U.S. patent application number 10/253054 was filed with the patent office on 2004-03-25 for processing of semiconductor components with dense processing fluids and ultrasonic energy.
This patent application is currently assigned to Air Products and Chemicals, Inc.. Invention is credited to Johnson, Andrew David, McDermott, Wayne Thomas, Schwarz, Alexander, Subawalla, Hoshang.
Application Number | 20040055621 10/253054 |
Document ID | / |
Family ID | 31977793 |
Filed Date | 2004-03-25 |
United States Patent
Application |
20040055621 |
Kind Code |
A1 |
McDermott, Wayne Thomas ; et
al. |
March 25, 2004 |
Processing of semiconductor components with dense processing fluids
and ultrasonic energy
Abstract
Method for processing an article with a dense processing fluid
in a processing chamber while applying ultrasonic energy during
processing. The dense fluid may be generated in a separate
pressurization vessel and transferred to the processing chamber, or
alternatively may be generated directly in the processing chamber.
A processing agent may be added to the pressurization vessel, to
the processing chamber, or to the dense fluid during transfer from
the pressurization vessel to the processing chamber. The ultrasonic
energy may be generated continuously at a constant frequency or at
variable frequencies. Alternatively, the ultrasonic energy may be
generated intermittently.
Inventors: |
McDermott, Wayne Thomas;
(Fogelsville, PA) ; Subawalla, Hoshang; (Macungie,
PA) ; Johnson, Andrew David; (Doylestown, PA)
; Schwarz, Alexander; (Bethlehem, PA) |
Correspondence
Address: |
AIR PRODUCTS AND CHEMICALS, INC.
PATENT DEPARTMENT
7201 HAMILTON BOULEVARD
ALLENTOWN
PA
181951501
|
Assignee: |
Air Products and Chemicals,
Inc.
|
Family ID: |
31977793 |
Appl. No.: |
10/253054 |
Filed: |
September 24, 2002 |
Current U.S.
Class: |
134/1.3 ;
134/184; 134/26; 134/34; 134/95.1; 257/E21.255 |
Current CPC
Class: |
C11D 7/264 20130101;
C11D 7/261 20130101; H01L 21/67057 20130101; B08B 7/0021 20130101;
C11D 7/5013 20130101; C11D 7/5022 20130101; B08B 3/12 20130101;
C11D 7/3209 20130101; C11D 7/241 20130101; C11D 7/263 20130101;
C11D 7/3281 20130101; G03F 1/82 20130101; H01L 21/31133 20130101;
C11D 11/007 20130101; C11D 7/3263 20130101 |
Class at
Publication: |
134/001.3 ;
134/026; 134/034; 134/095.1; 134/184 |
International
Class: |
C25F 001/00 |
Claims
1. A method for processing an article comprising: (a) introducing
the article into a sealable processing chamber and sealing the
processing chamber; (b) preparing a dense fluid by: (b1)
introducing a subcritical fluid into a pressurization vessel and
isolating the vessel; and (b2) heating the subcritical fluid at
essentially constant volume and essentially constant density to
yield a dense fluid; (c) transferring at least a portion of the
dense fluid from the pressurization vessel to the processing
chamber, wherein the transfer of the dense fluid is driven by the
difference between the pressure in the pressurization vessel and
the pressure in the processing chamber, thereby pressurizing the
processing chamber with transferred dense fluid; (d) introducing
one or more processing agents into the processing chamber either
before (c), or during (c), or after (c) to provide a dense
processing fluid; (e) introducing ultrasonic energy into the
processing chamber and contacting the article with the dense
processing fluid to yield a spent dense processing fluid and a
treated article; and (f) separating the spent dense processing
fluid from the treated article.
2. The method of claim 1 wherein the dense fluid is generated in
(b2) at a reduced temperature in the pressurization vessel below
about 1.8, wherein the reduced temperature is defined as the
average absolute temperature of the dense fluid in the
pressurization vessel after heating divided by the absolute
critical temperature of the fluid.
3. The method of claim 2 wherein the contacting of the article with
the dense processing fluid in the processing chamber in (d) is
effected at a reduced temperature in the processing chamber between
about 0.8 and about 1.8, wherein the reduced temperature is defined
as the average absolute temperature of the dense processing fluid
in the processing chamber during (d) divided by the absolute
critical temperature of the dense processing fluid.
4. The method of claim 1 wherein the dense fluid comprises one or
more components selected from the group consisting of carbon
dioxide, nitrogen, methane, oxygen, ozone, argon, hydrogen, helium,
ammonia, nitrous oxide, hydrogen fluoride, hydrogen chloride,
sulfur trioxide, sulfur hexafluoride, nitrogen trifluoride,
monofluoromethane, difluoromethane, trifluoromethane,
trifluoroethane, tetrafluoroethane, pentafluoroethane,
perfluoropropane, pentafluoropropane, hexafluoroethane,
hexafluoropropylene, hexafluorobutadiene, and octafluorocyclobutane
and tetrafluorochloroethane.
5. The method of claim 1 wherein the dense fluid comprises one or
more hydrocarbons having 2 to 6 carbon atoms.
6. The method of claim 1 wherein the total concentration of the one
or more processing agents in the dense processing fluid is between
about 0.1 and 20 wt %.
7. The method of claim 1 wherein the dense processing fluid
comprises one or more processing agents selected from the group
consisting of ethyl acetate, ethyl lactate, propyl acetate, butyl
acetate, diethyl ether, dipropyl ether, methanol, ethanol,
isopropanol, acetonitrile, propionitrile, benzonitrile, ethylene
cyanohydrin, ethylene glycol, propylene glycol, ethylene glycol
monoacetate, propylene glycol monoacetate, acetone, butanone,
acetophenone, trifluoroacetophenone, triethyl amine, tripropyl
amine, tributyl amine, 2,4, dimethyl pyridine,
dimethylethanolamine, diethylethanolamine, diethylmethanolamine,
dimethylmethanolamine, dimethylformamide, dimethylacetamide,
ethylene carbonate, propylene carbonate, acetic acid, lactic acid,
butane-diol, propane-diol, n-hexane, n-butane, hydrogen peroxide,
t-butyl hydroperoxide, ethylenediaminetetraacetic acid, catechol,
choline, and trifluoroacetic anhydride.
8. The method of claim 1 wherein the dense processing fluid
comprises one or more processing agents selected from the group
consisting of hydrogen fluoride, hydrogen chloride, chlorine
trifluoride, nitrogen trifluoride, monofluoromethane,
difluoromethane, trifluoromethane, trifluoroethane,
tetrafluoroethane, pentafluoroethane, perfluoropropane,
pentafluoropropane, hexafluoroethane, hexafluoropropylene,
hexafluorobutadiene, octafluorocyclobutane tetrafluorochloroethane,
fluoroxytrifluoromethane (CF.sub.4O), bis(difluoroxy)methane
(CF.sub.4O.sub.2), cyanuric fluoride (C.sub.3F.sub.3N.sub.3),
oxalyl fluoride (C.sub.2F.sub.2O.sub.2), nitrosyl fluoride (FNO),
carbonyl fluoride (CF.sub.2O), and perfluoromethylamine
(CF.sub.5N).
9. The method of claim 1 wherein the dense processing fluid
comprises one or more processing agents selected from the group
consisting of organometallic precursors, photoresists, photoresist
developers, interlayer dielectric materials, silane reagents, and
stain-resistant coatings.
10. The method of claim 1 which further comprises reducing the
pressure of the spent dense processing fluid to yield at least a
fluid phase and a residual compound phase, and separating the
phases to yield a purified fluid and recovered residual
compounds.
11. The method of claim 10 which further comprises recycling the
purified fluid to provide a portion of the subcritical fluid in
(b1).
12. The method of claim 10 which further comprises reducing the
pressure of the purified fluid to yield a further-purified fluid
phase and an additional residual compound phase, and separating the
phases to yield a further-purified fluid and additional recovered
residual compounds.
13. The method of claim 12 which further comprises recycling the
further-purified fluid to provide a portion of the subcritical
fluid in (b1).
14. The method of claim 1 wherein the subcritical fluid in the
pressurization vessel prior to heating in (b2) comprises a vapor
phase, a liquid phase, or coexisting vapor and liquid phases.
15. A method for processing an article comprising: (a) introducing
the article into a sealable processing chamber and sealing the
processing chamber; (b) preparing a dense processing fluid by: (b1)
introducing a subcritical fluid into a pressurization vessel and
isolating the vessel; (b2) heating the subcritical fluid at
essentially constant volume and essentially constant density to
yield a dense fluid; and (b3) introducing one or more processing
agents into the pressurization vessel before introducing the
subcritical fluid into the pressurization vessel, or after
introducing the subcritical fluid into the pressurization vessel
but before heating the pressurization vessel, or after introducing
the subcritical fluid into the pressurization vessel and after
heating the pressurization vessel; (c) transferring at least a
portion of the dense processing fluid from the pressurization
vessel to the processing chamber, wherein the transfer of the dense
processing fluid is driven by the difference between the pressure
in the pressurization vessel and the pressure in the processing
chamber, thereby pressurizing the processing chamber with
transferred dense processing fluid; (d) introducing ultrasonic
energy into the processing chamber and contacting the article with
the transferred dense processing fluid to yield a spent dense
processing fluid and a treated article; and (e) separating the
spent dense processing fluid from the treated article.
16. An apparatus for processing an article which comprises: (a) a
fluid storage tank containing a subcritical fluid; (b) one or more
pressurization vessels and piping means for transferring the
subcritical fluid from the fluid storage tank to one or more
pressurization vessels; (c) heating means to heat the contents of
each of the one or more pressurization vessels at essentially
constant volume and essentially constant density to convert the
subcritical fluid into a dense fluid; (d) a sealable processing
chamber for contacting an article with the dense fluid; (e)
ultrasonic generation means for introducing ultrasonic energy into
the sealable processing chamber; (f) piping means for transferring
the dense fluid from the one or more pressurization vessels into
the sealable processing chamber; and (g) one or more processing
agent storage vessels and pumping means to inject one or more
processing agents (1) into the one or more pressurization vessels
or (2) into the piping means for transferring the dense fluid from
the one or more pressurization vessels to the sealable processing
chamber or (3) into the sealable processing chamber.
17. The apparatus of claim 16 which further comprises pressure
reduction means and phase separation means to separate a spent
dense processing fluid withdrawn from the processing chamber to
yield at least a purified fluid and one or more recovered residual
compounds.
18. The apparatus of claim 17 which further comprises recycle means
to recycle the purified fluid to the fluid storage tank.
