the material substrate such that the
entire surface of the material substrate is uniformly treated with
the reactive species from the ionized gas plasma.
[0029] An exemplary method embodiment of the invention comprises
directing gas flow comprising argon and one or more molecular gases
from an inlet through a flow path within a housing to an outlet for
argon plasma comprising reactive neutral species, directing the gas
flow within the housing between a powered electrode surface of a
power electrode and a grounded electrode surface of a ground
electrode, the grounded electrode surface closely spaced from the
powered electrode surface, delivering radio frequency power coupled
to the power electrode and the ground electrode from a power supply
to ionize the gas flow and produce the argon plasma comprising the
reactive neutral species, disposing the housing with the argon
plasma within an enclosure including an enclosure gas flow wherein
the enclosure gas flow has been filtered to remove particles from
the flow, and disposing a material substrate within the enclosure
near the outlet of the housing to receive the reactive neutral
species. The method embodiment of the invention can be modified
consistent with any apparatus embodiment described herein.
[0030] In further embodiments, the molecular gas can be selected
from the group comprising oxygen and nitrogen and the material
substrate is a semiconductor wafer and the surface of the
semiconductor wafer is cleaned of organic contamination with the
plasma. The molecular gas can be hydrogen and the material
substrate can be a copper substrate and copper oxide on the copper
substrate is etched away with the plasma. The copper substrate can
be a lead frame strip.
[0031] These and other embodiments of the invention will become
apparent to those skilled in the art from the following description
including the preferred embodiments. Embodiments of the invention
includes methods to clean surfaces, methods to increase surface
energy and improve adhesion, methods for etching materials, and
methods for depositing thin films, wherein each of these processes
are carried out in such a way as to prevent the accumulation of
particles on the substrate surface.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] Referring now to the drawings in which like reference
numbers represent corresponding parts throughout:
[0033] FIG. 1 is a schematic diagram of an exemplary apparatus
embodiment of the invention, in which the plasma device is scanned
over the substrate wafer using a robot;
[0034] FIG. 2 is a schematic diagram of an exemplary apparatus
embodiment of the invention, in which the substrate wafer is
rotated underneath a stationary plasma device;
[0035] FIGS. 3A and 3B show cross sections of an example argon
plasma device;
[0036] FIG. 4 is a depiction of the particle map for a silicon
wafer in the cleanroom;
[0037] FIG. 5 is a histogram of the particle size distribution on
the silicon wafer exposed to the cleanroom;
[0038] FIG. 6 is a depiction of the particle map on a silicon wafer
after cleaning with the vacuum oxygen plasma;
[0039] FIG. 7 is a histogram of the particle size distribution on
the silicon wafer exposed to the vacuum O.sub.2 plasma;
[0040] FIG. 8 is a depiction of the particle map on a silicon wafer
after cleaning with atmospheric pressure argon and oxygen
plasma;
[0041] FIG. 9 is a histogram of the particle size distribution on
the silicon wafer exposed to the atmospheric pressure Ar/O.sub.2
plasma;
[0042] FIG. 10. Dependence of the natural log of the copper oxide
etch rate on inverse temperature (K); and
[0043] FIG. 11 shows a schematic of the attachment to the
self-contained plasma housing for depositing coatings onto a
substrate using the invention.
DETAILED DESCRIPTION INCLUDING THE PREFERRED EMBODIMENTS
Overview
[0044] As described above, plasma applications are disclosed that
process electronic materials without significant contamination of
the substrate surface with particles. The plasma apparatus and the
electronic material substrate are placed inside an enclosure,
including a cabinet or a cleanroom, with gas flow that is free of
substantial numbers of particles. The plasma apparatus consists of
a self-contained housing, which is supplied with radio frequency
power and a flow of gas comprising argon and a molecular gas in the
range of 0.1 to 5.0 volume %. Application of RF power to the
electrodes inside the housing causes the gas to be ionized at
atmospheric pressure and at low temperature. A high concentration
of reactive species, including for example O, N, H, F, Cl, C and S
atoms, is generated by free electron collisions with the gas
molecules inside the plasma. Laminar flow is maintained as the gas
flows into the housing, through the plasma, and out of the housing.
One of the electrodes may be heated which can help to stabilize the
plasma. The gas containing the reactive species is directed onto a
substrate placed a short distance downstream, wherein said
substrate is cleaned, activated, etched or coated with a thin film.
Throughout processing the substrate in the enclosure with the
plasma apparatus, few if any particles are deposited onto the
substrate.
[0045] It should be noted that there are some important
requirements of atmospheric plasma used with embodiments of the
present invention. In order to minimize the production of
particles, the atmospheric plasma must be struck and maintained as
a capacitive discharge without generating any streamers or
micro-arcs. In addition, the plasma device must employ a gas flow
path that is clean and devoid of any silicone grease, which can
lead to the production of particles. For example, a mass flow
controller used in the operation of the plasma device can use
Apiezon M vacuum grease or any similar silicone-free grease. Those
skilled in the art will understand techniques and devices for
producing suitable atmospheric plasma through a capacitive
discharge process without any streamers or micro-arcs and without
using any silicone grease based on the examples described
herein.
