U.S. patent application number 10/393872 was filed with the patent office on 2004-01-08 for precision surface treatments using dense fluids and a plasma.
Invention is credited to Jackson, David P..
Application Number | 20040003828 10/393872 |
Document ID | / |
Family ID | 30002924 |
Filed Date | 2004-01-08 |
United States Patent
Application |
20040003828 |
Kind Code |
A1 |
Jackson, David P. |
January 8, 2004 |
Precision surface treatments using dense fluids and a plasma
Abstract
The present invention is a method, process and apparatus for
selective cleaning, drying, and modifying substrate surfaces and
depositing thin films thereon using a dense phase gas solvent and
admixtures within a first created supercritical fluid antisolvent.
Dense fluids are used in combination with sub-atmospheric,
atmospheric and super-atmospheric plasma adjuncts (cold and thermal
plasmas) to enhance substrate surface cleaning, modification,
precision drying and deposition processes herein. Moreover,
conventional wet cleaning agents such as hydrofluoric acid and
ammonium fluoride may be used with the present invention to perform
substrate pre-treatments prior to precision drying and cleaning
treatments described herein. Finally, dense fluid such as solid
phase carbon dioxide and argon may be used as a follow-on treatment
or in combination with plasmas to further treat a substrate
surface.
Inventors: |
Jackson, David P.; (Saugus,
CA) |
Correspondence
Address: |
Jeffrey F. Craft
SONNENSCHEIN NATH & ROSENTHAL
SEARS TOWER, WACKER DRIVE STATION
P.O. BOX 061080
Chicago
IL
60606
US
|
Family ID: |
30002924 |
Appl. No.: |
10/393872 |
Filed: |
March 21, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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60365788 |
Mar 21, 2002 |
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Current U.S.
Class: |
134/1 ; 134/26;
134/30; 257/E21.228; 257/E21.259; 257/E21.26; 257/E21.279;
257/E21.576; 257/E21.577; 257/E21.581 |
Current CPC
Class: |
B08B 7/0021 20130101;
H01L 21/76814 20130101; B08B 2203/005 20130101; H01L 21/3121
20130101; H01L 21/31612 20130101; H01L 21/02274 20130101; H01L
21/02052 20130101; H01L 21/312 20130101; C23G 5/00 20130101; C23C
16/0227 20130101; H01L 21/02164 20130101; H01J 2237/335 20130101;
H01L 21/02216 20130101; H01L 21/76826 20130101; B08B 7/0035
20130101; H01J 37/3244 20130101 |
Class at
Publication: |
134/1 ; 134/26;
134/30 |
International
Class: |
B08B 003/12 |
Claims
I claim:
1. A method for treating a portion of substrate surface comprising
the steps of: (a) placing a substrate having a surface to be
treated in a pressure vessel; (b) introducing a supercritical
anti-solvent into the pressure vessel and establishing a first
supercritical fluid anti solvent phase in contact with the
substrate, the anti-solvent phase having a pressure and temperature
above the critical points of the anti-solvent; (c) introducing a
supercritical solvent into the pressure vessel and contacting a
portion of the substrate surface to be treated within the pressure
vessel; then (d) altering the physiochemistry of the dense fluid
solvent to create a treating phase in contact with the portion of
the substrate surface to be treated; (e) then removing the treating
phase, from the pressure vessel.
2. The method in accordance with claim 1 wherein the supercritical
anti-solvent is supercritical nitrogen, supercritical argon,
supercritical oxygen, supercritical xenon, supercritical helium or
mixtures thereof and the supercritical solvent is supercritical
carbon dioxide, supercritical nitrous oxide, a supercritical
hydrocarbon or mixtures thereof.
3. The method on accordance with claim 1 further comprising
energizing an electrode positioned in the pressure vessel at a
predetermined distance from the substrate at a voltage between 500
volts and 250,000 volts and at a frequency between 100 KHz and 10
GHz and a power of between 50 and 5000 watts.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This invention claims the benefit, under Title 35, United
States Code 119 (e), of Provisional Application No. 60/365,788,
filed Mar. 21, 2002 entitled "Precision surface treatments using
dense fluids and a plasma" which is hereby incorporated by this
reference.
BACKGROUND OF INVENTION
[0002] Smaller electronic, optical and micromechanical devices,
with nano-scale device manufacturing already on the horizon, are
driving the need for improved cleaning and drying technology.
Smaller circuits and surface features are becoming increasingly
affected by smaller surface particles and other residues
(contaminations) during manufacturing operations. Moreover, many
conventional wet cleaning techniques are not compatible with the
shrinking device geometries and new manufacturing materials. Still
moreover, the transition to larger wafer substrates such as the 300
mm platform is driving the need for increased performance and
productivity in cleaning and drying processes and tools. Precision
cleaning and drying are common and often repeated process steps
performed prior to or following almost every other non-cleaning
step of wafer manufacturing, for example photoresist deposition,
curing, acid etching, plasma etching and patterning. As such, a
present need exists for novel substrate dry cleaning and drying
technology, as well as the capability of integrating new dry
cleaning and drying technology with the aforementioned conventional
wafer manufacturing processes, wet or dry, to provide a more
efficient and integrated wafer processing tool.
[0003] Much interest exists to develop alternative wafer cleaning,
drying and photoresist deposition methods to replace hazardous
cleaning and drying chemicals such as alcohols, hydroxylamines, and
organic solvents, or to produce surfaces, films or coatings with
increased quality, planarity and thinness. The presence of organic
contaminants or particles on a substrate surface with thicknesses
on the order of 0.1 microns or greater generate considerable
cleaning difficulties. Often it is necessary to remove all gross
contamination first by solvent immersion cleaning before applying
precision spray cleaning procedures. In another example, vacuum
plasma cleaning using argon-oxygen or fluorine chemistry is used to
treat a layer of patterned photoresist. Often this procedure leaves
behind trace amounts of recalcitrant reactive ion etched (RIE)
polymeric residues comprising a mixture of metals, carbon and
fluorine. A second precision cleaning step using acids, alkalies,
peroxides, ozonated water, or hydroxylamine chemistries, followed
by deionized water rinses and alcohol drying are required to
produce precision clean and dry surfaces.
[0004] The trend towards miniaturization of silicon, germanium and
gallium arsenide microprocessors in the electronics industry and
the emergence of new microelectromechanical systems (MEMS)
manufacturing, which uses much the same microprocessor
manufacturing technology, is creating new material and process
challenges. Conventional cleaning, drying, etching, and deposition
technologies are being pushed to their limits as contamination
removal issues become more important with each new device
generation. For example, the smaller dimensions create new cleaning
challenges due to increasing capillary force pressures which hold
process fluids within cavities, more prevalent electrostatic forces
which hold micromechanical structures together, porous or complex
surface topography which preclude the use of aqueous chemistries,
and high aspect ratio cavities and vias which hide etch residues
and particles, among others. Moreover, new materials such as low-k
films and copper lines used to fabricate smaller device geometries
(line widths) are not compatible with many conventional wet
processing techniques described above.
[0005] For example, surface micromachining of polysilicon films
deposited on silicon wafers is an emerging technology in the
fabrication of microactuators and microsensors. These. miniaturized
components include microengines, microlever actuators,
accelerometers, optical switches, biomedical sensors, and pressure
sensors which have potential uses in a variety of applications for
mechanical and electrical devices both in industry and in
government.
[0006] Similar to integrated circuits (IC) fabrication, surface
machined microstructures are formed using a combination of masking,
dry plasma etching of polysilicon film deposited on the wafer, and
wet etching done in a liquid-phase acid solution such as
hydrofluoric acid (HF). A final HF etch is followed by a water
rinse. In some cases the HF etch is followed by an ammonium
fluoride (NH.sub.4F) treatment. After etching with acid and rinsing
with deionized water, the part is dried, for example using
evaporative methyl alcohol drying, to yield the released
micromachined sample.
[0007] Silicon is a very practical micromechanical material in that
it is capable. of a great amount of flexibility before fracturing.
However, the compliant nature of the silicon makes it susceptible
to fabrication problems. A significant problem in the fabrication
of the micromachined components is sticking of released structures
to the substrate after they are dried using conventional air drying
techniques. The sticking, combined with static friction which these
parts experience has been termed stiction, a phenomenon commonly
seen in magnetic storage media. A number of phenomena may
potentially cause microdynamic stiction of suspended
microstructures, several of which will be identified here.
Electrostatic forces due to electrostatic charging may cause
sticking. These forces can be generated on the wafer due to
etching, rinsing and drying operations. This is a non-equilibrium
condition which usually dissipates over time or with contact
between conducting surfaces. Second, a smooth surface finish may
cause stiction. Smooth surfaces are more likely to stick, while
surface roughness effectively increases the nominal separation
between micromachined surfaces. Third, a phenomenon called solid
bridging occurs when non-volatile impurities present in the drying
liquid are deposited on the surfaces of the microstructures. The
impurities in narrow gaps formed by the suspended microstructures
essentially bridge the gaps, causing the structures to stick.
Obviously, avoiding impurities in the liquid cleaning and drying
fluids will minimize solid bridging.
[0008] Perhaps the most troublesome cause of surface stiction is
liquid bridging. Liquid bridging is due to the surface tension
effects of trapped capillary liquids upon drying. The liquid,
usually water, used to rinse the microstructures is trapped in the
narrow gaps between the silicon wafer and the suspended structures.
Interfacial forces generated when the trapped capillary fluid dries
can cause the microstructures to collapse and stick. Moreover,
conventional thermal or solvent drying of silicon IC structures
such as microvias cause the cavity walls to crack as the sidewalls
are pulled together during extraction or evaporation of water or
high surface tension drying solvents such as methyl alcohol.
[0009] The meniscus force (Fm) between two flat, polished surfaces
(or wafer microvia sidewalls) with a liquid bridge is given by
LaPlace's law and is calculated using the following equation;
Fm=(.gamma.A/h)(cos.sub.100 1+cos.sub.100 2,
[0010] where;
[0011] .sub..phi.1--contact angles of the liquid with the surface
1;
[0012] .sub..phi.2--contact angles of the liquid with the surface
2;
[0013] A--shared surface area of the two parallel surfaces,
assuming the gap between them is flooded with capillary liquid;
[0014] h--average thickness of the liquid bridge;
[0015] .gamma.--surface tension, 73 dynes/cm for water at 25 C.
[0016] If a liquid such as water is present in small capillaries
during the drying process, the surface tension exerts tremendous
pressure on the sidewalls. This stress can be high enough to cause
smoothe flat interfaces to stick, or in the case of IC fabrication,
microvia sidewalls to collapse. Thus it is very beneficial to
reduce or eliminate surface tension to lessen or eliminate surface
stiction due to liquid bridging and prevent sidewall collapse of
very small trenches during drying.
[0017] Supercritical carbon dioxide (SCCO.sub.2) extraction has
been demonstrated to remove capillary liquids (e.g. methanol) from
micromachined structures, eliminating sticking caused by surface
tension effects. Carbon dioxide has long been known to be a good
solvent for many organic compounds and methanol, in particular, is
known to be very soluble in SCCO.sub.2. A conventional wafer drying
process using carbon dioxide has been to displace the rinse water
first with methyl alcohol, and then to dissolve the methanol with
liquid or supercritical carbon dioxide. If extremely small
capillaries are present, the process may be performed above the
critical temperature of methanol, which can be over 200 C., to
insure that the interfacial surface tension of the drying and
extraction fluids within these small capillaries is approximately 0
dynes/cm. After the methanol has been dissolved and carried away by
the supercritical fluid, the vessel is depressurized to yield dry,
released microstructures or dry vias and trenches. Surface tension
effects are eliminated since SCCO.sub.2 has negligible surface
tension like a gas. Furthermore, SCCO.sub.2 exhibits gas-like
properties of diffusivity and viscosity which allow the
supercritical fluid to access narrow gaps under the microfeatures
for removal of trapped capillary fluid. Other commercial uses of
dense fluids for drying include the extraction of solvents from
phase-separated polymer gels to produce microcellular foams and the
extraction of solvents from silica aerogels. Also, researchers at
the University of California at Berkeley have removed drying
solvent from micromachined samples using liquid carbon dioxide
which requires the additional step of increasing to supercritical
conditions before depressurizing to avoid the sticking problems
caused by a liquid/vapor interface.
[0018] The most significant drawbacks with the aforementioned
conventional dense fluid drying techniques are very long process
cycle times and the use of excessive amounts of supercritical or
liquid carbon dioxide in completely flooded pressure vessels to
remove only trace amounts of surface contamination (i.e., water and
drying solvents). Another drawback is that these drying methods do
not effectively remove small particles and in fact can easily
re-contaminate substrates which are completely bathed in the
reactor fluid. Moreover, these methods are not effective or
selective for removing other liquid contaminations present on the
substrate surface or trapped within pores of substrates. Still
moreover, solid contaminants such as carbon residues are not
effectively removed using these conventional techniques, even when
modified with organic solvents. Most often extreme pressures are
required to achieve separation.
[0019] Water, solid particles and residues (contaminations) must be
removed from critical surfaces. If left on critical surfaces, these
may bridge circuits, obscure light or produce other deleterious
side-effects which reduce yields, that is clean dry surfaces for
subsequent processing steps. For example, surface residues,
particles and liquids such as water must be removed prior to
placement or in-situ formation of thin coatings such as low k
dielectrics, or patterning, in preparation for subsequent
lithographic processes. Moreover, processes described above such as
cleaning, etching, drying and application of coatings are most
often performed as separate operations, which greatly increases the
risk of device contamination during manufacture.
[0020] With respect to cleaning wafers to remove trace organic and
particle contamination, commercial wet and dry cleaning systems
have been developed which employ ozone and water to replace
dangerous or ecologically-unsafe chemical processes such as
sulfuric acid-hydrogen peroxide mixtures, toxic organic solvents,
and amine-based cleaning agents. One such system, called the SMS
DIO3 photoresist strip process (Legacy Systems Inc., Fremont,
Calif.), uses an ozone generator and diffuser located in a tank of
chilled (5 C.) deionized water which is circulated into a tank
containing the wafers. This system suffers from an inability to
apply thermal energy to the substrate because it lowers the
solubility of ozone in solution and is essentially time-dependent
and concentration-dependent solid-ozone gas interfacial reaction.
Another commercialized process, called HydrOzone (trademark of
Semitool Inc.), diffuses ozone gas through a thin film of heated
water which is spreading over a spinning wafer. The HydrOzone
process is claimed to be more efficient than the DIO3 process above
because the water component may be heated to provide thermal
cleaning energy and RPM may be varied to control boundary layer
thickness. However, similar to the DIO3 process, transport of ozone
of any significant concentration into micron features on the wafer
surface is very limited due to the solid-ozone gas interface.
Moreover excessive agitation caused by rapid movement of water over
the spinning wafer accelerates the decomposition of the ozone gas
as it diffuses through the thin film boundary. Moreover, complete
drying of the substrate following cleaning by both methods is also
limited due to hydration of small capillaries, vias and interstices
present on the wafer. Finally, a lack of solvent selectivity can be
limiting in many resist removal applications. Commercial cleaning
of textiles using ozonated water is also known. Ozone acting as a
cleaning agent additive is used to destroy soils contained on
fabrics. This method is similar to ozonated water treatment of
wafer and suffers from the same solubility and selectivity
problems.
[0021] Following these ozonated processes, a technique often used
to rinse wafers is the "quick dump" method. The quick dump method
relies upon the rapid deployment of water from the rinse tank to
remove water and impurities from the semiconductor wafer. A
limitation with this method is its inability to actually remove
particles from the wafer. In fact, the rapid deployment of water
from the tank often transfers more particles onto the wafer. In
addition, the wafers from the quick dump tank must still undergo a
drying operation, further increasing the number of particles on the
wafer. As previously noted, an increase in particles on a surface
often relates to lower die yields on the semiconductor wafer.
[0022] A further technique used to both rinse and dry wafers relies
upon a spin rinse/dryer. The spin rinse/dryer uses a combination of
rinse water spray to rinse and centrifugal force to remove water
from the semiconductor wafer. The dry step removes water from the
semiconductor wafer substantially by centrifugal force and
evaporation. However, the spin rinse/dryer often introduces more
particles onto the wafer. In fact, initially dissolved or suspended
contaminants such as particles in the water are often left on the
semiconductor wafer, thereby reducing the number of good dies on
the wafer. Another limitation with the spin rinse/dryer is its
complex mechanical design with moving parts and the like. The
complex mechanical design often leads to problems such as greater
downtime, wafer breakage, more spare parts, and increased cost of
ownership, among other issues. A further limitation is static
electricity often builds up on the wafers during the spin cycle,
thereby attracting even more particles onto the surface of the
semiconductor. Accordingly, the spin rinse/drying does not clean or
remove particles from the wafer.
[0023] Other techniques used to dry wafers include an isopropyl
alcohol (IPA) vapor dryer, full displacement IPA dryer, and others.
These IPA-type dryers often rely upon a large quantity of a solvent
such as isopropyl alcohol and other volatile organic liquids to
facilitate drying of the semiconductor wafer. An example of such a
technique is described in U.S. Pat. No. 4,911,761, and its related
applications, in the name of McConnell et al. and assigned to CFM
Technologies, Inc. McConnell et al. Generally describes the use of
a superheated or saturated drying vapor as a drying fluid. This
superheated or saturated drying vapor often requires the use of
large quantities of a hot volatile organic material. The
superheated or saturated drying vapor forms a thick organic vapor
layer overlying the rinse water to displace (e.g., plug flow) such
rinse water with the drying vapor. The thick vapor layer forms an
azeotropic mixture with water, which will condense on wafer
surfaces, and will then evaporate to dry the wafer.
