Abstract:
An isolated semiconductor wafer platen is disclosed for use in high pressure processing. The use of vacuum chucking for holding a semiconductor wafer during processing is well known in the art and can be applied to high pressure systems as well, but some difficulties can arise under high pressure processes. Small deflections in even very thick metal support platens can lead to backside wafer wear, platen abrasion, and even breakage of semiconductor wafers. This invention discloses a method to eliminate the transfer of flexure inherent in high pressure vessel walls, yet still retain the vacuum chucking method.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a method and system for clamping a substrate in a high pressure processing system and, more particularly, in a supercritical processing system.  
       BACKGROUND OF THE INVENTION  
       [0002]     During the fabrication of semiconductor devices for integrated circuits (ICs), a sequence of material processing steps, including both pattern etching and deposition processes, are performed, whereby material is removed from or added to a substrate surface, respectively. During, for instance, pattern etching, a pattern formed in a mask layer. of radiation-sensitive material, such as photoresist, using for example photolithography, is transferred to an underlying thin material film using a combination of physical and chemical processes to facilitate the selective removal of the underlying material film relative to the mask layer.  
         [0003]     Thereafter, the remaining radiation-sensitive material, or photoresist, and post-etch residue, such as hardened photoresist and other etch residues, are removed using one or more cleaning processes. Conventionally, these residues are removed by performing plasma ashing in an oxygen plasma, followed by wet cleaning through immersion of the substrate in a liquid bath of stripper chemicals.  
         [0004]     Until recently, dry plasma ashing and wet cleaning were found to be sufficient for removing residue and contaminants accumulated during semiconductor processing. However, recent advancements for ICs include a reduction in the critical dimension for etched features below a feature dimension acceptable for wet cleaning, such as a feature dimension below approximately 45 to 65 nanometers (nm). Moreover, the advent of new materials, such as low dielectric constant (low-k) materials, limits the use of plasma ashing due to their susceptibility to damage during plasma exposure.  
         [0005]     At present, interest has developed for the replacement of dry plasma ashing and wet cleaning. One interest includes the development of dry cleaning systems utilizing a supercritical fluid as a carrier for a solvent, or other residue removing composition. The use of supercritical carbon dioxide, for example, in processing semiconductor wafers has been shown in the art.  
         [0006]     Certain challenges occur when attempting to process silicon wafers under high pressure. One such issue is how to hold the wafer in place during processing. It has been shown that a wafer can be supported at discrete locations around its edge, with high pressure supercritical carbon dioxide (SCCO2) surrounding the entire wafer.  
         [0007]     A different approach is to hold the wafer down on a platen using vacuum or reduced pressure from the top surface of the wafer, which has also been shown. In such a case, bias in pressure keeps the wafer in place during processing, which may include violent events such as sudden decompressions, high surface velocity jets for cleaning, etc. One of the significant drawbacks of vacuum holding is the restraining of the wafer against the platen. With such a large surface area of 300 millimeter (mm) wafers, for example, the exposed area of the platen, is subjected to loads that can exceed half a million pounds. Even very thick steels platens will deflect under this kind of load. Typical pressures encountered in SCCO2 processing are a minimum of 1,031 psi, but 3,000 psi is not uncommon, and upwards of 10,000 psi has been reported in the literature.  
         [0008]     If a wafer is held against a platen, typically of stainless steel, the resulting static pressure load can force the wafer against the platen, which can cause damage to the backside of the wafer. Particulates that may be present can then get embedded into the platen or into the backside of the wafer. This can cause irreparable harm to the wafer for subsequent process steps.  
         [0009]     Another effect of these high forces is the flexing of the platen under the pressure load. As the pressure increases, the wafer becomes restrained against the platen. As pressure continues to increase, the platen can bow due to the load. The wafer may or may not be able to follow the new shape that the platen is forced into due to the pressure load. Once the pressure is released or reduced, the wafer must again readjust for the change in shape of the platen. If multiple pressure cycles are applied, this effect can be repeated many times on a single wafer.  
         [0010]     Results of this flexing can break wafers, because they are brittle and fragile and cannot elastically deform like stainless steel. It can also cause a grinding or fretting effect between the wafer and the platen, due to the high forces and small displacements which take place. This can create metal or silicon particles to be interspersed between the wafer and platen, which in turn can damage the current wafer, and be present on the platen to damage subsequent wafers that are processed.  