19. A method for processing an article comprising: (a) introducing
the article into a sealable processing chamber and sealing the
processing chamber; (b) providing a dense processing fluid in the
processing chamber; (c) introducing ultrasonic energy into the
processing chamber and varying the frequency of the ultrasonic
energy while contacting the article with the dense processing fluid
to yield a spent dense processing fluid and a treated article; and
(e) separating the spent dense processing fluid from the treated
article.
20. The method of claim 19 wherein the frequency of the ultrasonic
energy is increased during (c).
21. The method of claim 19 wherein the frequency of the ultrasonic
energy is decreased during (c).
22. The method of claim 19 wherein the dense processing fluid is
prepared by: (a) introducing a subcritical fluid into a
pressurization vessel and isolating the vessel; (b) heating the
subcritical fluid at essentially constant volume and essentially
constant density to yield a dense fluid; and (c) providing the
dense processing fluid by one or more steps selected from the group
consisting of (1) introducing one or more processing agents into
the dense fluid while transferring the dense fluid from the
pressurization vessel to the processing chamber, (2) introducing
one or more processing agents into the pressurization vessel to
form a dense processing fluid and transferring the dense processing
fluid from the pressurization vessel to the processing chamber, (3)
introducing one or more processing agents into the derise fluid in
the processing chamber after transferring the dense fluid from the
pressurization vessel to the processing chamber, (4) introducing
one or more processing agents into the pressurization vessel before
introducing the subcritical fluid into the pressurization vessel,
(5) introducing one or more processing agents into the
pressurization vessel after introducing the subcritical fluid into
the pressurization vessel but before heating the pressurization
vessel, and (6) introducing one or more processing agents into the
pressurization vessel after introducing the subcritical fluid into
the pressurization vessel and after heating the pressurization
vessel.
23. The method of claim 19 wherein the dense processing fluid is
prepared by: (a) introducing a subcritical fluid into the sealable
processing chamber and isolating the chamber; (b) heating the
subcritical fluid at essentially constant volume and essentially
constant density to yield a dense fluid; and (c) providing the
dense processing fluid by one or more steps selected from the group
consisting of (1) introducing one or more processing agents into
the sealable processing chamber before introducing the subcritical
fluid into the sealable processing chamber, (2) introducing one or
more processing agents into the sealable processing chamber after
introducing the subcritical fluid into the sealable processing
chamber but before heating the subcritical fluid therein, and (3)
introducing one or more processing agents into the sealable
processing chamber after introducing the subcritical fluid into the
sealable processing chamber and after heating the subcritical fluid
therein.
24. A method for processing an article comprising: (a) introducing
the article into a sealable processing chamber and sealing the
processing chamber; (b) providing a dense fluid in the processing
chamber; (c) introducing ultrasonic energy into the processing
chamber and varying the frequency of the ultrasonic energy while
contacting the article with the dense fluid to yield a spent dense
fluid and a treated article; and (e) separating the spent dense
fluid from the treated article.
25. A method for processing an article comprising: (a) introducing
the article into a sealable processing chamber and sealing the
processing chamber; (b) providing a dense processing fluid in the
processing chamber; (c) introducing ultrasonic energy into the
processing chamber intermittently while contacting the article with
the dense processing fluid to yield a spent dense processing fluid
and a treated article; and (e) separating the spent dense
processing fluid from the treated article.
26. A method for processing an article comprising: (a) introducing
the article into a sealable processing chamber and sealing the
processing chamber; (b) providing a dense fluid in the processing
chamber; (c) introducing ultrasonic energy into the processing
chamber intermittently while contacting the article with the dense
fluid to yield a spent dense fluid and a treated article; and (e)
separating the spent dense fluid from the treated article.
Description
BACKGROUND OF THE INVENTION
[0001] Small quantities of contamination are detrimental to the
microchip fabrication process in the manufacturing of semiconductor
electronic components. Contamination in the form of particulates,
films, or molecules causes short circuits, open circuits, silicon
crystal stacking faults, and other defects. These defects can cause
the failure of finished microelectronic circuits, and such failures
cause significant yield reductions, which greatly increases
manufacturing costs.
[0002] Microelectronic circuit fabrication requires many processing
steps. Processing is performed under extremely clean conditions and
the amount of contamination needed to cause fatal defects in
microcircuits is extremely small. For example, an individual
particle as small as 0.01 micrometer in size can result in a killer
defect in a modern microcircuit. Microcontamination may occur at
any time during the many steps needed to complete the microcircuit.
Therefore, periodic cleaning of the wafers used for microelectronic
circuits is needed to maintain economical yields. Also, tight
control of purity and cleanliness of the processing materials is
required.
[0003] Numerous cleaning methods have been used in the manufacture
of semiconductor electronic components. These include immersion in
liquid cleaning agents to remove contamination through dissolution
and chemical reaction. Such immersion may also serve to reduce the
van der Waals adhesive forces and introduce double layer repulsion
forces, thereby promoting the release of insoluble particles from
surfaces. A standard wet cleaning process in common use begins with
exposure to a mixture of H.sub.2SO.sub.4, H.sub.2O.sub.2, and
H.sub.2O at 110-130.degree. C., and is followed by immersion in HF
or dilute HF at 20-25.degree. C. Next a mixture of NH.sub.4OH,
H.sub.2O.sub.2, and H.sub.2O at 60-80.degree. C. removes particles,
and a mixture of HC.sub.1, H.sub.2O.sub.2, and H.sub.2O at
60-80.degree. C. removes metal contamination. Each of these steps
is followed by a high purity H.sub.2O rinse. This wet cleaning
process reaches fundamental barriers at dimensions less than 0.10
micrometer. As the device geometries shrink and gate oxide
thickness decreases, sub-micrometer particle removal becomes
increasingly difficult.
[0004] Stripping/removal of primarily organic photoresist may be
performed using dilute aqueous mixtures containing H.sub.2SO.sub.4
and H.sub.2O.sub.2. Alternatively, the stripping/removal may be
performed using a two-step plasma, or reactive ion etching process,
followed by wet chemical cleaning of the residue material. Ozonated
H.sub.2O has been used for the decomposition of hydrocarbon surface
contaminants on silicon wafers.
[0005] Brush scrubbing has been used to enhance the liquid
immersion process by introducing hydrodynamic shear forces to the
contaminated surfaces. A typical application uses a wafer cleaning
apparatus comprising two opposed brushes for brushing a vertically
disposed wafer in a tank that can contain a process liquid.
[0006] The addition of ultrasonic energy can increase the
effectiveness of the liquid immersion process. Sound waves
vibrating at frequencies greater than 20,000 cycles per second (20
KHz), i.e., beyond the range of human hearing, have been used to
transmit high frequency energy into liquid cleaning solutions.
[0007] Wet processing methods may become problematic as
microelectronic circuit dimensions decrease and as environmental
restrictions increase. Among the limitations of wet processing are
the progressive contamination of re-circulated liquids,
re-deposition from contaminated chemicals, special disposal
requirements, environmental damage, special safety procedures
during handling, reduced effectiveness in deeply patterned surfaces
due to surface tension effects and image collapse (topography
sensitivity), dependence of cleaning effectiveness on surface
wet-ability to prevent re-adhesion of contaminants, and possible
liquid residue causing adhesion of remaining particles. Aqueous
cleaning agents that depend upon chemical reaction with surface
contaminants may also present compatibility problems with new thin
film materials, or with more corrosion-prone metals such as copper.
In addition, the International Technology Roadmap for
Semiconductors has recommended a 62% reduction in water use by the
year 2005 and an 84% reduction by the year 2014 to prevent water
shortages. With the continuing trend toward increasing wafer
diameters having a larger precision surface area, larger volumes of
liquid chemicals will be required in the fabrication process.
[0008] In view of these problems, methods for dry (anhydrous)
surface cleaning of semiconductor electronic components are being
developed. Among these are gas jet cleaning to remove relatively
large particles from silicon wafers. However, gas jets can be
ineffective for removing particles smaller than about 5 micrometers
in diameter because the forces that hold particles on the surface
are proportional to the particle size, while the aerodynamic drag
forces generated by the flowing gas for removing the particles are
proportional to the particle diameter squared. Therefore, the ratio
of these forces tends to favor adhesion as the particle size
shrinks. In addition, smaller particles are not exposed to strong
drag forces in the jet since they normally lie within the surface
boundary layer where the gas velocity is low.
[0009] Exposure to ozone combined with ultraviolet light can be
used to decompose contaminating hydrocarbons from surfaces, but
this technique has not been shown to remove inorganic contaminants
or particles effectively.
[0010] Other alternatives to wet cleaning include the use of jets
containing snow or pellet projectiles comprising frozen Ar,
N.sub.2, H.sub.2O or CO.sub.2 which are used to "sandblast"
contaminated surfaces. In these processes, pressurized gaseous or
gas/liquid mixtures are expanded in a nozzle to a pressure near or
below atmospheric pressure. The resulting Joule-Thomson cooling
forms solid or liquid aerosol particles, which traverse the
boundary layer and strike the contaminated surface. This technique
requires extremely clean and pure processing materials. Trace
molecular contaminants (e.g., hydrocarbons) in the feed gases can
condense into solid particulates or droplets upon expansion,
causing deposition of new contaminants on the surface. Although
useful in providing removal of many surface contaminants, these
processes cannot remove all of the important contaminants present
on a wafer surface, and have not yet found wide acceptance in the
semiconductor industry.
[0011] Immersion in supercritical fluids is another alternative to
wet cleaning. The effectiveness of supercritical fluids in various
cleaning and extraction applications is well established and
extensively documented. Supercritical fluids have solvent power
much greater than the corresponding gaseous state, and can
effectively dissolve and remove unwanted films and molecular
contaminants from a precision surface. The contaminants can be
separated from the cleaning agent by a reduction in pressure below
the critical value, which concentrates the contaminants for
disposal and permits recovery and re-use of the cleaning fluid.
[0012] Supercritical CO.sub.2 in particular has been used as a
versatile and cost effective method to overcome the above-mentioned
problems in wafer cleaning. Supercritical CO.sub.2 effectively
cleans parts with increasingly smaller dimensions and lowers water
usage, thereby yielding improvements in performance and
environmental benefits. Preliminary Cost of Ownership (CoO) studies
have shown that supercritical CO.sub.2 cleaning is also more cost
effective when compared to aqueous cleaning. CO.sub.2 in the
supercritical state has particularly good solvent properties and
has been found to be effective in removing organic impurities. It
can be modified with added co-solvents or entrainers to widen the
range of contaminants that can be removed, including particles,
native or chemical oxides, metallic contaminants, and other
inorganic materials. Ultrasonic energy can be introduced into
supercritical fluid cleaning reactors to enhance the efficiency of
the cleaning process.