[0024] A limitation with this type of dryer is its use of the large
solvent quantity, which is hot, highly flammable, and extremely
hazardous to health and the environment. Another limitation with
such a dryer is its cost, which is often quite expensive. In fact,
this dryer needs a vaporizer and condenser to handle the large
quantities of hot volatile organic material.
[0025] Still another technique relies upon a hot deionized (DI)
process water to rinse and promote drying of the semiconductor
wafer. By way of the hot DI water, the liquid on the wafer
evaporates faster and more efficiently than standard room
temperature DI water. However, hot water often produces stains on
the wafer, and also promotes build-up of bacterial and other
particles.
[0026] U.S. Pat. No. 6,240,936, DeSimone et al teaches a method for
applying liquid carbon dioxide onto a portion of spinning substrate
to clean and deposit solutes contained within the liquid carbon
dioxide spray agent in a carbon dioxide atmosphere. The '936
teaches using inert gases with liquid carbon dioxide and a pool of
liquid carbon dioxide within the process chamber, both methods
being taught to maintain a differential pressure for selectively
evaporating the liquid carbon dioxide from the substrate surface.
However, '936 does not teach first establishing an inert
supercritical fluid atmosphere into which a liquefied gas or
supercritical fluid such as carbon dioxide may be much more
selectively applied and controlled. Moreover, the '936 method as
well as very similar prior art by the present inventor, for example
U.S. Pat. No. 5,368,171, and practiced commercially since about
1992 by Deflex Corporation using a commercial centrifugal liquid
carbon dioxide spray cleaning device (Deflex SuperFuge.TM.) suffer
from an inability to fully exploit the delivery agent (dense fluid)
chemistry, as the liquid-state carbon dioxide chemistry is not a
variable geometry dense fluid. Moreover, the application
temperature must be maintained below 30 C. to maintain liquid phase
and solvent power. This is a significant disadvantage as elevated
temperatures improve spray cleaning performance, lowering particle
adhesion and increasing contaminant solubility, and increases the
solubility of high molecular weight polymers such as photoresist
resins. For example, in '936 the liquid phase carbon dioxide
surface tension, density and viscosity cannot be varied to any
significant degree, which prevents optimization of solute chemistry
and substrate cleaning and deposition processes. Moreover, the
substrate is contained in the saturated dense fluid vapor
atmosphere which requires large quantities of dense fluid and extra
processing steps to remove. Still moreover, it has been the
experience of the present inventor using the exemplary commercial
SuperFuge cleaning system described above that using liquid carbon
dioxide cleaning solvent at a pressure and temperature on the
vapor-liquid equilibrium boundary produces an inferior surface
cleanliness to liquid carbon dioxide compressed above its
liquid-vapor boundary. However, to achieve this later state the
entire pressure vessel--substrates, baskets, pressure walls,
centrifuge assembly--must be in liquid phase. Using this method,
most of the cleaning fluid is wasted and cross contamination
problems arise.
[0027] By contrast, the present invention provides a means for
producing variable geometry supersaturated liquids (liquids
compressed above their normal liquid-vapor pressure-temperature
boundaries) and supercritical fluids which are selectively
contacted with a substrate surface. Using an anti-solvent
atmosphere to first to create the proper temperature-pressure
conditions within which a much small quantity of dense fluid is
selectively introduced, used, and captured. The present invention
is more selective, uses less reagent (dense fluids), and the
techniques taught herein extend themselves to cleaning, surface
modification and deposition processes.
[0028] Moreover, in contrast to '936, the present invention teaches
selectively applying preferably a supercritical fluid, but a
liquefied gas may be applied, which may contain one or more
substances, to a substrate surface which is either colder or hotter
than the applied dense fluid. The substrate is made warmer or
colder than the dense fluid using an inert supercritical fluid
anti-solvent atmosphere which first bathes the substrate before
selective application of the dense fluid. Still moreover, and in
contrast again to '936, the substrate and reactor temperature of
the present invention is controlled much more precisely during
depressurization processes. This is accomplished because the
joule-thompson (J-T) coefficient of the supercritical atmosphere is
much lower than the J-T coefficient of the dense fluids, the
volumetric ratio of supercritical atmosphere to dense fluids is
very large, and the extraction of dense fluids during processing is
performed in such a manner as to prevent excessive amounts of dense
fluid from `boiling off` on substrate surfaces.
[0029] U.S. Pat. No. 5,908,510, McCullough, et al teaches a method
for residue removal from etched wafers using supercritical or
liquefied carbon dioxide, followed by a jet spray of solid
cryogenic particles. '510 teaches using surfactants and additives
under a contact time of between 30 minutes to 2 hours to remove CFx
residues from vias and trenches using stirred fluid at about 500 to
2500 rpm. The additives are water and a fluorinated surfactant. As
discussed above in '936, '510 suffers from lack of selectivity,
with the entire substrate processed in essentially carbon dioxide.
'510 does not teach creating reactive cleaning agents in-situ nor
selectively applying said instantaneous cleaning agents to
contaminated surfaces to achieve a desired substrate surface
cleanliness in a much more, rapid period of time. Moreover, in
contrast with '510, the present invention is a much more effective
surface cleaning (i.e., particles, residues) technique called
condensation shear surface cleaning which itself is greatly
enhanced using supercritical plasma techniques described
herein.
[0030] U.S. Pat. No. 5,013,366, Jackson et al (present inventor)
teaches stepping the temperature between the liquid state and
supercritical state in a series of steps. The entire substrate is
bathed in a dense fluid environment which is then changed
physicochemically by changing the chemistry of the bulk fluid in a
series of steps over time using bulk' fluid temperature changes
from above and below the critical temperature of the dense fluid.
The process of '366 is inefficient because it changes the
properties of an entire fluid environment to process a substrate
surface. Also '366 will not remove small surface particles because
it does not produce sufficient shear stress energy during the phase
transition. By contrast, the present invention uses a much smaller
fraction of dense fluid which is selectively applied to a substrate
surface and is changed from supercritical fluid phase to liquid
phase, or vice versa, in a single sweeping and radial step, and
preferably in a pulsed application while traversing the surface of
a substrate surface--producing both shear stress and a change in
chemistry. As such and by contrast, the object of the present
invention is to provide a shearing action across the surface of a
substrate, as well as change the viscosity, cohesive energy and
density of the fluid while it traverses from the center to the
perimeter of a planar substrate.
[0031] In U.S. Pat. No. 5,403,621, Jackson, et al teaches
non-selective deposition process of coating an entire substrate
surface by stepping the temperature between the liquid state and
supercritical state to cause a coating to drop out of solution. The
entire substrate is bathed in a dense fluid environment which is
then changed physicochemically by changing the chemistry of the
bulk fluid in a series of steps over time using bulk fluid
temperature changes from above and below the critical temperature
of the dense fluid. The process of '366 is inefficient because it
changes the properties of an entire fluid environment to process a
substrate surface--causing the coating to coat all surfaces of the
reactor and substrate simultaneously. By contrast, the object of
the present invention is to provide a selective method for
producing varying thickness of a coating over a portion of a
substrate surface while a dense fluid mixture traverses from the
center to the perimeter of a planar substrate.
[0032] In all of the prior art cited, none suggests the use of a
condensation shear cleaning technique whereby a condensable solvent
phase is selectively contacted with a substrate surface or portion
thereof to cause the solvent phase to "locally" condense due to
heat exchange with the substrate surface, while the substrate is
dynamic or static. Moreover, the prior art does not propose the
novel and beneficial aspects of combining plasmas with dense
fluids.
[0033] Another novel and beneficial aspect of the present invention
is that a supercritical fluid anti-solvent such as nitrogen or
argon provides enough vapor pressure to essentially condense and
contain most of the carbon dioxide compound as a liquid in a
hemisphere which is below the substrate. This allows for easy
recovery and reuse of most of the carbon dioxide through headspace
compression and minimizes joule-thompson cooling of the substrate
being treated during post-processing depressurization operations.
Because of a comparatively small ratio of dense
fluid-to-supercritical atmosphere (i.e., approx. 5 parts in 100) a
small quality of condensed dense fluid (liquid) is continuously
discharged from the bottom of the process chamber and only inert
supercritical atmosphere, having a much lower joule-thompson
coefficient than the dense fluid cleaning/deposition agent, remains
following each spray cleaning cycle.
[0034] An excellent description of a conventional coating process
is the photoresist deposition process and is detailed in VLSI
Fabrication Principles, Second Edition, Sorab K. Ghandi, summarized
as follows. In a conventional process, a film of photoresist
material is placed on a surface of the wafer which is covered by:a
masking film. It is desirable to have the photoresist film be very
uniform, highly adherent, and completely free of fine particulate
matter or pinholes. For example, a positive resist is applied using
a suitable organic carrier solvent. A pre-filtered resist-solvent
mixture is applied to the center point of a spinning wafer,
whereupon the coating mixture spreads outward in all directions
from the center to the perimeter of the wafer. The film thickness
is inversely proportional to the square root of the spin rate;
typically, spinning speeds range from 1000 to 6000 rpm and result
in film thickness that ranges from 0.5 to 3.0 um. Consistent
results are maintained only if the viscosity of the resist-solvent
mixture is maintained on a run-to-run basis. Extreme care must be
taken to clean and dry a wafer prior to photoresist deposition to
prevent adhesion problems and incorporation of surface particles.
Where surface adhesion is an issue due to, for example, the
presence of certain functional groups, clean and dry wafers may be
pre-wetted with a suitable organic coupling agent such as
hexylmethyldisilizane (HMDS), trichlorophenylsilane,
trichlorobenzene or xylene using a dipping or vapor-plating
technique. Ultra-clean conditions must be maintained during the
coating step to prevent the inclusion of atmospheric particles into
the coating matrix, caused by air turbulence from the spinning
wafer. The photoresist is sticky at this point and great care must
be taken to prevent particle contamination. Following deposition,
the coated wafer is softbaked, preferably from below to remove
excess organic solvent and vapors contained in the thin sticky
film, without forming a "skin" on top. The bake temperature may be
between 90 C. to 100 C. and causes the film to shrink to about 85%
of its original thickness.
[0035] Following cleaning, coating and softbake procedures, the
coated wafer is moved to a mask alignment, exposure with collimated
UV light, postbake, development, etching and stripping. Plasma
cleaning processes are widely used to remove patterned photoresist,
however radiation damage to MOS circuits can be a problem as well
as an inability to remove carbon-fluorine sidewall contaminants
produced by deep reactive ion etching (RIE) processes. These etch
residues must be removed prior to repeating the coating, softbake
and other lithographic processes described above. One aspect of the
present invention teaches removing RIE residues from plasma treated
substrates.
[0036] Following is a more detailed discussion of conventional
substrate cleaning, preparation and coating techniques in relation
to the present invention. This discussion will expose additional
novel and beneficial aspects of the present invention.
[0037] High Pressure Solvent Spray Cleaning
[0038] Solvent spray cleaning provides an alternative process for
removing particulate contaminants from flat surfaces. This
technique effectively removes submicron particles proportionally
with increasing spray pressures due to the production of high shear
stress within the surface boundary layer where small particles hide
out. To avoid recontamination of the surfaces, spray cleaning
should be practiced in a closed environment or under a flow of
clean inert atmosphere. Solvent spray cleaning techniques include
high pressure alcohol and fluorocarbon solvent spray, steam sprays,
dry ice pellet blasting and particle ice snow spray cleaning. These
techniques are useful for cleaning 2-D surfaces only and cannot
permeate of penetrate substrate pores, even at extremely high spray
pressures due to access problems. However, techniques such as
carbon dioxide snow cleaning, pellet cleaning, argon ice cleaning
are excellent polishing techniques for removing sub-micron
particles (particle cleans).
[0039] An aspect of the present invention is to optimize and
control particle drag or shear stress as a means for cleaning
and/or depositing a film on a planar substrate surface. Moreover,
cohesion energy of the same fluid spray may be controlled to
optimize penetration into a porous substrate surface to remove
trapped contaminants therein. Another novel aspect of the present
invention is to combine an atmospheric plasma process with a solid
phase spray head to combine the benefits of atmospheric plasma
cleaning with solid-phase dense fluid spray cleaning. A patented
and patents-pending TIG-Snow carbon dioxide spray technique
developed by the present inventor is used in an example final
particle cleaning device and technique, however any dielectric
spray technique described above such as argon ice, pellet carbon
dioxide and liquid nitrogen cleaning will work with an atmospheric
plasma to produce an ultraclean and particle free surface.
[0040] Plasma Cleaning
[0041] Plasma cleaning is a type of so called dry cleaning
procedure which has the advantage of being able to clean large
quantities of samples with little or no waste products and little
operator input. It also has the distinct advantage of directly
generating a clean, uniformly wettable and dry surface on the
substrate. The cleaning may involve an argon-oxygen plasma to
directly oxidize hydrocarbons, or an argon-only plasma to degrade
them and desorb them. Such processes are suitable for cleaning
surfaces and for activating polymer surfaces. The use of a plasma
bears the distinct advantage of being able to penetrate inside
complex structures. Generating plasma requires a specifically
designed reactor. Generally, the specific gas to be used is
injected to form an atmosphere at reduced pressure. Electromagnetic
radiation (RF or microwave frequency) is then coupled into the
enclosure, generating the plasma.
[0042] Some plasma systems operate at atmospheric pressure,
removing the need for a vacuum-sealed enclosure. Plasma cleaning
has an interesting feature, whereby its "effective" temperature
(its k.sub.bT equivalent thermal energy) is of several thousand
degrees. This is achieved by coupling energy into the gaseous phase
without strongly increasing the substrate temperature. As an
example, this process may clean carbon residue from surfaces
without excessive heating. The plasma cleaning techniques also find
application in the activation of other inorganic substrate surfaces
such as metals and organic polymer surfaces.
[0043] A beneficial aspect of the present invention is that one or
a combination of sub-atmospheric, atmospheric and super-atmospheric
plasma-aided processes can be performed. This gives a range of
beneficial plasma effects, for example producing supercritical
ozone' in high pressure plasmas produces a powerful in-situ
cleaning agent.
[0044] Plasma is loosely defined as a partially or wholly ionized
gas with a roughly equal number of positively and negatively
charged particles. It has been dubbed the "fourth state" of matter
with its properties being similar to those of a gas and liquid.
[0045] Various types of plasmas may be created depending upon
pressure, temperature, gas type, and frequency, among other facors.
Moreover, low and high temperature plasmas are formed using either
a low pressure or high pressure, respectively. High temperature
plasma is found at atmospheric or super-atmospheric pressures
between 1 atm and 100 atm or more with the beneficial cleaning
effect due primarily to high energy oxygen radicals or ozone. Low
temperature plasmas, used for surface modification and thin film
organic cleaning, are ionized gases generated at pressures between
0.1 and 2 torr. Low temperature plasmas work within a vacuum
chamber where atmospheric gases have been evacuated typically below
0.1 torr. These low pressures allow for a relatively long free path
of accelerated electrons and ions. Since the ions and neutral
particles are at or near ambient temperatures and the long free
path of the electrons, which are at high temperature or
electron-volt levels, have relatively few collisions with molecules
at this pressure the reaction remains at low temperature. By
contrast, atmospheric plasmas of nitrogen, oxygen and carbon
dioxide tend to dominated by reactive neutral species such as O
atoms, metastable 0.sub.2, and some 0.sub.3. High pressure plasma
reactions tend to be dominated by ozone or supercritical ozone. It
has been suggested that atmospheric plasmas are more similar to
low-pressure plasmas.
[0046] The ionization of the gases is accomplished by applying an
energy field using one of two frequencies regulated by the federal
government:
1 1. Radio frequency (RF) frequency - 13.56 MHZ 2. Microwave (MW)
frequency - 2.45 GHz.
[0047] RF plasmas exhibit significantly higher levels of
ultraviolet radiation (UV), which in part explains the higher
concentrations of electronically charged particles than found in
other plasma sources. RF plasmas have also been noted to be more
homogeneous, a trait that is critical in treating irregularly
shaped and overly large objects. MW source plasmas are generated
downstream or in a secondary environment. Downstream is defined as
the plasma generated in one chamber and drawn by a vacuum
differential into the work area or another chamber. Though this can
be advantageous for organic removal from ion sensitive components
it also produces a less homogeneous process resulting in the
compromising of uniformity across the work area. In surface
modification the effective depth of the modification is tens of
nanometers so the uniformity of the process becomes increasingly
important, rendering MW source plasmas a less desirable choice. The
voltage which must be exceeded to ignite a plasma is given by the
following equation:
V.sub.b=B(P d)/[In[A(P d)]-In[In(1+1/.gamma..sub.se))]]
[0048] Where
[0049] V.sub.b--breakdown voltage,
[0050] P--pressure,
[0051] d--gap distance,
[0052] .gamma.se--secondary emission coefficient of the cathode,
and
[0053] A and B are constants found experimentally.
[0054] Also, the type of dielectric fluid influences V.sub.b. When
a plasma interacts with a substrate surface and contaminants
thereon, four primary effects can occur:
[0055] 1. Removal of organic materials;
[0056] 2. Cross-linking via activated species of inert gases;
[0057] 3. Ablation; and
[0058] 4. Surface chemical restructuring.