[0046] The example apparatus and method produces a low-temperature,
atmospheric pressure argon plasma by flowing a mixture of argon and
molecular gases through a housing containing two closely spaced
electrodes, applying radio frequency power to one of the electrodes
(grounding the other) sufficient to strike and maintain the ionized
gas plasma, and flowing reactive neutral species out of the
housing, while keeping the free electrons and ions inside the
housing between the electrodes. Further details for operating a
suitable plasma delivery device for implementing an embodiment of
the invention can be found in U.S. Pat. Nos. 9,406,485 and
10,032,609, which are both incorporated by reference herein.
[0047] FIG. 1 shows a schematic of an exemplary apparatus for
plasma processing of electronic materials, in a way that is free of
the deposition of substantial numbers of particles on the
substrate. The apparatus includes an enclosure 300 that is equipped
with a filtration system 301 that introduces a flow of gas,
including, but not limited to, air, that is substantially free of
entrained particles. In one embodiment, the flow of gas inside the
enclosure is laminar. An argon plasma device 100 is mounted inside
the enclosure on a robot 302 with the ability to scan the plasma
device 100 in the x and y directions at a distance z over a
substrate 303. The substrate 303 is secured onto the robotic stage
304. The enclosure 300 may be a cabinet, cleanroom, or another
suitable three-dimensional space for mounting the robot and plasma
device inside. The substrate 303 may be any material, including,
but not limited to, a silicon wafer, a compound semiconductor
wafer, a silicon carbide wafer, a sapphire wafer, a glass sheet, a
plastic sheet, a molded plastic part, a metal lead frame strip, a
printed circuit board, a display, or a flexible circuit.
[0048] FIG. 2 shows a schematic of another exemplary apparatus for
plasma processing of electronic materials, in a way that is free of
the deposition of substantial numbers of particles on the
substrate. The apparatus includes an enclosure 300, a gas
filtration system 301, a plasma device 100, and a substrate 303.
The plasma device 100 is mounted on a fixture 305 that keeps the
outlet of the device 100 at a fixed distance from the substrate 303
between 1.0 and 10.0 mm. The substrate 303 is placed on a spinning
stage 306. The plasma device is configured so that the outlet
plasma beam extends over the radius of the circular substrate.
During plasma processing, the substrate is spun underneath the beam
at speeds ranging from 1 to 10,000 rpm.
[0049] FIGS. 3A and 3B are schematic cross-section diagrams of an
exemplary argon plasma device 100 according to an embodiment of the
invention for producing a low-temperature, atmospheric pressure
argon plasma. The device 100 comprises a housing 102 which supports
an inlet 104 for gas flow comprising argon and one or more
molecular gases 106 and an outlet 108 for argon plasma comprising
reactive neutral species. Typically, the molecular gas is added to
the argon gas flow at a concentration between 0.1 to 5.0 volume %.
The molecular gas dissociates into atoms (O, N, H, F, C or S atoms)
inside the argon plasma and then flows out of the outlet, e.g. onto
a substrate. In this example, the outlet 108 comprises a linear
opening.
[0050] A flow path within the housing 102 directs the gas flow to
become laminar as it moves from the inlet 104 toward a power
electrode 110. The power electrode 110 disposed within the housing
has a powered electrode surface 112 exposed to the laminar gas
flow. A ground electrode 114 is disposed adjacent to the power
electrode 110 such that a grounded electrode surface 116 is closely
spaced from the powered electrode surface and the laminar gas flow
is directed there between. In this example, the entire housing 102
is the ground electrode 114. However, those skilled in the art will
understand that the ground electrode 114 can be implemented as a
separate component in the region near the grounded electrode
surface 116. It is only necessary that the power and ground
electrodes 110 and 114 are electrically isolated from one another
as will be readily understood by those skilled in the art.
[0051] A power supply 118 for delivering radio frequency power is
coupled to both the power electrode and the ground electrode to
ionize the laminar gas flow and produce the argon plasma comprising
the reactive neutral species as it passes between the electrode
surfaces 112, 116. In addition, a heater 128 may or may not be
coupled to the device 100 for heating one or both of the power
electrode 110 and the ground electrode 114 as the laminar gas flow
is directed between the surfaces 112, 116. The heater 128 heats to
a temperature between 40 and 200.degree. C., but preferable between
40 and 80.degree. C. Heating can be implemented through any
suitable means however, in the example device 100, the heater 128
comprises heated liquid circulated through a hollow space within
the power electrode 110. Further details for operating a power
supply and heater in a suitable plasma delivery device for
implementing an embodiment of the invention can be found in U.S.
Pat. Nos. 9,406,485 and 10,032,609, which are both incorporated by
reference herein.
[0052] The powered electrode can be coated with a non-metallic,
non-conducting material between 1 and 100 microns thick. The
dielectric coating on the power electrode can be a hard, high
temperature, non-porous coating, including glass (SiO.sub.2),
alumina (Al.sub.2O.sub.3), aluminum nitride (AlN), or similar
inorganic electrical insulator. Note that reference to the "powered
electrode surface" is still applicable if such a coating exists on
the power electrode; direct physical contact between the conducting
electrodes and the gas flow is not required as will be understood
by those skilled in the art.
[0053] The example device 100 may employ an optical sensor for
receiving optical spectroscopy information of the argon plasma
comprising the reactive neutral species at the outlet 108. In this
example, the optical spectroscopy information is from a line of
sight 122 along the linear opening of the outlet 108 allowing for
measurement of the plasma afterglow. In addition, the device 100
employs a mirror 124 at one end of the linear opening for
reflecting the optical spectroscopy information into the fiber
optic feed 126 to the sensor 120. Further details for operating an
optical sensor to capture optical spectroscopy information in a
suitable plasma delivery device for implementing an embodiment of
the invention can be found in U.S. Pat. Nos. 9,406,485 and
10,032,609, which are both incorporated by reference herein.