[0013] Future microcircuits will have smaller feature sizes and
greater complexities, and will require more processing steps in
their fabrication. Contamination control in the process materials
systems and processing environment will become even more critical.
In view of these anticipated developments, there is a need for
improved wafer cleaning methods to maintain or improve economical
yields in the manufacture of these smaller and more complex
microelectronic systems. In addition, the advent of smaller feature
sizes and greater complexities will require improved fabrication
processes steps including etching, thin film deposition,
planarization, and photoresist development. Embodiments of the
present invention, which are described below and defined by the
following claims, address this need by improved processing methods
utilizing dense processing fluids with the application of
ultrasonic energy.
BRIEF SUMMARY OF THE INVENTION
[0014] A first embodiment of the invention includes a method for
processing an article comprising:
[0015] (a) introducing the article into a sealable processing
chamber and sealing the processing chamber;
[0016] (b) preparing a dense fluid by:
[0017] (b1) introducing a subcritical fluid into a pressurization
vessel and isolating the vessel; and
[0018] (b2) heating the subcritical fluid at essentially constant
volume and essentially constant density to yield a dense fluid;
[0019] (c) transferring at least a portion of the dense fluid from
the pressurization vessel to the processing chamber, wherein the
transfer of the dense fluid is driven by the difference between the
pressure in the pressurization vessel and the pressure in the
processing chamber, thereby pressurizing the processing chamber
with transferred dense fluid;
[0020] (d) introducing one or more processing agents into the
processing chamber either before (c), or during (c), or after (c)
to provide a dense processing fluid;
[0021] (e) introducing ultrasonic energy into the processing
chamber and contacting the article with the dense processing fluid
to yield a spent dense processing fluid and a treated article;
and
[0022] (f) separating the spent dense processing fluid from the
treated article.
[0023] The dense fluid may be generated in (b2) at a reduced
temperature in the pressurization vessel below about 1.8, wherein
the reduced temperature is defined as the average absolute
temperature of the dense fluid in the pressurization vessel after
heating divided by the absolute critical temperature of the fluid.
The contacting of the article with the dense processing fluid in
the processing chamber in (d) may be effected at a reduced
temperature in the processing chamber between about 0.8 and about
1.8, wherein the reduced temperature is defined as the average
absolute temperature of the dense processing fluid in the
processing chamber during (d) divided by the absolute critical
temperature of the dense processing fluid.
[0024] The dense fluid may comprise one or more components selected
from the group consisting of carbon dioxide, nitrogen, methane,
oxygen, ozone, argon, hydrogen, helium, ammonia, nitrous oxide,
hydrogen fluoride, hydrogen chloride, sulfur trioxide, sulfur
hexafluoride, nitrogen trifluoride, monofluoromethane,
difluoromethane, trifluoromethane, trifluoroethane,
tetrafluoroethane, pentafluoroethane, perfluoropropane,
pentafluoropropane, hexafluoroethane, hexafluoropropylene,
hexafluorobutadiene, and octafluorocyclobutane and
tetrafluorochloroethane. The dense fluid may comprise one or more
hydrocarbons having 2 to 6 carbon atoms.
[0025] The total concentration of the one or more processing agents
in the dense processing fluid may be between about 0.1 and 20 wt %.
In one embodiment, the dense processing fluid may comprise one or
more processing agents selected from the group consisting of ethyl
acetate, ethyl lactate, propyl acetate, butyl acetate, diethyl
ether, dipropyl ether, methanol, ethanol, isopropanol,
acetonitrile, propionitrile, benzonitrile, ethylene cyanohydrin,
ethylene glycol, propylene glycol, ethylene glycol monoacetate,
propylene glycol monoacetate, acetone, butanone, acetophenone,
trifluoroacetophenone, triethyl amine, tripropyl amine, tributyl
amine, 2,4, dimethyl pyridine, dimethylethanolamine,
diethylethanolamine, diethylmethanolamine, dimethylmethanolamine,
dimethylformamide, dimethylacetamide, ethylene carbonate, propylene
carbonate, acetic acid, lactic acid, butane-diol, propane-diol,
n-hexane, n-butane, hydrogen peroxide, t-butyl hydroperoxide,
ethylenediaminetetraacetic acid, catechol, choline, and
trifluoroacetic anhydride.
[0026] In another embodiment, the dense processing fluid may
comprise one or more processing agents selected from the group
consisting of hydrogen fluoride, hydrogen chloride, chlorine
trifluoride, nitrogen trifluoride, monofluoromethane,
difluoromethane, trifluoromethane, trifluoroethane,
tetrafluoroethane, pentafluoroethane, perfluoropropane,
pentafluoropropane, hexafluoroethane, hexafluoropropylene,
hexafluorobutadiene, octafluorocyclobutane tetrafluorochloroethane,
fluoroxytrifluoromethane (CF.sub.4O), bis(difluoroxy)methane
(CF.sub.4O.sub.2), cyanuric fluoride (C.sub.3F.sub.3N.sub.3),
oxalyl fluoride (C.sub.2F.sub.2O.sub.2), nitrosyl fluoride (FNO),
carbonyl fluoride (CF.sub.2O), and perfluoromethylamine
(CF.sub.5N).
[0027] In yet another embodiment, the dense processing fluid may
comprise one or more processing agents selected from the group
consisting of organometallic precursors, photoresists, photoresist
developers, interlayer dielectric materials, silane reagents, and
stain-resistant coatings.
[0028] The pressure of the spent dense processing fluid may be
reduced to yield at least a fluid phase and a residual compound
phase, and the phases may be separated to yield a purified fluid
and recovered residual compounds. The purified fluid may be
recycled to provide a portion of the subcritical fluid in (b1). The
pressure of the purified fluid may be reduced to yield a
further-purified fluid phase and an additional residual compound
phase, and the phases may be separated to yield a further-purified
fluid and additional recovered residual compounds. The
further-purified fluid may be recycled to provide a portion of the
subcritical fluid in (b1).
[0029] The subcritical fluid in the pressurization vessel prior to
heating in (b2) may comprise a vapor phase, a liquid phase, or
coexisting vapor and liquid phases.
[0030] Another embodiment of the invention includes a method for
processing an article comprising:
[0031] (a) introducing the article into a sealable processing
chamber and sealing the processing chamber;
[0032] (b) preparing a dense processing fluid by:
[0033] (b1) introducing a subcritical fluid into a pressurization
vessel and isolating the vessel;
[0034] (b2) heating the subcritical fluid at essentially constant
volume and essentially constant density to yield a dense fluid;
and
[0035] (b3) introducing one or more processing agents into the
pressurization vessel
[0036] before introducing the subcritical fluid into the
pressurization vessel, or
[0037] after introducing the subcritical fluid into the
pressurization vessel but before heating the pressurization vessel,
or
[0038] after introducing the subcritical fluid into the
pressurization vessel and after heating the pressurization
vessel;
[0039] (c) transferring at least a portion of the dense processing
fluid from the pressurization vessel to the processing chamber,
wherein the transfer of the dense processing fluid is driven by the
difference between the pressure in the pressurization vessel and
the pressure in the processing chamber, thereby pressurizing the
processing chamber with transferred dense processing fluid;
[0040] (d) introducing ultrasonic energy into the processing
chamber and contacting the article with the transferred dense
processing fluid to yield a spent dense processing fluid and a
treated article; and
[0041] (e) separating the spent dense processing fluid from the
treated article.
[0042] A further embodiment of the invention includes an apparatus
for processing an article which comprises:
[0043] (a) a fluid storage tank containing a subcritical fluid;
[0044] (b) one or more pressurization vessels and piping means for
transferring the subcritical fluid from the fluid storage tank to
one or more pressurization vessels;
[0045] (c) heating means to heat the contents of each of the one or
more pressurization vessels at essentially constant volume and
essentially constant density to convert the subcritical fluid into
a dense fluid;
[0046] (d) a sealable processing chamber for contacting an article
with the dense fluid;
[0047] (e) ultrasonic generation means for introducing ultrasonic
energy into the sealable processing chamber;
[0048] (f) piping means for transferring the dense fluid from the
one or more pressurization vessels into the sealable processing
chamber; and
[0049] (g) one or more processing agent storage vessels and pumping
means to inject one or more processing agents (1) into the one or
more pressurization vessels or (2) into the piping means for
transferring the dense fluid from the one or more pressurization
vessels to the sealable processing chamber or (3) into the sealable
processing chamber.
[0050] The apparatus may further comprise pressure reduction means
and phase separation means to separate a spent dense processing
fluid withdrawn from the processing chamber to yield at least a
purified fluid and one or more recovered residual compounds. The
apparatus may further comprise recycle means to recycle the
purified fluid to the fluid storage tank.
[0051] Another embodiment of the invention relates to a method for
processing an article comprising:
[0052] (a) introducing the article into a sealable processing
chamber and sealing the processing chamber;
[0053] (b) providing a dense processing fluid in the processing
chamber;
[0054] (c) introducing ultrasonic energy into the processing
chamber and varying the frequency of the ultrasonic energy while
contacting the article with the dense processing fluid to yield a
spent dense processing fluid and a treated article; and
[0055] (e) separating the spent dense processing fluid from the
treated article. The frequency of the ultrasonic energy may be
increased during (c). Alternatively, the method of claim 19 wherein
the frequency of the ultrasonic energy may be decreased during
(c).
[0056] The dense processing fluid may be prepared by:
[0057] (a) introducing a subcritical fluid into a pressurization
vessel and isolating the vessel;
[0058] (b) heating the subcritical fluid at essentially constant
volume and essentially constant density to yield a dense fluid;
and
[0059] (c) providing the dense processing fluid by one or more
steps selected from the group consisting of
[0060] (1) introducing one or more processing agents into the dense
fluid while transferring the dense fluid from the pressurization
vessel to the processing chamber,
[0061] (2) introducing one or more processing agents into the
pressurization vessel to form a dense processing fluid and
transferring the dense processing fluid from the pressurization
vessel to the processing chamber,
[0062] (3) introducing one or more processing agents into the dense
fluid in the processing chamber after transferring the dense fluid
from the pressurization vessel to the processing chamber,
[0063] (4) introducing one or more processing agents into the
pressurization vessel before introducing the subcritical fluid into
the pressurization vessel,
[0064] (5) introducing one or more processing agents into the
pressurization vessel after introducing the subcritical fluid into
the pressurization vessel but before heating the pressurization
vessel, and
[0065] (6) introducing one or more processing agents into the
pressurization vessel after introducing the subcritical fluid into
the pressurization vessel and after heating the pressurization
vessel.