[0059] A major problem that prevents adequate adhesion is the
presence of organic contamination on the surface. Contamination may
exist in the form of residues, mold release agents, anti-oxidants,
carbon residues or other organic compounds. Oxygen plasma is
excellent for removing organics and is commonly used for this
purpose. Oxygen plasma causes a chemical reaction with surface
contaminants resulting in their volatilization and removal from the
plasma chamber. Care must be taken in selection of cleaning process
parameters to ensure that organics are completely removed. It is
possible to "surface modify" the contamination instead of removing
it and thus still have a barrier layer which will cause adhesion to
fail. Critical parameters may include sufficient power density to
remove but not polymerize the organics or the addition of other
gases to facilitate the prevention of polymerization. When exposed
to the RF energy field, oxygen (02) is broken down into free
radicals at pressures above 0.1 torr is the most reactive element
in the plasma and will readily combine with any organic
hydrocarbon. The resultant combination is water vapor, CO and C02
which is carried away in the vacuum or fluid stream. The reaction
is by its nature complete with no residual surface products,
however non-organics such as salts are not so readily removed.
Sufficient RF energy must be applied to produce a high plasma
density. Lower power densities not only remove contamination at a
slower rate but also can actually impede the removal process. While
the top layers of organic are being removed with low power density,
underlying layers may cross-link in three dimensions creating a
stable but un-removed new structure.
[0060] When generating plasma using inert noble gases such as
helium or argon, the plasma will break C--C or C--H bonds by ion
and UV bombardment. These free radicals in turn recombine on the
surface causing a stable cross-linking of the surface structure.
The improved bond strength of the surface can be a very desirable
effect. In the case of PTFE (Teflon) treatment, it has been found
that pretreatment with helium followed by the plasma of ammonia
(NH3) will facilitate the bonding of a barrier layer to the PTFE,
which in turn will be receptive to adhesion.
[0061] Etching of surfaces also can also be accomplished by plasma.
Roughening of the surface can play a significant part in adhesion
by increasing the total contact area between the adhesive and the
subsurface. Etching is a result of gas selection or the length of
time the surface is exposed to the plasma. Ablation can be
accomplished with either active or inert gases and can be run to
excess causing extremely porous surfaces by too long of an exposure
to the plasma. The semiconductor manufacturing industry has used
plasma etching as a primary treatment method for over 20 years. In
addition the circuit board industry has used plasma as a means of
etching polymers smeared in the drilling process. Hole smears
prevent contact with plated through holes on multi-layer circuit
boards. Smears are easily removed by plasma ablation regardless of
how small the holes are.
[0062] Perhaps the plasma treatment effect offering the greatest
potential is modification of the surface structure. By adding polar
functional groups to the surface structure of the polymer we can
greatly increase the surface energy and thus aid the adhesion to
other substrate materials. Plasma treatment can be used to provide
for the oxidation of the surface in much more uniform methods than
by corona discharge or flame treatment. Large irregular surfaces
also can be treated with little possibility of over-treatment, a
drawback of both flame and corona methods. Elimination of adhesion
primers or promoters, typically organic chemicals, is a benefit of
using plasma prior to deposition of coatings.
[0063] Plasma reactions fall into two categories: chemical and
mechanical. Chemical reactions result from a chemical interaction
of the plasma with the surface of the product or contaminants
attached to the surface. These reactions include oxidation and
ablating the surface with such gases as oxygen, fluorine or
chlorine. Mechanical reactions are generated with the use of noble
gases such as argon or helium. Since these inert gases exist in
their monatomic state the reaction is a kinetic energy transfer or,
in simple terms a molecular scale sand blast. Dislodged
contaminants can be swept away in the vacuum stream before they
redeposit on the product or recombine on the product surface by
selecting the proper process parameters. Inert gas plasma is also
used to remove organic films and particulate matter from surfaces,
which might readily oxidize such as silver or copper.
[0064] Conventional plasma systems have the following basic
configuration:
[0065] 1. A vacuum chamber for the reaction.
[0066] 2. An energy source for gas ionization.
[0067] 3. Control circuitry to regulate the time, gas flow and
amount of energy.
[0068] 4. A vacuum system to provide the low-pressure
environment.
[0069] Chambers are manufactured in either metal or glass depending
on the application and the method of ionization. Quartz chambers
are used in highly critical environments where sub-micron
particulate generation is an issue. This includes the
semiconductor, hybrid and medical analysis industries. For
industrial applications, metal chambers are more prevalent and
allow for the rougher handling environment accompanying that
industry. Systems are even produced with tumbling chambers for
surface modification of a large volume of small parts. Aluminum
chambers offer an advantage over stainless steel chambers in that
aluminum will develop a natural oxide layer that becomes a tough
barrier to secondary reactions. Even the best stainless steel has
been known to oxidize in a plasma environment and over time the
oxidized surface can be a source of undesirable particulate.
[0070] Precision cleaning procedures using plasma are performed so
that the cleaned surface will be coated or otherwise used as soon
as possible after cleaning. Plasma surface treatment produces a
molecular clean and water wettable surface with high surface
energy. Thus, it has a tendency to adsorb particulate and organic
contamination from the ambient environment. Storing the surfaces
closed containers will reduce the adsorption of organic contaminant
molecules from the ambient air. Particulate contamination will
generally create unacceptable defects. Adsorbed organic contaminant
molecules, generally less than a full monolayer coverage, of order
nm thickness, will generate a heterogeneous wettability. This may
lead to non-uniform coatings, in particular if deposited from
liquid media. Surfaces can never be perfectly clean in ambient air.
It is generally recommended that the glass surface be used within a
few minutes following its cleaning. As such, it is desirable to
minimize the risk of coating defects caused by random ambient
contamination.
[0071] As noted, plasmas will crosslink an organic contaminant.
Crosslinking of thick films of organic contaminants such as oils
will hinder cleaning action because it creates a barrier (i.e.,
carbon) to subsequent plasma reactions. As such, plasma cleaning is
generally effective only when the surface is first gross cleaned to
remove contaminant film down to 100 microns or less, for example by
combining plasma with dense fluid cleaning--an important aspect of
the present invention.
[0072] Conventional plasma processing thus described is more
commonly called dielectric barrier discharge (DBD) plasmas which
are of the variety comprising silent discharge, corona, transferred
arc, plasma torch and low-pressure glow discharge. These plasmas
can be created and used under various pressures, temperatures and
using a variety of electrode configurations. Moreover, DBD plasma
behavior is observed in solid, liquid and gas phases. Atmospheric
pressure glow discharge (APGD) is getting attention today for
various applications ranging from air pollution treatment, ozone
production and more recently surface cleaning. However, high
pressure plasma processing (i.e., P>1 atm) is still a relatively
unexploited area. No known prior art exists for creating and using
dense fluid plasmas (i.e., supercritical fluid plasmas, solid spray
plasmas, liquid spray plasmas) taught herein.
[0073] The fundamental properties of DBD plasmas--for example the
production of excited species, radicals, ozone and ultraviolet
radiation--lead to photophysical and photochemical processes which
readily lend themselves to the development of new enhanced surface
cleaning, coating or modification processes, especially when used
in combination with the variable solvent geometry and many other
unique characteristics of dense fluids.
[0074] As such, novel and beneficial aspects of the present
invention are derived from the combination of dense fluid cleaning
solvents such as solid, liquid and supercritical carbon dioxide and
application techniques with various types of DBD plasmas (i.e.,
using different fluid combinations, fluid pressures, temperatures,
frequencies, voltages, DBD electrode configurations)--called dense
fluid plasma herein. Dense fluid plasma technology enables new
surface treatment possibilities not possible using either
technology alone. The unique properties of dense fluids, for
example being rather inert, having very low dielectric constants
(i.e., k<3), and behaving as cleaning solvents allow them to be
uniquely exploited herein as dielectric barrier discharge
compounds, cleaning agents, extraction agents, drying agents,
surface modification agents, deposition agents, plasma,
atmospheres, and anti-solvent agents within a full range of
temperatures and pressures. Moreover, exemplary plasma-aided dense
fluid processes taught herein are performed simultaneously or
sequentially in a single process reactor which keeps the substrate
surface clean, or un-compromised, in-between each process or
operation.
[0075] Another novel aspect of the present invention is that the
plasma process itself may be engineered to provide variable ion
concentration--a selective plasma surface treatment. Since ion flux
increases with decreasing pressure, adjusting pressure during
various uses of plasma in the present invention is a way of
controlling selectivity. This is very important in surface
treatment applications where a substrate surface, for example a
semiconductor wafer, contains both organic and inorganic
structures. These structures interact with and react to different
plasmas in different ways and at different reaction rates. For
example, a less energetic plasma (i.e., atmospheric or
superatmospheric plasma) may be more suitable for an organic
photoresist residue removal, when used with a dense fluid cleaning
agent, because it decreases the chance for ion damage of adjacent
inorganic or organic surface structures. Thus, having a selective
plasma treatment is very beneficial--having a selective dense fluid
plasma surface treatment technology is enabling.
[0076] Finally in another example the present aspect can uniquely
produce a 3-dimensionally ultraclean sterile implantable medical
device wherein its subsurface pores have been treated to remove as
much as 8% by weight of unreacted monomer oils, during which its
exposed surfaces are bathed in a sterilizing and surface energy
modifying plasma--all in a single operation. This new and improved
medical device exhibits enhanced performance such as increased
tensile strength which increases its longevity under stress,
extremely low potential for leaching of interstitial contaminants
(i.e., silicone fluids) into body fluids, its water-wettable
surfaces can be more readily adhesively bonded.
[0077] Spin Coating
[0078] Spin coating processes are used throughout many different
industries including, semiconductor wafer fabrication, optical
glasses, and LCD manufacturing under clean room conditions and with
fully automated handling. The coating thickness may vary between
several hundreds of nanometers and up to 10 micrometers. Even with
nonplaner substrates very homogeneous coating thickness can be
obtained. The quality of the coating depends on the rheological
parameters of the coating liquid. Another important parameter is
the Reynolds number of the surrounding atmosphere. If the rotation
velocity is in a range, that the atmospheric friction leads to high
Reynolds numbers (turbulences), disturbances in the coating quality
are observed. The final thickness of a spin coated layer on the
processing and materials parameters like angular velocity,
viscosity and solvent evaporation rate by the semi-empirical
formula;
H=A.omega..sup.-B
[0079] where
[0080] H--coating thickness,
[0081] .omega.--angular velocity; and
[0082] A and B are constants which are determined empirically.
[0083] Spin coating and spin cleaning technology as practiced by
the present inventor and others using dense fluids as noted herein
suffer from a lack of control of the various surfaces such as
atmosphere-dense fluid-substrate and other factors such as drag
force, viscosity, fluid chemistry and surface chemistry--all of
which are major components in optimizing and controlling the
production of thin or thick films and coatings and removing
undesirable substances from surfaces. The present invention teaches
methods and processes which overcome these limitations.
[0084] Chemical Coating
[0085] Chemical coating is a process where a chemical reaction,
e.g. the reduction of a metal is involved. The most common process
is the fabrication of mirrors where the glass surface acts as a
nucleating agent for the reduction of Ag.sup.+ to Ag.sup.0 in
presence of reducing agent. The vast majority of all mirrors still
are fabricated using this process. Another technology, which is
suitable as an example for precipitating copper layers on glass, is
the currently metalization process with commercially available
liquids after seeding of the surface.
[0086] The present invention is teaches methods and processes for
producing chemical coatings on substrate surfaces using a process
of first coating a substrate with an unreacted substance and then
selectively reacting said first coating with a secondary agent such
as plasma, hydrogen, and others to produce the desired chemical
coating.
[0087] Patterning Drying and Curing Techniques
[0088] Another aspect of the present invention is precision
patterning, drying and curing of coated critical substrates such as
IC, optical and MEMS wafers. Drying and curing techniques are
important for obtaining the appropriate coating properties, as in a
lithographic patterning or developing operations. The process may
be made very selective through patterned laser exposure.
Furthermore, organic polymer or organic-inorganic hybrid coating
materials can be cured by a low temperature IR treatment or
UV-curing, or reacted with a plasma. The present invention provides
methods for selective removal of resists and resist solvents from
patterned wafers as well as plasma etch clean-up techniques.
[0089] As line sizes becomes smaller and the complexity of
semiconductor integrated circuits increases, it is clearly
desirable to have a wet processing technique, including a method
and apparatus, that actually removes unwanted organic films and
particles, prevents additional particles, and does not introduce
stains on the wafers. The complete cleaning technique may also
include a step of gross drying the wafers, without other adverse
results. A further desirable characteristic includes reducing or
possibly eliminating the residual water absorbed on wafer surfaces
and edges when the gross water phase is removed. The water left
absorbed on such surfaces and edges often attracts and introduces
more particles onto the semiconductor wafer and is a outgas and
adhesion contamination in subsequent photoresist deposition
following cleaning and drying operations. The aforementioned
conventional techniques fail to provide such desired features,
thereby reducing the die yield on the semiconductor following
optical printing or lithographic processes.
[0090] There is a present need to provide an alternative and
integrated surface preparation and deposition technique which
overcomes the limitations of conventional technology described
above. As such, the present invention relates to the fields of
surface cleaning, surface modification, and deposition. The present
invention is illustrated in several examples including surface
cleaning, surface modification and deposition of an organic or
metal-organic film or coating upon a substrate surface. The
exemplary substrate surface is a wafer containing a surface mask
film such as silica, polysilicon, silicon nitride, silicides, and
metals. However it will be recognized that the invention has a
wider range of applicability. Merely by way of example, the
invention can also be applied to deposition of organic precursors,
lubricants, bactericidal agents and other substances wherein a thin
film is desired.
[0091] The present invention incorporates a unique isobaric dense
fluid condensation shear cleaning process, a novel plasma-assisted
drying process, plasma-based pre-cleaning and post-treatment steps,
a dense fluid condensation shear deposition process, and
post-cleaning and deposition reaction steps such as thermal
treatment, plasma cleaning, and reactive gas treatments, among many
other aspects. For example, the present invention teaches
performing multi-step processing of dense fluid cleaning and plasma
cleaning in a single process tool. In still another example, the
present invention teaches a method, process and apparatus which
embodies precision cleaning, drying, surface modification, and
deposition within a closed controlled environment
SUMMARY OF THE PRESENT INVENTION
[0092] The term `dense fluid` is used herein to describe
physicochemical states of carbon dioxide, ozone, nitrogen, xenon,
argon, and helium, and other fluids, wherein these physicochemical
states have densities that are within the range of liquid-like or
near-liquid substances; or which behave like liquids in that they
will mix or separate due to similar or dissimilar cohesive energy
properties, respectively. Giddings gives an equation for
calculating cohesive energy for non-polar compressed gases;
.delta.=2.56(.rho..sub.c).sup.1/2(.rho..sup.r.sub.g/.rho..sup.r.sub.l);
[0093] where
[0094] .delta.--cohesive energy,
[0095] P.sub.c--critical pressure of compressed gas,
[0096] .rho..sup.r.sub.g--reduced density of compressed gas,
and
[0097] .rho..sup.r.sub.g--reduced density of compressed gas,
liquid.
[0098] Liquids and compressed gases used in the present invention
will generally have cohesion energies within the range of 5
MPa.sup.1/2 (i.e., supercritical argon at 25 C. and 200 atm) to 47
MPa.sup.1/2 (i.e., water at 25 C. and 1 atm). As a general rule,
where polar energy and hydrogen bonding energy contributions are
insignificant, substances with dispersive energy differences of 5
MPa.sup.1/2 or more are considered to have limited or no
miscibility and will exist as two phases. This is the case for
compounds used to form a supercritical fluid anti-solvent and a
dense fluid solvent phase for the present invention. Liquid phase
compounds having energy contributions due to polarity, complex
formation or hydrogen bonding, for example water and methyl
alcohol, will exhibit a range of solubility behavior and equations
by Hildebrand and others may be used to compute a solubility
parameter for these. Organic solids such polymers will exhibit
cohesion energies in the general range of 18 MPa.sup.1/2 to 28
MPa.sup.1/2 whereas inorganic solids such as silicon and copper
exhibit cohesion energies of 300 MPa.sup.1/2 or more. More solid
phase carbon dioxide exhibits a cohesion energy of approximately 22
MPa.sup.1/2. As such dense fluids herein describe liquid,
supercritical and solid phase dielectric solvents.
[0099] As such, conducting cleaning, etching, drying, surface
modification and coating reactions under dense fluid conditions and
in mixed phase treatments of the present invention affords
opportunities to manipulate the reaction environment (interfacial
solvent and solvent-substrate surface properties). The reaction
environment can be manipulated, via pressure, temperature and type
of fluids present, to enhance, or reduce, solubility of reactants
and solutes, to modify reaction rates, or descrease particle
adhesion on surfaces. Moreover, supercritical cleaning and coating
treatments of the present invention affords much improved
selectivity by altering the physicochemistry of interfacial
constituents intimate with the substrate surface and atmosphere
above and below the substrate being treated.
[0100] For example, removal of submicron particles requires
significant drag or shearing energy, sometimes described as
particle shear stress, at the particle-substrate surface interface.
In the present invention, a shear stress sufficient to overcome
small particle adhesive forces may be applied by the viscous drag
force generated by a high velocity dense fluids flowing over the a
substrate surface. Dense fluid velocity and density are manipulated
in the present invention through phase transitions at the substrate
surface to increase viscous drag. Viscous drag force is calculated
according to the following equation:
F.sub.d=(C.sub.p)(V.sup.2/2)(A);
[0101] where
[0102] F.sub.d--viscous drag force,
[0103] C--drag force coefficient,
[0104] .rho.--dense fluid density,
[0105] V--local fluid velocity, and
[0106] A--projected frontal area of small particle.