[0054] In the device 100, the flow path is formed by a laminar flow
insert 130 disposed within the housing 102. The laminar flow insert
130 directs the gas flow from the inlet 104 to two opposing walls
132A, 132B of the chamber (while spreading each half of the gas
flow to be the width of the outlet 108) and then to two opposite
sides 134A and 134B of the powered electrode surface 112. The flow
insert can be manufactured of a high temperature, insulating
material that is resistant to plasma etching including
thermoplastics, including PEEK, perfluoroelastromers, Kalrez,
Viton, fluoropolymers, Teflon, or alumina and other ceramics. The
power electrode surface 112 comprises part of a cylindrical surface
and the laminar gas flow is directed circumferentially along the
part of the cylindrical surface toward the outlet 108. In this case
the bifurcated gas flow converges at the outlet 108 as plasma after
being ionized between the electrode surfaces 112 and 116. Further
details for using a flow insert in a suitable plasma delivery
device for implementing an embodiment of the invention can be found
in U.S. Pat. Nos. 9,406,485 and 10,032,609, which are both
incorporated by reference herein.
[0055] The power supply 118 can also include an auto-tuning network
that impedance matches the radio frequency power supply to the
argon plasma. In addition, the auto-tuning network follows a logic
algorithm that drives towards a forward power set point while
minimizing reflected power, and does so as the argon plasma moves
from strike conditions at a higher voltage to run conditions at a
lower voltage. For example, 50-ohm impedance matching can be
employed. Further details for operating an auto-tuning network in a
suitable plasma delivery device for implementing an embodiment of
the invention can be found in U.S. Pat. Nos. 9,406,485 and
10,032,609, which are both incorporated by reference herein.
[0056] Finally, the plasma device 100 can also be utilized with a
precursor device 136 external to the housing 102. The precursor
device 136 introduces a linear beam 138 of volatile chemical
precursor(s) into the reactive plasma flow near the outlet 108,
e.g. so as to enable the deposition of a thin film onto a substrate
placed a short distance downstream. The shape of the precursor
outlet can match that of the plasma outlet. For example, for a 4''
linear plasma source outlet, the precursor outlet is also a 4''
slit, but oriented such that the gas exiting from the precursor
outlet enters into the plasma gas stream exiting the source (e.g.
can be perpendicular to, or at 45 degrees to it, etc.). Typical
chemical precursors include tetraethyl-orthosilicate,
tetramethyl-cyclotetrasiloxane, trimethylsilane, and other
organosilanes or organometallics.
[0057] The example device 100 may be further modified or used in
process according to the detailed examples in the following
sections as will be understood by those skilled in the art. Some
example, applications for the devices and methods described herein
include, without limitation, cleaning a material surface,
activating a material surface for wetting, activating a material
surface for adhesion, depositing a thin film onto the substrate,
depositing a thin glass film onto the substrate, etching a thin
layer of material off of a substrate, and etching a metal oxide
layer, including copper oxide, off of a substrate.
Methods of Plasma Processing of Electronic Materials without
Particle Contamination.
[0058] The invention is further embodied by methods of processing
electronic materials without significant contamination of the
substrate with particles. The reactive gas exits the argon plasma
apparatus as described in FIGS. 3A and 3B and impinges on a
substrate where cleaning, activation, etching, and/or deposition
take place. The low-temperature, atmospheric pressure argon plasma
device generates the reactive gas flow without adding substantially
any particles to the gas steam. The radio frequency power applied
to the closely spaced electrodes, 110 and 114, generates a
capacitive discharge without substantially any streamers,
micro-arcs, or sparks. The capacitive discharge has a protective
sheath next to the electrode surfaces, 112 and 116. Ions entering
the sheath undergo many collisions, and lose their excess kinetic
energy before striking the electrode surface. This prevents
sputtering of the surface with ejection of particulate matter into
the gas stream.
[0059] Embodiments of the invention can be practiced with a mixture
of argon and other molecular gases at concentrations up to 5.0
volume %. Depending on the desired application, the molecular gas
may be oxygen, nitrogen, hydrogen, methane, carbon tetrafluoride
(CF.sub.4), octafluorobutane (C.sub.4F.sub.8), nitrogen trifluoride
(NF.sub.3), sulfur hexafluoride (SF.sub.6), ammonia, water,
hydrocarbons with carbon-carbon chain lengths from 2 to 6, and
other molecules that would be obvious to those skilled in the art.
The temperature of the gas exiting the plasma source generally
ranges from 40 to 80.degree. C., although temperatures higher than
80.degree. C. may be used, depending on the particular embodiment
of the invention. The temperature of the substrate is important for
the desired process, and this can be independently adjusted by the
temperature of the fluid recirculating through the plasma housing,
or by a separate heater placed underneath the substrate.
[0060] Examples are given below of methods of processing materials
without depositing substantial numbers of particles on the
substrate. These examples are not intended to limit the embodiments
of the invention, but to illustrate methods in which they can be
practiced. The apparatus and methods of the invention may be used
for many other purposes, which will be understood by those skilled
in the art.