[0066] Alternatively, the dense processing fluid may be prepared
by:
[0067] (a) introducing a subcritical fluid into the sealable
processing chamber and isolating the chamber;
[0068] (b) heating the subcritical fluid at essentially constant
volume and essentially constant density to yield a dense fluid;
and
[0069] (c) providing the dense processing fluid by one or more
steps selected from the group consisting of
[0070] (1) introducing one or more processing agents into the
sealable processing chamber before introducing the subcritical
fluid into the sealable processing chamber,
[0071] (2) introducing one or more processing agents into the
sealable processing chamber after introducing the subcritical fluid
into the sealable processing chamber but before heating the
subcritical fluid therein, and
[0072] (3) introducing one or more processing agents into the
sealable processing chamber after introducing the subcritical fluid
into the sealable processing chamber and after heating the
subcritical fluid therein.
[0073] In an alternative embodiment of the invention, an article
may be processed by a method comprising:
[0074] (a) introducing the article into a sealable processing
chamber and sealing the processing chamber;
[0075] (b) providing a dense fluid in the processing chamber;
[0076] (c) introducing ultrasonic energy into the processing
chamber and varying the frequency of the ultrasonic energy while
contacting the article with the dense fluid to yield a spent dense
fluid and a treated article; and
[0077] (e) separating the spent dense fluid from the treated
article.
[0078] Another embodiment of the invention may include a method for
processing an article comprising:
[0079] (a) introducing the article into a sealable processing
chamber and sealing the processing chamber;
[0080] (b) providing a dense processing fluid in the processing
chamber;
[0081] (c) introducing ultrasonic energy into the processing
chamber intermittently while contacting the article with the dense
processing fluid to yield a spent dense processing fluid and a
treated article; and
[0082] (e) separating the spent dense processing fluid from the
treated article.
[0083] In a further embodiment, a method for processing an article
may comprise:
[0084] (a) introducing the article into a sealable processing
chamber and sealing the processing chamber;
[0085] (b) providing a dense fluid in the processing chamber;
[0086] (c) introducing ultrasonic energy into the processing
chamber intermittently while contacting the article with the dense
fluid to yield a spent dense fluid and a treated article; and
[0087] (e) separating the spent dense fluid from the treated
article.
BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS
[0088] FIG. 1 is a density-temperature phase diagram for carbon
dioxide.
[0089] FIG. 2 is a generalized density-temperature phase
diagram.
[0090] FIG. 3 is a process flow diagram illustrating an embodiment
of the invention.
[0091] FIG. 4 is a schematic drawing of a pressurization vessel
used in the embodiment of FIG. 3.
DETAILED DESCRIPTION OF THE INVENTION
[0092] Cleaning is the most frequently repeated step in the
manufacture of integrated circuits. At the 0.18-micrometer design
rule, 80 of the approximately 400 total processing steps are
cleaning steps. Wafers typically are cleaned after every
contaminating process step and before each high temperature
operation to ensure the quality of the circuit. Exemplary cleaning
and removal applications include photoresist stripping/removal,
particle/residue removal for post-chemical mechanical planarization
(post-CMP cleaning), particle/residue removal for post-dielectric
etching (or post-metal etching), and removal of metal
contaminants.
[0093] A wide variety of contamination-sensitive articles
encountered in the fabrication of microelectronic devices and
micro-electromechanical devices can be cleaned or processed using
embodiments of the present invention. Such articles may include,
for example, silicon or gallium arsenide wafers, reticles,
photomasks, flat panel displays, internal surfaces of processing
chambers, printed circuit boards, surface mounted assemblies,
electronic assemblies, sensitive wafer processing system
components, electro-optical, laser and spacecraft hardware, surface
micro-machined systems, and other related articles subject to
contamination during fabrication. Typical contaminants to be
removed from these articles in a cleaning process may include, for
example, low and high molecular weight organic contaminants such as
exposed photoresist material, photoresist residue, UV- or
X-ray-hardened photoresist, C-F-containing polymers and other
organic and inorganic etch residues, ionic and neutral, light and
heavy inorganic (metal) species, moisture, and insoluble materials,
including post-planarization particles.
[0094] Dense fluids are well-suited to convey processing agents to
articles such as microelectronic components undergoing processing
steps and for removing undesirable components from the
microelectronic components upon completion of the process steps.
These process steps typically are carried out batchwise and may
include, for example, cleaning, film stripping, etching,
deposition, drying, photoresist development, and planarization.
Other uses for dense fluids include precipitation of nano-particles
and suspension of metallic nano-crystals.
[0095] Dense fluids are ideal for these applications because these
fluids characteristically have high solvent power, low viscosity,
high diffusivity, and negligible surface tension relative to the
articles being processed. As pointed out above, the processing
fluids used in microelectronic processing must have extremely high
purity, much higher than that of similar fluids used in other
applications. The generation of extremely high purity dense fluids
for these applications must be done with great care, preferably
using the methods described herein.
[0096] A single-component supercritical fluid is defined as a fluid
above its critical temperature and pressure. A related
single-component fluid having similar properties to a supercritical
fluid is a single-phase fluid which exists at a temperature below
its critical temperature and a pressure above its liquid saturation
pressure. In the present disclosure, the term "dense fluid" as
applied to a single-component fluid is defined to include both a
supercritical fluid and a single-phase fluid which exists at a
temperature below its critical temperature and a pressure above its
saturation pressure. A single-component dense fluid also can be
defined as a single-phase fluid at a pressure above its critical
pressure or a pressure above its liquid saturation pressure. The
term "component" as used herein means an element (for example,
hydrogen, helium, oxygen, nitrogen) or a compound (for example,
carbon dioxide, methane, nitrous oxide, sulfur hexafluoride).
[0097] A single-component subcritical fluid is defined as a fluid
at a temperature below its critical temperature or a pressure below
its critical pressure.
[0098] A dense fluid alternatively may comprise a mixture of two or
more components. In this case, the dense fluid is defined as a
single-phase multi-component fluid of a given composition which is
above its saturation or bubble point pressure, or which has a
combination of pressure and temperature above the mixture critical
point. The critical point for a multi-component fluid is defined as
the combination of pressure and temperature above which the fluid
of a given composition exists only as a single phase. In the
present disclosure, the term "dense fluid" as applied to a
multi-component fluid is defined to include both a supercritical
fluid and a single-phase fluid that exists at a temperature below
its critical temperature and a pressure above its bubble point or
saturation pressure. A multi-component dense fluid also can be
defined as a single-phase multi-component fluid at a pressure above
its critical pressure or a pressure above its bubble point or
liquid saturation pressure. A multi-component dense fluid differs
from a single-component dense fluid in that the liquid saturation
pressure, critical pressure, and critical temperature are functions
of composition. As described below, dense fluids may be prepared
according to embodiments of the invention from an initial
subcritical fluid having a fixed density and composition.
[0099] A multi-component subcritical fluid is defined as a
multi-component fluid of a given composition which is at or below
its saturation or bubble point pressure, or which has a combination
of pressure and temperature below the mixture critical point.
[0100] The generic definition of a dense fluid thus includes a
single component dense fluid as defined above as well as a
multi-component dense fluid as defined above. Similarly, a
subcritical fluid may be a single-component fluid or a
multi-component fluid.
[0101] The definition of a dense fluid for a single component is
illustrated in FIG. 1, which is a representative
density-temperature phase diagram for carbon dioxide. This diagram
shows saturated liquid curve 1 and saturated vapor curve 3, which
merge at critical point 5 at the critical temperature of
87.9.degree. F. and critical pressure of 1,071 psia. Lines of
constant pressure (isobars) are shown, including the critical
isobar of 1,071 psia. Line 7 is the melting curve. The region to
the left of and enclosed by saturated liquid curve 1 and saturated
vapor curve 3 is a two-phase vapor-liquid region. The region
outside and to the right of liquid curve 1, saturated vapor curve
3, and melting curve 7 is a single-phase fluid region. The dense
fluid as defined herein is indicated by cross-hatched region 9.
[0102] A generic density-temperature diagram can be defined in
terms of reduced temperature, reduced pressure, and reduced density
as shown in FIG. 2. The reduced temperature (T.sub.R) is defined as
the absolute temperature divided by the absolute critical
temperature, reduced pressure (P.sub.R) is defined as the absolute
pressure divided by the absolute critical pressure, and reduced
density (P.sub.R) is defined as the density divided by the critical
density. The reduced temperature, reduced pressure, and reduced
density are all equal to 1 at the critical point by definition.
FIG. 2 shows analogous features to FIG. 1 including saturated
liquid curve 201 and saturated vapor curve 203, which merge at
critical point 205 at a reduced temperature of 1, a reduced density
of 1, and a reduced pressure of 1. Lines of constant pressure
(isobars) are shown, including critical isobar 207 for which
P.sub.R=1. The region to the left of and enclosed by saturated
liquid curve 201 and saturated vapor curve 203 is the two-phase
vapor-liquid region. The crosshatched region 209 above the
P.sub.R=1 isobar and to the right of the critical temperature
T.sub.R=1 is a single-phase supercritical fluid region. The
crosshatched region 211 above saturated liquid curve 201 and to the
left of the critical temperature T.sub.R=1 is a single-phase
compressed liquid region. The dense fluid as defined herein
includes both single-phase supercritical fluid region 209 and
single-phase compressed liquid region 211.
[0103] The generation of a dense fluid in embodiments of the
present invention is illustrated in FIG. 2. In one embodiment, a
saturated liquid at point a is introduced into a vessel and sealed
therein. The sealed vessel is heated isochorically, i.e., at
essentially constant volume, and isopycnically, i.e., at
essentially constant density. The fluid moves along the line as
shown to point a' to form a supercritical fluid in region 209. This
is generically a dense fluid as defined above. Alternatively, the
fluid at point a may be heated to a temperature below the critical
temperature (T.sub.R=1) to form a compressed liquid. This also is a
generic dense fluid as defined above. In another embodiment, a
two-phase vapor liquid mixture at point b is introduced into a
vessel and sealed therein. The sealed vessel is heated
isochorically, i.e., at essentially constant volume, and
isopycnically, i.e., at essentially constant density. The fluid
moves along the line as shown to point b' to form a supercritical
fluid in region 209. This is generically a dense fluid as defined
above. In another embodiment, a saturated vapor at point c is
introduced into a vessel and sealed therein. The sealed vessel is
heated isochorically, i.e., at essentially constant volume, and
isopycnically, i.e., at essentially constant density. The fluid
moves along the line as shown to point c' to form a supercritical
fluid in region 209. This is generically a dense fluid as defined
above.
