Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR CLEANING AN ARTICLE
RELATED APPLICATIONS
This application claims the benefit of U.S. Provisional Application No.
60/346,507, filed January 7, 2002. The entire teachings of the above
application is
incorporated herein by reference.
BACKGROUND OF THE INVENTION
The manufacture of contaminant-sensitive articles often requires the use of
one or more solvents to remove impurities from the article. Traditionally,
those
solvents have been used in liquid phase. Recently, the use of supercritical
carbon
dioxide as a replacement for liquid solvents has been growing in popularity.
Employing supercritical carbon dioxide often results in reduced water
consumption,
reduced waste streams, reduced emissions, and/or enhanced solubility
characteristics. In the area of semiconductor manufacturing, supercritical
carbon
dioxide has'been utilized for many applications, such as photo-resist
developing,
photo-resist stripping, wafer cleaning, and wafer drying.
Generally, supercritical fluids are fluids which are above their critical
pressure and temperature, and have both gas- and liquid-like properties.
Solvent
properties of a supercritical fluid, such as supercritical carbon dioxide,
depend on the
fluid density, which in turn dependents on the pressure/temperature conditions
of the
fluid. For many organic impurities, the solvating properties of carbon dioxide
decrease as the pressure of the fluid is reduced from supercritical to a lower
pressure,
for instance to atmospheric pressure, as occurs during depressurizing a
chamber
employed in a cleaning operation. For high purity cleaning operations, such as
found in wafer manufacture or processing or during fabrication or processing
of
2~ optics and other workpieces or substrates, impurities that precipitate out
of the
carbon dioxide solvent, as the pressure is reduced, can impact the surface
being
cleaned, thereby contaminating it and thus reducing the effectiveness of the
cleaning
process.
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Therefore, a need exists for a method for cleaning an article, such as a wafer
or another workpiece that reduces or minimizes the above-referenced problems.
SUMMARY OF THE INVENTION
The invention is generally related to a method for cleaning an article by
contacting an article with a solvent fluid that includes carbon dioxide,
thereby
removing impurities from the article, and displacing the solvent fluid with a
displacing fluid. The displacing fluid is other than carbon dioxide. In one
embodiment, the article is a wafer and displacing is conducted at a
temperature and
pressure sufficient to prevent forming a second phase in the solvent fluid. In
another
embodiment, displacing is conducted at a temperature and pressure sufficient
to
prevent forming a second phase in the solvent fluid being displaced and carbon
dioxide is recycled to the fluid.
In yet another embodiment, the invention is directed to a method for reducing
deposition of non-volatile residues during a workpiece cleaning operation. The
method includes the steps of contacting the workpiece with a solvent fluid at
a first
pressure, the solvent fluid including carbon dioxide, whereby contaminants on
the
workpiece are removed by the solvent fluid; reducing the pressure of the
solvent
fluid, whereby non-volatile residues become insoluble in the solvent fluid;
and
displacing the solvent fluid at the reduced pressure with a displacing gas
that is other
than carbon dioxide, whereby the time that the workpiece is exposed to
insoluble
non-volatile residues is reduced, thereby reducing deposition of insoluble non-
volatile residues on the workpiece.
In a further embodiment, the invention is directed to a method for supplying
cleaning fluids to a vessel. The method includes the steps of supplying a
solvent
fluid stream at a first pressure to a vessel housing a article, wherein the
solvent fluid
includes carbon dioxide and is capable of dissolving contaminants on an
article in
the vessel; supplying a displacing fluid stream to the vessel, wherein the
displacing
fluid stream has a pressure sufficient to displace the solvent fluid from the
vessel and
the displacing fluid is other than carbon dioxide; and exhausting the solvent
fluid
from the vessel.
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The invention has numerous advantages. For example, practicing the method
of the invention results in preparing ultra-clean surfaces, as required in
semiconductor manufacturing and other industries. The method of the invention
is
economical and can be easily integrated into existing manufacturing
facilities. For
instance, in one aspect, the method of the invention employs nitrogen
displacing gas,
as nitrogen lines generally are available at the facility. In one embodiment,
low
pressure (e.g., 80-100 psig), available nitrogen can be employed. In another
embodiment, spent carbon dioxide is recycled thereby reducing carbon dioxide
consumption and associated costs. In a further embodiment recycling can be
conducted without requiring compression of the spent fluid. Furthermore, the
invention takes into account non-volatile residue impurities that can be
present in
even high purity grades of carbon dioxide and addresses problems caused by
their
precipitation upon chamber depressurization.
