Patent Publication Number: US-6666928-B2

Title: Methods and apparatus for holding a substrate in a pressure chamber

Description:
FIELD OF THE INVENTION 
     The present invention relates to methods and apparatus for holding a substrate in a pressure chamber. 
     BACKGROUND OF THE INVENTION 
     Integrated circuits (ICs), optoelectronic devices, micromechanical devices and other precision fabrications are commonly formed using thin films applied to substrates. As part of the fabrication process, it is often necessary to remove or clean a portion or all of the thin film from the substrate. For example, in the manufacture of semiconductor wafers including ICs, a thin photoresist layer may be applied to the semiconductor substrate and subsequently removed. 
     Contaminants removed from surface features of microelectronic substrates after various manufacturing steps (e.g., after post-ion implant, ‘back end of the line’ (BEOL) cleans, ‘front end of the line’ (FEOL) cleans, and post chemical mechanical planarization (CMP) steps) vary in nature and composition dramatically. Accordingly, cleaning and treating steps must address these contaminants with the appropriate chemistries and solvents to either react with, ionize, dissolve, swell, disperse, emulsify, or vaporize them from the substrate. As such, a variety of water and solvent-based systems, and dry cleaning processes have been developed to address the broad variety of waste materials. 
     SUMMARY OF THE INVENTION 
     According to method embodiments of the present invention, a method for cleaning a microelectronic substrate includes placing the substrate in a pressure chamber. A process fluid including dense phase CO 2  is circulated through the chamber such that the process fluid contacts the substrate. The phase of the CO 2  is cyclically modulated during at least a portion of the step of circulating the process fluid. 
     According to further method embodiments of the present invention, a method for cleaning a microelectronic substrate includes placing the substrate in a pressure chamber. A process fluid including dense phase CO 2  is sprayed onto the substrate in a chamber. The phase of the CO 2  is cyclically modulated during at least a portion of the step of spraying the process fluid. 
     According to further method embodiments of the present invention, a method for cleaning a microelectronic substrate includes providing the substrate in a pressure chamber containing a process fluid including dense phase CO 2  such that the substrate is exposed to the CO 2 . The phase of the CO 2  is cyclically modulated by alternating CO 2  mass flow between a supply of CO 2  and the chamber and between the chamber and a low pressure source. The supply of CO 2  is at a higher pressure than the chamber and the low pressure source is at a lower pressure than the chamber. 
     According to further method embodiments of the present invention, a method for cleaning a microelectronic substrate includes placing the substrate in a pressure chamber. A process fluid including dense phase CO 2  is introduced into the chamber such that the process fluid contacts the substrate to thereby clean the substrate. A portion of the process fluid is removed from the chamber. The portion of the process fluid is re-introduced into the chamber. 
     According to further method embodiments of the present invention, a method for cleaning a microelectronic substrate includes placing the substrate in a pressure chamber. A process fluid including dense phase CO 2  is introduced into the chamber such that the process fluid contacts the substrate to thereby clean the substrate. A portion of the process fluid is removed from the chamber. The portion of the process fluid removed from the chamber is distilled to separate CO 2  from other components of the process fluid. The separated CO 2  is re-introduced into the chamber. 
     According to further method embodiments of the present invention, a method for cleaning a microelectronic substrate includes cleaning a substrate in a process chamber using a process fluid including CO 2 . The used process fluid is removed from the process chamber. CO 2  is separated from the used process fluid. The separated CO 2  is reused in the process chamber or a further process chamber. 
     According to embodiments of the present invention, an apparatus for cleaning a microelectronic substrate includes a pressure chamber and means for circulating a process fluid including dense phase CO 2  through the chamber such that the process fluid contacts the substrate. The apparatus further includes means for modulating the phase of the CO 2  while the process fluid is being circulated. 
     According to further embodiments of the present invention, an apparatus for cleaning a microelectronic substrate using a process fluid including dense phase CO 2  includes a pressure chamber. A spray member is operative to spray the process fluid onto the substrate in the chamber. The apparatus further includes means for cyclically modulating the phase of the CO 2 . 
     According to embodiments of the present invention, an apparatus for cleaning a microelectronic substrate includes a pressure chamber containing a process fluid including dense phase CO 2 . A supply of CO 2  is fluidly connectable to the chamber. The supply of CO 2  is at a higher pressure than the chamber. A low pressure source is fluidly connectable to the chamber. The low pressure source is at a lower pressure than the chamber. Fluid control devices are operable to cyclically modulate the phase of the CO 2  in the chamber by alternating CO 2  mass flow between the supply of CO 2  and the chamber and between the chamber and the low pressure source. 
     According to embodiments of the present invention, an apparatus for cleaning a microelectronic substrate includes a pressure chamber and a supply of a process fluid including dense phase CO 2  fluidly connected to the chamber. A distilling system includes a still fluidly connected to the chamber and operative to separate CO 2  from the process fluid. The distilling system is operative to reintroduce the separated CO 2  into the chamber or a further chamber. 
     According to embodiments of the present invention, an apparatus for cleaning a microelectronic substrate includes a process chamber containing a process fluid including CO 2  and means for removing used process fluid from the process chamber. The apparatus further includes means for separating CO 2  from the used process fluid and means for returning the separated CO 2  to the process chamber or a further process chamber for subsequent use. 
     According to embodiments of the present invention, a process chamber assembly for use with a substrate includes a vessel and a substrate holder. The vessel defines a chamber. The substrate holder has a rotational axis and includes front and rear opposed surfaces. The front surface is adapted to support the substrate. At least one impeller vane extends rearwardly from the rear surface and radially with respect to the rotational axis. The impeller vane is operative to generate a pressure differential tending to hold the substrate to the substrate holder when the substrate holder is rotated about the rotational axis. Preferably, the process chamber assembly includes a plurality of the impeller vanes extending rearwardly from the rear surface and radially with respect to the rotational axis. 
     According to further embodiments of the present invention, a substrate holder for use with a substrate has a rotational axis and further includes front and rear opposed surfaces. The front surface is adapted to support the substrate. At least one impeller vane extends rearwardly from the rear surface and radially with respect to the rotational axis. The impeller vane is operative to generate a pressure differential tending to hold the substrate to the substrate holder when the substrate holder is rotated about the rotational axis. Preferably, the substrate holder includes a plurality of the impeller vanes extending rearwardly from the rear surface and radially with respect to the rotational axis. 
     According to method embodiments of the present invention, a method for rotating a substrate holder about a rotational axis includes providing a substrate holder. The substrate holder includes front and rear opposed surfaces. The front surface is adapted to support the substrate. At least one impeller vane extends rearwardly from the rear surface and radially with respect to the rotational axis. The substrate holder is rotated about the rotational axis such that the impeller vane generates a pressure differential tending to hold the substrate to the substrate holder. 
     According to embodiments of the present invention, a pressure chamber assembly for use with a substrate includes a vessel and a substrate holder assembly. The vessel defines a pressure chamber. The substrate holder assembly includes a substrate holder disposed in the pressure chamber, the substrate holder including a front surface adapted to support the substrate, and a housing defining a secondary chamber. At least one connecting passage provides fluid communication between the front surface of the substrate holder and the secondary chamber. The connecting passage is adapted to be covered by the substrate when the substrate is mounted on the front surface of the substrate holder. A passive low pressure source is fluidly connected to the secondary chamber. 
     According to further embodiments of the present invention, a pressure chamber assembly for use with a substrate includes a vessel and a substrate holder assembly. The vessel defines a pressure chamber. The substrate holder assembly includes a substrate holder disposed in the pressure chamber, the substrate holder including a front surface adapted to support the substrate, and a housing defining a secondary chamber. A restrictive passage provides fluid communication between the pressure chamber and the secondary chamber. At least one connecting passage provides fluid communication between the front surface of the substrate holder and the secondary chamber. The connecting passage is adapted to be covered by the substrate when the substrate is mounted on the front surface of the substrate holder. A low pressure source is fluidly connected to the secondary chamber. 
     According to method embodiments of the present invention, a method for holding a substrate to a substrate holder in a pressure chamber includes providing a first pressure in the pressure chamber. A substrate holder assembly is provided including a substrate holder disposed in the pressure chamber, the substrate holder including a front surface adapted to support the substrate, and a housing defining a secondary chamber. At least one connecting passage provides fluid communication between the front surface of the substrate holder and the secondary chamber. The substrate is mounted on the substrate holder such that the substrate covers the connecting passage. A second pressure is provided in the secondary chamber that is lower than the first pressure using a passive low pressure source. 
     According to further method embodiments of the present invention, a method for holding a substrate to a substrate holder in a pressure chamber includes providing a first pressure in the pressure chamber. A substrate holder assembly is provided including a substrate holder disposed in the pressure chamber, the substrate holder including a front surface adapted to support the substrate, and a housing defining a secondary chamber. A restrictive passage provides fluid communication between the pressure chamber and the secondary chamber. At least one connecting passage provides fluid communication between the front surface of the substrate holder and the secondary chamber. The substrate is mounted on the substrate holder such that the substrate covers the connecting passage. A second pressure is provided in the secondary chamber that is lower than the first pressure. 
     According to embodiments of the present invention, a pressure chamber assembly for retaining a fluid includes first and second relatively separable casings defining an enclosed chamber and a fluid leak path extending from the chamber to an exterior region. An inner seal member is disposed along the leak path to restrict flow of fluid from the chamber to the exterior region. An outer seal member is disposed along the leak path between the inner seal member and the exterior region to restrict flow of fluid from the chamber to the exterior region. The inner seal member is a cup seal. 
     According to further embodiments of the present invention, a pressure chamber assembly for retaining a fluid includes first and second relatively separable casings defining an enclosed chamber and a fluid leak path extending from the chamber to an exterior region. An inner seal member is disposed along the leak path to restrict flow of fluid from the chamber to the exterior region. An outer seal member is disposed along the leak path between the inner seal member and the exterior region to restrict flow of fluid from the chamber to the exterior region. The inner seal member is a cup seal. The inner seal member is adapted to restrict flow of fluid from the chamber to the exterior region when a pressure in the chamber exceeds a pressure of the exterior region. The outer seal member is adapted to restrict flow of fluid from the exterior region to the chamber when a pressure in the chamber is less than a pressure of the exterior region. 
     According to embodiments of the present invention, a pressure chamber assembly for processing a substrate includes a pressure vessel defining an enclosed pressure chamber. A substrate holder is disposed in the pressure chamber and is adapted to hold the substrate. A drive assembly is operable to move the substrate holder. The drive assembly includes a first drive member connected to the substrate holder for movement therewith relative to the pressure vessel and a second drive member fluidly isolated from the first drive member and the pressure chamber. A drive unit is operable to move the second drive member. The drive unit is fluidly isolated from the first drive member and the pressure chamber. The second drive member is non-mechanically coupled to the first drive member such that the drive unit can move the substrate holder via the first and second drive members. 
     According to further embodiments of the present invention, a pressure chamber assembly for processing a substrate includes a pressure vessel defining an enclosed pressure chamber. A substrate holder is disposed in the pressure chamber and is adapted to hold the substrate. A magnetic drive assembly is operable to move the substrate holder relative to the pressure vessel. 
     According to further embodiments of the present invention, a pressure chamber assembly for processing a substrate includes a pressure vessel defining an enclosed pressure chamber and an exterior opening in fluid communication with the pressure chamber. A substrate holder is disposed in the pressure chamber and is adapted to hold the substrate. A drive assembly is operable to move the substrate holder relative to the pressure vessel, the drive assembly including a housing covering the exterior opening of the pressure chamber so as to seal the exterior opening. 