[0104] The final density of the dense fluid is determined by the
volume of the vessel and the relative amounts of vapor and liquid
originally introduced into the vessel. A wide range of densities
thus is achievable by this method. The terms "essentially constant
volume" and "essentially constant density" mean that the density
and volume are constant except for negligibly small changes to the
volume of the vessel that may occur when the vessel is heated.
[0105] A dense fluid for practical application in the present
invention may be either a single-component fluid or a
multi-component fluid, and may have a reduced temperature in the
range of about 0.8 to about 1.8. The reduced temperature is defined
here as the absolute temperature of the fluid divided by the
absolute critical temperature of the fluid.
[0106] The dense fluid may comprise, but is not limited to, one or
more components selected from the group consisting of carbon
dioxide, nitrogen, methane, oxygen, ozone, argon, hydrogen, helium,
ammonia, nitrous oxide, hydrocarbons having 2 to 6 carbon atoms,
hydrogen fluoride, hydrogen chloride, sulfur trioxide, sulfur
hexafluoride, nitrogen trifluoride, chlorine trifluoride,
monofluoromethane, difluoromethane, trifluoromethane,
trifluoroethane, tetrafluoroethane, pentafluoroethane,
perfluoropropane, pentafluoropropane, hexafluoroethane,
hexafluoropropylene, hexafluorobutadiene, octafluorocyclobutane and
tetrafluorochloroethane.
[0107] A dense processing fluid is defined as a dense fluid to
which one or more processing agents have been added. A processing
agent is defined as a compound or combination of compounds that
promote physical and/or chemical changes to an article or substrate
in contact with the dense processing fluid. These processing agents
may include, for example, film strippers, cleaning or drying
agents, entrainers, etching or planarization reactants, photoresist
developers, and deposition materials or reactants. The total
concentration of these processing agents in the dense processing
fluid typically is less that about 50 wt % and may be in the range
of 0.1 to 20 wt %. The dense processing fluid typically remains a
single phase after a processing agent is added to a dense fluid.
Alternatively, the dense processing fluid may be an emulsion or
suspension containing a second suspended or dispersed phase
containing the processing agent. The dense processing fluid may be
used in processing such as film stripping, cleaning, drying,
etching, planarization, deposition, extraction, photoresist
development, or formation of suspended nano-particles and
nano-crystals.
[0108] The term "processing" or "processed" as used herein means
contacting an article with a dense fluid or a dense processing
fluid to effect physical and/or chemical changes to the article.
The term "article" as used herein means any article of manufacture
which can be contacted with a dense fluid or a dense processing
fluid. Representative articles may include, for example, silicon or
gallium arsenide wafers; reticles; photomasks; flat panel displays;
internal surfaces of processing chambers; printed circuit boards;
surface mounted assemblies; electronic assemblies; sensitive wafer
processing system components; electro-optical, laser and spacecraft
hardware; surface micro-machined systems; and other related
articles subject to contamination during fabrication. The term
"processing" may include, for example, film stripping, cleaning,
drying, etching, planarization, deposition, extraction, photoresist
development, or formation of suspended nano-particles and
nano-crystals.
[0109] An embodiment of the invention can be illustrated by the
generation and use of a dense processing fluid for use in the
cleaning or processing of an article such as a microelectronic
component. An exemplary process for this embodiment is shown in
FIG. 3, which illustrates an isochoric (constant volume) carbon
dioxide pressurization system to generate a carbon dioxide dense
fluid for an ultrasonic electronic component cleaning chamber or
processing tool, and includes a carbon dioxide recovery system to
recycle carbon dioxide after separation of extracted contaminants.
Liquid carbon dioxide and its equilibrium vapor are stored in
carbon dioxide supply vessel 301, typically at ambient temperature;
at 70.degree. F., for example, the vapor pressure of carbon dioxide
is 854 psia. At least one carbon dioxide pressurization vessel is
located downstream of the supply vessel 301. In this embodiment,
three pressurization vessels 303, 305, and 309 (described in more
detail below) are shown in flow communication with carbon dioxide
supply vessel 301 via manifold 311 and lines 313, 315, and 317
respectively. These lines are fitted with valves 319, 321, and 323,
respectively, to control flow of carbon dioxide from supply vessel
301 to the pressurization vessels. Fluid supply lines 325, 327, and
329 are connected to manifold 331 via valves 333, 335, and 337
respectively.
[0110] A detailed illustration of pressurization vessel 303 is
given in FIG. 4. Pressurization vessel 303 comprises outer pressure
casing 401, inner vessel 403, and thermal insulation 405 between
the inner vessel and the outer pressure casing. The thermal mass of
inner vessel 403 is preferably minimized to minimize the cool-down
time when the vessel is initially filled from carbon dioxide supply
vessel 301. Inner vessel 403 is in fluid communication with thermal
insulation 405 via opening 407 to ensure that the pressures inside
and outside of inner vessel 403 are approximately equal, which
allows the wall thickness and thermal mass of inner vessel 403 to
be minimized. Opening 407 may contain a de-misting medium, such as
metal mesh or porous sintered metal (not shown), to prevent liquid
carbon dioxide droplets from migrating into thermal insulation
405.
[0111] The level of liquid in the pressurization vessel may be
monitored conveniently by differential pressure sensor 409, which
is in fluid communication with the interior of inner vessel 403 via
lines 411, 413, and 415. A typical liquid level is shown between
liquid 417 and vapor 419 in inner vessel 403. Inner vessel 403 is
in fluid communication with lines 313 and 325 of FIG. 3 via line
420.
[0112] Heat may be supplied to inner vessel 403 by any desired
method. In one embodiment, hot heating fluid 421 is supplied via
line 423 to heat exchanger 425, which heats liquid 417 and vapor
419 by indirect heat exchange. Cooled heating fluid is withdrawn
via line 427. Heat exchanger 425 can be any type of heat exchange
assembly. One type of useful heat exchange assembly is a
longitudinally-finned pipe as shown in which a plurality of fins
429 are brazed or welded to pipe 431. The temperature and flow rate
of heating fluid 421 may be regulated to control the heating rate
during pressurization and the final temperature and pressure of the
dense fluid formed within inner vessel 403.
[0113] Returning now to FIG. 3, carbon dioxide supply vessel 301 is
connected via two-way flow line 339 to carbon dioxide liquefier 341
located above the carbon dioxide supply vessel 301. Heat exchanger
343, which may be a plate and fin or other type of heat exchanger
such as heat exchanger 425 of FIG. 4, is used to cool the interior
of liquefier 341. A cooling fluid is supplied via line 330 and may
be, for example, cooling water at an ambient temperature of
70.degree. F., which will maintain the pressure in carbon dioxide
supply vessel 301 at the corresponding carbon dioxide vapor
pressure of 854 psia.
[0114] In this illustration, valve 319 is open while valves 321,
323, and 333 are closed. Valve 335 or 337 may be open to supply
dense fluid carbon dioxide to manifold 331 from pressurization
vessel 305 or 309, which previously may have been charged with
carbon dioxide and pressurized as described below. Liquid carbon
dioxide from supply vessel 301 flows downward into pressurization
vessel 303 via manifold 311, valve 319, and line 313. As the liquid
carbon dioxide enters pressurization vessel 303, which was warmed
in a previous cycle, initial liquid flashing will occur. Warm flash
vapor returns upward into the carbon dioxide supply vessel 301 via
line 313 and manifold 311 as liquid flows downward into
pressurization vessel 303. The warm flash vapor flows back into
carbon dioxide supply vessel 301 and increases the pressure
therein. Excess vapor flows from supply vessel 301 via line 339 to
carbon dioxide liquefier 341, wherein the vapor is cooled and
condensed to flow downward via line 339 back to supply vessel
301.
[0115] After initial cooling and pressurization, liquid carbon
dioxide flows from supply vessel 301 into pressurization vessel
303. When the pressurization vessel is charged with liquid carbon
dioxide to a desired depth, valve 319 is closed to isolate the
vessel. The carbon dioxide isolated in vessel 303 is heated by
indirect heat transfer as described above and is pressurized as
temperature increases. The pressure is monitored by pressure sensor
345 (pressure sensors 347 and 349 are used similarly for vessels
305 and 309 respectively). As heat is transferred to the carbon
dioxide in vessel 303, the temperature and pressure rise, the
separate liquid and vapor phases become a single phase, and a dense
fluid is formed. This dense fluid may be heated further to become a
supercritical fluid, which by definition is a fluid at a
temperature above its critical temperature and a pressure above its
critical pressure. Conversely, the subcritical fluid is defined as
a fluid at a temperature below its critical temperature or a
pressure below its critical pressure. The carbon dioxide charged to
pressurization vessel 303 prior to heating is a subcritical fluid.
This subcritical fluid may be a saturated vapor, a saturated
liquid, or a two-phase fluid having coexisting vapor and liquid
phases.
[0116] As additional heat is transferred, the temperature and
pressure quickly rise to supercritical levels to form a
supercritical fluid having a desired density. The final carbon
dioxide pressure in the pressurization vessel of a known volume can
be predicted from the volume of the initial liquid charge. For
example, at 854 psia and 70.degree. F. the density of liquid carbon
dioxide in the vessel is 47.6 lb/ft.sup.3 and the density of the
coexisting carbon dioxide vapor is 13.3 lb/ft.sup.3. If the liquid
carbon dioxide charge occupies 46.3% of the volume of the vessel,
then the carbon dioxide vapor occupies the remaining 53.7% of the
volume. In this example, the average density of all carbon dioxide
in the vessel can be calculated as 0.463 (47.6)+0.537 (13.3), or
29.2 lb/ft.sup.3.
[0117] Since the internal volume of the vessel and the mass of
carbon dioxide in the vessel remain essentially unchanged during
the heating step, the average density of the captured carbon
dioxide will remain essentially unchanged at 29.2 lb/ft.sup.3
regardless of the temperature and pressure. In this example,
heating the selected initial charge of carbon dioxide isochorically
(at constant volume) at a fixed density of 29.2 lb/ft.sup.3 will
pass through the critical point at the critical temperature of
87.9.degree. F. and the critical pressure of 1,071 psia. Additional
heating will form a supercritical fluid at the desired temperature
and pressure having a fixed density of 29.2 lb/ft.sup.3. Using a
smaller initial quantity of liquid carbon dioxide in the vessel
will result in a lower density supercritical fluid; conversely,
using a greater initial quantity of liquid carbon dioxide in the
vessel will result in a higher density supercritical fluid. Heating
a higher density supercritical fluid to a given temperature will
generate a higher pressure than heating a lower density
supercritical fluid to the same temperature.
