Abstract:
A carbon dioxide recovery apparatus and process for supercritical extraction includes providing a process stream from a supercritical extraction procedure in which the process stream includes pressurized carbon dioxide, extraction process waste and optionally at least one co-solvent; reducing the pressure of the process stream below critical pressure; venting low pressure carbon dioxide vapor to exhaust; cooling the process stream to form a two phase mixture; separating the two phase mixture into a process liquid, containing co-solvent if present, and a process vapor phase stream; collecting the process liquid; filtering the process vapor phase stream to remove particulates and optionally residual co-solvent; passing the filtered process vapor stream through an adsorber to remove trace impurities to form a purified carbon dioxide vapor stream; and, drying the purified carbon dioxide vapor stream to remove residual water vapor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims priority from Provisional Patent Application No. 60/415,655 filed Oct. 2, 2002, which is incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The present invention relates to a process and apparatus for recovering carbon dioxide used for supercritical extraction in an industrial process, such as semiconductor wafer cleaning. 
   BACKGROUND 
   Supercritical mixtures of high purity CO 2  and organic co-solvents, such as ethanol, are used in the production of semiconductor wafers. This includes post-etch cleaning, residue removal, and metal and dielectric deposition. For a commercial process, CO 2  flows are large and CO 2  needs to be recycled after each wafer production step. CO 2  recycle for semiconductor requirements can only be done if the co-solvents and dissolved impurities can be substantially removed from the recycle stream. 
   For a large-scale chemical production system, the amounts of CO 2 /co-solvent waste generated are significant. To make the process more economical, CO 2  can be recycled, as long as the co-solvent (and other impurities) can be separated from the CO 2 . To accomplish this, the prior art discloses the use of distillation columns, where the CO 2  and the co-solvents are separated. This system will work as long as there are only two major components in the system, namely CO 2  and the co-solvent. If trace amounts of other components are present, this process has to be substantially altered with additional equipment, design changes and added expense. Still, the separation of all measurable traces is not always possible with known systems. 
   Previous processes for recovery of carbon dioxide for reuse in industrial processes such as semiconductor wafer manufacturing require the use of accessory equipment such as boilers and condensers for distilling and condensing the solvent waste stream, together with significant energy requirements. 
   U.S. Pat. No. 4,349,415 discloses a process for separating organic liquid solutes from their solvent mixtures, wherein the extractant can be carbon dioxide used in the supercritical state. 
   SUMMARY 
   A carbon dioxide recovery process for supercritical extraction is provided comprising: 
   providing a process stream from a supercritical extraction procedure wherein the process stream includes pressurized carbon dioxide, extraction process waste and optionally at least one co-solvent; 
   reducing the pressure of the process stream below critical pressure; 
   venting low pressure carbon dioxide vapor to exhaust; 
   cooling the process stream to form a two phase mixture; 
   separating the two phase mixture into a process liquid, containing co-solvent if present, and a process vapor phase stream; 
   collecting the process liquid; 
   filtering the process vapor phase stream to remove particulates and optionally residual co-solvent; 
   passing the filtered process vapor stream through an adsorber to remove trace impurities to form a purified carbon dioxide vapor stream; and, 
   drying the purified carbon dioxide vapor stream to remove residual water vapor. 
   The process may further comprise optionally supplementing the dried, purified carbon dioxide vapor stream with a distilled carbon dioxide vapor stream to form a feed carbon dioxide vapor stream; filtering the feed carbon dioxide vapor stream to remove condensable vapors and particulates; and cooling the filtered feed carbon dioxide stream to form an intermediate carbon dioxide liquid stream. 
   The process may still further comprise filtering the intermediate carbon dioxide liquid stream; and pressurizing the intermediate carbon dioxide liquid stream. A supercritical fluid may be formed from the pressurized carbon dioxide liquid stream and delivered to the extraction procedure. 
   In one embodiment, wherein the adsorber is a plurality of adsorber beds, the process may further comprise isolating at least one adsorber bed and flushing the isolated adsorber bed with vaporized refrigerant from at least one condenser. 