[0107] In the present invention, fluid density and hence viscous
drag force is increased by a factor of 2 or more by changing a
stream of dense fluid flowing over a substrate surface from
supercritical phase to liquid phase. Moreover, the reverse
transition, that is liquid phase to supercritical phase, also
increases drag force significantly through a decrease in local
fluid viscosity. In fact, the predominant factors which increase
viscous drag force are to decrease fluid viscosity and increase
free stream velocity.
[0108] Carbon dioxide exists as a low-density gas at standard
temperature and pressure conditions and possesses phase boundaries
with a triple point (Solid-Liquid-Gas coexist in equilibrium like a
glass of ice cubes and water) and a critical point (Liquid-Gas have
identical molar volumes). Through pressure or temperature
modification, carbon dioxide can be compressed into a dense gas
state. Compressing carbon dioxide at a temperature below its
critical temperature (C.T.) liquefies the gas at approximately 70
atm. Cooling liquid-state or gas-state carbon dioxide to its
freezing point causes a phase transition into solid-state carbon
dioxide. Compressing carbon dioxide at or above its critical
temperature and critical pressure (C.P.) also increases its density
to a liquid-like state, however there is a significant difference
between compression below and above the critical point. Compressing
carbon dioxide above its critical point does not effect a phase
change. In fact, carbon dioxide at a temperature at or above 305 K.
(88 F.) cannot be liquefied at any pressure, yet the density for
the gas may be liquid-like. At the critical point the density is
approximately 0.47 g/ml. At or above this point carbon dioxide is
termed a supercritical fluid (SCF). Carbon dioxide, and other dense
fluids such as nitrous oxide, sulfur hexafluoride, ammonia, xenon
and various hydrocarbon gases, exhibit significant cohesive energy
(>12 MPa.sup.1/2) near the critical point, which is around
ambient temperature for most of the compounds cited above.
[0109] Similarly, nitrogen exists as an inert gas phase at
atmospheric temperature and pressure. Nitrogen can be compressed to
near or above its critical point through pressure and temperature
adjustment. The critical pressure and temperature for nitrogen are
33.5 atm and -147 C., respectively. Nitrogen, and compounds such as
argon and helium, are chosen as supercritical fluid ant-solvents
for use in the present invention because they can be compressed to
supercritical conditions. However nitrogen, argon and helium do not
exhibit strong cohesive energy (<5 MPa.sup.1/2) under the
supercritical carbon dioxide temperature conditions of the present
invention because they are so far above their own critical
temperatures.
[0110] Supercritical carbon dioxide and supercritical nitrogen can
be compressed to a range of liquid-like densities, yet it they
retain the diffusivity of a gas. For example, compression of carbon
dioxide above its critical temperature causes a progressive
increase in density, approaching that of its liquid phase.
Combining supercritical fluids such as carbon dioxide and nitrogen
or argon provides unique stratification and interfacial behavior
which is very different from conventional standard
temperature-pressure solvent mixtures or using liquefied gases as
solvents. Cohesion energy and density differences between the dense
fluids can be varied using pressure and/or temperature. This
provides the ability to manipulate interfacial chemistry near the
substrate surface and within the feed solvent phase.
[0111] The present invention exploits the ability to separate two
dense gases based upon divergent cohesive energy characteristics.
Using pressure and temperature control, carbon dioxide is
compressed and heated to a liquid or supercritical fluid solvent
phase--in a range of pressures and temperatures from just below to
above the critical point. Similarly, nitrogen or argon may be
pressurized and heated to form a supercritical fluid phase
anti-solvent phase--in a range of pressures and temperatures below,
equal to or above the critical point for carbon dioxide. The
atmospheric and fluid phases each exhibit divergent cohesive
energy--the nitrogen phase exhibits anti-solvent behavior with the
carbon dioxide phase. Solvent-Anti-solvent supercritical fluid
systems can be produced using other gas mixtures, for example
nitrous oxide (solvent)--argon (nonsolvent), carbon dioxide
(solvent)--argon (non-solvent).
[0112] The present invention utilizes preferably liquid or
supercritical carbon dioxide as a cleaning or deposition agent and
solvent phase to deliver solutes such as organic solvent modifiers,
surface cleaning enhancement agents, coatings, wetting agents,
surface treatment agents or reaction precursors, into an
established atmosphere comprising an anti-solvent such as
supercritical nitrogen or argon. Other suitable inert gases may be
used if they can be compressed to a supercritical fluid condition
within a closed environment and exhibit non-solvent or anti-solvent
selective behavior with the applied dense fluid solvent phase.
Thus, a supercritical fluid anti-solvent environment (i.e., SCAr,
SCHe, SCN.sub.2) is used to selectively `precipitate` solutes
contained or entrained within a liquid or supercritical dense fluid
solvent phase, and/or to selectively change the density and
cohesive energy of the dense fluid solvent phase applied to a
substrate. For example, the supercritical anti-solvent can be
controlled to cause a quantity of supercritical fluid to
selectively condense to a liquid phase or vapor phase upon contact
with a substrate surface.
[0113] Supercritical carbon dioxide, which may entrain or dissolve
one or more solutes such as organic solvents, coatings, ozone, is
injected into a supercritical anti-solvent atmosphere in close
proximity to but above a rotating substrate, whereupon through
various control parameters such as, for example, solvent phase
chemistry, substrate spin rate, supercritical atmosphere
(non-solvent) pressure and temperature, and electric field, unique
and beneficial substrate-solvent-solute interfacial phenomenon can
be produced.
[0114] A first aspect of the present invention utilizes a novel
isobaric and super-atmospheric vapor condensation mechanism for
both cleaning and deposition processes and methods described
herein: An examination of the following commercial cleaning and
coating processes utilizing atmospheric and sub-atmospheric
condensation processes will aid in understanding the novel features
and benefits of the present invention.
[0115] Vapor Degreasing--Under constant atmospheric pressure, a
condensable cleaning vapor is formed above a sump by boiling a
cleaning solvent such as trichloroethane within a tank. A
substrate, which is cooler than the vapor, is lowered into the
clean condensable vapor above the boiling liquid, whereupon the
clean vapor condenses uniformly over the substrate, dissolving
contaminants and shearing particles from the substrate surfaces.
This occurs because the vapor is saturated and all that is
necessary is a temperature shift to cause it to change phase to
liquid. Dirty condensed liquid flows downward under the influence
of gravitational force, drips from the substrate and returns back
into the boiling tank below. This process can continue until the
substrate temperature equals the vapor temperature.
[0116] Chemical Vapor Deposition--Similar to vapor degreasing above
and under constant subatmospheric pressure (i.e., vacuum), a heated
atmosphere containing a condensable component is contacted with a
substrate, which is at a temperature lower than the vapor
temperature. This causes a uniform fraction of chemical vapor to
condense onto the substrate. Vapor concentration, vacuum pressure,
substrate-vapor temperature differential, and contact time are
typical process control parameters in this analog condensation
process.
[0117] As such, the first aspect of the present invention is a
process called critical fluid condensation shear cleaning. While a
substrate is spun on an axis (either a horizontal and/or vertical
orientation respective to earth plane), supercritical carbon
dioxide, which may contain one or more cleaning enhancement
additives such as supercritical ozone, organic solvents or trace
water, is selectively contacted, under vapor saturation conditions,
on a portion of a rotating substrate. Under the action of
centripetal force, the supercritical fluid solvent phase rapidly
moves outward over the substrate surface filling all surface
features with supercritical fluid cleaning agent. During this, heat
transfer between the supercritical fluid solvent phase and the
substrate surface, both below and above, is selectively controlled
to manipulate the solid-solvent-antisolvent interfacial
physicochemistry.
[0118] For example, if under relatively constant supercritical
argon pressure conditions and which is above or equal to the
critical pressure but below the temperature of the supercritical
carbon dioxide solvent being applied and if the substrate surface
temperature (Ts) is less than the supercritical fluid solvent phase
temperature (Tdf) being applied to it, that is Ts<Tdf, the
interfacial density, viscosity and cohesive energy will increase
and the fluid phase will transition to liquid phase--or condense
following contact or during spreading. During this process, the
supercritical fluid is moving over a substrate surface outwardly,
first contacting and filling microscopic voids, cracks and vias and
then condensing within these surface features. Under constant
centripetal force, a very beneficial and unique particle wetting
and surface shear stress cleaning action is constantly being
produced and applied--called critical fluid condensation shear
cleaning, or deposition, processes herein.
[0119] The entire substrate surface may be heated from a lower
temperature to the temperature of the supercritical fluid solvent
being applied over the surface, in which case condensation shear
cleaning ceases. The substrate surface temperature heats from the
center contact point in a radial direction outward toward the
perimeter of the substrate and reaches equilibrium with the spray
solvent temperature in this manner. This is much more efficient
than changing the temperature of the dense fluid surrounding the
substrate. Alternatively, the substrate may be cooled from below
using the condenser to produce a continuous condensation shear
cleaning action over the substrate being treated. Thus, by varying
the cooling rate below the substrate, the topside surface
condensation shear process may be controlled.
[0120] A particular advantage of the present invention as opposed
to prior art is the application of a relatively small quantity of
dense fluid (solvent) to the topside surface of a rotating
substrate within an established and substantially inert
(anti-solvent) supercritical nitrogen (or other suitable dense gas
anti-solvent) environment. For example, dense fluid solvents chosen
for the present invention have higher densities, and very divergent
cohesion energies, than the dense fluid anti-solvents. Because of
the differences in dielectric properties between and overall poor
thermal conductivities of the dense fluids chosen for the present
invention, the processing environment can be segregated into
distinct zones which have different properties and are used to
control certain aspects of the exemplary treatment processes
described herein.
[0121] For example, selectively applying heat energy to an upper
portion or zone of the antisolvent atmosphere (Ta), where
Ta>Tdf, and extracting heat from the lower portion or zone of
the anti-solvent atmosphere (T<Tscf), the supercritical carbon
dioxide may be selectively condensed to a liquid-phase within a
lower hemisphere of the process chamber and collected
simultaneously during the application of the supercritical fluid
spray to the substrate surface above. Due to the poor thermal
conductivity between the anti-solvent phase and solvent phases of
the present invention, a temperature gradient can be established in
the process chamber under relatively constant pressure. Thus, a
thermal gradient and a constant pressure (isobaric) process
condition can be maintained within the chamber and between a dense
fluid supply tank during processing. An anti-solvent supercritical
fluid zone is established above and surrounding the substrate to
selectively alter the interfacial physicochemistry (change cohesion
energy or change phase) of a dense fluid solvent spray and
substrate surface.
[0122] In addition, and simultaneously with the application of the
supercritical fluid solvent, a lower temperature zone may be
established below the substrate surface to collect reacted dense
fluid as a `condensed phase`, which separates readily within the
process chamber due to both density and cohesive energy increases.
Thus, another advantage of the present invention is that a variety
of dense fluid solvent pressures, temperatures and conditions can
be applied to and reacted with the surface of a substrate under
isobaric conditions and using pulsation, gravity flow, or high
pressure spray techniques. Moreover, another advantage of the
present invention is that the process chamber does contain a
significant quantity of liquefied dense fluid phase carbon dioxide
or other spent dense fluid solvent at any given time. Contaminants,
water and particles contained within a hemisphere below a substrate
and in a very small quantity of dense fluid relative to the volume
of the process chamber are much less likely to become entrained
within the anti-solvent phase due to significant differences in
cohesion energy, density, and viscosity. Therefore the use of a
condensed-phase zone as taught herein serves as a liquid trap for
reaction by-products and prevents re-deposition of contaminants
onto substrate surfaces above.
[0123] In another aspect of the present invention, a substrate is
plasma cleaned in a supercritical argon atmosphere at a temperature
which is below the critical temperature and greater than the
critical pressure of carbon dioxide. Under these conditions, the
substrate is rotated in a clockwise or counterclockwise direction.
The impingement spray, in this case liquid carbon dioxide, is
introduced at a pressure that is equal to or greater than the
internal anti-solvent atmosphere. In fact, the dense fluid spray
pressure and temperature can be applied at much greater pressure
than the internal pressure of the anti-solvent atmosphere. During
application of the dense fluid jet spray, a high voltage high
frequency showerhead electrode is energized. The supercritical
argon and liquid carbon dioxide acts as a dielectric barrier fluid,
through which a uniform plasma discharge is created. The presence
of oxygen (carbon dioxide is a source of oxygen) in the dense fluid
fluid flow over the substrate results in the generation of oxygen
radicals and supercritical ozone which serve as powerful surface
cleaning adjuncts.
[0124] Moreover, plasma adjuncts of the present invention may be
used under sub-atmospheric and atmospheric conditions which
provides a capability producing clean or etched substrates using
combinational cleaning under different pressure, temperature, fluid
conditions and with different gaseous and liquid admixtures. This
capability allows the operator to develop any variety of highly
selective substrate treatment processes incorporating cold and
thermal plasma treatments with liquid and supercritical phase dense
fluid treatments. Still moreover, plasma-aided deposition processes
may be developed with the dense fluid cleaning treatments.
[0125] Process variables for the present invention include, among
others, dense fluid solvent spray pressure, temperature, and
flowrate, substrate rotational velocity, and supercritical fluid
anti-solvent pressure, chemistry (type of gas or mixture), pressure
and temperature, and electric field strength (plasma density).
These variables must be optimized for each type of substrate
treated and for each type of treatment process developed--for
example a "dry-clean-deposit" or a "clean-etch-clean" treatment
combination. For example, it may be optimal to slowly flow
supercritical carbon dioxide containing a deposition compound over
a cold substrate (e.g. 15 C.) which is rotated at a low angular
velocity (e.g. 10 rpm) to produce a desired coating thickness. In
another example, it may be optimal to spray a substrate with high
pressure liquid carbon dioxide over a superheated substrate (e.g.
100 C.) rotating at very high angular velocity (e.g. 2000 rpm) to
remove tenacious surface contaminants.
[0126] Following is an example method which illustrates a first
aspect of the present invention. It can be used as a replacement
for conventional organic solvent vapor degreasing and drying
techniques.
[0127] An exemplary critical fluid condensation shear cleaning
method comprising the following steps:
[0128] 1. Place a substrate into a pressure vessel in any fixture
and at any orientation horizontal or vertical and relative to the
earth plane and optionally rotate the substrate about its central
axis at between 1 and 5000 rpm clockwise, counter-clockwise or
bi-directionally, and;
[0129] 2. Establish a first supercritical fluid anti-solvent phase
within said pressure vessel which completely surrounds and contacts
said substrate surfaces at pressures and temperatures above said
anti-solvent phase critical points, and;
[0130] 3. Contact said substrate surfaces or a portion of said
substrate surfaces thereof contained in said first supercritical
fluid anti-solvent phase with a continuous or pulsed stream of
pre-determined quantity of condensable second phase of
supercritical fluid solvent phase, which is of substantially less
volume than that of the first anti-solvent phase, and which may
contain one or more cleaning enhancement additives, to produce a
film of advancing supercritical fluid cleaning solvent and
additives on said substrate surface or portion of said substrate
surface, and;
[0131] 4. Selectively alter the physicochemistry of said second
supercritical solvent phase at said substrate surfaces or portion
thereof (cleaning zone) at relatively constant pressure to form a
third advancing and condensing solvent phase on said substrate
surfaces or portion of said substrate surfaces to separate
substrate surface contaminates to form a clean substrate, and;
[0132] 5. Control said first supercritical fluid anti-solvent phase
contained within said pressure vessel to have a temperature which
is less than, equal to or greater than the temperature of said
substrate surfaces in a zone above said substrate surface, and;
[0133] 6. Control said first supercritical fluid anti-solvent phase
contained within said pressure vessel to have a temperature which
is less than the temperature of said substrate in a zone below said
substrate surface, and;
[0134] 7. Capture said third condensed solvent phase, contaminates
and additives at a constant pressure within a zone of said pressure
vessel which is below said substrate, and;
[0135] 8. Immediately remove said third condensed solvent phase,
contaminates, and additives from said pressure vessel to prevent
re-contaminating the process chamber and substrate, and;
[0136] 9. Repeat steps 3 through 9 as required to produce a clean
substrate, and;
[0137] 10. Remove first supercritical fluid anti-solvent phase from
said pressure vessel, and;
[0138] 11. Remove said cleaned substrate from said pressure
vessel.
[0139] Exemplary Definitions for First Aspect
[0140] Substrates and substrate surfaces and portions thereof may
include any one or combination of wafers, semiconductors, pores,
vias, trenches, planes, interfaces, optical components, MEMS,
metals, ceramics, glasses, polymers, organics, inorganics,
biomedical, optoelectronic and others.
[0141] Substrate fixtures may include one or a combination of spin
processor, temperature controlled plates, drums, baskets, rack or a
shelf.
[0142] First supercritical anti-solvent phase compounds include one
or a combination of supercritical nitrogen, supercritical argon,
supercritical oxygen, supercritical xenon, and supercritical
helium.
[0143] Second supercritical solvent phase compounds include one or
a combination of supercritical carbon dioxide, supercritical
nitrous oxide, and supercritical hydrocarbons.
[0144] Third condensed solvent phase compounds include one or a
combination of liquid carbon dioxide, liquid nitrous oxide, and
liquid hydrocarbons.