[0118] The highest theoretically achievable pressure is obtained
when the pressurization vessel initially is completely filled with
liquid carbon dioxide, leaving no vapor head space in the vessel.
For example, the average density of the saturated carbon dioxide
liquid in the vessel at 70.degree. F. is 47.6 lb/ft.sup.3. Initial
heating of the liquid carbon dioxide will change the saturated
liquid into a dense fluid in a region of the phase diagram
sometimes termed a compressed liquid or subcooled liquid. As the
fluid is heated above the critical temperature of 87.9.degree. F.,
it becomes a supercritical fluid by definition. In this example,
the carbon dioxide may be heated at a constant density of 47.6
lb/ft.sup.3 to a temperature of 189.degree. F. to yield a
supercritical fluid at a pressure of approximately 5,000 psia.
[0119] By using the method illustrated in the above examples, a
dense fluid can be prepared at any selected density, temperature,
and pressure. Only two of these three parameters are independent
when the composition is fixed; the preferred and most convenient
way to prepare a dense fluid is to select an initial charge density
and composition in the pressurization vessel and then heat-the
charge to a desired temperature. Proper selection of the initial
charge density and composition will yield the desired final
pressure.
[0120] When carbon dioxide is used for a single-component dense
processing fluid, the carbon dioxide may be heated to a temperature
between about 100.degree. F. and about 500.degree. F. to generate
the desired dense fluid pressure in the pressurization vessel. More
generally, when using any component or components for the dense
fluid, the fluid may be heated to a reduced temperature in the
pressurization vessel of up to about 1.8, wherein the reduced
temperature is defined as the average absolute temperature of the
fluid in the pressurization vessel after heating divided by the
absolute critical temperature of the fluid. The critical
temperature is defined for a fluid containing any number of
components as that temperature above which the fluid always exists
as a single fluid phase and below which two phases may form.
[0121] Returning now to FIG. 3, valve 333 is opened and dense fluid
prepared as described above passes through manifold 331 under flow
control through metering valve 351. Optionally, one or more
entrainers or processing agents from entrainer storage vessels 353
and 355 may be introduced by pumps 357 and 359 into the dense fluid
in line 361 to provide a dense processing fluid, which in a
cleaning application may be described as a dense cleaning fluid.
The dense processing fluid is introduced into sealable processing
chamber or process tool 362 which holds one or more articles 363 to
be cleaned or processed, and valve 333 is closed. These articles
were previously placed on holder 365 in process tool 362 via a
sealable entry port (not shown). The temperature in process tool
362 is controlled by means of temperature control system 367. Fluid
agitator system 369 mixes the interior of process tool 362 to
promote contact of the dense processing fluid with articles
363.
[0122] Processing chamber or process tool 362 is fitted with
ultrasonic generator 370, which is an ultrasonic transducer array
connected to high frequency power supply 371. The ultrasonic
transducer may be any commercially available unit such as, for
example, an ultrasonic horn from Morgan Electro Ceramics of
Southampton, England. Ultrasonic generator 370 typically may be
operated in a frequency range of 20 KHz to 2 MHz. In the present
disclosure, the term "ultrasonic" refers to any wave or vibration
having a frequency above the human audible limit of about 20 KHz.
High frequency power supply 371 typically provides power in an
ultrasonic power density range of about 20 W/in.sup.2 to about 40
W/in.sup.2. The interior of process tool 362 typically is exposed
to ultrasonic waves for 30 to 120 seconds during the cleaning
step.
[0123] Ultrasonic transducers can be constructed from piezoelectric
or magnetostrictive structures. Piezoelectric transducers contain
crystals that oscillate at ultrasonic frequencies when alternating
current is applied. Sturdier magnetostrictive transducers consist
of a piece of iron or nickel surrounded by an electric coil. Such
transducers are commonly built into a "probe" assembly, which
includes an acoustically designed booster and a horn (not shown).
Such probes can be used to maximize power transmission into the
fluid, pressure vessel wall, or substrate holder.
[0124] Ultrasonic transducer array 370 may be mounted horizontally
above and facing the articles being processed, as is shown in FIG.
3, such that the sonic waves are generated in a downward direction
and impinge directly on articles 363. Alternatively, the transducer
array may be mounted vertically on either side of the articles
being cleaned (not shown) such that the ultrasonic waves are
generated in a horizontal direction across the articles being
cleaned.
[0125] In another alternative, the transducer array may be mounted
horizontally below and in contact with holder 365 (not shown) such
that the ultrasonic waves are generated in a generally vertical
direction and transmitted upward through holder 365. This
configuration can be used, for example, to apply the maximum
ultrasonic energy to the surface of a wafer, principally when
chemical reactions such as thin film deposition, etching, or
electropolishing occur at the wafer surface. The wafer can be
positioned in any orientation, i.e., facing up, facing down, or
facing sideways. In this case, acoustic streaming carries reaction
products and contaminants away from the surface. The flow sweeps in
from the sides and away from the surface. Dissolved materials and
suspended particles tend to migrate away from areas of sonic energy
concentration, and this arrangement would tend to carry
concentrated materials away from the surface and away from the
ultrasonic source.
[0126] While ultrasonic transducer array ultrasonic generator 370
as shown in FIG. 3 is mounted within process tool 362, it may be
mounted alternatively on the outside surface of the process tool
vessel such that the generated sonic energy is transmitted through
the walls of the vessel.
[0127] The initial pressure in pressurization vessel 303 and the
temperature in process tool 362 may be selected so that the dense
cleaning fluid in process tool 362 after the transfer step
typically is a single-phase dense fluid as defined above, whether
or not an entrainer or other processing agent is added to the
original dense fluid. Alternatively, the dense processing fluid may
be an emulsion or suspension containing a second suspended or
dispersed phase containing the processing agent.
[0128] A wide variety of contamination-sensitive articles may be
encountered in the fabrication of microelectronic devices, and
these micro-electromechanical devices can be cleaned or processed
using the present invention. Such articles may include, for
example, silicon or gallium arsenide wafers, reticles, photomasks,
flat panel displays, internal surfaces of processing chambers,
printed circuit boards, surface mounted assemblies, electronic
assemblies, sensitive wafer processing system components,
electro-optical, laser and spacecraft hardware, surface
micro-machined systems, and other related articles subject to
contamination during fabrication. Typical contaminants that can be
removed from these articles in a cleaning process may include, for
example, low and high molecular weight organic contaminants such as
exposed photoresist material, photoresist residue, UV- or
X-ray-hardened photoresist, C-F-containing polymers and other
organic and inorganic etch residues, ionic and non-ionic inorganic
metal-containing compounds, moisture, and insoluble materials
including post-planarization particles.
[0129] Sealed process tool 362 may be pressurized with the dense
cleaning fluid to a typical supercritical pressure of 1,100 to
10,000 psia, preferably 1,500 to 7,500 psia. The tool typically
operates at a supercritical temperature of up to 500.degree. F.,
and may operate in a range of 100.degree. F. to 200.degree. F. The
temperature in process tool 362 is controlled by means of
temperature control system 367. Typically, the contacting of
articles 363 with the dense cleaning fluid in process tool 362 may
be effected at a reduced temperature above 1.0 and typically below
about 1.8, wherein the reduced temperature is defined as the
average absolute temperature of the fluid in the cleaning chamber
divided by the absolute critical temperature of the fluid.
[0130] Several alternatives to the introduction of entrainer or
processing agent into line 361 to mix with the dense fluid prior to
flowing into process tool 362 are possible. In one alternative,
entrainer may be introduced directly into process tool 362 before
the tool is charged with dense fluid from pressurization vessel
303. In another alternative, entrainer may be introduced directly
into process tool 362 after the tool is charged with dense fluid.
In yet another alternative, entrainer may be introduced directly
into pressurization vessel 303 before the vessel is charged from
supply vessel 301. In a further alternative, entrainer may be
introduced directly into pressurization vessel 303 after the vessel
is charged from supply vessel 301 but before the vessel is heated.
In a final alternative, entrainer may be introduced directly into
pressurization vessel 303 after the vessel is charged from supply
vessel 301 and after the vessel is heated. Any of these
alternatives can be accomplished using the appropriate lines,
manifolds, and valves in FIG. 3.
[0131] In addition to the intense agitation provided by ultrasonic
transducer 370, the interior of process tool 362 may be mixed by
fluid agitator system 369 to enhance contact of the dense cleaning
fluid with articles 363. Additional fluid agitation may be provided
by a recirculating fluid system consisting of pump 372 and filter
373. Filter 373 serves to remove particulate contamination from the
recirculating fluid, and the resulting fluid agitation mixes the
dense fluid and promotes removal of contaminants or reaction
products from the contaminated articles by increasing convective
fluid motion.
[0132] When the cleaning cycle is complete, process tool 362 is
depressurized by opening valves 375 and 377 whereby the
contaminated dense fluid flows through heat exchanger 379, where it
is cooled to a temperature of 70.degree. F. to 150.degree. F. This
reduction in pressure and temperature condenses the dissolved
contaminants and entrainers in the dense fluid, and the resulting
fluid containing suspended contaminants and entrainers flows via
line 381 into separator 383. Condensed contaminants and entrainers
are removed via line 385 and the purified fluid flows via line 387
to intermediate fluid storage vessel 389. The pressure in storage
vessel 389 is between the supercritical extraction pressure in
process tool 362 and the pressure of carbon dioxide supply vessel
301. Typically, process tool 362 is depressurized in this step to a
pressure of 900 to 1,100 psia.
[0133] During the depressurization step, valve 333 optionally may
be opened so that carbon dioxide from pressurization vessel 303
also flows through cooler 370 and separator 383 with the
contaminated depressurization fluid. Optionally, after process tool
362 is initially depressurized, carbon dioxide from pressurization
vessel 303 may be used to partially pressurize and rinse process
tool 362 to dilute and remove residual contaminants and entrainers
therefrom, after which the process tool would be depressurized
through cooler 379 and separator 383 to a pressure of 900 to 1,100
psia. After closing valves 375 and 377, the remaining carbon
dioxide in process tool 362 then is vented through valve 391 to
reduce the pressure to atmospheric. Process tool 362 optionally
then may be evacuated to a subatmospheric pressure. At this point,
the sealable entry port (not shown) of process tool 362 is opened,
the processed articles are removed, and another group of
contaminated articles is loaded for the next cleaning cycle.