         [0011]     The magnitude of this flexing may be considered trivial under ordinary industrial circumstances. Unfortunately with semiconductor wafers, flexing of less than 0.0010 inches, or even as little as 0.0005 inches, have been shown to cause significant damage to wafers, or wafer breakage.  
         [0012]     At present, the inventors have recognized that if the force holding the wafer to the platen is reduced, the wafer can slip in relation to the platen, and the likelihood of breakage can be reduced. If the holding force is reduced even further in magnitude, then wear can also be eliminated because there would not be enough frictional force to create wear or particles.  
         [0013]     Misalignment of features on the wafer platen, or poor flatness of the platen surface can also result in wafer breakage if the holding load is high. If the wafer is required to span over holes or slots in the platen, then the wafer becomes a “bridge” with the entire pressure load bearing down on an unsupported region of thin silicon. It doesn&#39;t take a very large span to break a wafer when subjected to 3,000 psi or higher pressures.  
         [0014]     Accordingly, there is a need to overcome the above described problems.  
       SUMMARY OF THE INVENTION  
       [0015]     An objective of the present invention is to provide a pressure biased wafer holder for a high pressure processing system, and a system and method for pressure biased wafer holding in high pressure processing systems. A further objective of the invention is to provide a high pressure processing system and wafer holder therefor that overcomes the problems of the prior art. A more particular objective of the present invention is to provide a vacuum chuck for a high pressure processing system, for example a supercritical fluid processing system, that will avoid platen deformation and other problems of high pressure processing of substrates, for example, of semiconductor wafers.  
         [0016]     According to principles of the present invention, a method is provided for holding a semiconductor wafer to a platen in a high pressure fluid processing chamber. The method includes providing a rigid pressure plate between a high pressure fluid and an ambient atmosphere in the bottom of a processing chamber, supporting a platen disk on the pressure plate in the processing chamber, supporting a wafer on an upwardly facing surface of the platen disk, applying a vacuum to the backside of the supported wafer through passages within the platen disk, and substantially surrounding the underside of the platen disk with fluid at the pressure of high pressure fluid in the chamber.  
         [0017]     According to certain embodiments of the present invention, a wafer holder is provided for holding a semiconductor wafer by fluid pressure bias when processing the wafer in a processing chamber with a high pressure fluid. The wafer holder includes a platen disk having a wafer supporting surface on its frontside and passages therein that communicate with the wafer supporting surface and located to apply relatively low pressure to the backside of a wafer supported on the wafer supporting surface to hold the wafer to the platen from the pressure of the high pressure fluid. A rigid pressure plate holds the disk in a cavity that is configured to receive the platen disk while forming a gap between the disk and the pressure plate to communicate high pressure fluid behind the disk to substantially isolate the platen disk from a substantial pressure gradient between its front and back sides.  
         [0018]     Further according to some embodiments of the present invention, a processing system is provided for treating a substrate and includes a processing chamber configured to treat said substrate therein with a high pressure fluid, a platen coupled to said processing chamber and configured to support said substrate, a fluid supply system, a chuck coupled to said platen and configured to hold said substrate against said platen by a pressure gradient between said high pressure fluid and said platen; and means for substantially surrounding said platen with the high pressure fluid.  
         [0019]     These and other objectives and advantages of the present invention are set forth in the detailed description of the exemplary embodiments below. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]     In the accompanying drawings:  
         [0021]      FIG. 1  is a simplified schematic representation of one embodiment of a processing system according to principles of the present invention;  
         [0022]      FIG. 2  is a disassembled top perspective view of a platen assembly according to one embodiment of the system of  FIG. 1 ;  
         [0023]      FIG. 3  is a disassembled bottom perspective view of the platen assembly of  FIG. 2 ;  
         [0024]      FIG. 4  is a cross-sectional elevational view taken along line  4 - 4  of  FIG. 3 ;  
         [0025]      FIG. 4A  is an enlarged view of the encircled portion  4 A of  FIG. 4 .;  
         [0026]      FIG. 4B  is an enlarged view of the encircled portion  4 B of  FIG. 4 .; and  
         [0027]      FIG. 5  is a cross-sectional view taken along line  5 - 5  of  FIG. 4 .  
         [0028]      FIG. 6  is a bottom view of the platen of the assembly of  FIGS. 1-3 . 