Example 1
Apparatus and Method of Cleaning a Substrate
[0061] The atmospheric pressure argon plasma may be used to remove
organic compounds from surfaces, thereby cleaning the substrate.
The method of cleaning surfaces is accomplished by flowing argon
gas containing reactive molecules through the plasma to convert the
molecules into atoms and other reactive species. This gas flow,
that is free of a substantial number of particles, is directed onto
the surface to be cleaned. The contaminated surface is exposed to
the reactive species generated in the plasma for a sufficient
period of time to cause organic contamination to be removed without
damage to it. A sufficient period of time can be an exposure to the
reactive gas for 0.1 second to 1.0 hour, and generally in the range
of 1.0 second to 1.0 minute. Since the atmospheric plasma may be
scanned over the surface, the total treatment time may be longer
than the aforementioned time periods for especially large
substrates. Moreover, it may be advantageous for the contaminated
surface to be scanned with the argon plasma device multiple times,
but each time without the addition of substantial numbers of
particles.
[0062] Gas molecules that are suitable for embodiments of the
invention include, but are not limited to, oxygen, carbon dioxide,
carbon monoxide, nitrogen, nitrous oxide, ammonia and water. These
molecules may be converted into atoms, ions or metastable molecules
that are effective for surface cleaning. Oxygen containing gas
molecules, including O.sub.2, CO.sub.2, and NO.sub.2, are
particularly well suited for embodiments of the invention, because
they may be converted into ground-state O atoms, which among other
beneficial properties, are effective at etching away organic
contamination, but do not react with inorganic surfaces.
Atmospheric pressure plasmas suitable for embodiments of the
invention include those that generate a high concentration of
ground-state atoms, radicals, or metastable molecules downstream of
the plasma zone, but without the addition of particles to the gas
stream, most likely caused by energetic ion bombardment of
electrode surfaces.
[0063] An example embodiment of the invention was carried out on
silicon wafers 200 millimeters (mm) in diameter. The self-contained
plasma housing was mounted on a scanning robot and placed inside a
class 100 cleanroom (refer to the drawing in FIG. 1 for the
experimental setup). The class 100 cleanroom contained no more than
100,000 particles larger than 0.1 micron in size per cubic meter of
air. The silicon wafers were obtained directly from the vendor
without any further cleaning. They had a thin native oxide layer on
their surfaces. Each wafer was placed onto the robot stage and
scanned with the low-temperature, atmospheric pressure argon
plasma. The outlet slit for the reactive gas to flow out of the
plasma source was 100 mm in width. The distance between the outlet
slit and the 200 mm silicon wafer was approximately 3 mm. In each
experiment, the robot scanned the plasma source over one half of
the wafer, then stepped it 100 mm to the right, and scanned it over
the second half of the silicon wafer in the opposite direction.
Different scan speeds were used ranging from 25 to 300 mm/s. The
100 mm linear beam plasma source was fed with 40.0 liters per
minute (LPM) of argon and 0.32 LPM of oxygen. Radio frequency power
in the amount of 550 W at 27.12 MHz was supplied to the plasma
source.
[0064] Shown in Table 1 is a summary of the results obtained for
processing 200 mm silicon wafers with a standard vacuum plasma and
with an example embodiment of the invention (as described in the
preceding paragraph). After processing the silicon wafers, they
were tested with a Kruss Mobile Surface Analyst (MSA). This device
determines the surface energy and the water contact angle (WCA). A
native oxide on silicon contaminated with organic compounds will
have a surface energy well below 77.8 milli-Newton/meter (mN/m),
and WCA above 30.degree.. The first test was to remove the Si wafer
from a plastic storage container and examine it with the MSA in the
class 100 cleanroom. These results are presented in the first line
in the Table. One sees that the surface energy was 64.2 mN/m and
the water contact angle was 37.9.degree.. After treating a wafer
for 2 minutes (120 seconds) in the vacuum oxygen plasma, the
surface energy was 77.0 mN/m and the WCA was 6.7.degree.. Treatment
with the low-temperature, atmospheric pressure argon and oxygen
plasma at scan speeds ranging from 25 to 200 mm/s yielded identical
results within the experimental error of the measurement. The
surface energy was 77.7.+-.0.1 mN/m and the WCA was 6.4.+-.0.8
degrees. Only at the highest scan speed of 300 mm/s was a slightly
lower surface energy achieved. These results demonstrate that the
organic contamination on the native oxide surface can be completely
removed with the atmospheric argon and oxygen plasma at scan speeds
of 200 mm/s. The total process time for cleaning the 200 mm silicon
wafer was approximately 2.5 seconds, which is 48 times faster than
the vacuum plasma treatment. This same process has been applied to
300 mm silicon wafers. The total process time for cleaning a 300 mm
silicon wafer was approximately 5.5 seconds.
TABLE-US-00001 TABLE 1 Comparison of vacuum plasma cleaning to
atmospheric pressure argon and oxygen plasma cleaning of 200 mm
silicon wafers. Scan Speed Process Time Surface Free Energy Water
Contact Angle Treatment (mm/s) (s) (mN/m) (.degree.) Cleanroom
exposure only N/A 60 64.2 37.9 Vacuum O.sub.2 plasma N/A 120 77.0
6.7 Atmospheric Ar--O.sub.2 plasma 25 20 77.7 6.5 Atmospheric
Ar--O.sub.2 plasma 50 10 77.8 5.5 Atmospheric Ar--O.sub.2 plasma
100 5 77.8 6.1 Atmospheric Ar--O.sub.2 plasma 200 2.5 77.6 7.3
Atmospheric Ar--O.sub.2 plasma 300 1.7 76.8 10.3
[0065] Particle detection on the wafer surface was performed with a
light scattering tool. This tool uses a laser beam that scans over
the Si wafer. Any particles present on the surface will scatter the
incident light. By measuring the reflected light, it is possible to
map out the number, size and location of the particles on the
substrate. In this way, the unexpected results of the invention can
be revealed.