[0134] Optionally, another cooler and separator (not shown) similar
to cooler 379 and separator 383 may be installed in line 387. The
use of this second stage of separation at an intermediate pressure
allows more efficient separation of contaminants and entrainers
from the carbon dioxide solvent, and may allow a degree of
fractionation between the contaminants and entrainers.
[0135] Carbon dioxide in intermediate fluid storage vessel 389,
typically at a pressure in the range of 900 to 1,100 psia, may be
filtered by filter system 393 before being recycled via line 395
and valve 397 to liquefier 341, where it is liquefied and returned
to carbon dioxide supply vessel 301 for reuse. Makeup carbon
dioxide may be added as a vapor through line 398 and valve 399 or
added as a liquid directly (not shown) to carbon dioxide supply
vessel 301.
[0136] Alternatively, the purified carbon dioxide in line 387 or
line 395 may be vented directly to the atmosphere (not shown)
without recycling as described above. In this embodiment, the
carbon dioxide is introduced via line 398 and valve 399 and is used
in a once-through mode.
[0137] Multiple pressurization vessels may be used in the exemplary
process as described above. For example, when pressurization vessel
303 of FIG. 3 is in the process of filling and heating,
pressurization vessel 305 (which was previously filled and heated
to provide dense fluid at the desired conditions) can supply
process tool 362 via line 327, valve 335, manifold 331, and line
361. A cycle can be envisioned in which the three pressurization
vessels 303, 305, and 307 operate in a staggered cycle in which one
supplies dense fluid to process tool 362, another is being filled
with carbon dioxide from carbon dioxide supply vessel 301, and the
third is being heated after filling. Utilizing multiple
pressurization vessels in this manner increases the productivity of
process tool 362 and allows for backup if one of the pressurization
vessels is taken off line for maintenance.
[0138] The exemplary process described above uses carbon dioxide as
the dense fluid, but other dense fluid components may be used for
appropriate applications. The dense fluid may comprise one or more
components selected from the group consisting of carbon dioxide,
nitrogen, methane, oxygen, ozone, argon, hydrogen, helium, ammonia,
nitrous oxide, hydrocarbons having 2 to 6 carbon atoms, hydrogen
fluoride, hydrogen chloride, sulfur trioxide, sulfur hexafluoride,
nitrogen trifluoride, chlorine trifluoride, and fluorocarbons such
as, but not limited to, monofluoromethane, difluoromethane,
trifluoromethane, trifluoroethane, tetrafluoroethane,
pentafluoroethane, perfluoropropane, pentafluoropropane,
hexafluoroethane, hexafluoropropylene (C.sub.3F.sub.6),
hexafluorobutadiene (C.sub.4F.sub.6), octafluorocyclobutane
(C.sub.4F.sub.8) and tetrafluorochloroethane.
[0139] A dense processing fluid is generically defined as a dense
fluid to which one or more processing agents have been added. A
processing agent is defined as a compound or combination of
compounds that promote physical and/or chemical changes to an
article or substrate in contact with the dense processing fluid.
These processing agents may include, for example, film strippers,
cleaning or drying agents, entrainers, etching or planarization
reactants, photoresist developers, and deposition materials or
reactants. The total concentration of these processing agents in
the dense processing fluid typically is less that about 50 wt % and
may be in the range of 0.1 to 20 wt %. The dense processing fluid
typically remains a single phase after a processing agent is added
to a dense fluid. Alternatively, the dense processing fluid may be
an emulsion or suspension containing a second suspended or
dispersed phase containing the processing agent.
[0140] The exemplary process described above with reference to FIG.
3 may utilize one or more entrainers mixed with a dense fluid to
provide a dense film stripping or cleaning fluid containing 0.1 to
20 wt % entrainer. An entrainer is defined as a processing agent
that enhances the cleaning ability of the dense fluid to remove
contaminants from a contaminated article. Entrainers generally may
include solvents, surfactants, chelators and chemical modifiers.
Some examples of representative entrainers include acetylenic
alcohols and diols, organosilicones, ethyl acetate, ethyl lactate,
propyl acetate, butyl acetate, diethyl ether, dipropyl ether,
methanol, ethanol, isopropanol, acetonitrile, propionitrile,
benzonitrile, ethylene cyanohydrin, ethylene glycol, propylene
glycol, ethylene glycol monoacetate, propylene glycol monoacetate,
acetone, butanone, acetophenone, trifluoroacetophenone, triethyl
amine, tripropyl amine, tributyl amine, 2,4, dimethyl pyridine,
dimethylethanolamine, diethylethanolamine, diethylmethanolamine,
dimethylmethanolamine, dimethylformamide, dimethylacetamide,
ethylene carbonate, propylene carbonate, acetic acid, lactic acid,
butane-diol, propane-diol, n-hexane, n-butane, hydrogen peroxide,
t-butyl hydroperoxide, and chelating agents such as
ethylenediaminetetraacetic acid (EDTA), catechol, choline,
beta-diketone and beta-ketoimine ligands, trifluoroacetic anhydride
(TFM), halogenated carboxylic acids, halogenated glycols, and
halogenated alkanes.
[0141] Dense processing fluids prepared and managed by the methods
of the present invention may be used in other processing steps in
the manufacture of electronic components in which material is
removed from a part (etching, drying, or planarization), in which
material is deposited on a part (thin film deposition), or in which
material on a part is chemically modified (photoresist
development).
[0142] Surface etching is a chemical reaction process, typically
performed using liquid mixtures or dry plasma processes. During
semiconductor substrate processing, such etching is used to reduce
surface thickness, remove unwanted layers such as surface oxide,
and create surface features such as trenches and via holes. Surface
etching can be performed in a dense phase fluid system using
ultrasonic wave enhancement.
[0143] Ultrasonic waves can be used to enhance the reaction speed
of thin metal film deposition. Such films typically are deposited
from metallic precursors that undergo a reduction reaction at a
heated surface using a reductant such as hydrogen. The use of
ultrasonic energy with a dense processing fluid increases the rate
of the reaction, thereby improving process efficiency and improving
the quality of the thin film.
[0144] Photoresist development is normally performed in a liquid
phase system using chemicals such as tetramethyl ammonium hydroxide
(TMAH) to develop exposed photoresist. This process can be
performed in a dense phase fluid system according to the present
invention using ultrasonic energy to enhance the surface chemical
reactions which occur in photoresist development. The application
of ultrasonic energy in a dense phase processing fluid can improve
the diffusion of chemical reactants and reaction products near the
surface of the articles being processed.
[0145] In these alternative processing steps, appropriate
processing agents or reactive compounds may be added to the dense
fluid to form a dense processing fluid. Some representative
reactive compounds that can be added to a dense fluid as processing
agents for etching or planarization processes may include, for
example, hydrogen fluoride, hydrogen chloride, hexafluoroethane,
nitrogen trifluoride, reactive polishing slurries (containing
alumina, silica, ceria or magnesium abrasive particles suspended in
an acidic, or alkaline, i.e., potassium hydroxide- or
ammonia-containing mixture), and electrolytic solutions for reverse
electroplating of metal surfaces. Some representative reactive and
non-reactive compounds that can be added to a dense fluid for
deposition processes may include, for example include
organometallic precursors, photoresists, photoresist developers,
interlayer dielectric materials, silane reagents and various
coating materials, including but not limited to stain resistant
coatings. A representative reactive compound that may be added to a
dense fluid for photoresist development processesis tetramethyl
ammonium hydroxide (TMAH). Methanol is a representative compound
that may be added to a dense fluid for drying processes. In these
alternative uses of dense processing fluids, process tool 362 of
FIG. 3 as described above for cleaning can be replaced with the
appropriate process tool for these alternative applications.
[0146] The present invention combines ultrasonic energy and dense
fluid immersion concurrently in the same processing tool. The
semiconductor substrate or article being processed thereby is
exposed to enhanced dense fluid processing comprising dissolution
and/or chemical reaction combined with a simultaneous, ultrasonic
energy enhancement of the process. The auxiliary mechanisms for
fluid agitation described above, namely fluid agitator system 369
and recirculating fluid system consisting of pump 372 and filter
373, may also be used to enhance ultrasonic agitation. In this
manner, the solvent or processing agent in a cleaning process can
achieve greater penetration into relatively thick contaminant
films, such as photoresist, and the inert insoluble residues are
removed through the energy imposed by fluid phase oscillations.
Insoluble particles may be removed through a combination of
oscillatory effects and acoustic streaming (induced flow in the
cleaning fluid).
[0147] The increased rate of solvent penetration into contaminant
films provided by ultrasonic energy is advantageous in
semiconductor substrate cleaning applications, where high
throughput is necessary in order to provide an economical process.
Ultrasonic agitation also tends to increase the uniformity of the
cleaning process and thereby provides better cleaning or surface
treatment performance than can be achieved using dense fluid
immersion alone.
[0148] Ultrasonic energy causes localized pressure fluctuations in
the dense fluid or dense processing fluid, which aids in cleaning
or processing performance. These pulsations in pressure created by
the ultrasonic waves cause corresponding oscillations in the
density of the dense fluid about a mean value, which in turn causes
corresponding oscillations in the solvent power of the fluid about
the mean value. The solvent power therefore varies cyclically
between maximum and minimum values during the process, and maximum
achieved solvent power therefore exceeds the mean value that would
be available without ultrasonic waves. This in turn increases the
overall effectiveness of the dissolution process without a
concomitant increase in the mean operating pressure. Conventional
wet ultrasonic cleaning utilizes transient cavitation of liquids
followed by bubble collapse, and the resulting energy release, to
dislodge contaminants. Such cavitation can damage the delicate
features of modem semiconductor devices. The process of the present
invention preferably is operated entirely in the dense fluid region
such that no phase change occurs, and therefore no cavitation can
occur. Embodiments of the invention instead utilize high frequency
fluid oscillations to excite adhered contaminants near their
natural frequencies, thereby producing dislodgement. Since
cavitation is suppressed, power dissipation is minimized, and
acoustic streaming is enhanced.
[0149] A further embodiment of the invention is the use of variable
frequency ultrasonic treatment in which the ultrasonic frequency is
varied during the processing period. Variable frequency ultrasonic
treatment eliminates static vibrational nodes on the surface of the
article being processed and ensures, for example, that particles
having wide-ranging natural frequencies are dislodged and suspended
in the dense processing fluid. Frequencies utilized in this
invention may span the range from typical ultrasonic to megasonic
values (approximately 20 KHz to 2 MHz). In one embodiment, the
variable frequency ultrasonic treatment may comprise starting the
cleaning or processing period at a frequency in the lower end of
this range and increasing the frequency gradually during the
cleaning period to a final frequency in the upper end of this
range. Alternatively, the variable frequency ultrasonic treatment
may comprise starting the cleaning or processing period at a
frequency in the higher end of this range and decreasing the
frequency gradually during the period to a final frequency in the
lower end of this range. In another alternative, the frequency can
be raised and lowered in this range multiple times during the
cleaning or processing period.