[0145] Additives may include any one or combination of gases,
liquids, solids, organics, inorganics, ionics, non-ionics, oxygen,
fluorinated compounds, polymers, organic coupling agents, solvents,
water, electromagnetic energy, acoustic energy, ultraviolet
radiation, microwaves, and infrared heating energy.
[0146] Contaminates may include any one or combination of carbon,
fluorine, water, ionics, nonionics, metals, oxides, hydrocarbons,
fluorocarbons, organics, inorganics, radioactive, biological,
particulate and others.
[0147] A second aspect of the present invention is the capability
to dissolve or entrain coatings and other substances in a
supercritical solvent phase such as supercritical carbon dioxide
and selectively deposit said solutes onto a substrate surface. The
process of selectively depositing a substance onto a substrate
surface is similar to the condensation shear cleaning mechanism
described above, and is called condensation shear deposition
herein.
[0148] Supercritical carbon dioxide or another suitable
supercritical solvent, entraining or dissolving various components
such as organic solvents and coating resins is injected onto a
portion of a rotating substrate, whereupon through the various
control parameters described above, the solutes can be selectively
deposited onto the substrate surface as the supercritical fluid
solvent phase is distributed across the substrate surface.
[0149] Under the action of centripetal force or the flow of the
applied dense fluid, the supercritical fluid solvent phase rapidly
moves outward over the substrate surface filling all substrate
surfaces with supercritical fluid deposition agent. During this,
heat transfer between the supercritical fluid solvent phase and the
substrate surface, both below and above, is selectively controlled
to manipulate the substrate solid-dense fluid interface.
[0150] For example, if under relatively constant supercritical
nitrogen pressure conditions in which the supercritical nitrogen
pressure is above or equal to the critical pressure of the
supercritical carbon dioxide fluid solvent and the supercritical
nitrogen temperature is greater than the temperature of the
supercritical carbon dioxide solvent being applied to the surface,
and if the substrate surface temperature (Ts) is greater than the
supercritical fluid solvent phase temperature (Tscf) being applied
to it, that is Ts>Tscf, the interfacial density, viscosity and
cohesive energy will decrease and the fluid phase cohesion energy
will decrease--or selectively condense solute from solution during
contact or during spreading.
[0151] During this process, the supercritical fluid is moving over
a substrate surface outwardly, first contacting and filling
microscopic voids, cracks and vias and then condensing within these
surface features. Moreover, under constant centripetal force, the
anti-solvent (supercritical nitrogen) diffuses into the film of
spreading supercritical fluid causing rapid changes in solvent
power. Also, as in the example for the cleaning aspect described
above, the supercritical fluid solvent phase containing the coating
agent may be adjusted to cause a phase transition from
supercritical fluid to liquid phase as the solvent phase flows over
the substrate being treated.
[0152] An example method which illustrates a second aspect of the
present invention follows. It can be used as a replacement for
conventional spin coating and chemical vapor deposition
techniques.
[0153] An exemplary critical fluid condensation shear deposition
method comprising the following steps:
[0154] 1. Place a substrate into a pressure vessel in any fixture
and at any orientation horizontal or vertical and relative to the
earth plane and optionally rotate the substrate about its central
axis at between 1 and 5000 rpm clockwise, counter-clockwise or
bi-directionally, and;
[0155] 2. Establish a first supercritical fluid anti-solvent phase
within said pressure vessel which completely surrounds and contacts
said substrate surfaces at pressures and temperatures above said
anti-solvent phase critical points, and;
[0156] 3. Contact said substrate surfaces or a portion of said
substrate surfaces thereof contained in said first supercritical
fluid anti-solvent phase with a continuous or pulsed stream of
predetermined quantity of condensable second phase of supercritical
fluid solvent phase, which is of substantially less volume than
that of the first anti-solvent phase, and which contains one or
more deposition compounds and agents or additives, to produce a
film of advancing supercritical fluid deposition solvent phase on
said substrate surface or portion of said substrate surface,
and;
[0157] 4. Selectively alter the physicochemistry of said second
supercritical deposition solvent phase at said substrate surfaces
or portion thereof (deposition zone) at relatively constant
pressure to form a third advancing and condensing deposition
solvent phase on said substrate surfaces or portion of said
substrate surfaces to selectively and uniformly deposit a fraction
of deposition compound or agent contained therein onto separate
substrate surfaces or portions thereof to form a coated substrate,
and;
[0158] 5. Control said first supercritical fluid anti-solvent phase
contained within said pressure vessel to have a temperature which
is less than, equal to or greater than the temperature of said
substrate surfaces in the zone above said substrate surface,
and;
[0159] 6. Control said first supercritical fluid anti-solvent phase
contained within said pressure vessel to have a temperature which
is less than the temperature of said substrate in a zone below said
substrate surface, and;
[0160] 7. Capture said third condensed deposition solvent phase
containing excess deposition compounds and agents at a constant
pressure within a zone of said pressure vessel which is below said
substrate surfaces and portions thereon, and;
[0161] 8. Immediately remove said third condensed deposition
solvent phase and excess deposition compounds and agents from said
pressure vessel to prevent re-contaminating the process chamber and
substrate, and (exemplary follow-on procedures 9,10,11,12);
[0162] 9. Repeat steps 3 through 9 as required to produce a desired
thickness of deposition compound or agent on said substrate
surfaces or portions thereon, and;
[0163] 10. Repeat steps 3 through 9 with deposition additives such
reducing agents to form a reduced coating on said substrate
surfaces or portions thereon and/or;
[0164] 11. Repeat steps 3 through 9 with deposition additives such
infrared heat to form a semirigid coating on said substrate
surfaces or portions thereon and/or;
[0165] 12. Repeat steps 3 through 9 with deposition additives such
plasma to form a coating on said substrate surfaces or portions
thereon and;
[0166] 13. Remove first supercritical fluid anti-solvent phase from
said pressure vessel, and;
[0167] 14. Remove said coated substrate from said pressure
vessel.
[0168] Exemplary Definitions of Second Aspect
[0169] Substrates and substrate surfaces and portions thereof may
include any one or combination of wafers, semiconductors, pores,
vias, trenches, planes, interfaces, optical components, MEMS,
metals, ceramics, glasses, polymers, organics, inorganics,
biomedical, optoelectronic and others.
[0170] Substrate fixtures may include one or a combination of spin
processor, temperature controlled plates, drums, baskets, rack or a
shelf.
[0171] First supercritical anti-solvent phase compounds include one
or a combination of supercritical nitrogen, supercritical argon,
supercritical oxygen, supercritical xenon, and supercritical
helium.
[0172] Second supercritical deposition solvent phase compounds
include one or a combination of supercritical carbon dioxide,
supercritical nitrous oxide, and supercritical hydrocarbons.
[0173] Third condensed deposition solvent phase compounds include
one or a combination of liquid carbon dioxide, liquid nitrous
oxide, and liquid hydrocarbons.
[0174] Deposition compounds and agents may include one or more
organic, inorganic, polymers, resins, wetting agents, adhesion
promoters, metal-organic compounds.
[0175] Additives and agents may include any one or combination of
gases, liquids, solids, organics, inorganics, ionics, non-ionics,
oxygen, reducing agents, oxidizing agents, fluorinated compounds,
polymers, solvents, water, electromagnetic energy, acoustic energy,
ultraviolet radiation, microwaves, and infrared heating energy.
[0176] A third aspect of the present invention is to form a
sub-atmospheric, atmospheric or superatmospheric plasma above the
substrate prior to, during or following condensation shear cleaning
and coating methods described above. Moreover, the third aspect of
the present invention may be used without first forming a
supercritical fluid anti-solvent atmosphere and without using the
condensation shear cleaning and deposition methods herein.
[0177] An example method which illustrates a third aspect of the
present invention follows. It can be used as a replacement for
conventional plasma treatment techniques. A pulsed or continuous
electric field of between 500 and 250,000 volts, and at a frequency
of between 100 KHz and 10 GHz, may be selectively applied to a
substrate surface within a dense fluid process chamber using a
suitable dielectric barrier discharge electrode configuration.
Various electrode arrangements are possible to establish an
electric field suitable for performing any one or a combination of
the following plasma-aided dense fluid processes, among others:
[0178] 1. Sub-atmospheric plasma cleaning and etching
[0179] 2. Atmospheric plasma cleaning and etching
[0180] 3. Super-atmospheric (supercritical plasma) cleaning,
extraction, drying and etching
[0181] 4. Plasma-aided deposition processes described herein.
[0182] An exemplary critical fluid condensation shear plasma
cleaning method comprising the following steps:.
[0183] 1. Place a substrate into a pressure vessel in any fixture
and at any orientation horizontal or vertical and relative to the
earth plane and optionally rotate the substrate about its central
axis at between 1 and 5000 rpm clockwise, counter-clockwise or
bi-directionally, and;
[0184] 2. Establish a first supercritical fluid anti-solvent phase
within said pressure vessel which completely surrounds and contacts
said substrate surfaces at pressures and temperatures above said
anti-solvent phase critical points, and;
[0185] 3. Contact said substrate surfaces or a portion of said
substrate surfaces thereof contained in said first supercritical
fluid anti-solvent phase with a continuous or pulsed stream of
predetermined quantity of liquid carbon dioxide to form a solvent
phase, which contains one or more cleaning enhancement compounds
and agents or additives, to produce a film of advancing liquid
carbon dioxide solvent phase on said substrate surface or portion
of said substrate surface, and;
[0186] 4. Pulse or continuously energize an electrode positioned at
a pre-determined distance from and orientation to said substrate at
a voltage of between 500volts and 250,000 volts at a frequency of
between 100 KHz and 10 GHz and a power of between 50 and 5000
watts, and;
[0187] 5. Selectively alter the physicochemistry of said solvent
phase at said substrate surfaces or portion thereof (plasma
cleaning zone) at relatively constant pressure to form a third
advancing plasma solvent phase on said substrate surfaces or
portion of said substrate surfaces to selectively and uniformly
clean or etch or react a substrate surface or portion thereof,
and;
[0188] 6. Control said first supercritical fluid anti-solvent phase
contained within said pressure vessel to have a temperature which
is less than, equal to or greater than the temperature of said
substrate surfaces in the zone above said substrate surface,
and;
[0189] 7. Control said first supercritical fluid anti-solvent phase
contained within said pressure vessel to have a temperature which
is less than the temperature of said substrate in a zone below said
substrate surface, and;
[0190] 8. Capture said third condensed plasma solvent phase
containing excess cleaning enhancement compounds and contaminants
at a constant pressure within a zone of said pressure vessel which
is below said substrate surfaces and portions thereon, and;
[0191] 9. Immediately remove said third condensed plasma solvent
phase and contaminants from said pressure vessel to prevent
re-contaminating the process chamber and substrate, and (exemplary
follow-on procedures 10,11);
[0192] 10. Repeat steps 3 through 9 as required to produce a
desired cleanliness or etched surface on said substrate surfaces or
portions thereon, and;
[0193] 11. Repeat steps 3 through 9 with deposition additives to
form a plasma-reacted coating on said substrate surfaces or
portions thereon and/or;
[0194] 12. Remove first supercritical fluid anti-solvent phase from
said pressure vessel, and;
[0195] 13. Remove dense fluid plasma cleaned substrate from said
pressure vessel.
[0196] Exemplary Definitions of Second Aspect
[0197] Substrates and substrate surfaces and portions thereof may
include any one or combination of wafers, semiconductors, pores,
vias, trenches, planes, interfaces, optical components, MEMS,
metals, ceramics, glasses, polymers, organics, inorganics,
biomedical, optoelectronic and others.
[0198] Substrate fixtures may include one or a combination of spin
processor, temperature controlled plates, drums, baskets, rack or a
shelf.
[0199] First supercritical anti-solvent phase compounds include one
or a combination of supercritical nitrogen, supercritical argon,
supercritical oxygen, supercritical xenon, and supercritical
helium.
[0200] Second plasma cleaning solvent phase compounds include one
or a combination of supercritical carbon dioxide, supercritical
nitrous oxide, and supercritical hydrocarbons.
[0201] Third condensed plasma solvent phase compounds include one
or a combination of liquid carbon dioxide, liquid nitrous oxide,
and liquid hydrocarbons. Cleaning enhancement agents may include
one or more oxygen, hydrogen peroxide, hydrogen, nitrous oxide.
[0202] Additives and agents may include any one or combination of
gases, liquids, solids, organics, inorganics, ionics, non-ionics,
oxygen, reducing agents, oxidizing agents, fluorinated compounds,
polymers, solvents, water, electromagnetic energy, acoustic energy,
ultraviolet radiation, microwaves, and infrared heating energy.
[0203] In another example, supercritical carbon dioxide is
selectively contacted with a substrate may be subjected to a strong
electric field to form a supercritical plasma. During this process,
the supercritical fluid is moving over a substrate surface. Plasma
conditions will produce in-situ radicals such as oxygen radicals
and supercritical ozone within the flowing stream of dense fluid
(dielectric fluid). These radicals significantly enhance the
removal of recalcitrant residues such as carbon-fluorine compounds
(CF.sub.x), common following photoresist removal processes using
fluorine-aided (CF.sub.4) vacuum plasma etching processes.
Moreover, the presence of strong electric fields assists with
decreasing the surface tension of water trapped in Vias and between
silicon interfaces such as those found in MEMS devices. As such, a
novel use for an electric field in the present invention is to
efficiently clean and precision dry semiconductor and MEMS devices
Thus, the third aspect of the present invention is to produce
different beneficial plasmas by varying temperatures, pressures,
and dense fluid combinations, and using dense fluids as dielectric
barrier materials which aids in removing or depositing compounds to
and from substrate surfaces.
[0204] A fourth aspect of the present invention is combination of
the methods described herein to provide a completely integrated
substrate cleaning, surface modification and deposition system.
This may include integrating conventional wet acid etching,
cleaning and rinsing procedures into the dense fluid system,
followed by dense fluid drying and cleaning methods described
herein. It may also include post-treatment methods for dense fluid
deposition methods described herein. For example, to produce a
soft-baked or heat cured coating or may include the step of
re-cleaning or dense fluid solvent developing (i.e., patterning)
the reacted and coated substrate. In another example, reactants may
be introduced, for example a fluorine gas, to substrate having a
uniform coating of polyalkylether or polyarylether film to produce
a fluoropolymeric coating having improved solvent resistance
properties or a very low dielectric constant. These methods may be
performed within the same process chamber but using different dense
fluid combinations, atmospheric pressures and temperatures, and
treatment sequencing. For example, dense fluid solvent washing and
developing may be performed using the condensation shear cleaning
process above. Heat curing can then be performed under atmospheric
or vacuum conditions using the present invention by heating the
atmosphere over and below the coated substrate using exemplary
substrate temperature control systems. Finally, a vacuum may be
created and a partial pressure of gaseous reactants may be
introduced over the substrate with a plasma to provide etching or
planarization.
[0205] Thus the present invention has extensive utility in that it
can be used to clean, etch, disinfect and sterilize, coat and react
substrates in the same process chamber exploiting the unique
properties of dense fluids. Moreover, the present invention allows
for the integration of other wet and dry processes and process
chemistries such as etching agents, acid treatments, alkaline
treatments as well as reactive gases under dense fluid, atmospheric
and vacuum conditions. This has the added benefit of clustering
many wet and dry processes into one integrated cleaning-substrate
treatment-drying production tool. This capability provides multiple
surface cleaning and treatment capabilities into a small footprint
in production environments--which lowers cost and increases
performance.
[0206] A further understanding of the nature and advantages of the
invention may be realized by reference to the latter portions of
the specification and attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0207] These and other objects and advantages of the present
invention will be obvious to those of ordinary skill in the art
after having read the following detailed description of the various
aspects, which are illustrated in the various figures.
[0208] FIG. 1a--Exemplary graphic showing the relationships between
a mixture of anti-solvent, solvent, substrate and contaminant.
[0209] FIG. 1b--Exemplary mixed phase diagram showing phase
relationships between a mixture of supercritical anti-solvent,
dense fluid solvent, and contaminant (water) under various
pressures and temperatures. This figures also shows the pressure
and temperature boundary for using the present invention.
[0210] FIG. 2--Exemplary graphic describing a condensation isobar
for supercritical fluid spray onto a substrate.
[0211] FIG. 3--Exemplary graphic describing a phase change isobar
for liquid spray onto a substrate.
[0212] FIG. 4a--Exemplary graphic describing selective critical
fluid condensation shear cleaning phenomenon for the
supercritical-to-liquid substrate surface transition.
[0213] FIG. 4b--Exemplary graphic describing selective critical
fluid condensation shear cleaning phenomenon for the
liquid-to-liquid substrate surface transition.
[0214] FIG. 4c--Exemplary graphic describing selective critical
fluid condensation shear cleaning phenomenon for the
liquid-to-supercritical substrate surface transition.
[0215] FIG. 5--Exemplary graphic describing the selective critical
fluid condensation shear deposition phenomenon.
[0216] FIG. 6a--Exemplary graphic describing drag forces exerted on
a substrate surface during condensation shearing phenomenon for a
supercritical-to-liquid phase transition.
[0217] FIG. 6b--Exemplary graphic describing drag forces exerted on
a substrate surface during condensation shearing phenomenon for a
liquid-to-supercritical fluid phase transition.
[0218] FIG. 7a--Exemplary graphic describing selective alteration
of cohesive energy of dense fluid during condensation shearing for
a supercritical-to-liquid phase transition.