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0029]     In the following description, to facilitate a thorough understanding of the invention and for purposes of explanation and not limitation, specific details are set forth, such as a particular geometry of the processing system and various descriptions of the system components. However, it should be understood that the invention may be practiced with other embodiments that depart from these specific details.  
         [0030]     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views,  FIG. 1  illustrates a processing system  100  according to an embodiment of the invention. In the illustrated embodiment, processing system  100  is configured to treat a substrate  105  with a high pressure fluid, such as a fluid in a supercritical state, with or without other additives, such as process chemistry. The processing system  100  comprises processing elements that include a processing chamber  110 , a fluid flow system  120 , a process chemistry supply system  130 , a high pressure fluid supply system  140 , and a controller  150 , all of which are configured to process substrate  105 . The controller  150  can be coupled to the processing chamber  110 , the fluid flow system  120 , the process chemistry supply system  130 , and the high pressure fluid supply system  140 .  
         [0031]     Alternately, or in addition, controller  150  can be coupled to a one or more additional controllers/computers (not shown), and controller  150  can obtain setup and/or configuration information from an additional controller/computer.  
         [0032]     In  FIG. 1 , singular processing elements ( 110 ,  120 ,  130 ,  140 , and  150 ) are shown, but this is not required for the invention. The processing system  100  can comprise any number of processing elements having any number of controllers associated with them in addition to independent processing elements.  
         [0033]     The controller  150  can be used to configure any number of processing elements ( 110 ,  120 ,  130 , and  140 ), and the controller  150  can collect, provide, process, store, and display data from processing elements. The controller  150  can comprise a number of applications for controlling one or more of the processing elements. For example, controller  150  can include a graphic user interface (GUI) component (not shown) that can provide easy to use interfaces that enable a user to monitor and/or control one or more processing elements.  100341  Referring still to  FIG. 1 , the fluid flow system  120  is configured to flow fluid and chemistry from the supplies  130  and  140  through the processing chamber  110 . The fluid flow system  120  is illustrated as a recirculation system through which the fluid and chemistry recirculate from, and back to, the processing chamber  110  via primary flow line  122 . This recirculation is most likely to be the preferred configuration for many applications, but this is not necessary to the invention. Fluids, particularly inexpensive fluids, can be passed through the processing chamber  110  once and then discarded, which might be more efficient than reconditioning them for re-entry into the processing chamber. Accordingly, while the fluid flow system or recirculation system  120  is described as a recirculating system in the exemplary embodiments, a non-recirculating system may, in some cases, be substituted. This fluid flow system  120  can include one or more valves (not shown) for regulating the flow of a processing solution through the fluid flow system  120  and through the processing chamber  110 . The fluid flow system  120  can comprise any number of back-flow valves, filters, pumps, and/or heaters (not shown) for maintaining a specified temperature, pressure or both for the processing solution and for flowing the process solution through the fluid flow system  120  and through the processing chamber  110 . Furthermore, any one of the many components provided within the fluid flow system  120  may be heated to a temperature consistent with the specified process temperature.  
         [0034]     Some components, such as a fluid flow or recirculation pump, may require cooling in order to permit proper functioning. For example, some commercially available pumps, having specifications required for processing perfonnance at high pressure and cleanliness during supercritical processing, comprise components that are limited in temperature. Therefore, as the temperature of the fluid and structure are elevated, cooling of the pump is required to maintain its functionality. Fluid flow system  120  for circulating or otherwise flowing the supercritical fluid through processing chamber  110  can comprise the primary flow line  122  coupled to high pressure processing system  100 , and configured to supply the supercritical fluid at a fluid temperature equal to or greater than 40 degrees C. to the high pressure processing system  100 , and a high temperature pump (not shown) coupled to the primary flow line  122 . The high temperature pump can be configured to move the supercritical fluid through the primary flow line  122  to the processing chamber  110 , wherein the high temperature pump comprises a coolant inlet configured to receive a coolant and a coolant outlet configured to discharge the coolant. A heat exchanger (not shown) coupled to the coolant inlet can be configured to lower a coolant temperature of the coolant to a temperature less than or equal to the fluid temperature of the supercritical fluid. Details regarding pump design are provided in co-pending U.S. patent application Ser. No. 10/987,066, entitled “Method and System for Cooling a Pump”; the entire content of which is herein incorporated by reference in its entirety.  