[0150] In another embodiment of the invention, ultrasonic energy is
introduced intermittently into the process tool during the cleaning
or processing period. In this embodiment, on-off actuation of the
ultrasonic transducer provides intermittent bursts of power in the
dense phase processing fluid. Such pulsing, for example, would
prevent contaminants from being trapped in standing waves during a
cleaning step. The frequency may be either constant or variable
when the ultrasonic transducer is on, and the frequency may be
either constant or variable among multiple pulse periods.
[0151] Variable frequency or intermittent ultrasonic treatment may
be used with dense fluids that contain no processing agents or with
dense processing fluids that by definition contain one or more
processing agents. Any combination or schedule of frequency changes
and/or intermittent treatment periods during the cleaning or
processing step may be used with dense fluids or with dense
processing fluids. The application of variable frequency and/or
intermittent ultrasonic energy is particularly useful in
combination with selected processing agents in dense processing
fluids for the removal of various types of contaminant particles
from the articles being cleaned.
[0152] The use of ultrasonic energy complements dense fluid
cleaning or processing since dissolution is more effective for
smaller, soluble particles while ultrasonic cleaning or processing
is more effective for larger or insoluble particles. Ultrasonic
cleaning or processing works well in deeply patterned surfaces,
i.e., it is not topography sensitive, and the method is adaptable
to automation. Ultrasonic dense fluid cleaning can provide
comparable performance to wet megasonic cleaning but without the
limitations of wet chemical processing. For example, either method
can provide 90% removal of particles having diameters of 0.5
micrometer and larger, resulting in a surface density of less than
0.1 particle/cm.sup.2.
[0153] The dense fluid or dense processing fluid used with variable
frequency and/or intermittent ultrasonic treatment may be provided
by the methods described earlier with reference to FIG. 3.
Alternatively, the dense fluid or dense processing fluid for use
with variable frequency and/or intermittent ultrasonic treatment
may be prepared directly in the processing vessel by introducing a
subcritical fluid into the sealable processing chamber and
isolating the chamber, heating the subcritical fluid at essentially
constant volume and essentially constant density to yield a dense
fluid, and providing the dense processing fluid by one or more
steps selected from the group consisting of
[0154] (1) introducing one or more processing agents into the
sealable processing chamber before introducing the subcritical
fluid into the sealable processing chamber,
[0155] (2) introducing one or more processing agents into the
sealable processing chamber after introducing the subcritical fluid
into the sealable processing chamber but before heating the
subcritical fluid therein, and
[0156] (3) introducing one or more processing agents into the
sealable processing chamber after introducing the subcritical fluid
into the sealable processing chamber and after heating the
subcritical fluid therein.
[0157] Dense fluids and dense processing fluids are well-suited for
ultrasonic processing. The relatively low viscosity of these fluids
tends to minimize the rate of viscous dissipation of ultrasonic
waves in the fluid. Therefore, the ultrasonic waves can be
delivered to the surface being processed with relatively little
reduction in intensity. This permits high process efficiency at
minimal power consumption. Low viscous dissipation also tends to
increase acoustic streaming in the dense fluid or dense processing
fluid in cleaning processes, thereby promoting the removal of
particulate and dissolved contaminants from the vicinity of the
surface through a flushing action. This tends to bring fresh
solvent into close proximity to the surface, thereby creating a
higher concentration gradient for dissolved contaminants near the
surface and increasing the rate of diffusion of dissolved
contaminants away from the surface. The result is a reduction in
the processing time required to produce a clean surface.
[0158] The relatively low viscosity of dense fluids also helps to
reduce the thickness of the fluid boundary layer near the surface.
The tendency toward thinner acoustic boundary layers can be seen
from the following equation for acoustic boundary layer thickness,
.delta..sub.ac: 1 a c = v f
[0159] where .nu. is the kinematic viscosity of the fluid and f is
the frequency of the waves. A thinner fluid boundary layer tends to
promote removal of adhered surface particles, since they are
exposed to a greater average fluid velocity in a thin boundary
layer than if they were shielded in a thicker, low velocity
boundary layer.
[0160] In applying the present invention, semiconductor substrates
may be cleaned or processed individually in order to provide direct
process integration with other, single substrate processing
modules. Alternatively, multiple substrates, or batches, may be
cleaned or processed simultaneously in a container or "boat" placed
within the cleaning or processing chamber, thereby providing high
throughput and reduced cost of operation.
[0161] Ultrasonic waves improve semiconductor substrate cleaning
through dense fluid immersion by providing a method for removing
insoluble contaminants from surfaces using fluid oscillations and
acoustic streaming and increasing the rate of penetration of
solvents and co-solvents into thick film contaminant layers. As a
result, the required processing time can be reduced. The thickness
of the concentration boundary layer of dissolved reactants or
contaminants may be decreased near the surface by acoustic
streaming. This increases the diffusion rate of dissolved reactants
to the surface or contaminants away from the surface, thereby
reducing the required processing time. This also reduces the
required density of the dense processing fluid necessary to achieve
effective dissolution of soluble reactants or contaminants. This in
turn reduces the required pressure of the dense processing fluid
and reduces the overall cost of processing equipment necessary to
achieve effective processing conditions. Also, this reduces the
amount of dense fluid necessary to achieve effective processing
performance and reduces the required concentrations and amounts of
entrainers or reactants necessary to achieve effective processing
performance in a dense processing fluid. As a result, the overall
cost of ownership of the process, chemical disposal requirements,
energy requirements, and environmental damage caused by the process
can be reduced.
[0162] The following Examples illustrate embodiments of the present
invention but do not limit the embodiments to any of the specific
details described therein.
EXAMPLE 1
[0163] An embodiment of the invention according to FIG. 3 is used
to treat a silicon wafer having a photoresist layer that has
undergone multiple processing steps including exposure,
development, etching and/or implantation with a dense processing
fluid as described below.
[0164] Step 1: Pressurization vessel 303 having a volume of 2.71
liters is filled completely with 4.56 lb of saturated liquid
CO.sub.2 at 70.degree. F. and 853.5 psia. The density of the
initial CO.sub.2 charge is 47.6 lb/ft.sup.3. The vessel is
sealed.
[0165] Step 2: The pressurization vessel is heated until the
internal pressure reaches 5,000 psia. The density of the contained
CO.sub.2 remains at 47.6 lb/ft.sup.3, and the temperature reaches
189.degree. F. The contained CO.sub.2 is converted to a dense fluid
in the supercritical region (see FIG. 1).
[0166] Step 3: A contaminated silicon wafer is loaded into process
tool 362 having an interior volume of 1 liter. The process tool is
evacuated and the vessel walls and wafer are held at 104.degree.
F.
[0167] Step 4: Valve 333 connecting pressurization vessel 303 via
manifold 331 and line 361 to the process tool 362 is opened,
CO.sub.2 flows from pressurization vessel 303 into process tool
362, and the wafer is immersed in dense phase CO.sub.2. The
temperature of pressurization vessel 303 remains at 189.degree. F.
The common pressure of the pressurization vessel and process module
is 2,500 psia. The temperature of the process tool, 362, remains at
104.degree. F. The dense phase CO.sub.2 remains in the
supercritical state in both vessels as 1.79 lb of CO.sub.2 flows
into 1 liter process tool 362 while the remaining 2.77 lb of
CO.sub.2 remains in 2.71 liter pressurization vessel 303. The
density of the CO.sub.2 in the cooler process tool reaches 50.6
lb/ft.sup.3.
[0168] Step 5: An entrainer, propylene carbonate, is pumped from
entrainer storage vessel 353 by pump 357 into process tool 362 and
the process tool is isolated. The concentration of propylene
carbonate in the dense fluid in the process tool is 1 wt %. The
dense fluid is agitated in process tool 362 for two minutes, during
which time the wafer is processed to remove contaminants.
Ultrasonic transducer 370 is operated during this period at an
ultrasonic frequency of 40 KHz while high frequency power supply
371 provides power at an ultrasonic power density of 40
W/in.sup.2.
[0169] Step 6: Valves 333, 351, 375, 377, and 397 are opened so
that fluid in process tool 362 and pressurization vessel 303 flows
through cooler 379 and phase separator 383 to carbon dioxide
liquefier 341 while the pressure in the system is held at 900 psia.
Entrainers, reaction products, and contaminants are separated from
the CO.sub.2 in the separator 383. The temperature of
pressurization vessel 303 remains at 189.degree. F. during this
step and the temperature of the process tool remains at 104.degree.
F. during this step. CO.sub.2 is in the vapor phase in both
vessels. Neglecting the relatively small effect of other mixture
constituents, the density of CO.sub.2 in process tool 362 is 10.32
lb/ft.sup.3. 0.36 lb of CO.sub.2 remains in the process tool
362.
[0170] Step 7: Pressurization vessel 303 is isolated by closing
valve 333 and the vessel is cooled to 70.degree. F., wherein the
pressure falls to 632 psia, and the density of the contained
CO.sub.2 vapor in the vessel remains at 7.07 lb/ft.sup.3.
[0171] Step 8: The remaining 0.36 lb of CO.sub.2 in the process
tool 362 is vented by closing valve 375 and opening valve 391, the
tool is evacuated, and the clean, processed silicon wafer is
removed.
[0172] The cycle is repeated by returning pressurization vessel 303
to Step 1 by refilling with liquid CO.sub.2.
EXAMPLE 2
[0173] The process of Example 1 is repeated except that ultrasonic
transducer system 370 is operated during the cleaning period of
step (5) at a sonic frequency which begins at 20 KHz and is
increased at a constant rate during the cleaning period such that
the sonic frequency is 200 KHz at the end of the cleaning
period.
EXAMPLE 3
[0174] The process of Example 1 is repeated except that ultrasonic
transducer 370 is operated during the cleaning period of step (5)
at a sonic frequency which starts at 200 KHz and is decreased at a
constant rate during the cleaning period such that the sonic
frequency is 20 KHz at the end of the cleaning period.
EXAMPLE 4
[0175] The process of Example 1 is repeated except that ultrasonic
transducer 370 is operated intermittently by turning the transducer
system on for 1 second and off for 1 second in an alternating
pattern during the cleaning period of step (5). The sonic frequency
is 40 KHz during the time the transducer system is on.
* * * * *