BRIEF DESCRIPTION OF THE DRAWINGS
Figs 1 A-1 C illustrate the stages in the formation of a second phase as
pressure is reduced in a chamber housing an article that is contacted with
carbon
dioxide solvent.
Figs.2A-2D illustrate method steps in an example of one embodiments of the
the invention.
Figs 3A-3E illustrate method steps in an example of another embodiment of
the invention.
DETAILED DESCRIPTION OF THE INVENTION
The foregoing and other objects, features and advantages of the invention
will be apparent from the following more particular description of preferred
embodiments of the invention, as illustrated in the accompanying drawings in
which
like reference characters refer to the same parts throughout the different
views. The
drawings are not necessarily to scale, emphasis instead being placed upon
illustrating
the principles of the invention.
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The invention generally is related to producing clean surfaces, such as,
required, for example, during semiconductor manufacturing or processing. The
invention is related to removing, preventing or minimizing deposition of
contaminants on wafers, such as wafers that include one or more
electromechanical
devices, one or more integrated circuits or combinations thereof, as known in
the art.
Other workpieces that can be processed using the invention include parts used
in
semiconductor manufacture, e.g., sputtering targets and others, optical parts,
for
example, optical lenses, filters, frequency doubling devices, lasing crystals,
light
splitting elements, optical cavities, fiber optics, mirrors, and others.
Articles such as
television, vedeo-recording and photograhic camera components, components used
in scientific and medical instruments, in satellite communication, in the
aerospace
industry and other workpieces also can be processed as described herein.
The article can be fabricated from of any material, including inorganic
matter, such as silicon, silicon dioxide, graphite, or metal; organic matter,
such as
polymers; or a material made from a combination of inorganic and organic
materials.
This method for cleaning can be applied to a single article, or it can be used
to clean
two or more articles at a time.
The invention relates to a method for removing contaminants or impurities
from an article or from the environment surrounding the article, such as found
in a
chamber housing the article during its manufacture or processing. The method
can
itself be a step in a larger manufacturing operation, such as a process for
depositing
or growing a film, a photolithographic process, an etching process, an ion
implantation process, a chemical mechanical planarization process, a diffusion
process, a photo-resist development process, a process for developing
photosensitive
materials, a process for cleaning optical parts, a process for cleaning
components
useful in aerospace applications, a photo-resist stripping process, a wafer
cleaning
process, a wafer drying process, a degreasing process, or an extraction
process.
Contaminants include organic and/or inorganic materials not desired on the
final product. They can be solid, liquid or in gaseous form. Examples include
polymers, greases, and other organic materials; silicon; carbon; and/or metals
and
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other impurities. They can be present on the surface of the article or
diffused
throughout at least a portion of a material comprising of the article.
Impurities can be generated from the article itself, and can include portions
of a wafer that are removed during wafer processing or debris produced during
an
etching process. Impurities also can be delivered to the article with a
process fluid.
Chemical compounds, such as employed in fabricating or processing an article,
also
can be left on the surface of the article or can be present in the process
chamber once
the operation is completed.
The invention is particularly suitable in removing non-volatile residues
(NVRs). During operations that employ carbon dioxide at high pressures and in
particular at or near critical or supercritical conditions, many NVRs are
soluble in
the carbon dioxide. As the pressure is reduced, the density and solvent
properties of
carbon dioxide change and NVRs precipitate generating a second phase,
generally in
the form of aerosol droplets and/or solid particulates. Once in the second
phase,
NVRs can impact a surface of the article, thereby contaminating it
Examples of non volatile residues include, but are not limited to heavy
organics, such as, for example, hydrocarbons, (e.g., C,o-,.), heavy
halocarbons and
others.
Sources of NVRs include compressor oils, paints, elastomeric materials that
have some solubility in the solvent and are commonly found in gasket and valve
seat
materials, sealants used in the solvent feed lines and others. NVRs can be
generated
during a process operation on a workpiece, for instance during wafer cleaning.