[0066] Presented in FIG. 4 is a particle map of the silicon wafer
after it was exposed to the cleanroom environment only. The
particle detection apparatus shows a total of 19 particles on the
wafer surface. FIG. 5 shows the particle size distribution on the
Si wafer. The 19 particles detected on the surface include 8
particles between 0.2 and 0.3 microns (.mu.m) in size, 3 particles
between 0.3 and 0.5 microns in size, 1 particle between 0.5 and 0.7
microns in size, and 7 particles between 0.7 and 1.0 micron in
size.
[0067] FIG. 6 shows the particle map on a silicon wafer which was
obtained after cleaning the surface with a prior art vacuum oxygen
plasma. The laser scattering instrument detected a total of 2,776
particles on the wafer surface after processing. The number of
particles present on the wafer surface has produced a surface with
a 146 fold increased particle count. FIG. 7 shows a histogram of
the particle size distribution on the Si wafer. The 2,779 particles
detected on the surface include 827 particles between 0.2 and 0.3
.mu.m in size, 465 particles between 0.3 and 0.5 .mu.m in size, 328
particles between 0.5 and 0.7 .mu.m in size, and 1,156 particles
between 0.7 and 1.0 .mu.m in size.
[0068] The vacuum plasma adds many thousands of particles to the
wafer, and it necessitates a subsequent wet cleaning step to remove
these particles before further processing can occur, including, but
not limited to, fusion bonding. Complicated and costly
modifications can be made to the vacuum plasma to reduce the number
of particle adders, but these modifications will not completely
eliminate them (see, for example, G. S. Selwyn, Jpn. J. Appl. Phys.
32, 3068 (1993)).
[0069] FIG. 8 shows the particle map obtained on a silicon wafer
after cleaning it with the apparatus depicted in FIG. 1. The
atmospheric pressure, argon and oxygen plasma scanned over the 200
mm Si wafer at a speed of 200 mm/s, yielding a total process time
of 2.5 seconds. The laser light scattering instrument shows a total
of 23 particles on the substrate surface. A histogram of the
particle distribution is presented in FIG. 9. The 23 particles
detected on the surface include 7 particles between 0.2 and 0.3
.mu.m in size, 2 particles between 0.3 and 0.5 .mu.m in size, 2
particles between 0.5 and 0.7 .mu.m in size, and 12 particles
between 0.7 and 1.0 .mu.m in size.
[0070] It is evident that embodiments of the invention can yield an
unexpected improvement over the prior art. Total particle count and
the particle size distribution are summarized in Table 2 below. The
embodied invention, labeled as "Surfx Ar/O.sub.2 plasma" adds only
4 total particles when compared to the silicon wafer exposed only
to the cleanroom environment. The vacuum O.sub.2 plasma well known
in the prior art adds 2,757 particles over that from the cleanroom.
The ratio of these two values demonstrates that the embodied
invention is 689 times cleaner than the prior art process.
TABLE-US-00002 TABLE 2 Summary of particle counts measured after
plasma processing silicon wafers. Particle Count 0.2-03 0.3-0.5
0.5-0.7 0.7-1.0 Total Product microns microns microns microns
particles Cleanroom only 8 3 1 7 19 Vacuum O.sub.2 plasma 827 465
328 1,156 2,776 Surfx Ar/O.sub.2 plasma 7 2 2 12 23 Additional
particle ratio vacuum/Surfx: 689
[0071] This example is only intended to illustrate one way in which
the invention may be practiced. This particle-free plasma cleaning
process has many useful applications, including, but not limited
to, wafer level packaging. Packaging at the wafer level enables the
stacking of multiple devices onto a single substrate. This can
significantly increase the functionality and complexity of
integrated circuits without greatly increasing their production
costs. Microelectronic devices are becoming ever more complex with
higher levels of integration, higher operating frequencies, more
functionality, and increased performance. Three-dimensional chips,
obtained through wafer level packaging, are a promising approach to
achieving these goals.
[0072] One of the main methods for producing 3D chips is fusion
bonding. In fusion bonding, two ultra-smooth (<10 .ANG.
roughness) wafers are fused together without using adhesives or an
external force. This technique requires surface preparation by one
of a few methods: O.sub.2-based plasma, hydration, or dipping in a
hydrofluoric acid solution. After cleaning, placing two wafers one
on top of the other, leads to hydrogen bonding between the cleaned
and oxidized surfaces. Annealing at 600-1200.degree. C. drives
water out of the interfacial region and chemically fuses the wafers
together through oxygen bridge bonding. This processes requires a
scrupulously clean surface because the presence of any particles or
physical debris will inhibit intimate contact between the
substrates, and thereby prevent the formation of hydrogen bonds
across the interface. The apparatus and method described in this
example is an advantageous way to clean the surface prior to fusion
bonding.