   In another embodiment, a process for carbon dioxide supercritical extraction and recovery is provided comprising: 
   distilling a feed stream comprising carbon dioxide vapor off of a liquid carbon dioxide supply; 
   filtering the feed carbon dioxide vapor stream to remove condensable vapors and particulates; 
   cooling the filtered feed carbon dioxide stream to form an intermediate carbon dioxide liquid stream; 
   filtering the intermediate carbon dioxide liquid stream; 
   pressurizing the intermediate carbon dioxide liquid stream; 
   forming a supercritical fluid from the pressurized carbon dioxide liquid stream and delivering the supercritical fluid and optionally a co-solvent for a supercritical extraction procedure; 
   obtaining a process stream from the supercritical extraction procedure wherein the process stream includes pressurized carbon dioxide, extraction process waste and optionally at least one co-solvent; 
   reducing the pressure of the process stream below critical pressure; 
   venting low pressure carbon dioxide vapor to exhaust; 
   cooling the process stream to form a two phase mixture; 
   separating the two phase mixture into a process liquid, containing co-solvent if present, and a process vapor phase stream; 
   collecting the process liquid; 
   filtering the process vapor phase stream to remove particulates and optionally residual co-solvent; 
   passing the filtered process vapor stream through an adsorber to remove trace impurities to form a purified carbon dioxide vapor stream; 
   drying the purified carbon dioxide vapor stream to remove residual water vapor; and, 
   optionally supplementing the dried, purified carbon dioxide vapor stream with additional distilled carbon dioxide vapor. 
   An apparatus is provided for the recovery of carbon dioxide from a supercritical extraction process producing a process stream containing pressurized liquid carbon dioxide, extraction process waste and optionally at least one co-solvent, comprising means for reducing the pressure of the liquid carbon dioxide; and a vent for passing low pressure carbon dioxide vapor resulting from the pressure reduction to exhaust; characterized by further comprising: 
   a separator for forming the process stream into two phases comprising a process liquid, containing co-solvent if present, and a process vapor phase stream; 
   a container for collecting the process liquid; 
   at least one filter to remove particulates and optionally residual co-solvent from the process vapor phase stream; 
   an adsorber to remove trace impurities from the filtered process vapor stream to form a purified carbon dioxide vapor stream; 
   at least one dryer to remove residual water vapor from the purified carbon dioxide vapor stream; 
   a flow network having conduits connecting the components of the apparatus; 
   the conduits of the flow network including a connection between the at least one dryer and a condenser associated with a supply of carbon dioxide to the extraction process; and, 
   the flow network having valves associated with said conduits to allow for isolation of components of the apparatus. 
   In certain embodiments, the separator is a second condenser for cooling the process stream to form the two phases. In certain embodiments, the adsorber is a plurality of adsorber beds. In certain embodiments, at least one condenser includes an external refrigeration circuit having a heat exchanger to condense the purified carbon dioxide vapor feed stream through indirect heat exchange with a refrigerant stream. In certain of those embodiments, a conduit is provided between the refrigeration circuit and the adsorber to permit used refrigerant to flush out at least one adsorber bed when isolated from the apparatus. 
   In another embodiment, an apparatus is provided for the supply and recovery of carbon dioxide for a supercritical extraction process comprising: 
   a bulk liquid carbon dioxide supply tank for distilling off a feed stream comprising carbon dioxide vapor; 
   at least one purifying filter to remove condensable vapors and particulates from the carbon dioxide vapor feed stream; 
   a first condenser for condensing the carbon dioxide vapor feed stream into an intermediate liquid carbon dioxide stream; 
   a low pressure accumulation vessel for accumulating the intermediate liquid carbon dioxide stream; 
   at least one particle filter to remove particulates from the intermediate liquid carbon dioxide stream; 
   means for pressurizing the intermediate liquid carbon dioxide stream to form a pressurized liquid carbon dioxide stream; 
   a high-pressure accumulation vessel for accepting the pressurized liquid carbon dioxide stream; 
   a supercritical extraction apparatus for receiving the pressurized liquid carbon dioxide stream and optionally a co-solvent, for carrying out the supercritical extraction and providing a process stream comprising pressurized liquid carbon dioxide, extraction process waste and optionally the at least one co-solvent; 
   means for reducing the pressure of the liquid carbon dioxide; 
   a vent for passing low pressure carbon dioxide vapor resulting from the pressure reduction to exhaust; 
   a separator for forming the process stream into two phases comprising a process liquid, containing co-solvent if present, and a process vapor phase stream; 
   a container for collecting the process liquid; 
   at least one filter to remove particulates and optionally residual co-solvent from the process vapor phase stream; 
   an adsorber to remove trace impurities from the filtered process vapor stream to form a purified carbon dioxide vapor stream; 
   at least one dryer to remove residual water vapor from the purified carbon dioxide vapor stream; 
   a flow network having conduits connecting the components of the apparatus; 
   the conduits of said flow network including a vapor vent line from the low pressure accumulation vessel to the condenser to facilitate introduction of the intermediate liquid carbon dioxide stream into the low pressure accumulation vessel; 
   the conduits of the flow network including a connection between the at least one dryer and the at least one purifying filter upstream from the first condenser; and, 
   the flow network having valves associated with said conduits to allow for isolation of components of the apparatus. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic view of an apparatus for carrying out a supercritical extraction and recovery process. 