NVRs also can be brought into contact with the surface of an article by a
fluid employed in the manufacture, processing or in cleaning the article.
In the semiconductor industry, for example, carbon dioxide is used during
photo-resist developing; photo-resist stripping; wafer cleaning; and wafer
drying.
Bulk liquid carbon dioxide is believed to contain NVRs in concentrations of up
to 10
parts per million (ppm) by weight. Some higher purity grades, available in
cylinders, can contain approximately 0.15 ppm NVRs by weight.
Even the higher purity grades can carry an unacceptable amount of NVRs for
sensitive processes which require that the finished article contain less than
a specific
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number of particles of a given size. Some manufacturing processes, for
instance,
require that gases have less than 100 particles above some critical size
(typically on
the order of 100 nanometers) per standard cubic meter. It is estimated that
vaporization of one liter of the higher purity grades of liquid carbon dioxide
(~10
S ppb) can result in millions of NVR particles. In order to reach such
cleanliness
levels, the purity of the carbon dioxide would need to be increased at least a
thousand fold over the highest purity carbon dioxide currently available.
The formation of the second phase NVRs during a cleaning process that
employs high pressure carbon dioxide is illustrated with respect to Figs. lA,
1B and
1C. Shown in Fig. lA is chamber 10 that houses wafer 12. Chamber 10 is a
vessel
or container such as encountered, for instance, at a tool or process station
in a
semiconductor fabrication facility. Chamber 10 is designed to receive and hold
high
pressure fluids such as supercritical carbon dioxide (i.e., carbon dioxide
above its
critical temperature and pressure, specifically, above 31°C and 1070
pounds per
square inch absolute (psia)). Chamber 10 is provided with ports) for
introducing
process fluids and other chemicals and with evacuation port(s), as known in
the art.
Means for introducing and for evacuating chamber 10 can be employed, as known
in
the art. Examples include compressors, pumps, vent valves and others.
As shown in Fig. lA, chamber 10 is filled with carbon dioxide at a pressure
of 2000 pounds per square inch gauge (psig). At this pressure, contaminants on
the
wafer are dissolved in the carbon dioxide solvent and the concentration of
second
phase (insoluble) NVRs is minimal. As chamber 10 is decompressed to a lower
pressure, e.g., 200 psig, as shown in Fig. 2B, and then to ambient pressure,
as shown
in Fig. 1C, the solvating properties of carbon dioxide towards the NVRs
diminish
and a second phase is formed. The second phase NVRs in chamber 10 can impact
the wafer, thereby contaminating it.
In one embodiment, the method of the invention includes contacting an
article, e.g., a wafer with a solvent fluid that includes carbon dioxide,
whereby
contaminants on the article dissolve in the solvent fluid. Higher purity
grades of
carbon dioxide are preferred. In other embodiments, the method of the
invention can
be employed with bulk carbon dioxide.
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Generally, the solvent fluid includes at least SO percent (%) by weight carbon
dioxide. Preferably, the solvent fluid includes at least 75%, more preferably
at least
90% and most preferably, at least 98% carbon dioxide by weight.
The solvent fluid can be 100% carbon dioxide. In other embodiments the
solvent fluid includes at least one addtional component such as, for instance,
a co-
solvent, surfactant or a chelating agent. Examples of components that can be
employed in addition to carbon dioxide, alone or in combination, include
ammonia,
halogenated hydrocarbons, hydrofluoric acid, sulfur dioxide and others. Other
examples of cosolvents, surfactants, and/or chelating agents include silane;
hydrocarbons, such as methane, ethane, propane, butane, pentane, hexane,
ethylene,
and propylene; halogenated hydrocarbons such as tetrafluoromethane,
chlorodifluoromethane, sulfur hexafluoride, and perfluoropropane; inorganics
such
as ammonia, helium, krypton, argon, and nitrous oxide; alcohols, such as
ethanol,
methanol, or isopropyl alcohol; propylene carbonate; atmospheric gasses, such
as
nitrogen, hydrogen, ozone, or oxygen; water; amines, such as hydroxylamine and
alkanolamines; acetone; pyrrolidones such as N-methylpyrrolidone, N-
ethylpyrrolidone, N-hydroxyethylpyrrolidone and N-cyclohexylpyrrolidone;
amides
including dimethylacetamide or dimethylformamide; phenols and derivatives
thereof; glycol ethers; 2-pyrrolidone; dialkyl sulfone; organic and inorganic
acids
and their derivatives, such as hydrofluoric acid, hydrochloric acid, acetic
acid,
sulfuric acid, gallic acid, or a gallic acid ester; tetraalkylammonium
hydroxides;
ammonium bifluoride; ammonium-tetramethylammonium bifluoride; alkali metal
hydroxides; tartarates; phosphates; ethylenediaminetetraacetic acid (EDTA);
ammonia with sodium sulfides and iron sulfate; and mixtures thereof.