[0219] FIG. 7b--Exemplary graphic describing selective alteration
of cohesive energy of dense fluid during condensation shearing for
a liquid-to-supercritical phase transition.
[0220] FIG. 8a--Exemplary diagram showing the relationship between
permittivity, cohesion energy, viscosity, and surface tension for
water (exemplary contaminant) within a supercritical nitrogen
anti-solvent processing atmosphere.
[0221] FIG. 8b--Exemplary diagram showing the relationship between
electric field strength, pressure, temperature and surface tension
for water (exemplary contaminant) exposed to a dense fluid carbon
dioxide solvent.
[0222] FIG. 8c--Exemplary diagram showing the synergistic
mechanisms employed in the present invention to enhance the
precision drying of a substrate surface.
[0223] FIG. 9--Exemplary diagram showing a dense fluid showerhead
reactor.
[0224] FIG. 10--Exemplary dense fluid reactor zones.
[0225] FIGS. 11a, 11b and 11c--Exemplary graphic showing a
substrate loading, closure actuation, and locking sequences for an
exemplary dense fluid spin processor reactor.
[0226] FIG. 12--Exemplary isobaric dense fluid delivery and
management system.
[0227] FIGS. 13a, 13b, 13c and 13d--Exemplary schematic of
operation of a Class 10 robotic pick-and-place system with FOUP
load and unload modules for practicing the present invention using
a single dense fluid spin processor reactor.
[0228] FIG. 14--Exemplary schematic of a Class 10 robotic
pick-and-place systems with FOUP load and unload modules for
practicing the present invention using a dense fluid spin processor
reactor followed by an exemplary atmospheric plasma snow spray
particle cleaning module.
[0229] FIG. 15--Exemplary cleaning method combining a wet acid
treatment followed by a dense fluid dry and clean.
[0230] FIG. 16--Exemplary cleaning method combining a condensation
shear cleaning with various in-between plasma treatments.
DETAILED DESCRIPTION OF THE INVENTION
[0231] The isobaric processing environment created for performing
the various exemplary aspects of the present invention is
established using a mixture of two dense fluids--one serving as an
anti-solvent and the other serving as the solvent.
[0232] Referring to FIG. 1a, the exemplary anti-solvent
phase-solvent phase cleaning and deposition environment is created
by first fluidizing a pressure vessel (2), shown schematically,
with a first dense fluid to create a supercritical processing
atmosphere (4). Into the supercritical processing atmosphere (4), a
second dense fluid is selectively introduced to form a
supercritical, liquid or plasma processing fluid (6) therein and
which is generally introduced at a location between the processing
atmosphere (4) and a substrate surface and sub-surfaces (8)
containing various contaminant layers (10) thereon. Thus as shown
in the graphic, the exemplary processing environment is developed
as discrete layers having surfaces located between each layer,
creating two important and general layers described as an
anti-solvent phase (12) and a solvent phase (14). Cohesive energies
for the various layers show a distinct pattern--generally being
very low, or non-solvent (<12 MPa.sup.1/2), in the anti-solvent
phase and very high in the solvent phase (<12 MPa.sup.1/2). An
exemplary processing environment is given in the graphic
showing:
2 Processing atmosphere (anti-solvent): supercritical argon (16);
Processing fluid (solvent): liquid carbon dioxide (18);
Contaminant: liquid water (20); Substrate/Surfaces: silicon,
oxides, metals (22)
[0233] FIGS. 1a and 1b graphically describe the variety and nature
of isobaric processing environments may be created using the
present invention. Moreover, any number of possible substrate,
substrate surfaces and contaminants may be processed using the
present invention.
[0234] FIG. 1b shows a mixed phase diagram for argon, carbon
dioxide and water. Shown in the figure are the critical point for
argon (24), critical point for carbon dioxide (26), and the
critical point for water (28). Although not the actual liquid-vapor
boundary for each fluid or fluid-mixture represented, the
approximate liquid-vapor boundaries are represented from the
critical point to the atmospheric boiling point for each fluid and
with the pure-component supercritical regions represented by the
vertical and horizontal axes originating at the critical points for
each pure component. The pressure-temperature (P-T) boundary (30)
in the present invention ranges in pressures from 100 mTorr (32) to
250 atm (36) and in temperatures ranging from 10 C. (38) to 300 C.
(38). This P-T boundary (30) allows for the establishment of
several different combinations of processing atmosphere (FIGS. 1a,
4) and processing fluid (FIGS. 1a, 6). For example, processing
atmospheres may comprise gaseous argon- gaseous carbon dioxide (40)
for sub-atmospheric and atmospheric plasma cleaning, supercritical
argon-gaseous carbon dioxide (42) for super-atmospheric plasma
cleaning, supercritical argon-liquid carbon dioxide (44) for plasma
and condensation shear cleaning, and supercritical
argon-supercritical carbon dioxide (46) for plasma and condensation
shear cleaning. Exemplary dense fluid anti-solvents suitable for
use with the present invention include helium, argon, nitrogen,
oxygen, and mixtures thereof. Exemplary dense fluid solvents
suitable for use with the present invention include carbon dioxide,
nitrous oxide, ammonia, fluorocarbons, xenon, and sulfur
hexafluoride.
[0235] FIG. 2 shows temperature profiles produced when contacting a
supercritical dense fluid to a substrate having an controlled
temperature and an uncontrolled temperature while contained in a
supercritical anti-solvent. Referring to FIG. 2, once a suitable
anti-solvent environment is established, a dense fluid solvent may
be selectively contacted with a substrate surface. A dense fluid
solvent such as supercritical carbon dioxide may be contacted with
a substrate which is held at a temperature (Ts) which is below the
critical temperature (Tc) of the dense fluid (48). During
application of the supercritical carbon dioxide solvent over a
period of time (50), the substrate surface having a starting
temperature (52) will rise (54) and reach a second equilibrium
temperature (56) equilibrium at some point during the application
time (50). This rate of increase and final equilibrium temperature
is determined by the rate of heat extraction from the substrate.
Under these conditions, the supercritical fluid is continuously
condensing along the substrate from supercritical fluid phase to
liquid phase. However if the substrate is not actively cooled and
starts under the conditions of Ts<Tc, and as the supercritical
fluid continues to bathe the substrate, the substrate temperature
will rise in temperature (58) until it reaches an equilibrium
temperature (60) with the supercritical carbon dioxide solvent.
When the temperature rises to or above the critical temperature of
the dense fluid (48), the solvent will cease condensation.
[0236] FIG. 3 shows temperature profiles produced when contacting a
liquid state dense fluid to a substrate having an controlled
temperature and an uncontrolled temperature while contained in a
supercritical anti-solvent. Referring to FIG. 3, once a suitable
anti-solvent environment is established, a dense fluid solvent may
be selectively contacted with a substrate surface. A dense fluid
solvent such as liquid carbon dioxide may be contacted with a
substrate which is held at a temperature (Ts) which is above the
critical temperature (Tc) of the dense fluid (48). During
application of the liquid carbon dioxide solvent over a period of
time (50), the substrate surface having a starting temperature (62)
will fall (64) and reach a second equilibrium temperature (66)
equilibrium at some point during the application time (50). This
rate of decrease and final equilibrium temperature is determined by
the rate of heating the substrate. Under these conditions, the
liquid carbon dioxide is continuously changing phase from liquid to
supercritical state with the substrate. However if the substrate is
not actively heated and starts under the conditions of Ts>Tc,
and as the liquid carbon dioxide continues to bathe the substrate,
the substrate temperature will decrease in temperature (68) until
it reaches an equilibrium temperature (70) with the liquid carbon
dioxide solvent. Once the temperature of substrate decreases to or
below the critical temperature of the dense fluid (48), the solvent
will remain as a liquid phase following contact with the substrate
surface.
[0237] FIGS. 4a, 4b and 4c are graphics showing exemplary
mechanisms involved with selective condensation shear cleaning.
[0238] Referring to FIG. 4a, an exemplary substrate semiconductor
wafer (72) containing a microvia structure (74) and a contaminant
(76) therein is contained within a supercritical argon processing
atmosphere (78) at 75 atm and 25 C. Supercritical carbon dioxide
(80) is applied over the substrate surface (82) which spontaneously
spreads and condenses (84) over the substrate surface (82), filling
the exemplary microvia structure (74), and causing a shear stress
force and change in cohesion energy to be developed at the
substrate surface-contaminant interface (75), entrains or dissolves
the contaminant (76) contained therein. Under the shear stress
induced by the condensing fluid flow and/or centripetal force (if
the substrate is rotating about an axis), the condensed liquid
phase and entrained or dissolved contaminant (86) and is separated
from the substrate surface (82). To maintain the continuous
condensation shear cleaning action thus described, the substrate
may be rotated about an axis at an angular velocity of between 10
and 5000 rpm and heat (88) is continuously extracted from the
substrate (72) or substrate surface (82).
[0239] Referring to FIG. 4b, an exemplary substrate semiconductor
wafer (90) containing a low-k porous film (92) and a contaminant
(94) therein is contained within a supercritical nitrogen
processing atmosphere (95) at 65 atm and 20 C. Liquid carbon
dioxide (96) is applied over the substrate surface (98) which
spontaneously penetrates and wets (100) the substrate surface (98),
filling the exemplary porous film (92), and causing a shear stress
force and change in cohesion energy to be developed at the
substrate surface-contaminant interface (102), entrains or
dissolves the contaminant (94) contained therein. Under the shear
stress induced by the condensing fluid flow and/or centripetal
force (if the substrate is rotating about an axis), the liquid
phase with entrained or dissolved contaminant (104), is separated
from the substrate surface (98) and porous film (92). To maintain
the continuous shear cleaning action thus described, the substrate
may be rotated about an axis at an angular velocity of between 10
and 5000 rpm.
[0240] Referring to FIG. 4c, an exemplary substrate semiconductor
wafer (106) having a porous coating (108) entraining a contaminant
(110) therein is contained within a supercritical argon processing
atmosphere (112) at 80 atm and 40 C. Liquid carbon dioxide (114) is
applied over the substrate surface (116) which spontaneously
penetrates, spreads and changes phase (118) over the substrate
surface (116), filling the exemplary porous coating (108) with
supercritical carbon dioxide, and causing a shear stress force and
change in cohesion energy to be developed at the substrate
surface-contaminant interface (120), entrains or dissolves the
contaminant (110) contained therein. Under the shear stress induced
by the supercritical fluid flow and/or centripetal force (if the
substrate is rotating about an axis), the solvent phase with
entrained or dissolved contaminant (122) is separated from the
substrate (106). To maintain the continuous and selective phase
change cleaning action thus described, the substrate may be rotated
about an axis at an angular velocity of between 10 and 5000 rpm and
heated (124) continuously at the substrate (106) or substrate
surface (116).
[0241] Having thus described the exemplary condensation shear
cleaning mechanisms, following is a discussion of an exemplary
condensation shear deposition mechanism.
[0242] Referring to FIG. 5, an exemplary substrate semiconductor
wafer (126) having a cleaned, treated or otherwise modified
substrate surface (128), which may have been produced using one or
more of the cleaning methods described in this invention, is
contained within a supercritical argon processing atmosphere (130)
at 80 atm and 25 C. Supercritical carbon dioxide (132) containing
an deposition compound or agent (134) is applied over said
substrate surface (128) which spontaneously spreads and condenses
(136) over the substrate surface (128), causing a shear stress
force and change in cohesion energy to be developed at the
substrate surface-deposition compound interface (138), causing the
entrained or dissolved deposition compound (134) to deposit onto
the substrate surface (128). Under the shear stress induced by the
condensing fluid flow and/or centripetal force (if the substrate is
rotating about an axis), the condensed liquid phase and remaining
fraction of entrained or dissolved deposition compound (140) is
separated from the substrate (126). To maintain the continuous
condensation shear deposition action thus described, the substrate
may be rotated about an axis at an angular velocity of between 10
and 5000 rpm and heat (142) is continuously extracted from the
substrate (126) or substrate surface (128).
[0243] Having thus described the exemplary condensation shear
cleaning and deposition mechanisms, following is a discussion of
selective control of both shear stress and cohesion energy at dense
fluid solvent-substrate surface interfaces.
[0244] FIGS. 6a and 6b show schematically, the influence on drag
force for a supercritical-to-liquid phase and a
liquid-to-supercritical phase transitions, respectively.
[0245] Referring to FIG. 6a, an advancing stream of supercritical
fluid (144) transfers heat (146) to a substrate surface (148)
which, according to FIG. 2, causes the supercritical fluid (144) to
condense to liquid carbon dioxide (150). Small contaminant or
deposition compound particles (152) experience an increasing shear
stress due to an approximately two-fold increase in fluid
density.(.rho.). Also, the free stream contact angle (154) is
increasing which exerts an additional removal force on a surface
particle. Moreover, an advancing and increasing contact angle aids
in physical separation of liquids such as water from a substrate
surface, in essence lifting a residue from the substrate
surface.
[0246] Referring to FIG. 6b, an advancing stream of liquid phase
solvent (156) extracts heat (158) from a substrate surface (160)
which, according to the FIG. 3, causes the liquid phase solvent
(156) to change phase to a supercritical fluid solvent (162). Small
contaminant or deposition compound particles (164) experience an
increasing shear stress due to an decrease in fluid
viscosity.(.mu.), which increases local fluid velocities (V). Also,
the free stream contact angle (166) is decreasing which exerts an
additional removal force on a surface particle. Moreover, an
advancing and decreasing contact angle aids in physical separation
of liquids such as particles from a substrate surface, in essence
pushing a residue along the substrate surface.
[0247] FIGS. 7a and 7b show schematically, the influence on
cohesion energy for a supercritical-to-liquid phase and a
liquid-to-supercritical phase transitions, respectively.
[0248] Referring to FIG. 7a, an advancing stream of supercritical
fluid (168) transfers heat (170) to a substrate surface (172) which
causes the supercritical fluid (168) to condense to liquid carbon
dioxide (170). Small contaminant or deposition compound particles
(174) experience an increase in solvent cohesion energy (.delta.).
An increase or decrease in the solubility of the particle (174) in
the dense gas solvent will be produced depending upon the
differences between the cohesion energy of the gas solvent (170)
and contaminant-deposition compound particle (174).
Moreover,substrate surface (172) coatings (not shown) may be
affected, for example they may experience an increase or decrease
in interaction (i.e., penetration or swelling) with the dense gas
solvent (170) due to a dramatic change in cohesion energy.
[0249] Referring to FIG. 7b, an advancing stream of liquid phase
solvent (176) absorbs heat (178) from a substrate surface (180)
which causes the liquid phase solvent (176) to change phase to a
supercritical fluid solvent (182). Small contaminant or deposition
compound particles (184) experience a decrease in solvent cohesion
energy (.delta.). An increase or decrease in the solubility of the
particle (184) in the dense gas solvent will be produced depending
upon the differences between the cohesion energy of the gas solvent
(182) and contaminant-deposition compound particle (184). Moreover,
substrate surface (180) coatings (not shown) may be affected, for
example they may experience an increase or decrease in interaction
(i.e., penetration or swelling) with the dense gas solvent (182)
due to a dramatic change in cohesion energy.
[0250] With reference to FIGS. 8a, 8b and 8c, following is a
discussion of various other physical and chemical mechanisms of the
present invention which, besides exemplary removal mechanisms
discussed above, enhance the removal of polar substrate
contaminants such as liquid water. Water is a common contaminant
found on substrate surfaces. As discussed herein, water is a
by-product of plasma reactions and is always present in microscopic
quanities in microvias or between microscopic structures following
deionized water rinsing and nitrogen drying operations. Moreover,
water is present in varying degrees in the atmosphere and is
readily condensed or adsorbed onto substrate surfaces. The present
invention is particularly suited to performing precision drying to
remove polar liquid contaminants such as water and alcohols.
[0251] Referring to FIG. 8a, liquid water, although not highly
soluble in carbon dioxide from a cohesion energy perspective, can
be altered to enhance its solubility in non-polar dense fluid
solvents. Heating liquid water (186) trapped in a capillary (188)
from 25 C. and 1 atm (190) to 250 C. and 100 atm (192), while
steadily increasing the vapor pressure using a supercritical
nitrogen atmosphere (194) to maintain liquid phase, causes profound
changes in the physicochemistry of water. Water's structure is
changed which is exhibited as lower cohesion energy, from 47
MPa.sup.1/2 to approximately 30 MPa.sup.1/2. Moreover, surface
tension decreases from 73 dynes/cm to 30 dynes/cm and permittivity
decreases from 78 to 35. Finally, viscosity decreases from
approximately 1 mN-s/m.sup.2 to less than 0.2 mN-s/m.sup.2. In
effect, the liquid water trapped within the capillary is exhibiting
a chemistry similar to a mixture of methanol and water. Thus, water
solubility in a dense fluid solvent may be increased by increasing
both the pressure and temperature of the anti-solvent phase.