         [0035]     Referring again to  FIG. 1 , the processing system  100  can comprise high pressure fluid supply system  140 . The high pressure fluid supply system  140  can be coupled to the fluid flow system  120 , but this is not required. In alternate embodiments, high pressure fluid supply system  140  can be configured differently and coupled differently. For example, the fluid supply system  140  can be coupled directly to the processing chamber  110 . The high pressure fluid supply system  140  can include a supercritical fluid supply system. A supercritical fluid as referred to herein is a fluid that is in a supercritical state, which is that state that exists when the fluid is maintained at or above the critical pressure and at or above the critical temperature on its phase diagram. In such a supercritical state, the fluid possesses certain properties, one of which is the substantial absence of surface tension. Accordingly, a supercritical fluid supply system, as referred to herein, is one that delivers to a processing chamber a fluid that assumes a supercritical state at the pressure and temperature at which the processing chamber is being controlled. Furthermore, it is only necessary that at least at or near the critical point the fluid is in substantially a supercritical state at which its properties are sufficient, and exist long enough, to realize their advantages in the process being performed. Carbon dioxide, for example, is a supercritical fluid when maintained at or above a pressure of about 1,070 psi at a temperature of 31 degrees C. This state of the fluid in the processing chamber may be maintained by operating the processing chamber at 2,000 to 10,000 psi at a temperature of approximately 40 degrees C. or greater.  
         [0036]     As described above, the fluid supply system  140  can include a supercritical fluid supply system, which can be a carbon dioxide supply system. For example, the fluid supply system  140  can be configured to introduce a high pressure fluid having a pressure substantially near the critical pressure for the fluid. Additionally, the fluid supply system  140  can be configured to introduce a supercritical fluid, such as carbon dioxide in a supercritical state. Additionally, for example, the fluid supply system  140  can be configured to introduce a supercritical fluid, such as supercritical carbon dioxide, at a pressure ranging from approximately the critical pressure of carbon dioxide to 10,000 psi. Examples of other supercritical fluid species useful in the broad practice of the invention include, but are not limited to, carbon dioxide (as described above), oxygen, argon, krypton, xenon, ammonia, methane, methanol, dimethyl ketone, hydrogen, water, and sulfuir hexafluoride. The fluid supply system can, for example, comprise a carbon dioxide source (not shown) and a plurality of flow control elements (not shown) for generating a supercritical fluid. For example, the carbon dioxide source can include a CO2 feed system, and the flow control elements can include supply lines, valves, filters, pumps, and heaters. The fluid supply system  140  can comprise an inlet valve (not shown) that is configured to open and close to allow or prevent the stream of supercritical carbon dioxide from flowing into the processing chamber  110 . For example, controller  150  can be used to determine fluid parameters such as pressure, temperature, process time, and flow rate.  
         [0037]     Referring still to  FIG. 1 , the process chemistry supply system  130  is coupled to the recirculation system  120 , but this is not required for the invention. In alternate embodiments, the process chemistry supply system  130  can be configured differently, and can be coupled to different elements in the processing system I  00 . The process chemistry is introduced by the process chemistry supply system  130  into the fluid introduced by the fluid supply system  140  at ratios that vary with the substrate properties, the chemistry being used and the process being performed in the processing chamber  110 . Usually the ratio is roughly 1 to 15 percent by volume in systems where the chamber, recirculation system and associated plumbing have a volume of about one liter. This amounts to about 10 to 150 milliliters of additive in most cases. The ratio may be higher or lower.  
         [0038]     The process chemistry supply system  130  can be configured to introduce one or more of the following process compositions, but not limited to: cleaning compositions for removing contaminants, residues, hardened residues, photoresist, hardened photoresist, post-etch residue, post-ash residue, post chemical-mechanical polishing (CMP) residue, post-polishing residue, or post-implant residue, or any combination thereof; cleaning compositions for removing particulate; drying compositions for drying thin films, porous thin films, porous low dielectric constant materials, or air-gap dielectrics, or any combination thereof; film-forming compositions for preparing dielectric thin films, metal thin films, or any combination thereof; healing compositions for restoring the dielectric constant of low dielectric constant (low-k) films; sealing compositions for sealing porous films; or any combination thereof. Additionally, the process chemistry supply system  130  can be configured to introduce solvents, co-solvents, surfactants, etchants, acids, bases, chelators, oxidizers, film-forming precursors, or reducing agents, or any combination thereof.  