[0073] Another application that will benefit from the invention is
glass frit bonding, which is widely used to cap and seal
micro-electromechanical systems on the wafer level. Glass frit
bonding, also referred to as glass soldering, or seal glass
bonding, describes a wafer bonding technique with an intermediate
glass layer. The glass layers must be cleaned and activated for
bonding without becoming contaminated with particles. The
atmospheric pressure, argon and oxygen plasma has several
advantages over the vacuum oxygen plasma in this application,
including faster processing speeds, and the avoidance of particle
deposition onto the substrate.
[0074] Many methods of producing electronic materials require
atomically clean and particle-free surfaces. The above descriptions
of wafer level packaging and glass frit bonding are several
examples. Other examples would be obvious to those skilled in the
art.
Example 2
Apparatus and Method of Etching a Substrate
[0075] Another embodiment of the invention is etching of materials,
including glass, metals, metal oxides and polymer films, wherein
particles are not deposited on the substrate during the etching
process. For example, organic films may be etched by exposure to
the atmospheric pressure, argon and oxygen plasma mounted inside
the particle-free enclosure. Glasses, metals and metal oxides may
be etched by exposure to the afterglow from the atmospheric
pressure plasma apparatus fed with mixtures of argon and
halogen-containing molecules, including, but not limited to,
nitrogen trifluoride, carbon tetraflouride, and sulfur
hexafluoride. Further details for operating a suitable plasma
delivery device for etching materials in an embodiment of the
invention can be found in U.S. Pat. Nos. 9,406,485 and 10,032,609,
which are both incorporated by reference herein.
[0076] In an embodiment of the invention, metal oxide materials are
etched away through a hydrogen reduction process. For example a
flux of hydrogen atoms is generated in the plasma by feeding
hydrogen gas mixed with argon. A metal or semiconductor substrate
is placed downstream of the plasma discharge, so that only
ground-state hydrogen atoms and neutral species impinge on the
sample surface. These hydrogen atoms rapidly react with the metal
oxide surface to generate a clean metal surface and water vapor as
a byproduct. Embodiments of the invention allow for unwanted oxides
to be removed from live electronic devices while avoiding ion
bombardment and electrical arcing, which may damage the substrate.
An alternative method of removing oxide layers from metals is to
carry out this process in open air, where the plasma source
generates a large flux of neutral hydrogen atoms allowing for rapid
oxide removal, and eliminating any unwanted side reactions with the
ambient air.
[0077] One embodiment of the invention is a method of removing
copper oxide from copper using the atmospheric pressure plasma fed
with argon and a forming gas mixture of hydrogen and nitrogen. This
embodiment was demonstrated on copper lead frames that are used in
the semiconductor industry. The copper substrates were placed on a
hot plate and heated to 180.degree. C. At this temperature, a
copper oxide film spread over the surface, which exhibits a
characteristic purple color. Process gas containing a mixture of 15
L/min argon and forming gas at 1 L/min (95% nitrogen and 5%
hydrogen) was fed to the one-inch-linear plasma source at
atmospheric pressure. This plasma source produces a linear beam of
reactive gas 25 mm wide. Radio frequency power at 160 W was applied
to the electrodes, causing the plasma to be ignited and sustained.
The plasma source was then mounted 2 to 3 mm above the oxidized
copper surface. During exposure to the outlet gas flow from the
argon and hydrogen plasma, the purple copper oxide film was
removed, leaving behind a shiny metallic copper surface. Further
details for removing copper oxide with an embodiment of the
invention can be found in U.S. patent application Ser. No.
16/042,905, which is incorporated by reference herein.
[0078] An additional embodiment of the invention is a method to
reduce copper oxide using atmospheric pressure plasma fed with a
mixed gas of argon and hydrogen without the presence of nitrogen
from a forming gas mixture. The copper lead frames were first
oxidized using a forced convection oven operating between 200 and
250.degree. C. for a duration of 5, 10, 20 and 30 minutes. After 5
minutes, the copper metal exhibits a reddish-brown color indicating
an oxide layer thickness of 25 nm. Table 3 summarizes the color and
corresponding oxide thickness at each time interval in the
oven.
TABLE-US-00003 TABLE 3 Summary of thermally grown copper oxide
thickness and corresponding color. Time in Oven (min) Surface Color
Oxide Thickness (nm) 5 Red/Brown 25 10 Purple/Blue 30 20 Pale Blue
60 30 Yellow/Gold 100
[0079] Copper oxide etching was performed at ambient temperature
and pressure using a 1-inch (25 mm) linear plasma head fed with a
gas mixture containing 1% hydrogen in argon. The plasma was driven
with at 150 W of radio frequency (RF) power at 27.12 MHz. The
plasma head outlet was placed 1 to 2 mm away from the sample and
scanned over it at speeds between 0.5 and 2.0 mm/s, depending on
the oxide layer thickness. A 50 nm thick copper oxide layer was
reduced to metallic copper with a single pass treatment at a scan
speed of 2.0 mm/s. Complete removal of a 100 nm thick copper oxide
layer was achieved by scanning the argon and hydrogen plasma over
the surface at 0.5 mm/s.
[0080] The embodied invention can consist of an apparatus that
measures the thickness of the copper oxide on a sample, using for
example, the color of the copper substrate, and then determines the
appropriate plasma head scan speed based upon the time needed to
reduce the oxide layer back to copper metal. In addition, surface
oxides of a non-uniform thickness which may be encountered in
semiconductor and electronics manufacturing can be reduced to bare
copper metal at low temperature and high throughput with no warping
or damage to electronic packages, such as those containing lead
frames, copper wires and bond pads, and dies with copper bond
pads.