   

   DETAILED DESCRIPTION 
   Although the subject apparatus and process are applicable to many processes for making industrial products which must be cleaned during or after production, for convenience the apparatus and process will be exemplified with respect to their use in connection with semiconductor wafer processing. According to a process for cleaning an industrial product, such as a semiconductor wafer, with supercritical carbon dioxide (SCCO 2 ) and a co-solvent, and recovering the carbon dioxide (CO2) for recycle, an apparatus and process are provided including introducing a feed stream comprising carbon dioxide vapor into a purifying filter, such as for carrying out gas phase purification; condensing the purified CO 2  stream, such as by use of mechanical refrigeration or cryogenic refrigerants, and compressing and heating the purified CO 2  to achieve the supercritical state. 
   After the high purity, supercritical CO 2  stream has gone through the wafer production process, the supercritical waste stream (which may contain co-solvent and impurities removed from the wafers) is depressurized and cooled such as by using mechanical or cryogenic refrigerants to form a two-phase mixture, with the majority of the impurities being in the liquid phase. The phase separation results in a vapor phase that contains primarily CO 2  with trace amounts of impurities. The vapor phase is passed through an adsorption system such that the trace impurities are removed. The now purified CO 2  stream is cooled to a temperature such that the CO 2  is entirely condensed, and the purified liquid CO 2  is returned to a storage tank or low pressure accumulator. 
   The goal for wafer cleaning is to remove all surface contaminants or residue, such as particles, organics, metallics and native oxides. Throughout certain wafer fabrication processes, it is estimated that the surface of an individual wafer is cleaned up to 100 times. The wafers are treated with supercritical fluids to clean, strip solvents or photo-resist resins, dehydrate or otherwise treat the wafers or structures on the wafers. 
   The semiconductor wafer to be processed, or cleaned, is contacted with supercritical CO 2  (SCCO 2 ) directly in a processing chamber, such that the target residue dissolves in the SCCO 2  or in the SCCO 2  and a co-solvent. Between about 2% to about 50% by weight of organic co-solvents, such as ethanol, methanol, are often dissolved in the SCCO 2  to increase the solubility of the residue in SCCO 2 . This dissolution may be carried out at pressures over 100 bar and temperatures between approximately 40° C. and 200° C. 
   The dissolved mixture is transferred to another vessel where the pressure and temperature are reduced, causing the residue material and co-solvent to condense out as a waste stream. Co-solvent material can be recovered using standard distillation methods. The CO 2 -soluble co-solvent mixture is exhausted as a vent stream. 
   With reference to  FIG. 1 , a carbon dioxide recovery and supply apparatus is shown generally at  1 . From a bulk supply of liquid carbon dioxide  10 , a feed stream  11  comprising carbon dioxide vapor is distilled in a first purification stage, and is introduced into a purifying filter  13  and a particle filter  14  which can be any of a number of known, commercially available filters, for a second stage purification. The first filter  13  may be a coalescing filter to remove condensable hydrocarbons. A single adsorber bed, including activated carbon, alumina, and carbon molecular sieve material may remove additional amounts of hydrocarbon impurities. 
   Valve  12  is provided to isolate the bulk CO 2  supply  10 . The bulk supply  10  may be a tank of liquid CO 2  maintained at about 300 psig (2.1 MPa) and about 0° F. (−18° C.). As carbon dioxide vapor is drawn out of the bulk supply tank, a portion of the liquid carbon dioxide in the bulk tank is drawn through conduit  16  and introduced to a pressure build device  17  such as an electric or steam vaporizer or the like, to maintain the pressure relatively constant within the bulk supply tank even though carbon dioxide vapor is being removed. The vaporizer takes liquid CO 2  from the supply tank and uses heat to change the CO 2  from the liquid phase to the gas phase. The resulting CO 2  gas is introduced back into the headspace of the supply tank. 