         [0039]     The process chemistry supply system  130  can be configured to introduce N-methyl pyrrolidone (NMP), diglycol amine, hydroxyl amine, di-isopropyl amine, tri-isoprpyl amine, tertiary amines, catechol, ammonium fluoride, ammonium bifluoride, methylacetoacetamide, ozone, propylene glycol monoethyl ether acetate, acetylacetone, dibasic esters, ethyl lactate, CHF3, BF3, HF, other fluorine containing chemicals, or any mixture thereof. Other chemicals such as organic solvents may be utilized independently or in conjunction with the above chemicals to remove organic materials. The organic solvents may include, for example, an alcohol, ether, and/or glycol, such as acetone, diacetone alcohol, dimethyl sulfoxide (DMSO), ethylene glycol, methanol, ethanol, propanol, or isopropanol (IPA). For further details, see U.S. Patent No.  6 , 306 , 564 , filed May  27 ,  1998 , and titled “Removal of Resist or Residue from Semiconductors Using Supercritical Carbon Dioxide”, and U.S. Pat. No. 6,509,141, filed Sep. 3, 1999, and titled “Removal of Photoresist and Photoresist Residue from Semiconductors Using Supercritical Carbon dioxide Process,” both incorporated by reference herein.  
         [0040]     Additionally, the process chemistry supply system  130  can comprise a cleaning chemistry assembly (not shown) for providing cleaning chemistry for generating supercritical cleaning solutions within the processing chamber. The cleaning chemistry can include peroxides and a fluoride source. For example, the peroxides can include hydrogen peroxide, benzoyl peroxide, or any other suitable peroxide, and the fluoride sources can include fluoride salts (such as ammonium fluoride salts), hydrogen fluoride, fluoride adducts (such as organo-ammonium fluoride adducts), and combinations thereof. Further details of fluoride sources and methods of generating supercritical processing solutions with fluoride sources are described in U.S. patent application Ser. No. 10/442,557, filed May 20, 2003, and titled “Tetra-Organic Ammonium Fluoride and HF in Supercritical Fluid for Photoresist and Residue Removal”, and U.S. patent application Ser. No. 10/321,341, filed Dec. 16, 2002, and titled “Fluoride in Supercritical Fluid for Photoresist Polymer and Residue Removal,” both incorporated by reference herein.  
         [0041]     Furthermore, the process chemistry supply system  130  can be configured to introduce chelating agents, complexing agents and other oxidants, organic and inorganic acids that can be introduced into the supercritical fluid solution with one or more carrier solvents, such as N,N-dimethylacetamide (DMAc), gamma-butyrolactone (BLO), dimethyl sulfoxide (DMSO), ethylene carbonate (EC), N-methyl pyrrolidone (NMP), dimethylpiperidone, propylene carbonate, and alcohols (such a methanol, ethanol and 2-propanol).  
         [0042]     Moreover, the process chemistry supply system  130  can comprise a rinsing chemistry assembly (not shown) for providing rinsing chemistry for generating supercritical rinsing solutions within the processing chamber. The rinsing chemistry can include one or more organic solvents including, but not limited to, alcohols and ketone. In one embodiment, the rinsing chemistry can comprise sulfolane, also known as thiocyclopentane-1,1-dioxide, (cyclo)tetramethylene sulphone and 2,3,4,5-tetrahydrothiophene-1,1-dioxide, which can be purchased from a number of venders, such as Degussa Stanlow Limited, Lake Court, Hursley Winchester SO21 2LD UK.  