[0081] The copper oxide etch rate is a function of the substrate
temperature during the plasma reduction process. Two methods can be
used to increase the copper substrate temperature. The first is to
suspend the copper sample in air, or a non-oxidizing gas, such as
argon or nitrogen, using thermally insulating material to hold the
sample at the edges. This method reduces thermal conduction away
from the substrate material thereby allowing localized heating of
the sample to build up rather than be dissipated away. In this
case, the plasma gas is the source of heating the substrate.
Another embodiment of the invention is to use an external heat
source placed under the suspended sample to control the substrate
temperature.
[0082] FIG. 10 shows a plot of the natural log of the etch rate
versus the reciprocal of the absolute temperature in Kelvin. Using
the relationship shown in the figure, the heating device can
control the temperature of the substrate to increase the rate of
etching. For a 300 mm long copper lead frame at 30.degree. C. (303
K), the time needed to reduce a 50 nm oxide layer back to bare
metal is 150 seconds at the scan speed of 2 mm/s. Heating the lead
frame to 115.degree. C. (388 K) decreases the time required to 8
seconds (equivalent to a scan speed of 38 mm/s).
[0083] Copper oxide etching is a reversible reaction. Heating the
copper sample increases the etch rate by the atmospheric pressure
argon and hydrogen plasma. However, if the process takes place in
open air, then re-oxidation can occur on the hot copper surface. To
prevent re-oxidation, the embodied invention can be performed in an
inert gas environment, such as in argon or in nitrogen. One example
is to insert the copper substrate, such as a lead frame strip,
inside an enclosure, and purge the enclosure with hot argon or
nitrogen gas while the substate is being scanned with the argon and
hydrogen plasma. After etching away the copper oxide, the substate
can be quickly cooled in flowing argon or nitrogen gas to ambient
temperature. Once at ambient temperature, the copper oxidation rate
is negligible, and the sample can be removed from the enclosure and
transferred to the next processing step. Analysis of copper lead
frame strips after removal from the purged environment did not show
any evidence of re-oxidation over 8 hours storage at ambient
conditions.
[0084] In the Description of Related Art, it was pointed out that
copper oxide layers on copper lead frame strips are the source of
delamination of the epoxy mold covering the die and wire bonds
(refer to L. C. Yung, et al., IEEE Proceedings of the International
Conference on Software Engineering, 2010, Kuala Lumpur, Malaysia,
p. 654; and C. T. Chong, et al., IEEE Proceedings of the 45th
Electronic Components and Technology Conference, Las Vegas, Nev.,
USA, p. 463). Removal of the copper oxide with the argon and
hydrogen plasma should eliminate this problem. Experiments were
conducted on populated copper lead frame strips with dimensions of
70 mm.times.250 mm. The strips had been subjected to oxidation
during previous processing steps in the semiconductor packaging
operation. The lead frame strips were scanned at ambient
temperature and pressure using a 4-inch (100 mm) wide linear plasma
head fed with argon and hydrogen. The distance from the plasma
source exit to the strip was about 1 mm. The 100 mm wide beam
extend across the entire width of the 70 mm wide lead frame. The
head was scanned down the length of the 250 mm long strip at scan
speeds of 5, 10 and 20 mm/s. This yielded process times of 50, 25
and 12.5 seconds, respectively. The plasma was operated at 400 W of
RF power, a gas feed rate of 30.3 liters per minute (LPM), and with
a mixture of 0.46% hydrogen in argon.
[0085] After plasma treatment, the lead frame strips were placed in
the mold machine, and the mold injected over all the die packages
on the strips and cured. The packages were then examined for
delamination at the die pad--mold interface. No delamination was
observed at any of the die pads. Next, the strips were allowed to
sit at 30.degree. C. and a relative humidity of 60% for 168 hours
before testing for delamination (MSL 3 test). Again, no
delamination was observed on any of the packages. If the lead frame
strips were not treated with the plasma, or were treated with an
argon and oxygen plasma instead, delamination at the die pad--mold
interface was observed.
[0086] In another embodiment of the invention, the copper lead
frame strips and dies were cleaned with the argon and hydrogen
plasma before wire bonding. The removal of the copper oxide from
all the bond pads allowed copper wires to be bonded to the die and
lead frame pads with strong adhesion. Such a process has many
advantages in semiconductor packaging, because it simplifies the
materials and processes needed to obtain reliable packages.
Example 3
Apparatus and Method of Depositing a Thin Film
[0087] Another embodiment of the invention is a method of
depositing thin films with the argon plasma at atmospheric pressure
and low temperature, wherein there is essentially no deposition of
particles on the substrate. The embodiment has been reduced to
practice by depositing glass-like films on silicon wafers. Here, a
volatile chemical precursor is fed downstream at a second gas inlet
located just after the exit of the plasma source. The volatile
chemical precursor then combines with the reactive species in the
afterglow of the plasma. The reactive species attack the chemical
precursors, causing them to decompose and deposit a thin film on a
substrate placed less than 1.0 centimeter downstream.
[0088] In FIG. 11, a schematic is presented of a Teflon attachment
to the atmospheric pressure plasma source (refer to FIG. 3B). This
apparatus is another embodiment of the invention. It is mounted
directly onto the plasma source housing, and provides a means of
uniformly distributing the volatile chemical precursors into the
reactive gas beam exiting from the linear argon plasma source.