   The feed stream  11  after having been purified in the second stage is introduced into a condenser  18  that is provided with a heat exchanger  21  to condense the carbon dioxide vapor into a liquid  19 . Such condensation is effected by an external refrigeration unit  22  that circulates a refrigeration stream through the heat exchanger, preferably of shell and tube design. Isolation valves  28  and  29  can be provided to isolate refrigeration unit  22  and its refrigerant feed line  26  and return line  27 . 
   The liquid carbon dioxide is temporarily stored in a receiver vessel  30 , that is, a low pressure accumulation vessel. The level of liquid in the low pressure accumulation receiver vessel  30  is controlled by a level sensor  53  (such as a level differential pressure transducer) which monitors the level of liquid carbon dioxide and a pressure sensor  54  (such as a pressure transducer) which monitors the pressure within the receiver vessel  30 , via a controller (not shown, such as a programmable logic computer). 
   An intermediate liquid stream comprising high purity CO 2  liquid  19  is introduced from the receiver vessel  30  into a high-pressure accumulation vessel  50  after further purification and pressurization. The intermediate liquid carbon dioxide from the receiver vessel  30  travels through outlet conduit  32  and is again purified in a further purification stage by one of two particle filters  41  and  42 . The particle filters  41  and  42  can be isolated by valves  35 , 36  and  37 , 38  respectively, so that one filter can be operational while the other filter is isolated from the conduit by closure of its respective valves, for cleaning or replacement. The low pressure, purified intermediate liquid carbon dioxide stream  39  emerges from the final filtration stage for pressurization, such as by a compressor  45 , and storage in the high pressure accumulation vessel  50  prior to use in the desired process as described above. 
   A valve network controls the flow within the apparatus. In this regard, fill control valve  25  controls the flow of the intermediate liquid stream from the condenser  18  to the receiver vessel  30 . Control of the flow of the low pressure intermediate liquid carbon dioxide stream through outlet conduit  32  is effected by product control valve  34 . Drain valve  33  also is connected to outlet conduit  32  for sampling or venting, as needed. The venting of the low-pressure accumulation receiver vessel  30  via vent line (conduit)  31  to the condenser  18  is controlled by vent control valve  24 . 
   An insulation jacket  20 , such as one formed of polyurethane or the equivalent, can be disposed about the condenser  18 , the conduit for carrying the liquid CO 2    19 , the receiver vessel  30 , and the outlet conduit  32  and associated valves to maintain the desired temperature of the liquid CO 2 . 
   The liquid CO 2  in the high pressure accumulation vessel  50  may be stored on the order of 300 bar (30 MPa) pressure and 80° C., that is, above the critical pressure (7.38 MPa) and critical temperature (31.1° C.) for carbon dioxide. Upon demand, the high purity supercritical carbon dioxide  51  is dispensed into a cleaning chamber  60  for the industrial part, such as semiconductor wafers, together with at least one co-solvent for the residue to be removed from the wafers. 
   After the high purity supercritical CO 2  stream has gone through the wafer production or cleaning process, the supercritical waste stream  61  is depressurized and passed through redundant valves  62 ,  63  to form a CO 2 /co-solvent stream  65  on the order of about 25 bar (2.5 MPa) pressure and 50° C. Low pressure CO 2  vapor  66  can be sent to a scrubber via valve  64 . The CO 2 /co-solvent stream  65  passes through valve  68  to be cooled using mechanical or cryogenic refrigerants in a condenser  70  to form a two-phase mixture, with the majority of impurities being in the primarily co-solvent liquid phase  71  that is collected in receiver  75 . Conventional coolers, condensers and phase separators may be used for the vapor cooling and phase separation. 
   After the phase separation, the vapor phase stream  72  (now on the order of about 25 bar (2.5 MPa) pressure and 5° C.) containing primarily CO 2  with trace amounts of impurities, passes through valves  73 , 74  to a filter  80  using a temperature controlled, packed column operating at a temperature close to the saturation temperature for CO 2  at the given pressure. Residual co-solvent (such as ethanol, etc.) and solid waste material cleaned off of the industrial part, such as target semiconductor wafer (i.e. photomask, etching chemicals, etc. used in the manufacture of semiconductor wafers) is removed from the carbon dioxide vapor phase stream. 