Generally, the solvent fluid includes carbon dioxide at conditions under
which contaminants such as NVRs are soluble in the solvent fluid. For instance
the
solvent fluid includes carbon dioxide having a pressure that is at least 800
psig.
Preferably, the solvent fluid includes carbon dioxide that is at or near its
critical state
or at supercritical conditions.
The carbon dioxide solvent may be directed to the container in either a vapor,
liquid, or supercritical phase. Once inside the container, the carbon dioxide
solvent
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contacts the article in order to remove the impurities. The removal of the
impurities
can be accomplished through a physical or chemical mechanism, for example the
carbon dioxide solvent may dissolve the impurities; the impurities may diffuse
into
the carbon dioxide solvent from the material making up the article; or the
impurities
may react with the carbon dioxide solvent in a way that results in their
removal from
the article. The removal can also be a mechanical mechanism, for example the
or
the pressure and/or temperature of the carbon dioxide solvent can be
manipulated so
that its specific volume increases and/or decreases, resulting in stresses
that break
the impurities from the article. The removal of the impurities can also be
accomplished through a combination of chemical and mechanical mechanisms.
Optionally, the carbon dioxide solvent can also be agitated in order to
enhance both a chemical and mechanical mechanism. For example, agitation can
increase the speed of chemical removal mechanisms (such as dissolution,
diffusion,
and reaction) by increasing concentration gradients across the surface of the
article,
thereby driving the chemical mechanism towards completion. Similarly,
agitation
can increase the rate of removal for mechanical removal mechanisms because the
agitation creates shear forces in the fluid which can assist in pulling the
impurities
away from the surface of the article.
The temperature and/or pressure of the carbon dioxide solvent can be
manipulated to also facilitate the removal of the impurities. These
manipulations of
process conditions can result in the carbon dioxide solvent undergoing one or
more
phase shifts between the vapor, liquid, and/or supercritical phases depending
on
whether the chosen manipulations cross the carbon dioxide solvent's critical
temperature and/or pressure and it's condensation pressure and/or temperature.
These manipulations are preferably done to enhance the removal of the
impurities.
If there are several different types of impurities on or in the article, the
carbon
dioxide solvent can be cycled through various process conditions to enhance
the
removal of each type of impurity. While the carbon dioxide solvent is
undergoing
these manipulations, NVRs or removed impurities may dissolve into and/or
precipitate out of the solvent fluid.
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Optionally, at least a portion of solvent fluid including contaminants can be
replaced with fresh solvent fluid or pure carbon dioxide in an intermediate
rinsing
step, whereby spent solvent fluid is pushed through and additional
contaminants can
be removed from the surface being cleaned.
The method includes displacing the solvent fluid with a displacing fluid, that
is other than carbon dioxide, at a temperature and pressure sufficient to
prevent
forming a second phase in the solvent fluid being displaced, whereby the
contaminants are separated from the wafer, thereby cleaning the wafer.
For example, the solvent fluid is displaced at its pressure in the chamber,
without partial or total chamber decompression. If decompression of the
chamber is
employed, it is to a pressure at which solubility of NVRs in the solvent fluid
is
maintained.
The displacing fluid can be a gas, a liquid or a supercritical fluid. Suitable
displacing fluids include inert gases such as nitrogen, helium, argon or
krypton,
other gases, such as oxygen, and any combinations thereof. Nitrogen is
preferred. In
one embodiment of the invention, the displacing fluid is a high purity gas. In
another embodiment, the displacing fluid is an ultrahigh purity gas, for
instance
having purity levels of sub parts per billion in all contaminants or as known
in the
industry. High purity and ultra-high purity gases, such as nitrogen and others
can be
obtained commercially.