Volatile chemical precursors are fed into the attachment 200 at a
top port 202, and out through a slit 204 in the side near the base
of the Teflon piece 206. After exiting the attachment, the
precursor chemicals efficiently mix with the reactive species in
the afterglow. These species attack the chemical precursor, causing
it to decompose and deposit a thin film onto a substrate located a
short distance downstream.
[0089] The organosilane precursor chemical used in this example is
tetramethyl-cyclotetrasiloxane (TMCTS) which is delivered just
below the plasma source fed with argon and oxygen. The plasma was
operating at 120 W RF power using 18 LPM argon and 0.2 LPM oxygen
and the plasma deposition system was scanned over the surface at 25
mm/s. Tetramethyl-cyclotetrasiloxane was dispersed into the carrier
gas and introduced to the apparatus through the attachment system
located 1.0 mm away from the gas exit from the plasma housing. The
precursor chemical was delivered to the attachment by flowing
helium through a bubbler filled with the liquid precursor. The flow
rate through the bubbler was set at 0.4 LPM and an additional
dilution of 3.0 LPM of helium was added to this gas stream before
entering the deposition attachment. Silicon wafers, 6 inches in
diameter, were placed on a holder 7 mm downstream of the
attachment. The pitch of the robotic painting program as it scanned
the wafer was fixed at 1 mm. The total dwell time of the
atmospheric plasma housing and attachment over the silicon wafers
was varied by altering the number of deposition cycles.
[0090] Differences in coating thickness are apparent by observing
the color of the thin glass film. Before deposition, the silicon
wafer has a uniform silver color. After deposition, one observes a
bright blue coating generated by the deposited glass film. A color
variation due to a thickness variation is observed at the edges of
the film. However, over 90% of the film area, no significant
variation in color is seen. This indicated a high degree of
uniformity is achieved with the embodiment of the invention
depicted in FIG. 11. Further details for operating a suitable
plasma delivery device for implementing an embodiment of the
invention can be found in U.S. Pat. Nos. 9,406,485 and 10,032,609,
which are both incorporated by reference herein.
Example 4
Apparatus and Method of Cleaning and Activating a Metal
Substrate
[0091] Another embodiment of the invention is an apparatus and
method of cleaning and activating metal substrates with the
low-temperature, atmospheric pressure, argon plasma apparatus,
without the addition of substantial amounts of particles onto the
metal substrates. One application of this embodiment is to improve
the adhesion of coatings and glues to metal surfaces. Copper
surface activation was accomplished using the atmospheric pressure,
argon and oxygen plasma. The plasma scan speed over the substrate
was varied from 5 to 100 millimeters per second, and the water
contact angle of the copper was measured after each scan. A 43%
reduction in water contact angle was observed at a speed of 50
mm/s. The water contact angle was reduced from 91.degree. to
26.degree. when the plasma scan speed was 5 mm/s. A low water
contact angle is indicative of a hydrophilic surface. Such a
surface should make strong bonds to coatings and glues.
[0092] The atmospheric pressure argon plasma removes organic
contaminants from metal surfaces, and thereby increases the metal
surface energy so that it will strongly bond to other materials. A
copper lead frame was exposed to an argon plasma that was
additionally fed with oxygen, nitrogen or hydrogen. After plasma
treatment, the surface free energy (SFE), along with the polar and
dispersive components of the SFE, was measured with a Kruss Mobile
Surface Analyst. Exposure to the argon and oxygen plasma increased
the polar component of the surface energy from <3 mN/m to 21
mN/m. Substantial increases in the polar component of the SFE was
observed with the argon and nitrogen plasma and the argon and
hydrogen plasma, although to a lesser extent than achieved with the
argon and oxygen plasma. A large increase in the polar component of
the surface energy is a good indication that the copper is
activated for bonding to other materials. Further details for
operating a suitable plasma delivery device for implementing an
embodiment of the invention can be found in U.S. Pat. Nos.
9,406,485 and 10,032,609, and U.S. patent application Ser. No.
16/042,905, which are incorporated by reference herein.
[0093] Apparatus and methods are disclosed for generating an argon
plasma for processing electronic materials that does not result in
the contamination of the substrate with significant numbers of
particles. A plasma apparatus with auto-tuning and temperature
control methods has been developed which produces stable argon
plasmas that may be used to process materials at atmospheric
pressure and low temperature. The device contains a means for
controlling the temperature of a flowing gas and a means for
partially ionizing said flowing gas such that uniform and stable
plasmas are generated without adding particles into the gas stream.
Embodiments of the invention include processes that employ the
argon plasma apparatus to treat materials at low temperature and
high throughput without contaminating the substrate with particles,
and at a cost which has been previously unavailable. One method
uses the argon plasma device to remove organic materials. The
methods further cover the robotic application of the
low-temperature, atmospheric pressure plasma to a substrate all of
which is contained in a particle-free enclosure. Another embodiment
of the invention uses the argon plasma with hydrogen gas feed to
remove metal oxide films from metals. In yet another embodiment,
the argon plasma is combined with a means of introducing chemical
precursors to the system thereby causing the plasma-enhanced
chemical vapor deposition of a thin film onto a substrate without
the additional co-deposition of particles. A further embodiment of
the invention can use the plasma to clean and activate a metal
substrate.
* * * * *