[0252] Referring to FIG. 8b, an investigation of the effect of
carbonating superheated water using the exemplary isobaric dense
fluid and plasma cleaning processes herein reveals that the
cohesion energy of water can be further altered to enhance its
solubility in a non-polar dense fluid solvent phase. Moreover,
research suggests that a strong electric field, present when using
high pressure plasma processes herein, can have a profound effect
on the surface tension of liquid water. Selective carbonation of
trace liquid water (186) trapped in a capillary (198) from 25 C.
and 1 atm (200) to 35 C. and 100 atm (202), while steadily
increasing the vapor pressure using a supercritical argon
atmosphere (204) to maintain liquid phase, causes profound changes
in the physicochemistry of water. As discussed herein, dense fluid
solvent phase, in this case supercritical carbon dioxide (206), may
be selectively contacted with the substrate surface (208) while
maintained under supercritical anti-solvent pressure and
temperature conditions. Water's structure is selectively changed
when the supercritical carbon dioxide (206) penetrates and reacts
with the water (196), for example forming Lewis acid-base
complexes. This is exhibited as lower surface tension, which
decreases from 73 dynes/cm to <35 dynes/cm due to high pressure
carbonation alone. Note that pressurizing water with gases such as
helium, nitrogen and argon do not produce an equivalent and
beneficial change in water's properties. Finally, the presence of a
strong electric field, created by the plasma processes of the
present invention, produces electric charges on the surface film of
water (210), which reduces the surface tension of water to less
than 5 dynes/cm, depending upon the strength of the electric field.
In effect, carbonated and electrified liquid water trapped within
the capillary is exhibiting properties similar to a supercritical
fluid. Thus, polar liquid contaminant solubility in a non-polar
dense fluid solvent may be increased by selectively contacting the
polar liquid with a more reactive dense fluid (i.e., carbon
dioxide) and can be further increased by simultaneously applying an
electric field or super-atmospheric plasma to the substrate
surface.
[0253] Thus referring to FIG. 8c, a polar contaminant such as water
can be selectively modified to enhance its solubility in a
non-polar dense fluid solvent phase by increasing temperature
(212), using a more reactive dense fluid such as supercritical or
liquid carbon dioxide (214), applying an electric field of several
thousand volts (216). Moreover, a small amount of polar additive
such as methanol may be added to the dense fluid, or alternatively
the substrate surface may be doped with a polar additive, prior to
dense fluid extraction processes herein.
[0254] Having thus described the exemplary condensation shear
cleaning and deposition mechanisms with consideration given to
optimizing contaminant separation, following is a discussion of
exemplary mechanical apparatus for use with the present
invention.
[0255] FIG. 9 is a cut-away graphical side view of exemplary
components and integration scheme for constructing a dense fluid
showerhead reactor for performing the methods and processes of the
present invention. Referring to FIG. 9, the exemplary reactor
comprises a pressure vessel (218) constructed for example of
stainless steel, for which the interior surfaces (220) exposed to
supercritical fluid anti-solvent, dense fluid solvent and plasma
energy, may be alternatively coated or lined with various
protective materials including silica, glass, fluorocarbon coating,
ceramics and anodized aluminum.
[0256] The exemplary reactor pressure vessel is constructed to have
interlocking closure (222) and base (224) sections. The closure
section (222) may house an insulated plasma showerhead (226), which
may contain coolant lines (228) or an active showerhead electrode
heating and cooling system (not shown), a dense fluid solvent inlet
line (230), a supercritical fluid anti-solvent inlet line (232),
and a sealed high voltage electrode assembly (234) designed
specifically for conveying high voltage electricity into pressure
or vacuum chambers (supplier: Conax Buffalo, Buffalo, N.Y.). The
exemplary electrode (234) communicates a high frequency and high
voltage energy to the exemplary showerhead electrode (226) using a
solid or coaxial conductor (236). The reactor base section (224)
may house a grounded plate (238) onto which a substrate such as a
semiconductor wafer (240) is mounted. Below the plate (238) is a
heating element (242) which is used to heat the plate (242), which
in turn heats the exemplary substrate (240). Alternatively, and not
shown in the figure, the heated electrode plate system thus
described may be replaced with a temperature controlled electrode
plate to provide either heating or cooling for performing various
aspects of the present invention. Below the exemplary heater (242)
is placed a circular and perforated attenuation plate (244) which
separates a condensation zone (246) from a reaction zone (248).
Within the condensation zone (246) is placed an exemplary spiral
condenser coil (250) or other cooling device (i.e., thermal
electric cooler). Finally, the exemplary reactor contains an
exhaust or drain port (252) which is used to convey purge gases,
evacuated atmospheres, spent dense fluid solvents and additives,
reaction by-products, and supercritical fluid anti-solvent from the
reactor. The exemplary closure (222) may be connected (254) and
disconnected (256) to the exemplary base section (224) using an
actuator and locking mechanism (both not shown). Moreover, the
exemplary showerhead electrode (226) may be perforated (258) to
provide an even flow (259) of dense fluid solvent and supercritical
fluid anti-solvent atmosphere over the substrate (240) or the dense
fluid inlet pipe (230) may be fed through the center of the
electrode (260). A perforated showerhead would be useful for
substrates which are fixed to heated manifold as shown. A
centralized dense fluid inlet pipe configuration would be useful
for depositing a quantity of dense fluid onto the centermost
portion of a substrate surface (262) which is rotated about its
axis using a spin processor (not shown). In either case, the
supercritical fluid anti-solvent inlet line (232) may be used with
a perforated electrode as described herein.
[0257] Moreover, the electrode (226) may be coated with a
dielectric barrier material such as Teflon or ceramic, or as a
novel aspect of the present invention may be an uncoated stainless
steel with the exemplary supercritical fluid anti-solvents and
dense fluid solvents as used herein acting as a dielectric barrier
material. Thus a number of planar and cylindrical dielectric
barrier discharge configurations, and hence plasmas, are possible
with the present invention.
[0258] An earth ground connection (264) to the substrate mounting.
plate electrode (238) completes the circuit necessary to establish
a working plasma within the reactor. The distance (266), denoted as
He herein, between the substrate surface (268) and the bottom
surface of the showerhead electrode (270) is critical as it relates
to plasma energy density, electric field strength, deposition
phenomenon and cleaning phenomenon associated with the present
invention. As such, Hc may be adjusted to have a value of 1 mm to
virtually any distance desired and required for a particular
substrate or substrate electrode configuration. Finally, the
showerhead electrode (226) should be completely sheathed (272) in
an electrical insulation material such as a fluorocarbon polymer or
ceramic such as aluminum oxide to prevent spurious electrical
discharges to the reactor walls. Moreover, the dense fluid solvent
line (230), supercritical fluid anti-solvent (232) fluid and
coolant line (228) connections to the showerhead electrode should
be made using non-conducting and non-contaminating tube materials
such as fluorocarbon or polyetheretherketone (PEEK). A more general
discussion of the important aspects of the design and operation of
the exemplary dense fluid reactor follows.
[0259] Plasma showerhead reactors such as the exemplary reactor of
FIG. 10 employ a perforated or porous planar surface to dispense
reactant gases more-or-less uniformly over a second parallel planar
surface. Such a configuration can be used for batch processing of
multiple substrates, but also lends itself to processing of single
round wafers. Wafers can be heated separately from the dense fluid
dispense and reactor chamber wall, so dense fluid showerhead
reactor may be a cold-wall: only the fixture surface holding the
substrate need be at the process temperature. This fact is often
helpful in keeping the substrate cleaner during spray cleaning
operations herein. Coolant may be provided to help in controlling
the temperature of the showerhead. Many conventional deposition
processes employ precursors which can exist as liquids at room
temperature (TEOS and related organosilanes and siloxanes, TDEAT
and TDMAT, most copper precursors, WF6, etc.). When working with
such materials it is often useful to warm the walls of the chamber
above ambient temperature to avoid any possibility that the
precursors may condense on the walls or inside the showerhead
plenum. The top and bottom electrodes may be constructed to be
about the same size. Note that if one wishes to ground the wafer
electrode, which is often convenient for safety and wafer loading,
the showerhead has RF power applied: it is then necessary to
incorporate appropriate insulating sections in the gas supply
system, to avoid creating a parasitic discharge in the gas feed
lines to the chamber.
[0260] The diameter of a showerhead reactor is determined by the
size of the wafer or substrate batch to be processed; in
particular, single-wafer reactors usually employ a bottom electrode
similar in size to the wafer.
[0261] A very important chamber design parameter is the ceiling
height or electrode gap, which is denoted as Hc in FIG. 9. As noted
above, changes in Hc can have important effects on the plasma
density and potential. This is extremely important in optimizing Hc
for a particular pressure. As noted herein, as pressure within the
reactor increases, an increase in voltage is required to ignite a
plasma. As such, the distance of Hc may be decreased or the voltage
increased.
[0262] Finally, changing Hc is often important in controlling the
radial uniformity of the deposited film thickness. For all these
reasons it is useful to be able to change Hc easily during process
development, either through simple hardware changes or by providing
adjustable mounting of the substrate electrode.
[0263] The fluid hole configuration of the perforated showerhead is
often important in determining, for example, uniformity of
deposition and plasma plume. It is therefore useful to design the
showerhead so that the perforated faceplate is easily removed and
another of possibly different design substituted. Changes in the
plenum, such as the addition of blocking plates or changes in the
gas inlet configuration, can also be used to improve uniformity.
Operation at high dense fluid pressures requires that the holes be
small, as otherwise a localized hollow cathode discharge may occur
in the holes, causing localized heating and erosion. Although not
shown in the FIG. 9, electrode configurations may include stainless
steel or aluminum electrodes with a perforated dielectric barrier
material such as a clean ceramic membrane sandwiched against the
metal surface. The ceramic may contain capillary gas channels which
produce plasma jets when a dense fluid or atmospheric fluid is
jetted through the capillary channel when plasma power is applied
to the metal plate above.
[0264] It is always necessary to be able to access the chamber area
for cleaning and maintenance. As such, the showerhead reactor
should be configured with a lift or hinge, so that the showerhead
may be lifted away from the process reactor vessel. A design option
is to introduce dense fluids and gases into the process chamber to
one or more holes in the faceplate where the top and bottom seal
together: a small o-ring mounted around the holes allows a
corresponding hole in the top plate to form a sealed passage for
gas supplies when the chamber is assembled, without impeding
disassembly. The consequent "dead space" between the sealing faces
within the o-ring formed when the chamber is reassembled must be
carefully purged before processing if moisture-sensitive processes
are to be performed.
[0265] Having thus described the exemplary dense fluid reactor for
use with certain aspects of the present invention, following is a
more generalized discussion of the design aspects of the exemplary
apparatus of FIG. 9.
[0266] FIG. 10 graphically shows various reactor zones created when
constructing and using the exemplary apparatus of FIG. 9. Shown in
the figure are the various major components discussed in FIG. 9.
These include dense fluid inlet ports (230,232), showerhead
electrode (226), substrate (240), substrate electrode/fixture
(238), cooling coils (250), and discharge port (252). Placement of
the various components as shown creates three distinct and
purposeful reactor zones. An atmospheric zone (274) is created in
the reactor which extends from the interior and topside surface of
the reactor (276) to the lower interior surface of the reactor wall
(278). The atmospheric zone of the present invention may have a
pressure of between 100 mTorr and 250 atm, a temperature of between
10 C. and 300 C., and is generally behaving as a non-solvent
reaction environment.
[0267] A reaction zone (248) is created which extends the area
bounded between the substrate surface (280) to the top of the
grounded substrate electrode surface (282). The reaction zone is a
centralized portion of the atmospheric zone which contains the
dense fluid solvent and processing plasma which is selectively
applied to a substrate surface, and is generally held at a
temperature which is less than, equal to or greater than the
atmospheric zone (274). Another aspect of the present invention is
that the grounded electrode (238) may be a perforated metal surface
which is positioned above the exemplary substrate (240)--not shown
in FIG. 10. This is a downstream configuration wherein the
substrate (240) is not present within the plasma sheath or glow
discharge zone. This may be beneficial if the substrate is
susceptible to ion damage or if the substrate is to exposed to
plasma-generated species such as oxygen radicals, ozone or
supercritical ozone carried to a substrate by a dense fluid. In
either case, the plasma may be applied in a pulsed or continuous
operation at a frequency of between 100 KHz and 10 GHz, and
voltages of between 50 and 5000 volts. Also, either a DC or AC
source may be used for power.
[0268] The condensation zone is created in the reactor which
extends from the top of the of the grounded substrate electrode
(282) to the bottom interior surface (278) of the reactor. The
condensation zone (246) is the bottommost portion of the
atmospheric zone (274) and is maintained at a temperature which is
generally less than the upper and middle hemispheres of the
atmospheric zone (274).
[0269] Establishing reactor zones are necessary to perform
condensation shear cleaning and deposition aspects of the present
invention, as well as plasma-aided adjuncts thereto. Moreover, an
important and novel aspect of the present invention is to maintain
the integrity of the substrate once cleaned, coated or otherwise
treated in accordance with methods described herein.
Compartmentalizing or segregating the dense fluid reactor into
distinct zones is accomplished using both thermal gradients as well
as porous structures. Moreover, a judicious application of the
dense fluid cleaning or deposition agents and solvents, usually at
a fraction of the volume of the supercritical fluid antisolvent
processing atmosphere, prevents flooding the entire reactor with
excess processing fluids and agents. Still moreover, the dense
fluid flow is always from top to bottom to insure that contaminants
are move away from both the processing atmospheric zone and
reaction zone surrounding a treated substrate. Thus, a number of
novel design considerations are combined to eliminate or minimize
re-contaminating or otherwise spoiling the integrity of a treated
substrate.
[0270] Again referring to FIG. 10, the exemplary method for
constructing, employing, and deconstructing the exemplary reactor
zones as described above is as follows:
[0271] 1. (280) Introduce a supercritical fluid anti-solvent fluid
such as supercritical helium, nitrogen, oxygen or argon through an
inlet port which is located at the uppermost portion of a dense
fluid reactor. Establish and maintain a pre-determined pressure and
temperature.
[0272] 2. (282) Selectively introduce and contact a substrate
surface, which is centrally located within the dense fluid reactor
and contained within same supercritical antisolvent atmosphere,
with a dense fluid solvent, additives and agents such as
supercritical or liquid carbon dioxide, nitrous oxide or ammonia. A
plasma or electric field may or may not be present during the
application of said dense fluid solvent.
[0273] 3. (284) Condense said dense fluid solvent in a condensation
zone which is below said substrate surface and immediately remove
said dense fluid through an outlet port which is located at the
bottommost portion of the dense fluid reactor.
[0274] 4. (286) Remove said supercritical fluid anti-solvent fluid
from reactor through an outlet port which is located at the
bottommost portion of the dense fluid reactor.
[0275] FIGS. 11a, 11b and 11c are schematic representation of an
exemplary spin processor reactor for use with the present
invention. The figures depict a novel substrate loading and spin
processor mounting design as well as an exemplary locking
mechanism.
[0276] Referring to FIG. 11a, an exemplary dense fluid spin
processor reactor for use with the present invention comprises a
high pressure stainless steel closure (288) and a stainless steel
base (290). The closure (288) contains a centrally-located dense
fluid inlet port and pipe (292), which extends (294) a distance
down into the closure portion, and a supercritical fluid
anti-solvent inlet port (296). The exemplary closure (288) also has
a mounting and dismounting device (298) affixed to the topside of
the closure as shown. The exemplary closure contains a substrate
mounting fixture (300) which accepts, centers and holds an
exemplary wafer substrate (302). As shown, the exemplary closure
(288) is connected and disconnected with the base (290) using the
mount and dismount device (298) in an up and down orientation
(304). The closure contains a smooth and flat sealing surface
(306).
[0277] The exemplary base contains a grooved sealing surface and
fluorocarbon o-ring (308), for example Tefzel, E. I. Dupont, and a
cavity (310) within which is placed a spin processor base (312),
having a shape consistent with securely holding the substrate for a
balanced high speed spin. For example a machined depression or
vacuum may be employed using a round spin processor plate for a
wafer substrate. Below the spin processor plate (312) is a spiral
condensing coil (314) and an outlet port (316) is located below the
condenser coil (314). A magnetic drive unit (318), available from
Autoclave Engineers, Erie, Pa., is centrally connected to the
bottom (320) of the base (290) and extends into the cavity (310)
and is connected to the bottom of the spin processor plate (312) as
shown. A motor (not shown) is attached to the magnetic drive and is
controlled to rotate the spin processor plate (312) in a clockwise
or counterclockwise direction and at a speed of between 10 and 5000
rpm.
[0278] Finally, locking rings (322) are used to mate and seal the
closure (288) and base (290) sections of the exemplary dense fluid
reactor. The locking rings may be moved along a horizontal motion
(305) to seal and lock, and unseal and unlock the exemplary
reactor. The interior surfaces of the exemplary closure and base
may be coated with Teflon or lined with an anodized surface
material to prevent corrosion if corrosive fluids are to be
used.
[0279] Referring to FIG. 11b, the exemplary closure (288), with
mounting fixture (300) and wafer substrate (302), is lowered
against the base, mating the sealing surface of the closure (FIG.
11a, 306) with the sealing surface of the base (FIG. 11a, 308).
During this process, the wafer substrate (302) lifts off and away
from the mounting fixture (300) as the wafer comes to rest atop the
spin processor plate (312). Also, the closure (288) has been
designed so that the dense fluid inlet port (292) is positioned at
a predetermined distance from the topside and central portion of
the exemplary substrate (302). Moreover, the mounting fixture cages
the substrate (302) and may be designed to have a solid ring
section (not shown) positioned circumferentially located at the
upper portion (324) of the mounting fixture (300) which surrounds
wafer in a barrier and which directs the dense fluid solvent
downward toward the condensing coil (314) as its flows away from
the substrate surfaces. Finally, as shown in the figure the
exemplary locking rings (322) remain in an unlocked position away
from the closure (288), as the closure is lowered to or raised from
the base (290).