The method of the invention can be conducted continuously or in batch-wise
manner.
An illustration of stages of this embodiment of the invention is shown in Figs
2A - 2D.
Fig 2A shows chamber 14, housing wafer 12. Chamber 14 can be a chamber
such as described above. In other embodiments chamber 14 can be designed so
that
fresh fluid fed to the container mixes to a significant degree with the carbon
dioxide
solvent already present (for instance, in the fashion of a continuously-
stirred tank
reactor), or so that the flow path that enhances the displacement of used
solvent,
impurities, and NVRs (e.g., in a plug-flow fashion). Preferably, the chamber
geometry is such that the exposure of the article to impurities and NVRs is
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minimized during the displacement of the carbon dioxide solvent, impurities,
and
NVRs. Ports and means for feeding and evacuating fluids from chamber 14 can be
provided as known in the art.
As shown in Fig 2A, chamber 14 is filled with carbon dioxide at about 2000
psig that includes dissolved contaminants.
An inert gas at a pressure higher than that of the carbon dioxide in the
chamber, e.g., higher than 2000 psig is directed to chamber 14, as shown in
Fig 2B.
Carbon dioxide together with dissolved contaminants is displaced from chamber
14,
as shown in Fig. 2C. Chamber 14, including displacing gas is then
depressurized to
atmospheric pressure, as shown in Fig. 2D.
The final number of impurities left on the article after evacuation can be
improved by using a higher purity displacing fluid and/or carbon dioxide
solvent, or
by increasing the volume of the displacing fluid used in order to more
thoroughly
displace the carbon dioxide solvent, impurities, and NVRs.
Once solvent fluid, NVRs and other impurities are displaced from the
container, the flow of displacing fluid can be stopped and the container can
be
evacuated.
Carbon dioxide displaced from the chamber can be exhausted as a waste
stream or can be directed to another operation or tool in the facility.
In a preferred embodiment, the fluid displaced from chamber 14 is purified,
for example by directing fluid exhausted from the chamber to one or more
purification units. As the pressure of fluid exhausted from chamber 14 is
high, (e.g.,
2000psig), the spent fluid generally can be directed to a purification unit
without
further compression. Examples of purification techniques that can be employed
include distillation, adsorption, absorption, chemical reactions, phase
separation and
other methods.
The solvent fluid displaced can be purified with respect to NVRs, co-
solvents, surfactants and chelating agents. In a further embodiment, the
resulting
stream can be further purified to separate carbon dioxide from the displacing
fluid,
e.g., nitrogen. A suitable method for separating carbon dioxide from nitrogen
includes distillation.
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In a preferred embodiment, carbon dioxide is recycled as described in U.S.
Patent Application No 10/274,302, Recycle for Supercritical Carbon Dioxide,
filed
on October 17, 2002, the teachings of which are incorporated herein by
reference in
their entirety.
S Displacing fluid also can be recycled. If helium is used as a displacing
fluid,
recycling is particularly attractive due to its cost and its significantly
light weight to
effect an easier removal process.
In another embodiment, the invention is related to a method for reducing
deposition of non-volatile residues during a workpiece cleaning operation. The
method includes contacting the workpiece with a solvent fluid, essentially as
described above. The method includes reducing the pressure of the solvent
fluid,
whereby contaminants, e.g., NVRs, become insoluble in the solvent fluid. The
method further includes displacing the solvent fluid, at the reduced pressure,
with a
displacing fluid, such as described above, whereby the time during which the
workpiece is exposed to insoluble contaminants, e.g., NVRs, is reduced,
thereby
reducing deposition of insoluble contaminants, e.g., NVRs on the workpiece.
In one embodiment, the pressure of the solvent fluid is reduced to less than
about 1000 psig. In another embodiment, the pressure is reduced to less than
200
psig. In further embodiment, the pressure of the solvent fluid is reduced to a
value
that is lower than the pressure of a nitrogen gas source or line available at
the
facility, e.g., to less than about 80-100 psig.
Preferably, the wafer is exposed to insoluble NVRs for less than about 30
seconds. More preferably, the wafer is exposed to insoluble NVRs for less than
3
seconds.