         [0043]     Furthermore, the process chemistry supply system  130  can be configured to introduce treating chemistry for curing, cleaning, healing (or restoring the dielectric constant of low-k materials), or sealing, or any combination thereof, or for applying low dielectric constant films (porous or non-porous). The chemistry can include hexamethyldisilazane (HMDS), chlorotrimethylsilane (TMCS), trichloromethylsilane (TCMS), dimethylsilyldiethylamine (DMSDEA), tetramethyldisilazane (TMDS), trimethylsilyldimethylamine (TMSDMA), dimethylsilyldimethylamine (DMSDMA), trimethylsilyldiethylamine (TMSDEA), bistrimethylsilyl urea (BTSU), bis(dimethylamino)methyl silane (B[DMA]MS), bis (dimethylamino)dimethyl silane (B[DMA]DS), HMCTS, dimethylaminopentamethyldisilane (DMAPMDS), dimethylaminodimethyldisilane (DMADMDS), disila-aza-cyclopentane (TDACP), disila-oza-cyclopentane (TDOCP), methyltrimethoxysilane (MTMOS), vinyltrimethoxysilane (VTMOS), or trimethylsilylimidazole (TMSI). Additionally, the chemistry may include N-tert-butyl- 1,1-dimethyl- 1-(2,3,4,5-tetramethyl-2,4-cyclopentadiene- 1-yl) silanamine, 1,3-diphenyl- 1,1,3,3-tetramethyldisilazane, or tert-butylchlorodiphenylsilane. For further details, see U.S. patent application Ser. No. 10/682,196, filed Oct. 10, 2003, and titled “Method and System for Treating a Dielectric Film,” and U.S. patent application Ser. No. 10/379,984, filed Mar. 4, 2003, and titled “Method of Passivating Low Dielectric Materials in Wafer Processing,” both incorporated by reference herein.  
         [0044]     Additionally, the process chemistry supply system  130  can be configured to introduce peroxides during, for instance, cleaning processes. The peroxides can include organic peroxides, or inorganic peroxides, or a combination thereof. For example, organic peroxides can include 2-butanone peroxide; 2,4-pentanedione peroxide; peracetic acid; t-butyl hydroperoxide; benzoyl peroxide; or m-chloroperbenzoic acid (mCPBA). Other peroxides can include hydrogen peroxide.  
         [0045]     The processing chamber  110  can be configured to process substrate  105  by exposing the substrate  105  to fluid from the fluid supply system  140 , or process chemistry from the process chemistry supply system  130 , or a combination thereof in a processing space  112 . Additionally, processing chamber  110  can include an upper chamber assembly  114 , and a lower chamber assembly  115 .  
         [0046]     The upper chamber assembly  114  can comprise a heater (not shown) for heating the processing chamber  110 , the substrate  105 , or the processing fluid, or a combination of two or more thereof. Alternately, a heater is not required. Additionally, the upper chamber assembly  114  can include flow components for flowing a processing fluid through the processing chamber  110 . In one embodiment, the high pressure fluid is introduced to the processing chamber  110  through a ceiling formed in the upper chamber assembly  112  and located above substrate  105  through one or more inlets located above a substantially center portion of substrate  105 . The high pressure fluid flows radially outward across an upper surface of substrate  105  beyond a peripheral edge of substrate  105 , and discharges through one or more outlets, wherein the spacing between the upper surface of substrate  105  and the ceiling decreases with radial position from proximate the substantially center portion of substrate  105  to the peripheral edge of substrate  105 .  
         [0047]     The lower chamber assembly  115  can include a platen  116  configured to support substrate  105  and a drive mechanism  118  for translating the platen  116  in order to load and unload substrate  105 , and sealing lower chamber assembly  115  with upper chamber assembly  114 . The platen  116  can also be configured to heat or cool the substrate  105  before, during, and/or after processing the substrate  105 . For example, the platen  116  can include one or more heater rods configured to elevate the temperature of the platen to approximately 31 degrees C. or greater. Additionally, the lower assembly  115  can include a lift pin assembly for displacing the substrate  105  from the upper surface of the platen  116  during substrate loading and unloading.  
         [0048]     Additionally, controller  150  includes a temperature control system coupled to one or more of the processing chamber  110 , the fluid flow system  120  (or recirculation system), the platen  116 , the high pressure fluid supply system  140 , or the process chemistry supply system  130 . The temperature control system is coupled to heating elements embedded in one or more of these systems, and configured to elevate the temperature of the supercritical fluid to approximately 31 degrees C. or greater. The heating elements can, for example, include resistive heating elements.  
         [0049]     A transfer system (not shown) can be used to move a substrate into and out of the processing chamber  110  through a slot (not shown). In one example, the slot can be opened and closed by moving the platen  116 , and in another example, the slot can be controlled using an on-off valve (not shown).  
         [0050]     The substrate can include semiconductor material, metallic material, dielectric material, ceramic material, or polymer material, or a combination of two or more thereof. The semiconductor material can include Si, Ge, Si/Ge, or GaAs. The metallic material can include Cu, Al, Ni, Pb, Ti, and/or Ta. The dielectric material can include silica, silicon dioxide, quartz, aluminum oxide, sapphire, low dielectric constant materials, TEFLON®, and/or polyimide. The ceramic material can include aluminum oxide, silicon carbide, etc.  