     According to embodiments of the present invention, a pressure chamber assembly includes a pressure vessel and a guard heater assembly. The pressure vessel defines an enclosed chamber. The guard heater assembly includes a guard heater disposed in the chamber and interposed between a surrounding portion of the pressure vessel and a holding volume. The guard heater is adapted to control a temperature of the holding volume. The guard heater is insulated from the surrounding portion of the pressure vessel. 
     According to some embodiments of the present invention, the guard heater and the surrounding portion of the pressure vessel define an insulating gap therebetween. Preferably, the insulating gap has a width of at least 0.1 mm. 
     According to some embodiments of the present invention, the guard heater assembly includes a layer of insulating material disposed between the guard heater and the surrounding portion of the pressure vessel. Preferably, the layer of insulating material has a thickness of at least 0.1 mm. 
     The guard heater assembly may further include a second guard heater disposed in the chamber and interposed between a second surrounding portion of the pressure vessel and the holding volume. The second guard heater is adapted to control the temperature of the holding volume. The second guard heater is insulated from the second surrounding portion of the pressure vessel. 
     A fluid spray bar may be mounted in the guard heater. A substrate holder may be disposed in the holding volume. 
     According to embodiments of the present invention, a process chamber assembly for use with a substrate and a flow of process fluid includes a vessel and a spray member. The vessel defines a chamber. The spray member includes at least one spray port formed therein adapted to distribute the flow of process fluid onto the substrate in the chamber. The spray member is operative to rotate about a rotational axis relative to the vessel responsive to a flow of the process fluid out of the spray member through the at least one spray port. 
     The spray member may include a distribution portion including a distribution channel therein, the at least one spray port extending from the distribution channel to exteriorly of the spray member. 
     The at least one spray port may extend at an angle with respect to the rotational axis. Preferably, the at least one spray port extends at an angle of between about 5 and 85 degrees with respect to the rotational axis. 
     The process chamber assembly may include a plurality of the spray ports formed in the spray member. 
     A bearing may be interposed between the spray member and the vessel to allow relative rotation between the spray member and the vessel. 
     According to further embodiments of the present invention, a spray member for distributing a flow of process fluid onto a substrate includes a spray member including at least one spray port formed therein adapted to distribute the flow of process fluid onto the substrate in the chamber. The spray member is operative to rotate about a rotational axis responsive to a flow of the process fluid out of the spray member through the at least one spray port. 
     The spray member may include a distribution channel therein, the at least one spray port extending from the distribution channel to exteriorly of the spray member. 
     The at least one spray port may extend at an angle with respect to the rotational axis. Preferably, the at least one spray port extends at an angle of between about 5 and 85 degrees with respect to the rotational axis. 
     The spray member may include a plurality of the spray ports formed in the spray member. 
     The spray member may include a bar-shaped distribution portion, the at least one spray port being formed in the distribution portion. Alternatively, the spray member may include a disk-shaped distribution portion, the at least one spray port being formed in the distribution portion. 
     According to method embodiments of the present invention, a method of applying a process fluid to a substrate includes placing the substrate in a chamber of a vessel. A spray member is provided including at least one spray port formed therein. The process fluid is distributed from the at least one spray port onto the substrate. The spray member is rotated about a rotational axis relative to the vessel by flowing the process fluid out of the spray member through the at least one spray port. 
     Objects of the present invention will be appreciated by those of ordinary skill in the art from a reading of the figures and the detailed description of the preferred embodiments that follow, such description being merely illustrative of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of an apparatus according to embodiments of the present invention; 
     FIG. 2 is a block diagram of a chemistry supply/conditioning system forming a part of the apparatus of FIG. 1; 
     FIG. 3 is a block diagram of an alternative chemistry supply/conditioning system forming a part of the apparatus of FIG. 1; 
     FIG. 4 is a block diagram of a further alternative chemistry supply/conditioning system forming a part of the apparatus of FIG. 1; 
     FIG. 5 is a block diagram of an alternative recirculation system forming a part of the apparatus of FIG. 1; 
     FIG. 6 is a block diagram of a further alternative recirculation system forming a part of the apparatus of FIG. 1; 
     FIG. 7 is a block diagram of a supply/recovery system according to embodiments of the present invention; 
     FIG. 8 is a cross-sectional view of a pressure chamber assembly according to embodiments of the present invention in a closed position; 
     FIG. 9 is a cross-sectional view of the pressure chamber assembly of FIG. 8 in an open position; 
     FIG. 10 is a cross-sectional view of an upper guard heater forming a part of the pressure chamber assembly of FIG. 8; 
     FIG. 11 is a top plan view of the upper guard heater of FIG. 10; 
     FIG. 12 is a bottom plan view of the guard heater of FIG. 10; 
     FIG. 13 is a cross-sectional view of a lower guard heater forming a part of the pressure chamber assembly of FIG. 8; 
     FIG. 14 is a bottom plan view of the lower guard heater of FIG. 13; 
     FIG. 15 is an enlarged, cross-sectional, fragmentary view of the pressure chamber assembly of FIG. 8; 
     FIG. 16 is a perspective view of a cup seal forming a part of the pressure chamber assembly of FIG. 8; 
     FIG. 17 is a fragmentary, perspective view of the cup seal of FIG. 16; 
     FIG. 18 is a cross-sectional view of a pressure chamber assembly according to further embodiments of the present invention; 
     FIG. 19 is a cross-sectional view of a pressure chamber assembly according to further embodiments of the present invention; 
     FIG. 20 is a top plan view of a chuck forming a part of the pressure chamber assembly of FIG. 19; 
     FIG. 21 is a bottom plan view of the chuck of FIG. 20; 
     FIG. 22 is a cross-sectional view of the chuck of FIG. 20 taken along the line  22 — 22  in FIG. 21; 
     FIG. 23 is a cross-sectional, schematic view of a pressure chamber assembly according to further embodiments of the present invention; 
     FIG. 24 is a top plan view of a chuck forming a part of the pressure chamber assembly of FIG. 23; 
     FIG. 25 is a cross-sectional view of the chuck of FIG. 24 taken along the line  25 — 25  of FIG. 24; 
     FIG. 26 is a cross-sectional view of a pressure chamber assembly according to further embodiments of the present invention; 
     FIG. 27 is a bottom view of a spray member forming a part of the pressure chamber assembly of FIG. 26; 
     FIG. 28 is a cross-sectional view of the spray member of FIG. 27 taken along the line  28 — 28  of FIG. 27; and 
     FIG. 29 is a bottom plan view of a spray member according to further embodiments of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention now is described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. 
     The present invention relates generally to, inter alia, the cleaning or treating of microelectronic substrates (such as semiconductor substrates) during or subsequent to the manufacturing of integrated circuits, microelectronic devices, MEM&#39;s, MEOM&#39;s and opto-electronic devices. Removal of surface contaminants and particulates is a key step in the integrated circuit fabrication process. There are numerous cleaning steps (commonly referred to as “cleans”) in the fabrication process. The different types of cleans include pre-diffusion cleans, front end of the line post-ash cleans, back end of the line post-etch cleans, pre-metal deposition cleans, front end of the line plasma strip, back end of the line clean/strip, post-ion implantation cleans and post-chemical mechanical planarization (CMP) cleans. There are many types and sources of particulates and contaminants in the fabrication process. The particles and contaminants may be molecular, ionic, atomic or gaseous in nature. The source may be inherent (e.g., redeposition of resist) or extrinsic to the process (e.g., wafer transport). 
     The shift of interconnect systems shift from Al/SiO 2  to Cu/low-k presents new challenges that may be effectively addressed using the methods and apparatus of the present invention. For example, a primary problem with the transition to Cu is the tendency of Cu to corrode when exposed to an oxidizing environment, because Cu does not have the self-passivating properties of Al. Corrosion of Cu during cleans of dual damascene structures can result in high contact resistance, undercutting and lift-off of the dielectric layers, thereby reducing circuit yields. Additional concerns have focused on the chemical compatibility of traditional cleans with low-k materials. As an example, it has been demonstrated that amine chemistries gas from OSG and other inorganic spin-on dielectric films, causes via poisoning. Aspects of the present invention may address the currently challenging cleans of these new interconnect systems. 
     With reference to FIG. 1, an apparatus  10  according to preferred embodiments of the present invention is shown therein. As illustrated, the apparatus  10  is adapted to clean a surface of a wafer substrate  5 . However, it will be appreciated by those of skill in the art from the description herein that various features and aspects of the apparatus and the methods described hereinbelow may be used for cleaning or otherwise treating wafers or other types of substrates or workpieces. Additionally, it will be appreciated by those of skill in the art from the description herein that various components and steps as described herein below may be omitted or replaced with other (for example, conventional) components or steps as appropriate. 
     The wafer  5  may be, for example, a wafer of semiconductor material such as silicon, silicon oxide, gallium arsenide, etc. The wafer  5  has a substantially planar work surface  5 A and an opposing substantially planar backside surface  5 B. A continuous or discontinuous layer of waste material is disposed on the work surface  5 A. The waste layer may be a layer of photoresist, reactive ion etch residue, chemical mechanical polishing residue or post-ion implantation residue. The waste material in the aforementioned layers may include inorganic or organic contaminants such as polymers based on stryenic, acrylic, novolac, cyclic olefinic maleic anhydride resins; etch residue based on ions of fluorine, chlorine, bromine or iodine; and slurry residue containing silica or alumina abrasives with other common slurry additives such as oxidizers, buffers, stabilizers, surfactants, passivating agents, complexing agents, corrosion inhibitors or other agents. Other types of workpieces may be cleaned or otherwise treated using the apparatus including, for example, MEMS, MEOMS, opto-electronic devices, and 3-D micro/nano-structures. 
     The apparatus  10  includes generally a flow/pressure control system  100 , a recirculation system  200 , a supply/recovery system  300 , a pressure chamber assembly  400 , and a substrate handling system  500  (FIG.  8 ). The pressure chamber assembly  400  includes a pressure chamber  410 . As discussed in greater detail below, the wafer  5  is held in the pressure chamber  410  for processing. The flow/pressure control system  100  conditions and applies a chemistry or chemistries (also referred to as adjuncts or modifiers), CO 2  (in the form of liquid, gas, and/or supercritical fluid (ScCO 2 )), and/or a mixture of chemistries and CO 2  to the working surface  5 A of the wafer  5 . The substrate handling system  500  holds the wafer  5  and, optionally, moves the wafer  5  to facilitate uniform cleaning. The recirculation system  200  may be used to filter and return process fluid to the pressure chamber  410 . The supply/recovery system  300  supplies the process fluids and may be employed to clean post-process effluent and, optionally, return a portion thereof (typically, recovered CO 2 ) for further use in the apparatus  10 . 
     Turning to the flow/pressure control system  100  in greater detail, the system  100  includes a tank T 1  containing CO 2  at high pressure. The pressure of the CO 2  in the tank T 1  is preferably between about 400 psi and 4000 psi, depending on the process(es) to be executed using the apparatus  10 . The volume of the tank T 1  is preferably at least 5 times the volume of the pressure chamber  410 . A temperature control device may be operatively connected to the tank T 1 . The temperature control device may be, for example, a temperature sensor and a heating coil or probe or heat exchanger. The temperature of the CO 2  in the tank T 1  is preferably between about 0° C. and 90° C., depending on the processes to be executed using the apparatus  10 . The CO 2  may be in liquid, gas or supercritical phase. 