Optionally, at least a portion of the solvent fluid, together with dissolved
contaminants, can be displaced using fresh solvent fluid or pure carbon
dioxide in an
intermediate rinsing step. The rinsing step can be conducted before or after
chamber
depressurization. Carbon dioxide and, optionally, displacing gas recycling can
be
employed, essentially as described above. Recycling can include a compression
step
prior to directing fluid that is to be recycled to a purifier.
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As discussed above, the method can be conducted continuously or in batch-
wise fashion.
An example of the stages of this embodiment of the invention is shown in
Figs 3A - 3E. Shown in Fig 3A is chamber 14 housing wafer 12, essentially as
S described above. Carbon dioxide at a pressure of about 2000 psig is present
in
chamber 14. Chamber 14 is depressurized to 200 psig, as shown in Fig. 3B. Upon
the pressure reduction, NVRs precipitate out of solution, forming a second
phase.
An inert gas, e.g., nitrogen at a pressure sufficient to push through solvent
fluid at
200 psig is directed into chamber 14, as shown in Fig. 3C, thereby displacing
carbon
dioxide and second phase impurities from chamber 14, as shwon in Fig. 3D.
Displacing the carbon dioxide and second phase impurities from the chamber
reduces the time that the wafer is exposed to insoluble NVRs, thereby reducing
deposition of insoluble NVRs on the wafer. As shown in Fig. 3E, chamber 14 is
then depressurized to ambient atmosphere.
In one embodiment, the invention is related to a method for producing an
ultraclean article. As used herein, the term "ultra-clean" refers to a
substrate
contaminated by less than about 2,000 particles per square meter of surface
areas,
the impurities having an effective diameter greater than about 0.1 micron and
being
measured by a light scattering technique. Light scattering methods for
measuring
particles having an effective diameter greater than about 0.1 micron on a
solid
surface are known in the art. For example, a suitable method is described in
Diaz,
R.E., et al., "On-Wafer Measurement of Particles in Contamination-Free
Manufacturing for Semiconductors and other Precision Products," in
Contamination-Free Manufacturing for Semiconductors and other Precision
Products, edited by R.P. Donovan (Marcell Dekker), p. 79 (2001).
The method includes contacting the article, in a chamber, with carbon
dioxide solvent, whereby impurities on the article dissolve in the carbon
dioxide
solvent and directing a displacing gas into the chamber to reduce the time
that the
article is exposed to non-volatile residues present in the carbon dioxide,
thereby
reducing the number of impurities on the article to less than about 2000
particles per
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square meter of surface area, wherein each of the impurities has an effective
diameter greater than 0.1 microns as measured by a light scattering technique.
In a further embodiment, the invention is directed to a method for supplying
cleaning fluids to a vessel. The method includes the steps of supplying a
solvent
fluid stream at a first pressure to a vessel housing a article, wherein the
solvent fluid
includes carbon dioxide and is capable of dissolving contaminants on an
article in
the vessel; supplying a displacing fluid stream to the vessel, wherein the
displacing
fluid stream has a pressure sufficient to displace the solvent fluid from the
vessel and
the displacing fluid is other than carbon dioxide; and exhausting the solvent
fluid
from the vessel.
Exhaust fluid from the vessel can be purified, essentially as described above,
and carbon dioxide can be recycled to the vessel. Purified carbon dioxide can
be
compressed prior to being returned to the vessel, by means known in the art.
In one embodiment the displacing fluid stream is at a pressure that is at
least
as high as the first pressure. In one example, the displacing fluid is no more
than
about 100 psi greater than the pressure of the solvent fluid that it
displaces. In
another embodiment, the first pressure is at least 1000 psig. The fluid
exhausted
from the vessel can be at a pressure higher than the operating pressure of the
purification unit.
In yet another embodiment, solvent fluid is exhausted from the vessel at a
pressure that is lower than the first pressure and the displacing fluid is at
a pressure
sufficient to push through the solvent fluid.
EQUIVALENTS
While this invention has been particularly shown and described with
references to preferred embodiments thereof, it will be understood by those
skilled in
the art that various changes in form and details may be made therein without
departing from the scope of the invention encompassed by the appended claims.