   The filtered vapor phase  81  is passed through an adsorption bed system  85  such that trace impurities are removed. The adsorption beds may contain activated carbon, alumina, or carbon molecular sieve material. For the separation of trace impurities, a 2-bed adsorption system  85  operated in a cyclic manner can be used. Each adsorption bed  82 , 83  can have a plurality of layers of adsorbents to ensure that all trace impurities are substantially removed. The purified CO 2  stream  90  is recovered, to be converted to a high-pressure product. When one adsorption bed  82  is saturated with trace impurities, the CO 2  stream is switched by closing valves  86  and  87  to the clean adsorption bed  83  by opening valves  88  and  89 . In one embodiment, a portion of the vaporized refrigerant from a condenser, such as nitrogen or another disposable refrigerant, can be used as a purge for regeneration of the dirty adsorption bed at low pressures. The purged impurities from the adsorption bed, such as the co-solvent, can be recovered separately, if desired. 
   The purified CO2 stream  90  is dried to remove residual water vapor, such as by being passed through a two bed desiccant dryer  95 . The purified stream can supplement or replace the first stage purity CO2 stream  11  from the bulk supply by the action of valves  92  and  12 . The purified CO 2  vapor passes through filters  13  and  14  as described above, and is cooled in the condenser  18  to a temperature such that the CO 2  is entirely condensed and returned to the low pressure accumulation vessel  30 . 
   EXAMPLE 
   A semiconductor wafer is processed using a mixture of supercritical CO 2  and 10% organic co-solvent. Residue removed from the wafer precipitates out as the pressure of the gas is reduced from about 300 bar (30 MPa) to about 25 to about 30 bar (2.5 to 3 MPa). The temperature of the gas at this stage is between about 30° to about 60° C. This CO 2  gas (containing 5% by weight organic co-solvent) is cooled to temperatures between about −10° C. and −5° C. A vapor-liquid phase system is formed where a majority of the organic co-solvents and other impurities are present in the liquid phase. The vapor phase contains about 300-1000 ppm of organic co-solvent material. The liquid phase is separated by gravity in a settling chamber, and the vapor phase is transported to an adsorbent bed containing an organic polar molecule-selective adsorbent. Activated alumina is suitable for this separation. 
   Additional adsorbents, such as activated carbons and molecular sieves, may also be present to adsorb any other trace organic impurities. The clean CO 2 , recovered at about 20 bar pressure (2 MPa) is then cooled to about −20° C. and liquefied. The liquefied CO 2  product is returned to the CO 2  accumulator tank. About 90% of the CO 2  is recovered by this process. The additional 10% requirement can be obtained from the bulk CO 2  supply as described above. 
   The present apparatus and process are advantageous over prior systems because they provide for the removal of trace contaminants without a major change in operation procedure, by changing adsorbents as compared to requiring distillation of CO 2 /so-solvent (such as ethanol) mixtures. The provide for cyclic operation, i.e., they do not require continuous feed for operation. Further, they provide a more economical design and operation, due to the absence of unnecessary accessory equipment such as boilers and condensers. 
   The entire process may be controlled by a programmable controller, and records data from the process can be sent to a computer which can be used to retrieve the data remotely. The apparatus and process may include a fully automated microprocessor controller which continuously monitors system operation providing fault detection, pressure control and valve sequencing, ensuring purifier reliability, while minimizing operator involvement. 
   By way of example and not limitation, level sensor  53 , pressure sensor  54 , and temperature sensors can provide information for the controller, in order to provide instructions to flow control valves  12 ,  24 ,  25 ,  34 , and  92 , or safety relief valves  55  and  56 . Isolation bypass valves  69  and  76  may be controlled automatically or manually. The valves in the apparatus may be actuated pneumatically, by pulling a tap off of the CO 2  vapor conduit such as at valve  93 , to supply gas for valve actuation. 
   The apparatus may include system alarms to detect potential hazards, such as temperature or pressure excursions, to ensure system integrity. Alarm and warning conditions may be indicated at the operator interface and may be accompanied by an alarm beeper. 
   It will be understood that the embodiment(s) described herein is/are merely exemplary and that a person skilled in the art may make many variations and modifications without departing from the spirit and scope of the invention. All such modifications and variations are intended to be included within the scope of the invention as described herein. It should be understood that any embodiments described above are not only in the alternative, but can be combined.