         [0051]     The processing system  100  can further comprise an exhaust control system. The exhaust control system can be coupled to the processing chamber  110 , but this is not required. In alternate embodiments, the exhaust control system can be configured differently and coupled differently. The exhaust control system can include an exhaust gas collection vessel (not shown) and can be used to remove contaminants from the processing fluid. Such exhaust control system can be used as an alternative to the recirculation system  120  that is provided to recycle the processing fluid.  
         [0052]     The processing system  100  can also comprise a pressure control system (not shown). The pressure control system can be coupled to the processing chamber  110 , but this is not required. In alternate embodiments, the pressure control system can be configured differently and coupled differently. The pressure control system can include one or more pressure valves (not shown) for exhausting the processing chamber  110  and/or for regulating the pressure within the processing chamber  110 . Alternately, the pressure control system can also include one or more pumps (not shown). For example, one pump may be used to increase the pressure within the processing chamber, and another pump may be used to evacuate the processing chamber  110 . In another embodiment, the pressure control system can comprise seals for sealing the processing chamber. In addition, the pressure control system can comprise an elevator for raising and lowering the substrate  105  and/or the platen  116 .  
         [0053]     The platen  116  is a platen assembly that includes a vacuum chuck for clamping the wafer to the platen. The platen  116  includes a semiconductor wafer platen disk that is isolated for use in high pressure processing, where the use of vacuum chucking for holding a semiconductor wafer during processing usually presents some difficulties, such as, for example, small deflections in the support platens that can lead to backside wafer wear, platen abrasion, and or breakage of semiconductor wafers. These difficulties are largely eliminated by platen assembly features to allow vacuum chucking to be used in high pressure processing.  
         [0054]      FIG. 2  is a top exploded perspective view of one embodiment  200  of a platen assembly  116  according to the principles of the present invention. The illustrated embodiment  200  of the wafer platen assembly  116  includes disk  201 , which may usually be of stainless steel. The disk  201  has a highly polished top face  205  on which the wafer  105  will rest. The wafer platen disk  201  has two concentric vacuum grooves  207 ,  208  on its face  205 . These grooves  207 ,  208  are connected through passages to one or more ports on the underside of the platen assembly  200 , which ports connect to a vacuum pump or other low pressure source remote from the processing chamber. The vacuum source applies negative pressure, typically maintained at at least  10  psi below atmospheric pressure, to apply a negative pressure to the underside of the wafer  105  that will hold the wafer  105  to the surface  205  whether the chamber  110  is pressurized or depressurized.  
         [0055]     The disk  201  may contain a variety of radial bores, including bores  211  ( FIGS. 2, 4  and  5 ) for the use of rod style heater cartridges (not shown), as well as bores  212  for mounting a control thermocouple (TC). The disk  201  also has three small axial through bores  214  extending through the top face  205  and through to the bottom of the assembly  200  for wafer lifting pins (not shown). On the backside of the disk  201  is a stepped circular recess  218  to accept isolation plate  220 . See  FIG. 3  and  FIG. 5  for bottom view of the disk  201  and  FIG. 4  for cross-section thru the disk  201  at the elevation of the heater rod bores  211 .  
         [0056]     Isolation plate  220  contains o-ring groove features (not shown) for holding  0 -rings for sealing between it and the disk  201  of the wafer platen assembly  200 , between the wafer platen  201  vacuum bore and the isolation plate  220 . Grooves for o-ring seals  224  are also provided on the underside of the isolation plate  220  between all the pass-thru bores and the pressure plate  204 . The isolation plate  220  has very small pads  230  (see  FIG. 4A ) that extend beyond the bottom plane of the isolation plate  220 . These pads  230  space off the isolation plate  220  and the wafer platen disk  201  from the pressure plate  204  to leave a narrow gap  232  between the pressure plate  204  and both the isolation disk  220  and platen disk  201 . By spacing off from the pressure plate  204 , the very small gap  232  is created underneath the entire wafer platen  201  and the isolation plate  220 , which will fill with pressurized fluid from the processing chamber and equalize pressure with the fluid filling up to the top side  205  of the wafer platen  201 , as shown in  FIG. 4B . This will create a situation where any increase in chamber pressure will result in no pressure gradient across the wafer platen of the wafer platen disk  201  that would produce deflection or distortion of the wafer platen disk  201 . This is because the disk  201  and the isolation plate  220  “float” in the pressurized gas that fills the gap  232 . Although there will be forces applied where the  0 -rings on the bottom side of the isolation plate  220  mount against the pressure plate  204  due to the differences in pressures, these loads will be “spot” loads and will not contribute substantially to, and bending of, the wafer platen disk  201 .  