[0280] Referring to FIG. 11c, the exemplary dense fluid reactor is
sealed and locked by sliding the locking rings (322) over the top
and bottom sides of the connected closure and base sections forming
a circumferential sealing clamp as shown. The dense fluid reactor
is now ready for processing.
[0281] A variety of dense fluid reactors may be designed,
constructed and used in accordance with the present invention. For
example a plasma barreling apparatus may be constructed to perform
bulk extraction of implantable devices wherein the substrates
(i.e., silicone pump bodies) rotate in and out of a liquid carbon
dioxide dense fluid solvent zone contained in a lower hemisphere of
the reactor and up into an anti-solvent supercritical fluid or
vapor plasma treatment zone in the upper hemisphere of the reactor.
Moreover, pre- and post-process plasma treatments may be performed
on dense fluid extracted substrates.
[0282] In another example, multiple wafers may loaded into a
multi-ported mounting fixture, similar that as shown in FIG. 11a
(300) and inserted into the reactor cavity. Moreover, the mounting
fixture may be designed to have interlaced high voltage and
grounding electrodes to perform plasma treatments described herein
with the multiple wafers.
[0283] Having thus described the exemplary dense fluid reactor
designs for use with various aspects of the present invention,
following is a discussion of the entire dense fluid management
system for producing and controlling the atmospheres and novel
isobaric processing conditions present in the exemplary dense fluid
reactor.
[0284] FIG. 12 is an exemplary isobaric dense fluid management
system for use with the present invention. Referring to FIG. 12,
the exemplary dense fluid management system comprises an exemplary
dense reactor (326) with its various connections shown including a
refrigeration system (328) for the internal condensing coil (330),
a high voltage generator (332) for delivering high frequency energy
to the internal electrode (not shown), a grounded spin processor
drive (334) to rotate the exemplary wafer substrate (336).
Connected to the exemplary dense fluid reactor (326) are three
pressure vessels or tanks; a supercritical fluid anti-solvent tank
(338), a dense fluid solvent tank (340), and a dense fluid
separation tank (342). Finally, a smaller ballast tank (344) may
also be connected to the reactor.
[0285] Again referring to FIG. 12, the following exemplary tank and
reactor connection scheme is required to establish reactor pressure
and fluid conditions required to perform the various aspects of the
present invention. The supercritical fluid anti-solvent inlet port
(346) of the exemplary reactor is connected by a high pressure
supercritical fluid pipe (348) to the topside of supercritical
fluid tank (338). The dense fluid inlet port (350) of the exemplary
reactor (326) is connected to a bottom port (352) of the dense
fluid solvent tank (340) by a high pressure dense fluid pipe (354).
The dense fluid connection pipe (354) may be connected to a ballast
tank (344), which is then connected via a connection pipe (356) to
the dense fluid inlet port (350). The discharge port (358) on the
exemplary reactor (326) is connected to a bottom port (360) on the
dense fluid separation tank (342) via a high pressure drain pipe
(362). Finally, to complete the pressure control circuit, a top
port (364) on the supercritical fluid tank (338) is connected to a
top port (366) on the dense fluid tank (340) via a inert gas vapor
connection pipe (368), and a top port (366) on the dense fluid tank
(340) is connected to a top port (370) of the dense fluid
separation tank (342) via a vapor return pipe (372).
[0286] Various check valves are used to insure that liquid and
vapors flow in pre-defined directions to and from each tank and
between the tanks and the reactor. A check valve (374) on the
supercritical tank vapor transfer pipe (348) insures that vapor
exchanges from the supercritical fluid tank to the supercritical
atmosphere inlet (346) on the reactor. A check valve (376) on the
supercritical vapor transfer line (368) insures that vapor flows
from the supercritical fluid tank (338) and into the top of the
dense fluid tank (340). A check valve (378) on the dense fluid
liquid transfer pipe (354) insures that liquefied gas flows from
the bottom of the dense fluid tank (340) to the dense fluid inlet
(350) of the reactor (326), or into the ballast tank (344) and
finally into the reactor as above. A check valve (380) insures that
effluents from the reactor discharge port (358) flow to either the
inlet port (360) of dense fluid separation tank (342) via a
separation tank valve (382), or to purge or vacuum valves described
later. Finally, a check valve (384) on the separation tank vapor
transfer pipe (372) insures that vapor flows to the top of the
dense fluid tank (340).
[0287] Various valves are used to connect the various holding tanks
to the atmosphere, the reactor, drains, vents, and vacuum pump
subsystems. The dense fluid tank contains two drain valves; a vapor
drain valve (384) and a liquid drain valve (388). The inlet ports
to the reactor (326) contains two valves; a supercritical fluid
anti-solvent inlet valve (390) and a dense fluid solvent inlet
valve (392). The reactor (326) contains three outlet valves; a
vacuum pump valve (394), a vent valve (396) and a separation tank
valve (382). The separation tank also contains a separated waste
discharge valve (398).
[0288] Various liquid and vapor treatments may be performed to
purify the liquids and supercritical fluids and gases. For example,
membrane filters are used to remove small particles entrained in
various fluid streams transferred between the holding tanks and the
dense fluid reactor. These include a dense fluid filter (400),
supercritical fluid inlet filter (402), and vapor return filter
(404). Vapor treatment modules, increasing adsorbent or other
techniques may be used to remove volatile and non-volatile
contaminants and water from the liquids and gases. These include a
vapor treatment system (406) on the supercritical fluid delivery
pipe (348) and a vapor treatment system on the recovered
supercritical fluid pipe (372).
[0289] A supply of pure dense fluid and supercritical fluid is
required to provide an initial charge for processing. Dense fluid
and dense gas supply tanks are connected to the holding tanks to
create and maintain the proper volume of liquid carbon dioxide, for
example. A pure liquid carbon dioxide supply (410) with a pump
(412) are connected to the dense fluid supply tank (340) via a
connection pipe (414). An optical sensor located on the dense fluid
tank (340), which is connected electrically (418) to a pilot switch
on the filling pump (412), maintains a pre-determined level within
the tank (340). A supply of pure nitrogen, argon or other suitable
source gas :(420) with a gas booster pump (422) is connected to the
supercritical fluid holding tank (338) via a supply connection pipe
(424).
[0290] Finally, a dense fluid liquid pump (426) is used to compress
liquid state dense fluids from the dense fluid tank (340) to a
pre-determined pressure and into the reactor (326) or into the
ballast tank (344). The ballast tank (344) may be heated to change
the phase of the compressed liquid to supercritical fluid solvent
phase prior to injection into the reactor (326). Moreover, the
ballast tank (344), shown schematically with a mixer apparatus
(428), is suitable for blending additives and agents into the dense
fluid solvent prior to delivery to the reactor (326). A vacuum pump
(430) is used to produce sub-atmospheric conditions for performing
cold plasma treatment operations herein. Finally, a vapor heater
(432) is used to heat supercritical fluid anti-solvent to a
predetermined temperature prior to injection into the reactor
(326).
[0291] The exemplary dense fluid management system as described
above using FIG. 12 can be used to produce and maintain isobaric
fluid conditions and reactor zone establishment necessary for
performing various aspects of the present invention.
[0292] FIGS. 13a, 13b, 13c and 13d are top views of a schematic
representation of the operation of an integrated and robotic
surface treatment system using the present invention. Referring to
FIG. 13a, the exemplary robotic processing system comprises a Fed.
Std. 209 Class 100 workstation (434) which contains the dense fluid
reactor (436) and a wafer transfer robot (438). Attached to the
front of the workstation is a front opening universal pod (FOUP)
load station (440) containing un-processed wafer substrates (442)
and a FOUP unload station (444) containing processed wafer
substrates (446). Referring to FIG. 13a, the exemplary robot (428),
using an appropriate pick-up tool (448), picks up an un-processed
wafer from the load FOUP station (440). Referring to FIG. 13b, the
robot (438) transfers the unprocessed wafer (442) to the front of
the dense fluid reactor closure (437). Referring to FIG. 13c, the
robot (438) inserts the un-processed wafer substrate (442) into the
exemplary mounting fixture (not shown here) as shown in FIG. 11a,
(300) located within the closure (437). Referring to FIG. 13d, the
robot sits the wafer onto the mounting fixture (not shown) within
the closure (437), withdraws from the closure and locking ring
area, whereupon the locking rings (444) are brought into the locked
position as shown. The system is now ready to process the loaded
wafer substrate. Upon completion of the process, the reverse of the
above procedure is performed with the processed after substrates
being off loaded into the unload FOUP.
[0293] FIG. 14 shows another exemplary dense fluid substrate
treatment system incorporating a second wafer processing tool
incorporating a solid carbon dioxide spray tool--representing for
example a combination of immersion drying and cleaning (gross
clean) and submicron particle removal (precision clean). The
exemplary robotic processing system comprises a Fed. Std. 209 Class
100 workstation (434) which contains the dense fluid reactor (436)
and a wafer transfer robot (438) on a robotic track system (450).
Attached to the front of the workstation is a front opening
universal pod (FOUP) load station (440) containing un-processed
wafer substrates (442) and a FOUP unload station (444) containing
processed wafer substrates (446). In this exemplary substrate
cleaning tool configuration, the un-processed wafer substrate (442)
is first processed using one or more of the exemplary cleaning,
surface modification, precision drying or deposition processes
herein within a first reactor (436). The substrate loading
procedure using this novel tool is identical to the tool loading
procedure of FIGS. 13a-d above. However, following processing in
the reactor (436), the robot off-loads the processed wafer to a
second tool, in this case a novel solid carbon dioxide spray plasma
cleaning tool (452) to perform high pressure CO.sub.2 snow spray
cleaning as well as surface plasma treatment as a final particle
clean. This may be done as a separate operation to increase
throughput of a dense fluid cleaning tool.
[0294] The exemplary plasma snow cleaning device (452) comprises
one or more dielectric solid spray cleaning nozzles (454) which are
aligned transverse and in communication with an atmospheric plasma
treatment electrode (456). Similar to the exemplary and perforated
dense fluid showerhead reactor herein and described in FIG. 9,
however this exemplary dense fluid spray plasma spray head contains
a linear array or bank of solid spray nozzles as shown which are
constructed from non-conductive materials such as PEEK tubing and
fittings. Moreover, the exemplary dense fluid spray head comprises
a patented and patents-pending solid carbon dioxide spray technique
(i.e., U.S. Pat. No. 5,725,154) which combines condensed liquid
carbon dioxide and additives (458) and an inert ionized and heated
propellant gas phase (460) to form a variable geometry spray when
mixed in a suitable nozzle (454). A power source is connected (462)
to the plasma electrode (456) using a connector (464). Thus the
present design shows the exemplary spray nozzles (454) being
machined into the solid body of a plasma electrode (456) to form a
plasma snow spray cleaning tool which may be developed into any
geometric shape, such as for example a rectangular or a circular
treatment tool. Thus the combinational plasma snow spray cleaning
head may be formed to treat any type of substrate. FIG. 14 shows an
exemplary spin processor cleaning tool wherein the plasma snow
cleaning head (452) is scanned (466) in a back and forth direction
over, and at a pre-determined distance from and angle to, an
exemplary wafer substrate (442b) which is being rotated at between
10 and 5000 rpm by a vacuum spin processor (below and not shown).
An exemplary ULPA-filtered gas manifold (468) flows clean inert
filtered and possibly heated atmosphere over the substrate (442b)
being treated while the exemplary plasma snow treatment head (452)
is moved over the substrate (442b). Moreover the exemplary spray
head (452) is integrated into a return manifold (470) which is an
airflow return for the clean air emanating from the clean air
manifold (468). As such, a laminar airflow maybe produced over the
substrate being treated in accordance with the present invention.
Moreover, this atmosphere may be composed from clean dry air,
argon, oxygen, nitrogen, and carbon dioxide and mixtures thereof.
Still moreover, plasma treatment gases may be employed as described
in exemplary plasma treatments discussed herein. Finally, the
exemplary plasma snow treatment tool may be used in combination
with a deposition, patterning, reactive ion etch, post-etch residue
removal described herein as well as many other substrate treatment
processes.
[0295] Following the second operation, the precision cleaned wafer
substrate (446) is unloaded into the unload FOUP (444). Virtually
any configuration of wet and dry substrate treatment tools may be
developed and integrated with the present invention to provide for
example multiple-clean tools, clean-with-inspection tools, acid
etching-drying-cleaning tools, and many other combinations.
[0296] FIG. 15 and FIG. 16 are flow diagrams for additional
exemplary methods with a schematic of the exemplary apparatus for
performing said method.
[0297] As shown in FIG. 15, a substrate is loaded (472) into the
exemplary reactor, in this instance a spin processing reactor as
described in FIGS. 11a-d herein, whereupon the substrate is rotated
(474) at a velocity of between 10 rpm and 5000 rpm. A conventional
wet acid spray treatment (476) is applied to the substrate
consisting of an application of hydrofluoric acid solution (478),
deionized water rinses (480), followed by a gross dry using
nitrogen (482), which is the anti-solvent supply in this example.
Following the gross nitrogen drying sequence, the exemplary reactor
of FIGS. 11a-d is pressurized to 65 atm and controlled at a
temperature of 25 C. to establish a first supercritical nitrogen
anti-solvent environment (482). Following this, liquid carbon
dioxide is withdrawn from a dense fluid tank (484), and using a
pump and heater (486), a small but pre-determined amount of liquid
carbon dioxide is compressed to supercritical carbon dioxide at a
pressure of 150 atm and 60 C. and applied (488) in a pulsed
(intermittent) or continuous spray to a substrate which initiates
the condensation shear cleaning process described herein and shown
in the isobar (490). The process continues until the substrate
temperature rises to or above the critical temperature of the dense
fluid. Alternatively, the condensation shear process may be
controlled using a coolant (492) to operate continuously with each
injection of, or a continuous application, of supercritical carbon
dioxide. During application of each aliquot of supercritical carbon
dioxide, an internal cooling coil condenses the spent cleaning
agent which is withdrawn immediately from the reactor using the
isobaric dense fluid management system design of FIG. 12. Following
the condensation shear cleaning step, the supercritical fluid
anti-solvent environment is withdrawn (494) and vented to the
atmosphere. At this point the substrate is warm and dry because a
heated supercritical anti-solvent is bathing the substrate, using
the vapor heater of FIG. 12, (432), as the reactor pressure is
reduced to ambient conditions. Alternatively, the substrate may be
bathed in heated ionized nitrogen to neutralize any residual
electrostatic charges accumulated during processing. Finally, the
clean, dry and neutralized substrate is unloaded (496) from the
reactor.
[0298] FIG. 16 is very similar to the method described in FIG. 15
but without a conventional wet processing and gross drying
pre-treatment step. The method shown in FIG. 16 teaches the option
of 1) establishing a sub-atmospheric or atmospheric plasma (498)
prior to or while establishing a supercritical anti-solvent
environment within a reactor; 2) establishing a super-atmospheric
plasma (500) prior to or while establishing a dense fluid solvent
cleaning agent within a reactor; 3) establishing a
super-atmospheric plasma (502) prior to or while removing first
supercritical fluid antisolvent from a reactor; and 4) establishing
a sub-atmospheric or atmospheric, plasma (504) prior to the next
process step. Thus as depicted in FIG. 16, plasma steps may augment
the condensation shear cleaning and deposition steps at various
steps and for various purposes to formulate and execute a variety
of surface treatment recipes.
EXAMPLES OF USE
[0299] Following are example applications for the present
invention.
Example 1
[0300] Precision cleaning a semiconductor wafer to remove submicron
particles from microscopic structures using a condensing flow of
supercritical carbon dioxide.
Example 2
[0301] Precision drying a semiconductor silicone device containing
optical switches to remove trace moisture and residues from
microscopic interfaces using a plasma and liquid carbon dioxide
spray.
Example 3
[0302] Precision cleaning a silicon semiconductor wafer containing
integrated circuits to remove post-plasma reactive ion etch
residues (carbon-fluorine compounds) from sidewalls of microscopic
vias using combinations of argon-oxygen plasma and a condensing
spray of supercritical carbon dioxide.
Example 4
[0303] Deposition of an organometallic coating on a silicon
substrate, heating said coated substrate and exposing same to a
supercritical atmosphere of argon and hydrogen reducing
atmosphere.
Example 5
[0304] Extracting a polydimethylsilicone implantable device in a
tumbling drum reactor to remove unreacted monomers from subsurface
pores and simultaneously plasma treating substrate surfaces to
destroy biological contaminants and to increase substrate surface
energy for adhesive bonding.
Example 6
[0305] Deposition of an organic coating on a silicon substrate,
softbaking said film, and exposing said film to a plasma.
Example 7
[0306] Precision drying a semiconductor silicone device containing
integrated circuits using a condensing supercritical carbon dioxide
spray containing methanol.
Example 8
[0307] Precision cleaning of a thin semiconductor wafer using
atmospheric plasma and dense fluids.
Example 9
[0308] Atmospheric plasma etching of a low-k photoresist layer
followed by supercritical fluid drying and cleaning
(extraction).
Example 10
[0309] Precision cleaning a wafer surface and plasma-enhance
chemical vapor deposition (PECVD) of silicon dioxide from
tetraethoxysilane (TEOS).
Example 11
[0310] Precision cleaning a CMOS image sensor and gold wire bonds
using a combination of atmospheric plasma and solid phase carbon
dioxide spray treatment.
[0311] Having thus described the various aspects of the present
invention, it should be understood by those skilled in the art that
many different method variations, apparatuses, and processes can be
developed using the present invention.
* * * * *