     A plurality of outlet lines L 3 , L 4  and L 5  are fluidly connected to the tank T 1 . In the event that it may be desired to supply liquid CO 2  from the tank T 1 , the lines L 3 , L 4  and L 5  preferably draw from a lower portion of the tank T 1  (e.g., via a lower outlet or a dip tube). The outlet lines L 3 , L 4  and L 5  fluidly connect the tank T 1  to a chemistry supply/conditioning system  120  (schematically illustrated in FIG.  1  and described in greater detail below), a feed line L 1 , and a feed line L 2 . Valves V 1 , V 2  and V 3  are provided to control flow in the lines L 3 , L 4  and L 5 , respectively. 
     A plurality of chemistry supplies S 1 , S 2 , S 3  are fluidly connected to the system  120 . Each supply S 1 , S 2 , S 3  may include a single chemistry or multiple compatible chemistries that are combined at or upstream of the respective supply S 1 , S 2 , S 3 . The supplies may include the respective chemistries disposed in suitable containers. Where feasible, the containers are preferably at atmospheric pressure to allow for convenient refilling. 
     The chemistries provided by the supplies S 1 , S 2 , S 3  may include, for example: water; oxidizers such as peroxides or permanganates; acids such as hydrofluoric, sulfuric, and nitric; bases such as secondary and tertiary amines; ammonium hydroxide; solvents such as organic carbonates, lactones, ketones, ethers, alcohols, sulfoxides, thiols, and alkanes; surfactants such as block copolymers or random copolymers composed of fluorinated segments and hydrophilic or lipophilic segments; surfactants with siloxane-based components and hydrophilic or lipophilic components; conventional ionic and non-ionic hydrocarbon-based surfactants; and salts such as ammonium fluoride and choline. Incompatible chemistries are chemistries which, when combined or exposed to one another, tend to react with one another in a manner that impedes the process and/or damages or unduly fouls the apparatus  10  or wafer  5 . Examples of incompatible chemistries include acids and bases. 
     Level sensors may be provided in each of the supplies S 1 , S 2 , S 3  to indicate that a refill is needed and/or to provide a metric of chemistry use in the process. Means such as a heating coil or jacket may be provided to control the temperatures of the supplies. A mixing device may be provided in each supply S 1 , S 2 , S 3 . 
     As discussed in more detail below, the system  120  is operable to provide one or more controlled volumes of chemistry (with or without CO 2 ), which volumes may be conditioned by the system  120 . The feed lines L 1  and L 2  are each fluidly connected to the system  120  to receive the volume or volumes of the chemistries. The feed line L 1  is fluidly connected to a nozzle  191  in fluid communication with the pressure chamber  410 . The feed line L 2  is fluidly connected to a spray member  190  in the pressure chamber  410 . Filters F 1  and F 2  are provided in the feed lines L 1  and L 2 , respectively. Preferably and as illustrated, the filters F 1 , F 2  are located downstream of all lines that feed into the feed lines L 1 , L 2 . 
     A vacuum line L 16  is fluidly connected to the pressure chamber  410 . A vacuum unit P 1  is operable to draw a full or partial vacuum in the pressure chamber  410  through the line L 16 . The vacuum unit P 1  may be a pump or one or more tanks that are maintained at or near vacuum at all times by a continuously operating vacuum pump. A vacuum tank may be advantageous in that the pressure chamber  410  may be evacuated more rapidly and the tank may be re-evacuated while wafer processing is occurring. If multiple vacuum tanks are used, they may be staged in their operation to generate greater vacuum in the pressure chamber  410  in less time. 
     The vacuum unit P 1  may be advantageous for managing the air (or ambient gas) introduced to the system. In each batch step, the pressure chamber  410  may be opened and closed to insert and/or remove a substrate. During the time when the pressure chamber  410  is open, the chamber may fill with ambient gas (typically, air). Active control and management using the vacuum unit P 1  may be used to prevent this insertion of ambient gas from building up over time in the process fluids (assuming some level of recycling of the process fluids is accomplished). 
     A circulation line L 6  fluidly connects the pressure chamber  410  and the system  120 . Preferably, the line L 6  draws from a lower portion of the pressure chamber  410 . 
     A secondary gas supply tank T 2  is fluidly connected to the pressure chamber  410  with a controllable valve V 15  provided therebetween. Preferably, the secondary gas has a higher saturated vapor pressure than CO 2 . Preferably, the secondary gas is an inert gas. More preferably, the secondary gas is helium, nitrogen or argon. 
     Pulsing Feature 
     A variable volume device or pulse generator  102  may be fluidly connected to the pressure chamber  410 . The pulse generator  102  includes a chamber  102 B and a pressurizing member  102 A movable in the chamber  102 B. The pulse generator  102  is operable to generate a rapid decrease and/or increase (i.e., pulse) in pressure in the pressure chamber  410 . Preferably, the swept volume of the pressurizing member  102 A is between about 0.1 and 5 times the volume of the pressure chamber  410 . Preferably, the pulse generator  102  is adapted to provide pressure pulsing cycles at a rate of between about 1 cycle/10 seconds and 50 cycles/second. Preferably, the pulse generator  102  is adapted to decrease and/or increase the pressure in the pressure chamber  410  by at least 100 psi, and more preferably by between about 300 psi and 1500 psi. 
     The pulse mechanism may be any suitable mechanism including, for example, a piston coupled to a linear actuator, a rotating shaft and a connecting rod, a magnetic piston movable by means of an external electric coil, and/or an electrically, pneumatically or hydraulically driven piston or diaphragm. In a hydraulic or pneumatic system, the pulse mechanism may be paired with valving to quickly admit and release pressure to the non-process side of the diaphragm thereby displacing the piston or diaphragm. In one embodiment, the high pressure tank T 1  and a low pressure vessel such as T 2  may be fluidly connected to provide motive force for the pulse mechanism (piston or diaphragm). 
     Suitable valving (not shown) may be added such that the pulse chamber  102 B is filled from one pathway, a valve in this pathway may be closed and the fluid may thereafter be driven back to the pressure chamber  410  through a second pathway including a filter. The second pathway may feed the returning fluid to the pressure chamber  410  through the spray member  190 . The multiple pathways may serve to prevent the reintroduction of contaminants just removed from the wafer or particles generated in the pulse chamber, if a piston is used. 
     While the pulse generator  102  is illustrated as connected to a bottom portion the pressure chamber  410 , the pulse generator  102  may draw from any height of the pressure chamber  410 . In particular, it may be desirable to configure the pulse generator  102  to draw from an upper portion when used to facilitate processes utilizing two-phase (liquid/gas) process fluids in the pressure chamber  410  or to affect fluid and particulate flow in the vicinity of the wafer. It may be advantageous to move fluid rapidly away from the substrate surface (vertically), rather than move it across (parallel to) the wafer&#39;s surface as a bottom nozzle would. A relatively large pulse chamber may be used to enable particle dislodgement from the wafer surface and also enable particle transport well away from the wafer, to prevent redeposition. A relatively large pulse chamber may also be used to enable phase changes through two phases—such as from supercritical to liquid to gas. 
     An outlet line L 10  and a valve V 6  are provided to selectively vent the pressure chamber  410  to a lower pressure region, such as to a low pressure tank T 2  as discussed below, a fluid transfer device (e.g., a pump), or atmosphere. Waste effluent from the pressure chamber  410  may be drawn off to the low pressure region. 
     In addition to allowing removal of waste from the pressure chamber  410 , the line L 10  and the valve V 6  may be used in tandem with the high-pressure tank T 1  to generate pressure pulses in the pressure chamber  410 . This may be accomplished by raising the pressure in the pressure chamber  410  using the tank T 1  (i.e., by controlling one or more of the valves V 1 , V 2 , V 3  and/or other valves to provide an open path between the tank T 1  and the pressure chamber  410 ), closing the valve V 6 , and then rapidly dropping the pressure in the pressure chamber  410  by opening the valve V 6 . The waste effluent may go to a low pressure tank, for example, such as the tank T 2 . This sequence may be repeated as needed. 
     Chemistry Supply/Conditioning System 
     The chemistry supply/conditioning system  120  is operable to provide a selected flow or amount of chemical adjuncts from the supplies S 1 , S 2 , S 3  (more or fewer supplies may be used) to the pressure chamber  410 . Moreover, the system  120  may be operable to selectively control the pressure, temperatures and flow rates of chemistries or chemistry/CO 2 . In accordance with the present invention, certain alternative configurations may be employed for the system  120  as described hereinbelow. It will be appreciated from the description herein that various aspects and features of the disclosed embodiments may be omitted or combined with or substituted for other aspects and features of the embodiments. 
     With reference to FIG. 2, a chemistry supply/conditioning system  120 A is schematically illustrated along with certain relevant portions of the apparatus  10 . A fluid transfer device P 3  selectively draws or permits gravity flow of chemistry fluid (“first flow”) from the supply S 1  to a reservoir R 1  at substantially ambient pressure. A level measuring device  122  measures the volume of the fluid in the reservoir and thereby the volume of the chemistry to be delivered to the pressure chamber  410 . The fluid transfer device P 3  may also serve to determine the volume of the fluid in the reservoir R 1  by metering the flow through the device P 3 . The chemical adjunct in the reservoir may thereafter drain under force of gravity through a conditioning unit C 1  (as discussed below), the filter F 1 , and the line L 1  into the pressure chamber  410 . 
     Alternatively, CO 2  (e.g., supercritical CO 2  (ScCO 2 ), liquid CO 2 , or compressed liquid CO 2  or gaseous CO 2 ) from the tank T 1  may be delivered to the reservoir R 1  through a line L 3 A by operation of a valve V 1 A. A pressurized mixture of the adjunct and CO 2  is thereby delivered to the pressure chamber  410  through the unit C 1 , the filter F 1 , and the line L 1 . 
     With further reference to FIG. 2, the system  120 A is adapted to deliver a second flow of chemistry-containing process fluid to the pressure chamber  410 , the second flow including chemistry from the supply S 2  which is not compatible with the supply S 1 . The system  120 A provides a flow path for the second flow that is separate from that used for the first flow. The second flow path includes elements P 4 , R 2 ,  122 , and C 2  corresponding generally to elements P 3 , R 1 ,  122 , and C 1 . 
     In the same manner as discussed above, the second flow may be a chemistry only stream (i.e., no CO 2 ) that is transferred to reservoir R 2  via P 4  and then through the conditioning unit C 2 , the filter F 2 , and the line L 2  to the pressure chamber  410 . Alternatively, CO 2  from the tank T 1  may be introduced into the reservoir R 2  through a line L 3 B by operation of a valve V 1 B such that the adjunct/CO 2  is delivered to the pressure chamber  410  under pressure. 
     FIG. 2 further illustrates the use of the circulation line L 6  to return process fluid from the pressure chamber  410  to the reservoir R 2  by using P 4  or a pressure differential. The returned fluid may be remixed with the second flow for reuse in the process. A further filter (not shown) may be provided in the line L 6 . 
     With reference to FIG. 3, a chemistry supply/conditioning system  120 B according to further embodiments of the present invention is shown therein. The system  120 B is particularly well-suited for delivering gaseous chemistries. The system  120 B corresponds to the system  120 A except that the reservoirs R 1 , R 2  are omitted and high pressure CO 2  is made directly available to the conditioning units C 1 , C 2  via lines L 3 A, L 3 B and valves V 1 A and V 1 B. By operation of the fluid transfer device P 3  (or P 4 ), the system  120 B may inject the adjunct S 1  (or S 2 ) through the conditioning unit C 1  (or C 2 ) and the filter F 1  (or F 2 ) and into the pressure chamber  410 . Alternatively, high pressure CO 2  may be added to and mixed with the chemistry S 1  or S 2  in the respective conditioning unit C 1 , C 2 . In this case, the volume of the chemistry delivered to the pressure chamber  410  may be measured by metering the flow of the chemistry through the fluid transfer device P 3  (or P 4 ) or by measuring the volume change in the supply vessels S 1  or S 2 . The flow rate(s) of chemistries and/or CO 2  to the conditioning units C 1  and C 2  may also be controlled to achieve a desired ratio of CO 2  to chemistry in the stream being delivered to chamber  410 . 
     With reference to FIG. 4, a chemistry supply/conditioning system  120 C according to further embodiments of the present invention is shown therein. The system  120 C includes a fluid transfer device P 5  operable to selectively draw alternatingly from each of the supplies S 1  and S 2  as well as the supply of high pressure CO 2  from the tank T 1  (via line L 3 A and valve VIA). The device P 5  forces the selected chemistry through a conditioning unit C 3  and one or both of the filters F 1  and F 2  (depending on the operation of valves V 9  and V 10 ) so that the fluid is ultimately injected into the pressure chamber  410  under pressure. Optionally, CO 2  from the tank T 1  may be added to the selected chemistry by introducing the CO 2  into the conditioning unit C 3  using the line L 3 B and the valve V 1 B. In order to prevent mixing of the incompatible chemistries S 1 , S 2 , CO 2  (preferably, pure ScCO 2 ) from the tank T 1  is introduced through the line L 3 A to flush the fluid transfer device P 5  and the remainder of the flow path to the pressure chamber  410  shared by the two chemistry flows. 
     Recirculation System 
     The recirculation system  200  includes an outlet line L 7  fluidly connected to a lower portion of the pressure chamber  410 . Lines L 8  and L 9  are in turn fluidly connected to the line L 7  and also to the feed lines L 1  and L 2 , respectively, upstream of the filters F 1  and F 2 . A fluid transfer device P 2  is operable to draw fluid from the pressure chamber  410  and force the fluid through the lines L 8  and L 9  and ultimately back into the pressure chamber  410 . The recirculated fluid flow may be combined with other fluid flow in the lines L 1  and L 2  (e.g., from the system  120  and/or from the lines L 3  or L 4 ). Valves V 4  and V 5  are provided in the lines L 8  and L 9 . 
     The recirculation system  200  may serve to provide additional fluid mechanical action to the wafer surface without requiring additional removal of CO 2  and/or chemistry and introduction of new CO 2  and/or chemistry. Moreover, the recirculation system  200  may serve to continuously clean (e.g., filter, distill, or separate components through density modulation) the process fluid during the cleaning process. 
     An alternative recirculation system  200 A according to the present invention is shown in FIG.  5 . The system  200 A includes an outlet line L 14 . Return lines L 15  and L 16  fluidly connect the line L 14  to a recirculation nozzle  193  and the spray member  190 , respectively, in the pressure chamber  410 . A fluid transfer device P 6  is operable to force fluid from the pressure chamber  410  through a filter F 3  and back into the pressure chamber  410  through the nozzle  193  and/or the spray member  190 . Valves V 7  and V 8  are provided to enable alternating delivery of fluid to the spray member or recirculation nozzle and to prevent unintended back flow through the nozzle  193 . 
     A further alternative recirculation system  200 B according to the present invention is shown in FIG.  6 . The system  200 B includes an outlet line L 30  fluidly connecting the pressure chamber  410  to a still  243  (having a heating element  245 ) through a transfer system  242 . The transfer system  242  converts the waste stream from the pressure chamber  410  from its starting state (e.g., liquid, compressed liquid, or supercritical fluid) to a liquid. Preferably, the transfer system  242  is also adapted to prevent backflow of fluids from the still  243  to the pressure chamber  410 . For this purpose, the transfer system  242  may include one or more shut-off valves and/or one-way/check valves. 
     If the waste stream from the pressure chamber  410  is a liquid, the transfer system  242  may not change the fluid or may merely change the temperature of the fluid (e.g., using a heater or chiller). If the waste stream from the pressure chamber  410  is a compressed liquid, the transfer system may provide a pressure let down (e.g., by means of a torturous path, an orifice, or control valve). The transfer system  242  may also include a temperature-altering element. If the waste stream from the pressure chamber  410  is a supercritical fluid, there is preferably a pressure let-down as discussed above as well as a temperature-altering step. In this case, it may be necessary or desirable to cool the fluid to cross into the 2-phase Liquid/Gas region of the phase diagram. 
     Once in the liquid state, the fluid is boiled/distilled in the still  243  to separate the fluid into two components: a lighter component, which will be predominantly CO 2  gas, and a heavier component which will be predominantly adjunct chemistry and entrained contaminants. The heavier component may be conveyed (e.g., by gravity) to a recycling/disposal system  244 . 
     The CO 2  gas (lighter) stream is directed to a heat exchanger  246  via a line L 31  where the CO 2  gas stream is converted (through manipulation of temperature and pressure) to the conditions of the processing fluid (i.e., liquid, compressed liquid or supercritical fluid). If the fluid starting condition was liquid, the exchanger may include a heat transfer coil  247  connected to the heating device so as to transfer heat from the condensing fluid to the still  243 . The CO 2  may be additionally cleaned through filtration, adsorption, absorption, membrane separation, physical separation (e.g., centrifugal force) or electrostatic separation. The conditioned CO 2  may then be provided back to additionally process the substrate or to process a subsequent substrate. Additional chemistries may be added to this incoming fluid (e.g., at a mixing reservoir  248 ). 
     The distilling recirculation system  200 B may be used to provide a continuous or intermittent flow of the process fluid through the pressure chamber  410 . The mass flow may serve to assist in the cleaning process by transporting particulates away from the wafer  5  (e.g., to prevent redeposit on the wafer) and/or providing mechanical action (agitation) on the wafer surface. The mass flow may be filtered or otherwise conditioned. The mass flow may be fully driven by the addition of heat in the still  243  so that no pumps or other potentially particulate-generating mechanical devices are required. Multiple transfer systems  242 , stills  243 , heat exchangers  246  may be used to provide increased continuous flow. 
     Each of the recirculation systems  200 ,  200 A,  200 B may be employed to provide mass flow through the chamber  410  without loss of process fluid mass from the process loop (except the relatively small quantities of adjuncts and particulates that are filtered or distilled out of the process fluid stream. Moreover, each of the recirculation systems  200 ,  200 A may be employed to provide mass flow through the chamber  410  without altering the chemical composition of the process fluid. 
     As depicted in FIGS. 1-5, the filters F 1 , F 2  as well as the filter F 3  are preferably adapted to provide filtration of at least particles in the range of 10 nm (as in nanometers) to 50 microns. Suitable filters may include sintered filters, bag-type filters, magnetic filters, electrostatic filters, and/or combinations thereof. Preferably, as in the illustrated embodiments, every fluid stream pathway into the pressure chamber  410  has a filter as its final element before the pressure chamber  410 . In particular, all valves and fluid transfer devices for delivering fluid to the pressure chamber  410  are disposed upstream of at least one filter. 
     The conditioning units C 1 , C 2 , C 3  may include means for mixing the chemistries of the adjunct or for mixing the adjunct and CO 2  (when present) to promote homogeneity and solvation of adjuncts. The conditioning units may also include means for controlling the temperature of the adjunct or adjunct/CO 2 . Suitable mixing devices or processes include mechanical mixers and flow mixing. Temperature control may be achieved using probes, internal coils, elements, and/or an external jacket, for example. An electrical heater or a fluid heat exchanger may be used, for example. 
     The fluid transfer devices P 3 , P 4 , P 5  are preferably capable of accurately and consistently metering a flow of fluid. Suitable devices may include diaphragm pumps, syringe pumps, or piston pumps, for example. 
     While particular arrangements have been illustrated and described herein, it will be apparent to those of skill in the art that various modifications may be made in keeping with the present invention. For example, in the system  120 A (FIG.  2 ), the circulation line L 6  may feed to the fluid transfer device P 3  such that the flow from the line L 6  is directed to the line L 1 . Valving (not shown) may be provided to allow selection of the feed line (i.e., L 1  or L 2 ) for each flow path, so that the chemistry (with or without CO 2 ) from the supply S 1 , for example, can be directed to either or both of the spray member  190  and the nozzle  191 , as desired. The apparatus  10  may include one or more chemistry supply paths that include an in-line reservoir (i.e., as in the system  120 A) and/or one or more parallel chemistry supply paths that are direct injection (i.e., as in the system  120 B) and/or one or more parallel chemistry supply paths that serve alternative supplies (i.e., as in the system  120 C). Additional filters, fluid transfer devices, reservoirs, conditioning units and valving may be provided as needed to provide additional flexibility. 
     Cleaning/Pulsing Process 
     The apparatus  10  may be used to execute a wide range of processes wherein the wafer  5  in the pressure chamber  410  is subjected to fluid streams, pools and atmospheres, including chemical adjuncts, CO 2  and mixtures thereof, in various states (e.g., liquid, gas, supercritical fluid). Such processes may serve to clean or otherwise treat (e.g., coat) the wafer surface  5 A. For example, the apparatus  10  may be used to conduct methods as disclosed in the following commonly owned U.S. patent applications, the disclosures of which are hereby incorporated herein by reference in their entireties: 
     1. U.S. patent application Ser. No. 09/951,259; inventors James P. DeYoung, James B. McClain, Michael E. Cole, and David E. Brainard; filed Sep. 13, 2001; and titled  Methods for Cleaning Microelectronic Structures with Cyclical Phase Modulation  (Attorney Docket No. 5697-45IP); 
     2. U.S. patent application Ser. No. 09/951,249; inventors James P. DeYoung, James B. McClain, Stephen M. Gross, and Joseph M. DeSimone; filed Sep. 13, 2001; and titled  Methods for Cleaning Microelectronic Structures with Aqueous Carbon Dioxide Systems  (Attorney Docket No. 5697-45IP2); 
     3. U.S. patent application Ser. No. 09/951,092; inventors James P. DeYoung, James B. McClain, and Stephen M. Gross; filed Sep. 13, 2001; and titled  Methods for Removing Particles from Microelectronic Structures  (Attorney Docket No. 45IP3); 
     4. U.S. patent application Ser. No. 09/951,247; inventor(s) James P. DeYoung, James B. McClain, and Stephen M. Gross; filed Sep. 13, 2001; and titled  Methods for the Control of Contaminants Following Carbon Dioxide Cleaning of Microelectronic Structures  (Attorney Docket No. 5697-45IP4). 
     The following are exemplary processes that may be practiced in accordance with the present invention. Preferably, the valving, fluid transfer devices, and sensors are operatively connected to a computerized controller to provide feedback and control as needed to conduct the desired process steps. 
     The wafer  5  is inserted into the pressure chamber  410  and secured to the chuck  510  by any suitable means such as adhesive or clamps. More preferably, the wafer  5  is secured to the chuck in one of the manners described below with regard to the wafer holding assemblies  520  (FIG. 19) and  550  (FIG.  23 ). The door of the pressure chamber is thereafter closed and sealed. 
     Air and any other gases in the pressure chamber  410  are evacuated from the pressure chamber  410  through the line L 16  using the vacuum unit P 1 . 
     Optionally, chemistry from one or more of the supplies S 1 , S 2 , S 3  may be applied to the wafer using the chemistry supply/conditioning system  120  prior to pressurizing the pressure chamber  410 . 
     The pressure chamber  410  is thereafter pressurized with CO 2  (preferably liquid CO 2  or ScCO 2 ) from the high-pressure tank T 1 . Preferably, the pressure chamber  410  is pressurized to a pressure of at least 400 psi, and more preferably, between about 800 psi and 3000 psi. Additionally, the atmosphere in the pressure chamber  410  is maintained at a selected temperature (preferably between about 10° C. and 80° C.), for example, using a guard heater as discussed below. 
     Once the pressure chamber  410  is pressurized to the selected pressure, dense-phase CO 2  is circulated through the line L 2  to the spray member  190  and/or the nozzle  191 . The spray member directs the dense-phase CO 2  onto the wafer surface  5 A. Optionally, chemistry, with or without liquid or supercritical CO 2  mixed therewith, from one or more of the supplies S 1 , S 2 , S 3  may be applied to the wafer using the chemistry supply/conditioning system  120 . 
     The pulse generator  102  and/or the high-pressure tank T 1  and the valve V 6  are then used to effectuate cyclical phase modulation (CPM). More particularly, the pulse generator  102  and/or the high-pressure tank T 1  and the valve V 6  are operated (with appropriate temperature control of the process fluid) to effect phase changes between liquid, supercritical, and gas states. Preferably, the phase changes are effected between supercritical and liquid states in a cyclical fashion. For example, CPM processes as disclosed in the commonly owned U.S. patent application Ser. No. 09/951,259; inventors James P. DeYoung, James B. McClain, Michael E. Cole, and David E. Brainard; filed Sep. 13, 2001; and titled  Methods for Cleaning Microelectronic Structures with Cyclical Phase Modulation  (Attorney Docket No. 5697-45IP), the disclosure of which is hereby incorporated herein by reference in its entirety, may be conducted. 
     During the CPM cycles, CO 2  or CO 2  with chemistry may be applied to the wafer  5  via the spray member  190 . Fluid and particulate matter from the pressure chamber  410  may be removed from the pressure chamber  410  and recirculated locally via the recirculation system  200  or  200 A and/or recirculated via the line L 6  and the system  120 . 
     The process fluid (dense-phase CO 2 , adjuncts and waste matter) is removed from the pressure chamber  410  via the line L 10 . As discussed below, CO 2  may be withdrawn from the pressure chamber  410  to a recovery tank. The process pathways (including the pressure chamber  410 ) may be flushed one or more times with pure liquid or supercritical CO 2  from the tank T 1 . 
     The foregoing steps of optionally applying one or more of the chemistries S 1 , S 2 , S 3  to the wafer (with or without ScCO 2 ), conducting CPM and removing the process fluid may be repeated as needed. Following the final CPM cycle, the process fluid is removed and optionally a rinsing fluid (e.g., a co-solvent or surfactant) is dispensed from the supplies S 1 , S 2 , S 3  onto the wafer  5  (preferably under pressure from the spray member  190 ). 
     The pressure chamber  410  and the process pathways (including the recirculation pathway) are thereafter flushed with ScCO 2  from the tank T 1  to remove adjuncts and remaining residues. If no rinse fluid is used, a pure CO 2  (liquid or supercritical) fluid is used to remove adjuncts and remaining contaminants from the substrate. The flushing dense-phase CO 2  may be recirculated, but is finally removed via the line L 10 . A final rinse of the wafer  5  and the pressure chamber  410  is preferably conducted using pure liquid or supercritical CO 2 . 
     Thereafter, the pressure chamber  410  is depressurized and the wafer  5  is removed. 
     Preferably, the apparatus  10  is operable to apply the process fluid from the spray member  190  onto the wafer surface at a pressure of at least 400 psi, and more preferably between about 800 psi and 3000 psi. The process may include applying the process fluid to wafer using the spray member  190  with the spray member  190  rotating relative to the wafer. Either or both the spray member (e.g., the spray member  190  or the spray member  602 ) and the chuck (e.g., the chuck  510 ,  522 , or  552 ) may be rotationally driven. 
     Moreover, a flow of process fluid may be provided across the wafer  5  by feeding the process fluid into the chamber  410  via a feed nozzle (e.g., the nozzle  191 ) and simultaneously removing process fluid through one or more of the outlet lines (e.g., the line L 7 , the line L 10 , the line L 11 , and/or the line L 6 ). Preferably, the apparatus  10  is operable to provide such a flow through the chamber  410  at a rate of at least 2 gpm. 
     As noted above, the process may include simultaneously pulsing the density of the CO 2  containing process fluid and spraying the process fluid onto the wafer  5 . Likewise, if the phase modulation is accomplished using the pulse generator  102 , a flow of the process fluid through the chamber  410  may be provided at the same time as the density modulation. The wafer  5  and/or the spray member  190  may be simultaneously rotated. 
     In each of the foregoing steps involving the application of chemistries, the chemistries may be any suitable chemistries. In particular, it is contemplated that the chemistries may include co-solvents, surfactants, reactants, chelants, and combinations thereof. Notably, the separate flow paths and/or flushing means of the chemistry supply system  120  may be used to safely and effectively add incompatible chemistries to the chamber  410 . 
     The apparatus may deliver process components in different states (e.g., liquid, gas, supercritical) to the chamber  410  and may allow for components in different states to coexist in the chamber  410 . The apparatus may provide heated CO 2  gas (e.g., from the tank T 1 ) to drain or flush process components from the cleaning chamber for cleaning steps using liquid CO 2 . Alternatively, the apparatus may deliver a secondary gas such as helium, nitrogen or argon from the secondary gas tank T 3  to displace process fluids during a cleaning step and preceding a rinse step when either liquid or supercritical CO 2  is used as the primary process fluid during the cleaning step. The apparatus may also provide heated ScCO 2  (e.g., supercritical CO 2 ) at a temperature higher than that of the primary processing fluid but at a density lower than that of the primary processing fluids used to displace processing fluids after a cleaning step, but prior to a rinse step for cleaning steps using ScCO 2 . 
     Supply/Recovery System 
     The supply/recovery system  300  is adapted to supply and/or recycle and re-supply CO 2  and/or chemistry to the cleaning process. As the process proceeds, some CO 2  will be lost. The process may include batch cycles where the pressure chamber  410  is pressurized and depressurized many times in succession as the substrates (e.g., wafers are moved through the CO 2 -based processing equipment). For example, some CO 2  will be lost to atmosphere when the pressure chamber is opened to remove and replace wafers. Some CO 2  will be lost from the system in the waste stream that is drained from the system. Substantial amounts of the CO 2  will be contaminated or otherwise rendered unsuitable or potentially unsuitable for further recirculation through the process loop. For these reasons, it is necessary to provide sources of additional CO 2  to replenish the CO 2  lost from the process. Additionally, it may be desirable to recycle CO 2  as well as chemistry for reuse in the apparatus  10  or elsewhere. 
     Stock CO2 Supply 
     With reference to FIG. 7, the supply/recovery system  300  includes a CO 2  stock supply  312 . The supply  312  may be, for example, CO 2  supplied in one or more liquid cylinders, carboys of sub-ambient liquid, or bulk supply systems of sub-ambient liquid. The storage method preferably allows for supply of either liquid or gaseous CO 2 . 
     The supply  312  is fluidly connected to the process chamber  410  via a line L 17 , which is provided with a valve V 11  to control the flow into the pressure chamber  410 . Preferably, the system  300  is adapted such that the CO 2  from the supply can be delivered directly (i.e., without aid of any fluid transfers devices, pressurizing tanks, or the like) into the pressure chamber  410  at a desired pressure (preferably between about 15 and 50 psig). The supply  312  may be from a gas or liquid source. 
     CO 2  as commonly distributed for industrial and commercial uses (e.g., food processing such as carbonation of beverages and freeze-drying, pH control, or dry ice) is not sufficiently clean for processing of micro-electronic substrates. Commonly, such CO 2  supplies include contaminants such as organic materials, other gases, water and particulate matter. Accordingly, the system  300  may include a purification unit D 1  between the supply  312  and the pressure chamber  410 . The purification unit D 1  is operative to purify the CO 2  supply to the requisite ultra-high cleanliness and purity. In this manner, the purification unit D 1  enables the effective use of food grade or industrial grade CO 2 , thereby allowing the use of existing supply chains and distribution chains for CO 2 . 
     The purification unit D 1  may include one or more of the following means for filtering gas or liquid CO 2 : 
     1. Distillation: The CO 2  may be drawn from a gaseous supply or a gaseous portion of the supply. Liquid CO 2  may be drawn, boiled, relocated to a collection volume and re-condensed; 
     2. Filtration; 
     3. Membrane separation (preferably paired with distillation); and 
     4. Absorption/adsorption (e.g., capture based on attractive forces or molecule size). 
     CO 2  may also be delivered to the process (and, more particularly, to the pressure chamber  410 ) by introducing additional CO 2  into the vapor-saver unit  320  discussed below. Preferably, this additional CO 2  is first purified using a purification unit corresponding to the purification unit D 1 . 
     Waste Stream Handling 
     As noted above in the discussion regarding the process, at various times (including, typically, at the end of each run), processing fluid may be removed from the pressure chamber  410  via the line L 10 . Such fluids may include liquid, gaseous, or supercritical CO 2 , chemistry, and various contaminants (e.g., particles dislodged from the wafer(s)). 
     The system  300  includes a low-pressure tank T 2  to receive the waste stream drawn removed from the pressure chamber  410 . The tank T 2  is preferably maintained at a pressure of between about ambient and 3000 psi. The volume of the tank T 2  is preferably at least 5 times the volume of the pressure chamber  410 . 
     Different compositions may be expelled to the tank T 2 , in which case the tank T 2  is a segmented tank or multiple tanks. The pressure in the tank T 2  is less than that of a pressure head upstream of and fluidly communicating with the pressure chamber  410  so that the pressure differential forces the waste stream into the tank T 2  from the pressure chamber  410 . Preferably, the high-pressure tank T 1  provides the pressure head so that no pump or other mechanical device is required. 
     The reduction in pressure of the CO 2  as it is transferred from the pressure chamber  410  to the tank T 2  may be used to facilitate separation. Supercritical CO 2  process fluid may be expanded through a pressure reduction device (e.g., a control valve or orifice) to a lower pressure. At this lower pressure, components of the processing fluid (e.g., chemical adjuncts or entrained contaminants) may be rendered insoluble, thereby facilitating the efficient separation of the expanded stream into a light-fluid CO 2  stream and a heavy-fluid (insoluble) alternate stream. 
     A supercritical CO 2  process fluid may also be expanded through a pressure reduction to the two-phase Liquid/Gas area of the phase diagram. This may enable the segmentation of different process fluids in different segmented volumes of a divided tank or multiple tanks. Such segmentation may be advantageous to could mitigate the generation of mixed waste streams, which may be more costly to manage than single component fluid streams. Segmentation may also enable the utilization of distillation for separation of the processing fluid components (e.g., separation of CO 2  for recycle from chemical adjuncts and entrained contaminants for disposal). 
     A liquid process fluid stream may be expanded and heated to the gas-state. This would allow a continuous distillation-like separation of components (i.e., evaporation of flash evaporation), for example, as described below with regard the distillation system  340 . 
     Recycling and Abatement 
     The waste stream received in the tank T 2  is thereafter transferred to a recycling/abatement station  310  through a line L 29  (which is provided with a valve V 12 ). The waste stream may be transferred by means of a pump or the like, but is preferably transferred using a non-mechanical means such as pressure differential and/or gravity. To the extent the waste stream has been separated in the tank T 2 , there may be two of more separate lines delivering the respective separated streams for separate handling by the unit  310 . These streams may be treated and directed by the system  300  in the following manners: 
     1. CO 2  may be disposed of through controlled venting or draining via a line L 27  to a safe atmospheric discharge and/or collection for unrelated use; 
     2. CO 2  may be directly supplied to the pressure chamber  410  via a line L 22 . The CO 2  is preferably purified by means of a purification unit D 3 . The CO 2  as delivered to the pressure chamber  410  through the line L 22  may be at greater than atmospheric pressure, in which case it may be used to perform or augment the pressurization of the main processing chamber at the beginning of each cycle; 
     3. CO 2  may be directed to the purification unit D 1  through the line L 23  and thereafter into the pressure chamber  410 ; 
     4. Gaseous CO 2  may be directed through a purification unit D 2 , through a liquefying unit  314  (which adjusts the pressure and chills the CO 2  gas), and supplied to the stock CO 2  supply  312  for further use in the manner described above; 
     5. CO 2  may be passed through a purification unit D 4  and re-pressurized and supplied to the high-pressure tank T 1  through a line L 25  using a pressurizing device (e.g., a pump) P 8 ; 
     6. CO 2  may be directed via a line L 26  through a purification unit D 5  to a vapor saver tank  320  as discussed below; and 
     7. Chemical adjuncts and contaminants may be treated and/or disposed of/recycled through a line L 28  and in accordance with good chemical stewardship. 
     Vapor Recovery 
     Following draining of the process fluid from the pressure chamber  410 , a pressurized CO 2  vapor will remain in the pressure chamber  410 . It is desirable and often necessary to remove this vapor prior to opening the pressure chamber  410  to remove the substrate(s) (e.g., wafer(s)). 
     One method for depressurizing the chamber is to vent the chamber using a controlled release. Alternatively, a compressor or pump may be used to draw down the pressure in the pressure chamber  410 . 
     The pressure of the CO 2  may also be reduced using a vapor recovery system  322  and method as follows. Such methods and apparatus may employ features and aspects of the methods and apparatus disclosed in U.S. patent application Ser. No. 09/404,957, filed Sep. 24, 1999 and in U.S. patent application Ser. No. 09/669,154, filed Sep. 25, 2000. 
     A vapor recovery tank or pressurized container  322  is used to rapidly capture CO 2  (typically, gas or SCF) at the end of a process cycle through a line L 18 . The captured CO 2  is typically a gas or supercritical fluid, but may be a liquid (in which case, the venting is preferably from the bottom of the chamber  410  to avoid formation of solid/dry ice). In this manner, the pressure chamber  410  may be depressurized very rapidly. Advantageously, the capturing method is not constrained by the volumetric throughput of a mechanical device (e.g., a compressor). The volume of the vapor recovery tank  322  is preferably on the order of one to 500 times the volume of the pressure chamber  410 . 
     The captured CO 2  may be handled in any desired manner, including: 
     a) it may be disposed of through a line L 21  having a valve V 10 , and preferably through a surge tank  324 ; 
     b) using the line L 21  and surge tank  324 , it may be recovered and recycled for use in another application (e.g., a CO 2 -based fire suppression system or a storage container for recycle for use in some other service); 
     c) it may be recovered and recycled for use in the same application (compressed and/or liquified, and/or converted into SCF) and re-supplied to the processing system or to the CO 2 -supply system; 
     d) it may be used in the next processing step to pressurize the pressure chamber  410  (which may be a prerequisite for pressurizing the pressure chamber  410  up to sufficient pressure to effectively add CO 2  based processing fluids). 
     The vapor recovery system may include a compressor P 7  for assisting the transfer of material from the pressure chamber  410  to the vapor recovery tank(s). For example, at the end of a processing cycle, the pressure chamber  410  may be at high pressure (CO 2 -gas at vapor pressure or a supercritical fluid, 300&lt;P (psia)&lt;3000) and the vapor recovery tank may be at a low pressure. In order to depressurize the pressure chamber  410  to a low (e.g., ambient) pressure very quickly (e.g., to allow opening of the chamber and removal of the substrate) while saving the majority of the CO 2 , the two chambers may be equalized, and then: 
     a) a compressor may be used to push more CO 2  from the main processing chamber to the vapor-saver tank; and 
     b) a second vapor recovery tank may be used (e.g., in cascading manner) to again rapidly equilibrate and additionally lower the pressure of the pressure chamber  410 . 
     A compressor may also be used to remove the material from the vapor recovery tank(s) between the end of a first run and the end of the next run at which time the vapor recovery tank(s) may be required to be at low pressure again. The captured CO 2  may be handled in any of the manners described above. 
     It will be appreciated that various valving and flow control apparatus in addition to that illustrated may be employed in the system  300 . The vapor-saver system  320  and the several options for handling the CO 2  from the waste stream of line L 10  are independent and any may be eliminated from the system  300  as desired. Each of the purification units D 2 , D 3 , D 4 , D 5  may correspond to the purification unit D 1  (i.e., may use any of the methods listed above—distillation, filtration, membrane separation, and absorption/adsorption). As an alternative to the several purification units D 2 , D 3 , D 4 , D 5 , two or more of these purification units may be combined so that the respective flow paths each have a common extent through the shared purification unit and thereafter diverge. 
     Pressure Chamber Assembly 
     With reference to FIGS. 8 and 9, the pressure chamber assembly  400  includes an upper casing  420  and a lower casing  430 . When in a closed position as shown in FIG. 8, the casings  420 ,  430  define a pressure chamber  410  therebetween and a sealing system  450  as described in more detail below seals the chamber  410 . When in a closed position as shown in FIG. 8, a pair of opposed clamps  440  surround end portions of the casings  420 ,  430  to limit separation of the casings  420 ,  430 . The clamps  440  can be displaced to allow the casings  420 ,  430  to be separated into an open position as shown in FIG.  9 . 
     Guard Heater 
     A guard heater assembly  460  is disposed in the chamber  410  and includes an upper guard heater  462  and a lower guard heater  472 . The guard heater assembly  460  defines a holding volume  411  between the heaters  462 ,  472 . A platen or chuck  510  is disposed in the holding volume  411  between the guard heaters  462 ,  472  and is adapted to support the wafer  5  for rotation about a vertical axis between the guard heaters  462 ,  472 . A spray member  190  is mounted in a groove  464 F the upper guard heater  462  and adapted to direct fluid through nozzles  192  onto the working surface  5 A of the wafer. 
     The casings  420 ,  430  are preferably each unitarily formed of stainless steel or other suitable metal. Passages  422 A,  422 B,  422 C are formed through the casing  420 . Passages  432 A,  432 B,  432 C are formed through the casing  430 . As best seen in FIG. 9, the casing  420  has an annular flange  424  with an outer, annular recess  425  formed therein and defined in part by a vertical wall  425 A. The casing  430  has an annular flange  434  with an annular groove  435  formed therein. The flange  434  has a vertical wall  434 A. The casings  420  and  430  have opposing annular abutment faces  426  and  436 , respectively. 
     With reference to FIGS. 10-12, the upper guard heater  462  includes an interior member  464  having a top wall  464 A and an annular side wall  464 B. A spiral flow channel  466 A is formed in the top wall  464 A. An outer plate  467  covers the top wall  464 A. An annular surrounding member  468  surrounds the side wall  464 B and defines an annular surrounding channel  466 B therewith. A channel  466 C fluidly connects the channels  466 A and  466 B. An inlet  466 D in the top plate  467  fluidly connects the passage  422 A to the channel  466 B, and an outlet  466 E fluidly connects the passage  422 B to the channel  466 A. The outer plate  467  and the wall  468  are secured to the interior member  464  by welds  8 , for example. The spray member  190  extends through an opening  467 A in the outer plate  467  and is retained (e.g., by an upstream nozzle or screws) in a groove  464 C in the top wall  464 A. The nozzles  192  of the spray member  190  are fluidly connected to the passage  422 C. The interior member  464 , the outer plate  467  and the surrounding wall  468  are preferably formed of stainless steel. The guard heater  462  may be secured to the casing  420  by screws with small standoffs holding the screws off of the walls. 
     With reference to FIGS. 13 and 14, the lower guard heater  472  includes an interior member  478  and an outer plate  474  secured thereto by welds  8 , for example. An opening  479  extends through the outer plate  474 , and an opening  476 D extends through the interior member  478 . A spiral flow channel  476 A is formed in the interior member  478 . An inlet passage  476 B in the outer plate  474  fluidly connects the passage  432 A to the channel  476 A, and an outlet passage  476 C fluidly connects the passage  432 B to the flow channel  476 A. The interior member  478  and the outer plate  474  are preferably formed of stainless steel or other suitable metal. The guard heater  472  may be secured to the casing  430  by screws with small standoffs holding the screws off of the walls. 
     Preferably, the guard heaters  462 ,  472  each have a surface area (i.e., the “interior”, inwardly facing surfaces) to volume ratio of at least 0.2 cm 2 /cm 3 . More preferably, the guard heaters  462 ,  472  each have a surface area to volume ratio of between about 0.2 and 5.0 cm 2/ cm 3 , and most preferably of about 0.6 cm 2 /cm 3 . 
     As discussed above, the temperature of the wafer environment (i.e., the chamber  410  and the fluid(s) therein) is preferably controlled during and between the cleaning and other process steps. The temperature in the chamber  410  is controlled using the guard heater assembly  460 . More particularly, a flow of temperature control fluid is introduced through the passage  422 A, through the inlet opening  466 D, through the channel  466 B, through the passage  466 C, through the passage  466 A, through the outlet opening  466 E and out through the passage  422 B. In this manner, heat from the temperature control fluid is transferred to the guard heater  462  to heat the guard heater  462  (when the fluid is hotter than the guard heater  462 ) or, alternatively, heat from the guard heater  462  is absorbed and removed by the fluid to cool the guard heater  462  (when the fluid is cooler than the guard heater  462 ). The lower guard heater  472  may be heated or cooled in the same manner by a temperature control fluid that flows through the passage  432 A, through the inlet opening  476 B, through the channel  476 A, through the outlet opening  476 C, and through the passage  432 B. 
     The temperature control fluids may be any suitable fluid, preferably a liquid. Suitable fluids include water, ethelyne glycol, propelyne glycol, mixtures of water with either ethelyne or propelyne glycol, Dowtherm A (diphenyl oxide and diphenyl), Dowtherm E, (0-dichlorobenzene), mineral oil, Mobiltherm (aromatic mineral oil), Therminol FR (chlorinated biphenyl). Most preferably, the temperature control fluids are a 50%/50% mixture of water and ethelyne glycol. The fluid may be heated by any suitable means such as an electric, gas-fired or steam heater. The fluid may be cooled by any suitable means such as fluid chiller, for example, of vapor compression refrigeration type or evaporative type. 
     The guard heater assembly  460  and the casings  420 ,  430  are spaced apart to define an insulating gap  470  therebetween that substantially envelopes the guard heaters  462 ,  472 . More particularly, an insulating gap  470 A is defined between the outer plate  467  and the adjacent surrounding wall portions of the casing  420  and preferably has a width A. An insulating gap  470 B is defined between the surrounding wall  468  and the adjacent wall of the casing  420  and has a width B. An insulating gap  470 C is defined between the outer plate  474  and the adjacent surrounding wall portion of the casing  430  and has a width C. Preferably, each of the widths A, B and C is at least 0.1 mm. More preferably, each of the widths A, B and C is between about 0.1 and 10 mm, and most preferably about 1.0 mm. 
     The insulating gap  470  may serve to substantially increase the efficiency, controllability and manufacturing throughput of the system  10 . The insulating gap  470  may substantially thermally insulate the guard heaters  462 ,  472  from the casings  420 ,  430  so that the effect of the temperatures of the casings  420 ,  430  on the atmosphere surrounding the wafer  5  is reduced or minimized. Restated, the insulation gap  470  may substantially limit the thermal mass that must be heated or cooled by the temperature control fluids to the thermal masses of the guard heaters  462 ,  472 . Accordingly, the temperature of the process fluid may be controlled such that it is substantially different than that of the casings  420 ,  430 . 
     While a fluid flow heating/cooling arrangement is illustrated and described above, other means for heating/cooling the guard heaters  462 ,  472  may be employed in addition to or in place of fluid heating. For example, electrical resistance coils (e.g., designed to radiate heat directly to the wafer) may be provided in the guard heaters  462 ,  472 . 
     With reference to FIG. 18, a pressure chamber assembly  400 A according to alternative embodiments of the present invention is shown therein. The assembly  400 A differs from the assembly  400  only in that the guard heater assembly  460 A thereof includes insulating layers  471 ,  473  in place of the insulating gap  470 . The guard heaters  462 ,  472  may be secured to the insulating layers  471 ,  473  which are in turn secured to the casings  420 ,  430 , respectively. 
     The insulating layers  471 ,  473  may be formed of crystalline fluoropolymers such as PCTFE (polychlorotrifluoroethylene), PTFE (polytetrafluoroethylene), or PVF2 (polyvinylidene difluoride). Preferably, the insulating layers  471 ,  473  are formed of bulk PTFE, virgin PTFE or glass-filled PTFE. The insulating layers  471 ,  473  may be honey-combed, open cellular, or otherwise constructed or configured to enhance the insulating performance thereof. 
     Preferably, the guard heater assemblies  460 ,  460 A are adapted to provide temperatures in the pressure chamber  410  ranging from about 0° C. to 90° C. Preferably, the guard heater assemblies  460 ,  460 A are adapted to provide heat to the atmosphere in the pressure chamber  410  at a maximum rate of at least 500 joules/second. 
     Pressure Chamber Sealing System 
     The casings  420 ,  430  which define the pressure chamber  410  also define a fluid leak path  3  (FIG. 15) at the interface from the pressure chamber  410  to an exterior region  7  (e.g., the ambient atmosphere (directly or indirectly)). The sealing system  450  is adapted to restrict (fully or partially) the flow of fluid along the fluid leak path  3 . 
     As best seen in FIG. 15, the sealing system  450  includes an O-ring  452 , an annular cup (or chevron) seal  454 , an annular spring  456  and an annular retaining ring  458 . As discussed below, the combination of the seals  452 ,  454  serves to improve the effectiveness and durability of the pressure chamber seal. 
     The retaining ring  458  is fixed to the flange  424  and extends radially outwardly toward the flange  434  and below the recess  425 . The retaining ring  458  may be formed of stainless steel or other suitable material. The retaining ring  458  may be secured to the flange  424  by any suitable means, for example, threaded fasteners. 
     The cup seal  454  is shown in FIGS. 16 and 17. “Cup seal” as used herein means any self-energized seal that has a concave portion and is configured such that, when the concave portion of the seal is pressurized (e.g., by a pressurized chamber on the concave side of the seal), the seal is thereby internally pressurized and caused to exert an outward force (e.g., against adjacent surfaces of a pressure vessel defining the pressure chamber), to thereby form a seal. The cup seal  454  includes an annular inner wall  454 B joined along an annular fold  454 C to an annular outer wall  454 A and defining an annular channel  454 D therein. 
     The cup seal  454  is preferably unitarily formed of a flexible resilient material. Preferably, the cup seal  454  is formed of a material that is resistant to swelling and damage when exposed to dense CO 2 . Suitable materials include fluorinated polymers and elastomers, such as PTFE (Teflon®, DuPont), filled PTFE, PTFE copolymers and analogs, such as FEP (fluorinated ethylene/propylene copolymers), Teflon AF, CTFE, other highly stable plastics, such as poly(ethylene), UHMWPE (ultra-high molecular weight poly(ethylene)), PP, PVC, acrylic polymers, amide polymers, and various elastomers, such as neoprene, Buna-N, and Epichlorohydrin-based elastomers. Suitable seal materials can be obtained from PSI Pressure Seals Inc., 310 Nutmeg Road South, South Windsor, Conn. 06074. 
     The cup seal  454  may be secured to the flange  424  by affixing at least one, and preferably both, of the inner wall  454 B and the fold  454 C to the adjacent portions of the flange  424  and/or the retaining ring  458 . The inner wall  454 B,  454 C may be secured to the flange  424  using adhesive, for example. Preferably, the cup seal  454  is retained by the retaining ring  458  without the use of adhesive or the like. 
     The spring  456  may be any suitable spring capable of repeatedly and reliably biasing the outer wall  454 A away from the inner wall  454 B (i.e., radially outwardly). Preferably, the spring  456  biases the cup seal  454  radially outwardly beyond the flange  424  when the casings  420 ,  430  are separated (see FIG.  9 ). Preferably, the spring  456  is a wound wire spring or a cantilever type spring having a shape similar to, but smaller than, the cup seal  454  and nested inside the cup seal  454 . The spring  456  is preferably formed of spring grade stainless steel. The spring  456  may be integrally formed with the cup seal  454 . In addition to or in place of the provision of the spring  456 , the cup seal  454  may be formed so as to have an inherent bias to spread the walls  454 A,  454 B apart. Moreover, the spring  456  may be omitted and the cup seal  454  may be provided with no inherent bias. 
     The O-ring  452  is disposed in the groove  435 . Preferably, the O-ring  452  is secured in the groove  435  by an interference fit. The O-ring is formed of a deformable, resilient material. Preferably, the O-ring  452  is formed of an elastomeric material. More preferably, the O-ring  452  is formed of bunna-n or neoprene, and most preferably of EDPM. The O-ring  452  is sized such that, when the O-ring  452  is in its unloaded state (i.e., when the casings  420 ,  430  are separated; see FIG.  9 ), a portion of the O-ring  452  will extend above the abutment face  436 . 
     When the casings  420 ,  430  are closed, the cup seal  454  is captured between the flanges  424  and  434  as shown in FIGS. 8 and 15. The spring  456  biases the walls  454 A and  454 B against the walls  434 A and  425 A, respectively. When the chamber  410  is pressurized above the ambient pressure, the pressure exerted in the channel  454 D forces the walls  454 A and  454 B apart and into tighter, more sealing engagement with the respective walls  434 A and  425 A. 
     In this manner, the cup seal  454  provides a secure, primary seal that prevents or substantially reduces the flow of the fluid from the chamber  410  to the O-ring  452  along the fluid leak path  3 . The O-ring  452  is thereby spared potentially damaging exposure to the process fluid. Such protection of the O-ring  452  may substantially extend the service life of the O-ring  452 , particularly where the process fluid includes high pressure CO 2 . Accordingly, the sealing system  450  may provide for a high throughput wafer manufacturing system with relatively long-lived seals. 
     Notably, when the chamber  410  is pressurized, the casings  420 ,  430  may be separated somewhat by the internal pressure so that the O-ring  452  is not well-loaded for sealing. Because the cup seal  454  serves as a primary seal, a secure sealing arrangement may nonetheless be provided. However, in the event of a partial or complete failure of the cup seal  454 , the O-ring  452  may serve to prevent or reduce leakage of the process fluid to the environment. According to certain embodiments, the assembly  400  may be adapted such that the O-ring  452  will allow fluid to pass along the fluid leak path  3  when the chamber  410  is at at least a selected pressure so that the O-ring is not pressurized and no damaging process fluid (e.g., CO 2 ) is in contact with the O-ring for extended periods of time. 
     When the fluid in the chamber  410  is at atmospheric pressure or vacuum, the sealing effectiveness of the cup seal  454  will typically be diminished (however, the bias of the spring  456  may provide some sealing performance). In this event, the O-ring  452  may serve as the primary seal to prevent or reduce leakage of atmospheric fluid into the chamber  410  through the fluid leak path  3 . Notably, the atmospheric fluid (typically air) typically will not include high concentrations of CO 2  or other components unduly harmful to the O-ring material. 
     Preferably, and as illustrated, the O-ring  452  sealing arrangement is a butt-type arrangement so that no sliding components are present. The pressure energizing mechanism of the cup seal  454  allows for use of a relatively low bias force for the spring  456 . These aspects of the invention assist in minimizing the generation of any particles that may be detrimental to the wafer  5 . The cup seal  454  may be otherwise oriented or located in the pressure chamber assembly. Two or more of the cup seals  454  may be arranged in series along the fluid leak path. 
     From the description herein, it will be appreciated that the combination of a cup seal and an elastomeric O-ring seal overcomes certain problems associated with high pressure sealing of CO 2  holding vessels that typically neither an elastomeric O-ring seal nor a cup seal can overcome. In particular, elastomeric O-rings are generally not long-lived when exposed to high-pressure CO 2  and then rapidly depressurized. Cup seals when used as pressure seals typically require a large pre-load spring to enable the same vessel for vacuum service. Such large pre-load may cause greater friction and wear and, thus, generation of damaging/contaminating particles. In accordance with the present invention, the elastomeric O-ring may be externally energized (compressed) when required to establish a vacuum within the chamber. 
     Wafer Holding Assembly 
     With reference to FIGS. 19-22, a wafer holding assembly  520  according to further embodiments of the present invention is shown therein. The assembly  520  may be used in place of the chuck  510  in a pressure chamber assembly  400 B (FIG. 19) otherwise corresponding to the pressure chamber assembly  400 . As will be better appreciated from the following description, the wafer holding assembly  520  includes a substrate holder or platen or chuck  522  and is adapted to retain the wafer on the chuck  522  by means of a pressure differential generated by rotation of the chuck  522 . 
     The chuck  522  has a front surface  524  and an opposing rear surface  528 . A plurality (as shown, eight) of impeller vanes  529  extend rearwardly from the rear surface  528  and radially outwardly with respect to a central rotation axis E—E (FIG.  19 ). A plurality (as shown, four) of passages  526 A extend fully through the chuck  522  from the rear surface  528  to a circumferential channel  526 B formed in the front surface  524 . A plurality (as shown, sixteen) of channels  526 C extend radially outwardly from and fluidly communicate with the channel  526 B. Additional circumferential channels (not shown) may fluidly connect the channels  526 C. 
     As shown in FIG. 19, the chuck  522  is mounted on a driven shaft  530  for rotation therewith about the rotational axis E—E. As the chuck  522  is rotated, the impeller vanes  529  tend to push or force the fluid between the rear surface  528  and the adjacent, opposing surface  412  of the pressure chamber  410  radially outwardly in the directions F toward the outer periphery of the chuck  522 . A pressure differential is thereby generated beneath the chuck  522  between the inner region (i.e., nearest the axis E—E) of the chuck  522  and the outer region of the chuck. More particularly, the pressure in the central region (including the pressure at the lower openings of the passages  526 A) is less than the pressure at the outer edges of the chuck  522  and the pressure in the chamber  410  on the side of the wafer  5  opposite the chuck  522 . As a result, a differential is created between the fluid pressure exerted on the top surface of the wafer  5  and the pressure of the fluid in the channels  526 B,  526 C. 
     In the foregoing manner, the wafer  5  is secured to the chuck  522  as the chuck  522  and the wafer  5  are rotated. In order to retain the wafer  5  on the chuck  522  prior to initiating rotation or during process steps without rotation, and/or in order to provide additional securement, supplemental holding means may be provided. Such supplemental means may include, for example, adhesive, clamps, and/or an externally generated pressure differential assembly such as the wafer holding assembly  550  described below. 
     With reference to FIGS. 23-25, a wafer holding system  551  according to further embodiments of the present invention is shown therein. The system  551  includes a wafer holding assembly  550  and may be used in place of the chuck  510  in a pressure chamber assembly  400 C (FIG. 23) otherwise corresponding to the pressure chamber assembly  400  (for clarity, certain elements of the assembly  400 C are not shown). The assembly  400 C is further provided with a magnetic drive assembly  580 . 
     As will be better appreciated from the following description, the wafer holding assembly  550  includes a substrate holder or platen or chuck  552  and is adapted to retain the wafer  5  on the chuck  552  by means of a pressure differential between the pressure in the pressure chamber  410  and the pressure at an outlet  564 . The magnetic drive system  580  is adapted to drive the chuck  552  relative to the pressure chamber  410  without requiring sealing directly between relatively moving elements (namely, a shaft  560  and the casing  430 ). It will be appreciated that the wafer holding system  551  may be used with other drive arrangements and that the magnetic drive assembly  580  may be used with other wafer holder mechanisms. 
     Turning to the magnetic drive assembly  580  in greater detail, the assembly  580  includes an upper housing  585  and a lower housing  584 . The upper end of the upper housing  585  is received in the casing  430  such that a gas-tight seal is provided therebetween (e.g., by means of a suitable sealing device such as a gasket). The shaft  560  extends through the housing  585  and is rotatably mounted thereon by upper and lower bearings  586  and  588 . A seal  561  is positioned between the shaft  560  and the housing member  585 . The seal  561  is preferably a non-contact seal. More preferably, the seal  561  is a gap seal (more preferably, defining a gap G having a width of between about 0.001 and 0.002 inch) or a labyrinth seal. The seal  561  may also be a lip seal or a mechanical seal. 
     An internal magnet holder  590  is mounted on the lower end of the shaft  560  for rotation therewith and has an inner magnet M 1  mounted on an outer portion thereof. The internal magnet carrier  590  is disposed in the lower housing member  584 . A pressure cap  596  surrounds the internal magnet carrier  590  and forms a gas-tight seal (e.g., by means of a suitable sealing device such as a gasket) with the lower end of the lower housing member  584 . In this manner, the pressure cap  596  and the upper housing member  585  together form a gas-tight reservoir for fluids that may enter the upper housing member  585  from the pressure chamber  410 . 
     A drive unit  582  is mounted on the housing member  584 . The drive unit  582  may be any suitable drive device such as a hydraulically driven unit or, more preferably, an electrically driven unit. The drive unit  582  is operable to rotate a shaft  594  that extends into the housing member  584 . An external magnet holder  592  is mounted on the shaft  594  for rotation therewith. The external magnet holder  592  is disposed in the housing member  584 , but is mechanically and fluidly separated from the internal magnet holder  590  and the pressure chamber  410  by the pressure cap  596 . An external magnet M 2  is mounted on the external magnet holder  592  for rotation therewith. 
     The magnets M 1  and M 2  are relatively constructed, arranged and configured to such that they are magnetically coupled to one another. In this manner, the magnets M 1 , M 2  serve to indirectly mechanically couple the external magnet holder  592  and the internal magnet holder  590 , and thereby the shaft  594  and the shaft  560 . Thus, the chuck  522  may be rotated by operation of the drive unit  582 . 
     The magnetic drive assembly  580  may be any suitable drive assembly with suitable modifications as described herein. Suitable magnetic drive assemblies include the BMD 150, available from Büchi AG of Uster, Switzerland. Moreover, other types of non-mechanically coupling drive units may be used. 
     As best seen in FIGS. 24 and 25, the chuck  552  has a front surface  554 . A countersunk passage  556 B extends fully through the chuck  552 . A plurality of channels  526 A extend radially outwardly from and fluidly communicate with the passage  556 B. Additional circumferential channels (not shown) may fluidly connect the channels  526 A. 
     As shown in FIG. 23, the chuck  552  is mounted on the driven shaft  560  by a nut  558  for rotation with the shaft  560  about a rotational axis F—F. The shaft  560  has an axially extending connecting passage  562  extending therethrough. The nut  558  has a central aperture that allows fluid communication between the passage  562  and the passage  556 B. A passage  563  extends radially through the shaft  560  and fluidly connects the passage  562  to the secondary chamber  565  defined between the housing  585  and the shaft  560 . Preferably, the seal  561  is a non-contact seal (e.g., a gap seal or a labyrinth seal) forming a restricted flow passage that provides fluid communication between the pressure chamber  410  and the secondary chamber  565 . 
     An outlet  564  in the housing member  585  fluidly connects the secondary chamber  565  with a line L 40 . A line L 41  having a valve V 30  fluidly connects a flow restrictor  566  and a storage tank  568  to the line L 40 . The flow restrictor  566  may be a throttling orifice or a suitable partial closure valve such as a needle valve adapted to provide a controlled limit on flow therethrough. A line L 42  having a valve V 31  fluidly connects a fluid transfer device P 20  (e.g., a vacuum pump) to the line L 40 . 
     The system  551  may be used in the following manner to secure the wafer  5  to the chuck  552 . A pressure is provided in the storage tank  568  that is less than the pressure of the atmosphere in the pressure chamber  410  under typical process conditions. During processing, the valve V 30  is opened so that the secondary chamber  565  is placed in fluid communication with the storage tank  568  which serves as a passive low pressure source (i.e., no pump, compressor or the like is employed to generate the pressure or vacuum). In this manner, the pressure in the chamber  565  (and, therefore, in the fluidly communicating channels  556 A) is less than the pressure in the pressure chamber  410 . A pressure differential is thereby generated between the upper surface of the wafer  5  and the backside of the wafer  5 , causing the wafer  5  to be drawn down onto the chuck  552  in the direction D. 
     The flow restrictor  566  serves to limit flow of fluid from the secondary chamber  565  to the storage tank  568 , thereby providing a controlled leak. The controlled leak serves to ensure a that sufficient differential pressure is provided across the wafer  5  to hold it in place without allowing undue loss of the fluid from the pressure chamber  410 . 
     Preferably, the pressure of the storage tank  568  is greater than atmospheric pressure, but less than the pressure of the pressure chamber  410  during the intended processes. The storage tank  568  may permit gas that is drawn from the pressure chamber  410  to be cleaned and recycled or otherwise disposed of. 
     Alternatively, the storage tank  568  may be omitted or bypassed such that the line L 41  vents directly to atmosphere when the valve V 30  is opened. 
     If the pressure of the atmosphere in the pressure chamber  410  is the same as or less than the pressure of the passive low pressure source (i.e., the storage tank  568  or the ambient atmosphere), the fluid transfer device P 20  may be operated to reduce the pressure in the chamber  565  to less than the pressure in the pressure chamber  410  to generate the desired amount of pressure differential across the wafer  5 . In this event, the valve V 30  is closed and the valve V 31  is opened. 
     Preferably, the system  551  is operable to generate a pressure in the channels  556 A that is at least 1 psi less than the pressure in the pressure chamber  410 , and more preferably, between about 5 and 20 psi less. 
     Rotating Spray Member 
     The spray member  190  as described above as well as the spray members  602 ,  652  described below provide dispersed inlets to deliver process fluids directly to the surface of the wafer. Moreover, the spray members provide a distributed stream of these fluids that incorporates mechanical action from the fluid/surface impingement. This mechanical action is generally the result of the momentum of the fluid stream coming out of the spray member. 
     Design of the spray member (including, for example, number, spacing and sizes of spray ports) may be used to selectively control the use of the energy transfer/mechanical action. Additionally, simultaneous rotation of the wafer may serve to generate shear (momentum) between the fluid and the wafer surface to further facilitate removal of materials from the surface. 
     With reference to FIG. 26, a pressure chamber assembly  400 D according to further embodiments of the present invention is shown therein. The assembly  400 D may be the same as the assembly  400  (certain aspects not shown in FIG. 26 for clarity), for example, except for the provision of a rotating spray member assembly  600 . The assembly  400 D may include a rotatively driven wafer holder  510  or the wafer  5  may be held stationary. The spray member assembly  600  may be used with any of the above-described pressure chamber assemblies. Notably, the spray member assembly  600  may be used to provide relative rotation between a spray member and a wafer without requiring a rotating wafer holder. 
     The spray member assembly  600  includes a spray member  602  as also shown in FIGS. 27 and 28. The spray member  602  includes a shaft portion  610  and bar-shaped distribution portion  620 . An axial passage  612  extends from an upper opening  614  and through the portion  610  and fluidly communicates with a lateral passage  622  in the portion  620 . A series of spray ports  624  extend from the passage  622  to the lower, outer edge of the distribution portion  620 . The spray member  602  may be formed of a highly oxidatively stable material such as  316  stainless steel. 
     A bearing  630  is fixed within a passage  427  in the casing  420  such that a flange  632  of the bearing  630  is received in an enlarged portion  427 A of the passage  427 . The bearing  630  is preferably a sleeve bearing as shown. The bearing  630  may be formed of PTFE, PE or PEEK. Preferably, the bearing  630  is formed of PTFE. 
     The shaft portion  612  extends through the bearing  630  and has a flange  616  overlying the flange  632 . An end cap  640  is securely mounted to the casing  420  in the portion  427 A and over the flange  616 , for example, by threading. Preferably, the end cap  640  forms a gas pressure tight seal with the casing  420 . 
     The end cap  640  is adapted to receive a supply of process fluid (e.g., from a supply line  9 ) such that the flow of process fluid is directed through a passage  642  and into the passage  612 . The fluid continues into the passage  622  and is dispensed through the ports  624 . 
     With reference to FIGS. 27 and 28, the ports  624 A are angled with respect to the intended rotational axis N—N (see FIG. 28) of the spray member  602 . Preferably, the ports  624  are disposed at an angle M (FIG. 28) of between about 0 and 85 degrees, and more preferably of between about 30 and 60 degrees. The ports  624  are angled opposite the direction R (FIG. 27) of intended rotation. 
     In use, the reaction force responsive the fluid exiting the ports  624  (i.e., the hydraulic propulsion) causes the spray member  602  to rotate about the axis N—N within the bearing  630 . Notably, because the bearing  630  is mounted internally (i.e., within the high pressure region) of the pressure chamber  410  separated from ambient pressure by the end cap  640 , the bearing is not subjected to loading from a substantial pressure drop thereacross. 
     Alternatively or in addition to the hydraulically driven rotation, the spray member  602  may be coupled to a drive unit. The spray member may be directly or indirectly mechanically coupled to the drive unit (e.g., using a bearing/seal/drive unit configuration) or may be non-mechanically coupled (e.g., using a coupling force for electromagnetic or magnetic (permanent, electro- or induction-driven) coupling). Some or all of the ports  624  may be oriented parallel to the axis of rotation N—N. 
     A spray member  652  according to further embodiments of the present invention may be used in place of the spray member  602  and with any of the foregoing modifications or features. The spray member  652  has a shaft portion  660  and corresponds to the spray member  602  except that the bar-shaped distribution portion  620  is replaced with a plate- or disk-shaped distribution portion  670  having a pattern of spray ports  674  formed therein. The pattern of the spray ports  674  may be modified. 
     It will be appreciated that various of the inventions described hereinabove and as reflected in the claims that follow may be used for processes other than those specifically discussed above with regard to the preferred embodiments. For example, the means and methods for holding a wafer to a chuck may be employed to hold other types of substrates, in other types of processes (e.g., processes not involving CO 2  or wafer fabrication). The supply/recovery system  300  and the subsystems thereof may be used in other systems and processes using CO 2  containing process fluids, such as chemical mechanical planarization (CMP) systems employing CO 2 . 
     The foregoing is illustrative of the present invention and is not to be construed as limiting thereof. Although a few exemplary embodiments of this invention have been described, 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. Therefore, it is to be understood that the foregoing is illustrative of the present invention and is not to be construed as limited to the specific embodiments disclosed, and that modifications to the disclosed embodiments, as well as other embodiments, are intended to be included within the scope of the invention.