         [0057]     Further, the pads  230  on the bottom of the isolation plate  220  are arranged in a circular pattern. The result of analysis and wear has shown that a thick platen assembly  200  that includes the pressure plate  204  will flex in an expanding and contracting radial pattern. By arranging other structures in a similar circular pattern, the flexing can be better followed by the isolation plate  220  and all the mounting fasteners. Accordingly, it is preferential not to bolt in a non-symmetric pattern when such movement can occur, as uneven stress and wear patterns can develop.  
         [0058]     A seal ring  203 , shown in  FIGS. 2 and 3 , fits into an annular groove  202  in the top face of the peripheral edge of the pressure plate  204 . The seal  203  creates a pressure tight seal when the pressure plate  204  is brought into intimate contact with the rest of the chamber assembly, such as inner wall  113  of  FIG. 1 , to enclose a cavity containing the processing space  212  directly above the top surface  205  of the wafer platen disk  201  into which the supercritical CO2 may be introduced for processing. Not shown for clarity are a number of other o-ring seals on the isolation plate  220 .  
         [0059]     Pressure plate  204  is a thick piece of stainless steel and can serve as a closure for one side of the pressure vessel for the processing chamber  110 . The thicker the pressure plate  204 , the less will be its deflection during high pressure processing. Typically, a range of thickness about 3 to 5 inches is usually sufficient for a 3,000 psi chamber design for a chamber for processing 200 millimeter and 300 millimeter diameter wafers. The top face of the pressure plate  204  contains a cylindrical recess  240  configured to accept the wafer platen disk  201  plus a very small additional space  242  at the periphery of the gap  232  all the way around the perimeter of the disk  201 . The gap thickness is not a critical dimension, but with a smaller gap, less gas and additional co-solvents will be required to fill the gap volume. Gaps on the order of 0.020 inches are usually acceptable, but gaps an inch or more may also be acceptable. Further, if the space  242  is not narrow, additional structure to hold the disk  201  on center is desirable.  
         [0060]     In the center portion of the pressure plate  204  are a series of mating holes  245  for retention bolts which will mount through holes  235  in the isolation plate  220  and into holes  219  in the wafer platen disk  201 . Bores  244  are also provided to allow for a set of three lift pins to pass through the pressure plate  204 , in alignment with the bores  214  in the disk  201  and bores  234  in the isolation plate  220 , to lift a wafer  105  from, and lower a wafer  105  onto, the surface  205 . The lift pin mechanism is not shown, but various mechanisms are well known in the art. Also provided in the pressure plate  204  are pass-through holes for wiring associated with heaters and thermocouples that are mounted in the bores  211  and  212 , respectively.  
         [0061]     Although one embodiment is depicted in the drawings, other versions and features are contemplated. For example, it is anticipated that an over temperature switch will be incorporated into the wafer platen disk  201 . This will require more bores or cavities in wafer platen  201 . The use of a patch type heating element may be employed instead of separate heating rods. A single center support or pad may also be used instead of multiple pads in a circular pattern as depicted. Each such variation or design may have its own merits based on specific circumstances.  
         [0062]     Further advantages include the ability to locate a vent beneath the wafer platen disk  201 , which may improve flow dynamics in some chamber designs. Typically, the disk  201  and isolation plate  220 , and all of the cavities and ports within them, may be machined into a single piece wafer platen disk  201 , or even a single piece assembly of platen disk  201  and pressure plate  204 . The fabrication of such large parts can be a complex task. If the wafer platen disk  201 , as illustrated in the assembly  200 , is separated from the rest of the wafer platen assembly  200 , the production and overall cost of the parts is reduced. Also, with separate parts, the refurbishment costs may be greatly reduced since only the wafer platen disk  201  is likely to need to be overhauled on most occasions.  
         [0063]     Although only certain exemplary embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention.