Abstract:
Provided is a novel method and system for supplying high-pressure carbon dioxide to an application having a variable carbon dioxide flow requirement. The method includes providing a high-pressure carbon dioxide feed stream to a buffer volume and determining the amount of carbon dioxide to be delivered to the application tool. The pressure in the buffer volume is maintained at a pressure that exceeds the pressure required by the application tool. The temperature in the buffer volume is further adjusted to modify the density of the carbon dioxide and based thereon the size of the buffer volume. Thereafter, the carbon dioxide from the buffer volume is delivered at a variable flow rate as required by the application tool.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a method for supplying high-pressure carbon dioxide to an application having a variable carbon dioxide flow requirement. In particular, the invention relates to supplying carbon dioxide to a semiconductor application tool that has an instantaneous demand, which is significantly higher than its average demand.  
         [0003]     2. Description of Related Art  
         [0004]     The need for the instantaneous delivery of a fluid has long been known. In related applications, high pressure gas sources have been utilized where an instantaneous demand for fluid is necessary. For example, U.S. Pat. No. 4,977,921 to Knight et al. discusses the use of a modified tube trailer to store high pressure air or nitrogen in order to provide a high instantaneous flow of gas to clean out large size lines. U.S. Pat. No. 5,417,615 to Beard concerns the use of a pressurized gas source to store high-pressure air that is used to propel an amusement park ride.  
         [0005]     U.S. Pat. No. 6,725,671 B2 to Bishop discloses the effect of temperature and pressure on the capacity of a natural gas storage system. The temperature and pressure and storage vessel wall thickness are chosen so as to minimize the cost of storing natural gas.  
         [0006]     In the semiconductor industry there has been a need to provide a high demand of fluid to an application, such as a cleaning tool. In this regard, U.S. Pat. No. 6,085,762 to Baton and U.S. Pat. No. 6,403,544 B1 to Davenhall et al. disclose the use of buffer volumes or ballast tanks in the supply of a supercritical fluid to a semiconductor wafer processing application. The process includes sending pressure pulses of the supercritical fluid to the application.  
         [0007]     U.S. Pat. No. 6,612,317 B2 to Costantini et al. describes the use of a buffer volume or ballast tank in the supply of a supercritical fluid to a semiconductor wafer processing application. Changing the pressure of the fluid in the buffer volume is disclosed as a means to controlling the fluid delivered to the application tool, and rapid discharge of the fluid in the buffer volume into the wafer-processing chamber is shown. It will be understood that the term “buffer volume”, as utilized throughout the specification and the claims refers to a pressure vessel such as a ballast tank utilized to deliver fluid to a semiconductor application.  
         [0008]     One of the disadvantages associated with the related art systems is that they do not recognize the selection of temperature in the buffer volume as an important factor in controlling the amount of carbon dioxide available for delivery to a semiconductor application tool. Another disadvantage is that the related art does not provide a means of controlling the release of the pressure in the buffer vessel.  
         [0009]     To overcome these disadvantages, it is an object of the present invention to provide a means for controlling the amount of carbon dioxide available for delivery to an application tool during high instantaneous demand periods.  
         [0010]     It is yet another object of the present invention to adjust the buffer volume capacity without modifying the physical dimension of the buffer volume.  
         [0011]     It is a further object of the present invention, to size the buffer volume by manipulating the temperature of the carbon dioxide once the maximum amount of carbon dioxide to be delivered to the semiconductor application tool has been ascertained.  
         [0012]     It is another object of the invention to replace expensive and oversized pumps that would be needed to deliver a high flow rate to the semiconductor cleaning tool.  
         [0013]     It is a further object to deliver the required carbon dioxide flow rate, instantaneously, and therefore eliminate the time lag that would be required when utilizing pumps to provide the required flow rate.  
         [0014]     Other objects and advantages of the invention will become apparent to one skilled in the art on a review of the specification, figures and claims appended hereto.  
       SUMMARY OF THE INVENTION  
       [0015]     The foregoing objectives are met by the process of the present invention.  
         [0016]     According to one aspect of the invention, a method for supplying high-pressure carbon dioxide to an application tool having a variable carbon dioxide flow requirement is provided. The method includes providing a high-pressure carbon dioxide feed stream to a buffer volume; maintaining the pressure in the buffer volume between a minimum pressure that exceeds the pressure required by the application tool and a maximum pressure such that the average density of carbon dioxide contained in the buffer volume at minimum pressure is different from the average density of carbon dioxide contained in the buffer volume at said maximum pressure.  
         [0017]     The average temperature in the buffer volume is adjusted to modify the difference between the average density of carbon dioxide contained in the buffer volume at the minimum pressure and the average density of carbon dioxide contained in the buffer volume at maximum pressure such that the flow requirement associated with the application tool is satisfied. The high-pressure carbon dioxide supply stream is delivered from the buffer volume at a variable flow rate as required by the application tool.  
         [0018]     In accordance with another aspect of the invention, a system for supplying high-pressure carbon dioxide to an application having a variable carbon dioxide flow requirement is provided. The system includes a buffer volume to receive a high-pressure carbon dioxide feed stream, wherein the pressure in the buffer volume is maintained between a minimum pressure that exceeds the pressure required by the application tool and a maximum pressure such that the density of the carbon dioxide at minimum pressure is different from the density of said carbon dioxide at maximum pressure, and the temperature in said buffer volume is adjusted.  
         [0019]     A carbon dioxide purification unit is disposed upstream of the buffer volume to deliver a high-pressure carbon dioxide feed stream, and an application tool is disposed downstream of the buffer volume to receive carbon dioxide from the buffer volume at a variable flow rate as required by the application tool. 
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0020]     The invention will be better understood by reference to the figures wherein like numbers denote same features throughout and wherein:  
         [0021]      FIG. 1  illustrates a schematic diagram of the overall system for delivery of carbon dioxide to a semiconductor application tool having variable flow requirement;  
         [0022]      FIG. 2  depicts an exemplary operational five minute cycle for delivery of carbon dioxide, in accordance with the invention;  
         [0023]      FIG. 3  illustrates a schematic diagram of the overall system having a single core heat exchanger;  
         [0024]      FIG. 4  illustrates a schematic diagram of the overall system, in accordance with another embodiment where the apparatus upstream of pressurization means has been modified;  
         [0025]      FIG. 5  depicts a schematic diagram of the overall system, in accordance with another embodiment having a single core heat exchanger disposed upstream of the pressurization means; and  
         [0026]      FIG. 6  illustrates a schematic diagram of the overall system, in accordance with another embodiment where the recirculation has been redesigned. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]     The manufacturing of integrated circuit devices requires numerous complicated steps necessary to form various features onto the wafer substrate. Some of the steps include cleaning the substrate with high-pressure carbon dioxide. As utilized herein the term “high-pressure carbon dioxide” refers to carbon dioxide at a pressure greater than its critical pressure of 1060 psig. In the event that the temperature associated with the carbon dioxide is below its critical temperature of 88° F. and is at high pressure it will be understood that the fluid is in liquid phase. On the other hand, if the temperature associated with the carbon dioxide is greater than 88° F., the fluid will be in supercritical phase. Therefore, at above 1060 psig, the carbon dioxide is in one phase only, whether it be liquid or supercritical (i.e. no discontinuous phase transition).  
         [0028]     The semiconductor cleaning process of the present invention provide an effective means of supplying high-pressure carbon dioxide to a semiconductor cleaning application which has a very high instantaneous demand for high-pressure carbon dioxide, but a much lower average demand for high-pressure carbon dioxide during a given operational cycle. Typically, the wafer cleaning application processes one wafer per cycle, and the carbon dioxide demand during that cycle varies from a very high rate to virtually zero.  
         [0029]     As illustrated in  FIG. 1 , the supply system includes a carbon dioxide bulk tank  10  for holding the carbon dioxide delivered to the semiconductor application tool  100 . The carbon dioxide removed from the bulk tank  10  is transferred through heat exchangers  29 , 30  and a carbon dioxide purification unit  40 , which is discussed in detail below, where particular embodiments are referenced. A high-pressure carbon dioxide stream  75  is generated and delivered to a buffer volume tank/vessel  80 , which is utilized to meet the tool instantaneous demand for high-pressure carbon dioxide.  
         [0030]     By way of explanation, the application tool  100  requires a maximum instantaneous flow rate (S max ) of carbon dioxide at a certain set pressure level (P). The pressure in buffer volume  80  (P bv ) must always be at least equal to the set pressure (i.e., P bv = or &gt;P). However, to compensate for some pressure drop in the line leading from buffer volume  80  to application tool  100 , the pressure in the buffer volume (P bv ) cannot drop below a minimum required pressure (P min ) which is greater than the pressure required by the application tool (i.e., P bv = or &gt;P min = or &gt;P).  
         [0031]     The average flow rate (F avg ) of high-pressure carbon dioxide feed stream  75  (F i ) delivered to buffer volume  80  must be equal or higher than the average flow rate (S avg ) of high-pressure carbon dioxide supply stream  85  (S i ) delivered to application tool  100 . Thus, when F i  is greater than S i , the pressure in buffer volume  80  increases. Conversely, when S i  is greater than F i , the pressure in buffer volume  80  will decrease. Accordingly, over the course of the processing cycle, when the flow rate to application tool  100  is greater than flow rate of high-pressure carbon dioxide feed stream  75 , the additional carbon dioxide required by application tool  100  must be delivered from the carbon dioxide stored in buffer volume  80 .  
         [0032]     In accordance with the present invention, applicants have found that properly sizing buffer volume  80 , without replacing it or modifying the physical characteristics of it, allows the requisite amount of carbon dioxide to be delivered to the application tool during the operating cycle. To properly size buffer volume  80 , the flow rate of carbon dioxide to the buffer volume and application tool  100 , can be integrated with respect to time in order to determine the net change amount of carbon dioxide in the buffer volume. The buffer volume supplies the net changed amount of carbon dioxide by undergoing a change in pressure from a maximum pressure to a minimum pressure when carbon dioxide is required by the application. The temperature in the buffer volume at maximum and minimum pressure will determine the density of the carbon dioxide in the buffer volume. Thus, the size of the buffer volume necessary to supply the net change amount of carbon dioxide can be determined. Once the buffer volume has been controlled, the amount of carbon dioxide supplied from the buffer volume to the application tool can be changed without changing its physical characteristics by modifying the volume temperature.  
         [0033]     With reference to  FIG. 2 , application tool  100  is shown to have a cycle of five minutes. The tool requires 14 lbs/min of high pressure carbon dioxide for the first minute of the cycle, 11 lbs/min for the third minute of the cycle, and no carbon dioxide during other periods of the five minute cycle. The average carbon dioxide demand of application tool  100 , S avg , is 5 lbs/min. The flow rate of high-pressure carbon dioxide feed stream  75 , F i , remains constant at 5 lb/min throughout the cycle. The largest carbon dioxide deficit that buffer volume  80  must make up is at the end of the third minute of the cycle. The deficit can either by calculated by integrating the difference between F i  and S i  for the entire cycle, or alternatively, by taking note of the visual representation on the graph.  
         [0034]     As shown in  FIG. 2 , at the third minute, 10 additional lbs of carbon dioxide have been sent to the application in high-pressure carbon dioxide supply stream  85  than have entered the buffer volume through high-pressure carbon dioxide feed stream  75 . Therefore, buffer volume  80  must have a carbon dioxide buffer capacity of 10 lbs.  
         [0035]     The buffer capacity required by any application tool  100  can be calculated in a similar manner, as shown in  FIG. 2 . Once the required buffer capacity for buffer volume  80  is known, buffer volume  80  can be sized. In addition to the required buffer capacity, one must know the density of carbon dioxide at the maximum and minimum pressure loading of buffer volume  80 . Turning back to the cycle depicted in FIG.  2 , the maximum pressure loading of buffer volume  80  occurs at 0 seconds and 300 seconds. The minimum pressure loading of buffer volume  80  occurs at 180 seconds. The pressure in buffer volume  80  at minimum loading is P min . If the temperature in buffer volume  80  is known, the carbon dioxide density can be determined.  
         [0036]     Preferably, the buffer volume is maintained at isothermal conditions during a particular cycle of operation. Therefore, the buffer volume temperature at maximum and minimum loading pressure is approximately the same. As aforementioned, the minimum pressure loading (P min ) must be greater than the pressure (P) in the line required by application tool  100 . In a preferred embodiment the maximum loading pressure is selected to be less than or equal to ninety percent (90%) of the maximum allowable operating pressure of buffer volume  80 . Operating the buffer volume under these conditions takes into consideration safety concerns, so that the safety relief devices built into the system need not be activated, save unforeseen emergency situations.  
         [0037]     Upon selecting the maximum loading pressure, the carbon dioxide density at this particular pressure, as well as at the minimum loading pressure can be determined. That allows for the calculation of the buffer volume, by dividing the buffer volume capacity by the difference in density at maximum and minimum loading pressures. Once a buffer volume has been constructed, the amount of carbon dioxide that can be delivered instantaneously, can be adjusted by modifying the buffer volume temperature.  
         [0038]     For instance, the operator of the system indicates that a processing cycle of five minutes is desired, where the application tool  100  will requires an instantaneous 1.35 lbs at a pressure of 2850 psig. Since the carbon dioxide demand of 1.35 lbs is instantaneous, a high-pressure carbon dioxide stream  75  is continuously delivered to buffer volume  80 , during the draw. The amount delivered, however, is insubstantial in comparison to the amount drawn by the application tool. An analysis similar to the one discussed and depicted in  FIG. 2  is made at different temperature points. If a 50 psi pressure drop is assumed to exist in the line leading from the buffer volume to the application tool, the minimum loading pressure is 2900 psig. The maximum loading pressure for the buffer volume is selected as 3100 psig at a temperature of 110° F. The density of carbon dioxide at 3100 psig and 110° F. is 52.238 lb/ft 3 . The density of carbon dioxide at 2900 psig and 110° F. is 51.418 lb/ft 3 . Thus, dividing the required buffer volume capacity of 1.35 lbs by the difference in density (i.e., 52.238-51.418)lb/ft 3  shows that the buffer volume is 1.65 ft 3 . Modifying this temperature modifies the amount of carbon dioxide delivered to the application tool.  
         [0039]     As illustrate in the Table, below, the temperature associated with buffer volume  80  was modified to a temperature other than 110° F., and the the calculations were carried out in the manner explained above.  
                                                     TABLE                               Carbon                       Dioxide   Carbon       Pressure       Density   Dioxide   Required       at   Temperature   at   Density   Size of       Maximum   in Buffer   Maximum   at 2900   Buffer       Loading   Volume 80   Loading   psig   Volume 90       (psig)   (° F.)   (lbs/ft 3 )   (lbs/ft 3 )   (ft 3 )                                3100   50   61.654   61.264   3.46       3100   70   58.767   58.277   2.76       3100   85   56.445   55.855   2.29       3100   100   53.973   53.256   1.88       3100   110   52.238   51.418   1.65       3100   120   50.433   49.492   1.43       3100   150   44.611   43.192   0.95       3100   200   34.502   32.377   0.64                  
 
         [0040]     As shown in the Table, the density of carbon dioxide decreases as buffer volume temperature increases, and less total carbon dioxide is contained in buffer volume  80 . The density difference between the pressure maximum and minimum loading as the buffer volume temperature increases. The required size of buffer volume  80  decreases as buffer volume temperature increases. Similarly, the amount of carbon dioxide available for delivery to the application tool increases for a given buffer volume as temperature increases, even though less carbon dioxide is contained in the buffer volume at maximum pressure. This finding is unexpected, as normal practice would be to decrease the buffer volume temperature at a given pressure to increase the amount of carbon dioxide contained therein. It is of utility to the present invention, as the volume of carbon dioxide that may be stored and delivered to application tool  100  is increased.  
         [0041]     Increasing the buffer volume temperature results in an increased ability of the buffer volume to deliver large volumes of carbon dioxide instantaneously. Therefore, buffer volume temperature must be selected on a case by case basis, and balanced against the other factors which suggest lower temperatures. These factors include: (a) the buffer volume/vessel requires thicker walls since the strength of the wall material decreases as the buffer volume temperature increases; (b) additional heat will be lost to the environment and the cost to maintain the buffer volume temperature will increase with the temperature; (c) more energy may be necessary to raise the temperature of the carbon dioxide in buffer volume  80  to the desired temperature.  
         [0042]     The carbon dioxide temperature desired at application  100  should also be a consideration in selecting buffer volume temperature. In a preferred embodiment the buffer volume temperature should be maintained slightly above ambient at approximately 110 to 120° F.  
         [0043]     Returning to  FIG. 1 , the entire carbon dioxide supply system in accordance with one embodiment is discussed in detail. The carbon dioxide (CO 2 ) source for the system is CO 2  bulk tank  10 . The tank can range in size from a two hundred liter Dewar to a tank holding three hundred tons of CO 2 . Typically, the capacity of the bulk tanks range from six tons to one hundred tons. For example, the CO 2  bulk tank  10  can be refilled from a CO 2  delivery trailer without interrupting the operation of the CO 2  supply system.  
         [0044]     The CO 2  bulk tank  10  normally contains liquid CO 2  with a vapor headspace maintained at a pressure between 250 and 350 psig and a temperature between −9° F. and 10° F. A saturated liquid carbon dioxide is drawn from the lower part of bulk tank  10  and routed to CO 2  booster pump  15 . The pump raises the pressure of the CO 2  by approximately 50 to 120 psi, but the temperature is increased only incrementally. The subcooled liquid CO 2  exiting booster pump  15  is conveyed, in part, to pressurization means  20 . The pressurization means can be a pump, selected from a diaphragm pump, a metal diaphragm pump, a piston pump or other pumping means.  
         [0045]     The CO 2  is pressurized in CO 2  pressurization means  20  to a pressure as high as the pressure at maximum loading in buffer volume  80  plus the line pressure drop between CO 2  pressurization means  20  and buffer volume  80 . The CO 2  normally increases in temperature by approximately 20° F. across CO 2  pressurization means  20 , and exits at a temperature ranging from about 10 to 30° F.  
         [0046]     Part of the subcooled CO 2  liquid exiting booster pump  15 , and which exceeds the maximum capacity of CO 2  pressurization means  20  is circulated through backpressure regulator  16  and returned to CO 2  bulk tank  10 . A small portion of the subcooled liquid CO 2  flashes to vapor as it passes through backpressure regulator  16 . Over time, this vapor would increase the pressure in CO 2  bulk tank  10 . To prevent this phenomenon, refrigeration system  12  conveys a refrigerant through cooling coil  11  in CO 2  bulk tank  10  to recondense the CO 2  vapor that forms from flashing across backpressure regulator  16 . This cooling also recondenses CO 2  vapor that forms due to heat leak into CO 2  bulk tank  10  from ambient.  
         [0047]     The pressurized CO 2  exiting pressurization means  20  is conveyed through heat exchangers  29  and  30  where it exchanges heat indirectly with some or all of the CO 2  returned from CO 2  purification unit  40 . Heat exchangers  29  and  30  allow for heat recovery from CO 2  purification unit  40  that takes place at a temperature greater than that of the CO 2  exiting CO 2  pressurization means  20 . The CO 2  flows through heater unit  35 , which raises the temperature of the CO 2  to the temperature required for CO 2  purification unit  40 . Heater  35  is necessary because the heat recovery in heat exchangers  29  and  30  is not totally efficient.  
         [0048]     CO 2  purification unit  40  can be selected from any purification device capable of operating at a temperature greater than the temperature of the CO 2  exiting pressurization means  20 . Preferably, the unit operates at temperatures between 100° F. and 1000° F., and most preferably at temperatures between 200° F. and 700° F. Exemplary purification units include adsorption, absorption, chemical reaction, catalytic oxidation, and filtration.  
         [0049]     Filtration at elevated temperature may be part of the CO 2  purification unit&#39;s function. Filters such as the sintered metal filters commonly used in the electronics industry are known to perform better in vapor phase than in liquid phase applications. This is largely due to increased diffusivity and Brownian motion of particles in the vapor phase. The behavior of filters in supercritical service (i.e., pressure greater than 1060 psig and temperature greater than 88° F.) has not been well-studied, but it is known that as the temperature of a supercritical fluid increases, its behavior, including its diffusivity, becomes similar to that of a gas.  
         [0050]     When the CO 2  purification unit  40  is operated at a temperature higher than the buffer volume  80  temperature, the CO 2  becomes more gas-like. Therefore, filtration will be more effective. When a filter is included as part of CO 2  purification unit  40 , it should be placed downstream of any particle generating means of purification such as adsorption and catalytic oxidation.  
         [0051]     CO 2  removed from the CO 2  purification unit  40  is re-routed through heat exchanger  30 , where it exchanges heat indirectly with CO 2  directed from CO 2  pressurization means  20  thereto. CO 2  that is required in buffer volume  80  flows into high pressure carbon dioxide feed stream heating or cooling means  74 . Based on the pressure sensed in buffer volume  80  at valve  60 , CO 2  exceeding a predetermined level is recirculated to the bulk tank  10 . CO 2  that is not needed in buffer volume  80  is routed through heat exchanger  29  where additional heat is recovered. CO 2  exiting heat exchanger  29  flows as recirculation stream  59  through valve  60  which releases the pressure to approximate the pressure of CO 2  bulk tank  10 . Some of the CO 2  will flash to vapor across valve  60 , but this vapor is recondensed in CO 2  bulk tank  10  by refrigerant supplied by refrigeration system  12  to cooling coil  11 .  
         [0052]     Recirculation of CO 2  is optional, as there are times when it is desirable to have an average flow through CO 2  pressurization means  20  and CO 2  purification  40  that is greater than the average demand, S avg , of application  100 . For instance, if application  100  stops drawing carbon dioxide because it requires maintenance, it is desirable to continue the operation of the carbon dioxide supply system to maintain the removal of energy/heat from CO 2  pressurization means  20  and provide energy/heat to CO 2  purification unit  40 .  
         [0053]     In this embodiment, heat recovery from CO 2  purification  40  is performed utilizing two heat exchangers  29  and  30  rather than one heat exchanger. Otherwise, if all the CO 2  from purification unit  40  exchanges energy/heat with the CO 2  from pressurization means  20 , it would be cooled to a temperature below the desired temperature in buffer volume  80 . The use of two heat exchangers  29  and  30  allows the CO 2  to be sent to high pressure carbon dioxide feed stream heating or cooling means  74  at a temperature close to that desired in buffer volume  80  upon exiting heat exchanger  30 . Accordingly, recirculation stream  59  is cooled further in heat exchanger  29 , prior to returning the stream to bulk tank  10 .  
         [0054]     The temperature of the CO 2  routed through high pressure carbon dioxide feed stream heating or cooling means  74  is adjusted depending on the inlet CO 2  temperature to that desired in buffer volume  80 . Exemplary equipment that may be utilized as heating or cooling means  74  includes an air-cooled coil, a water-cooled or refrigerant-cooled heat exchanger, and a heater. High pressure carbon dioxide feed stream  75  exits high pressure carbon dioxide feed stream heating or cooling means  74  and is routed to buffer volume  80 .  
         [0055]     The CO 2  in buffer volume  80  may be heated or cooled by buffer volume temperature adjustment means  81 . Since high pressure carbon dioxide feed stream heating or cooling means  74  adjusts the temperature of high pressure carbon dioxide feed stream  75  to match the temperature desired in buffer volume  80 , the duty required of buffer volume temperature adjustment means  81  is minimal. It is typically designed to make up for heat losses or gain between buffer volume  80  and ambient temperatures. Equipment that may be employed for buffer volume temperature adjustment means  81  includes heat trace, an internal heater, an external band or cable heater, external cooling coils and internal cooling coils. Buffer volume  80  is normally insulated to reduce the amount of heat exchange with the ambient surroundings. Alternatively, it may be advantageous for the buffer volume to be operated such that the temperature within the buffer volume/tank is lower at the bottom than at the top. Temperature of fluid contained in the upper portion of the buffer volume may be controlled by withdrawing fluid from a lower section of the buffer volume (not shown), heating the fluid utilizing temperature adjusting means and returning this stream to the upper portion of the buffer buffer volume  80 .  
         [0056]     When application tool  100  requires CO 2 , the fluid exits buffer volume  80  in high-pressure carbon dioxide supply stream  85 . The high-pressure carbon dioxide supply stream  85  may then be heated or cooled using a heating or cooling means  86  to produce the temperature required at the application tool  100 . It is routed through filter  88  and across valve  90 . Utilizing filter  88  facilitates supply of particle-free CO 2  even if buffer volume  80  does not have an electropolished internal surface. Valve  90  controls the flow of high-pressure carbon dioxide supply stream  85  and ensures that the pressure downstream of valve  90  is the pressure desired at application tool  100 . The Co 2  is then routed through filter  92  on its way to application tool  100 , where it is used in semiconductor wafer processing.  
         [0057]     Turning to the embodiment shown in  FIG. 3 , the heat exchangers can be combined into one heat exchanger core  31 . This arrangement may reduce the space required for the CO 2  supply system and it may also reduce cost compared to having two heat exchangers. One type of heat exchanger that would be suitable for this duty would be a microchannel heat exchanger such as a Printed Circuit Heat Exchanger (PCHE) supplied by the Heatric division of Meggitt (UK) Ltd.  
         [0058]      FIG. 4  exhibits another embodiment of the supply system. The CO 2  booster pump  15  and backpressure regulator  16  shown in  FIG. 1  are replaced by refrigeration system  13  and subcooler heat exchanger  14 . The modified system provides an alternate supply of subcooled liquid to CO 2  pressurization means  20 . The CO 2  routed from CO 2  bulk tank  10  is subcooled by 20° F. or more in subcooler heat exchanger  14 , prior to CO 2  pressurization means  20 .  
         [0059]     Refrigeration system  13  is employed to provide refrigerant to condenser heat exchanger  65 . Instead of recondensing the vapor introduced in bulk tank  10  through the use of cooling coil  11 , the CO 2  vapor that forms from the pressure drop across valve  60  is recondensed in condenser heat exchanger  65  prior to returning it to CO 2  bulk tank  10 .  
         [0060]     Although not shown, alternatively, the supply system could be disposed to include CO 2  booster pump  15 , backpressure regulator  16 , in conjunction with the condenser heat exchanger  65  and refrigeration system  13 . Further, the system could contain both refrigeration system  12  and cooling coil  11  along with refrigeration system  13  and subcooler heat exchanger  14  and/or condenser heat exchanger  65 .  
         [0061]     Turning to  FIG. 5 , an embodiment is depicted where an alternate heat exchanger configuration system is employed. Heat exchangers  29  and  30 , subcooler heat exchanger  14  and condenser heat exchanger  65  shown in  FIG. 4  may be replaced by one heat exchanger core, such as heat exchanger  21 . Heat exchanger  21  would include a high-pressure and a low-pressure side. The advantages associate with the employment of heat exchanger  21  would be reduced space and potentially reduced cost for the system. One type of heat exchanger that would be suitable for this duty would be a microchannel heat exchanger such as a Printed Circuit Heat Exchanger (PCHE) supplied by the Heatric division of Meggitt (UK) Ltd.  
         [0062]      FIG. 6  illustrates an alternative embodiment, where the recirculation stream  59  is taken after high pressure carbon dioxide supply stream heating or cooling means  86  and filter  88 . It will be recognized by those skilled in the art that recirculation stream  59  can be taken at other points in the system. Other locations include downstream of valve  90 , but upstream of high pressure carbon dioxide supply stream heating or cooling means  86 , between high pressure carbon dioxide feed stream heating or cooling means  74  and buffer volume  80 , and between CO 2  pressurization means and heat exchanger  33 .  
         [0063]     In addition to using a pump as CO 2  pressurization means  20 , a compressor could also be employed. In the event that a compressor is employed as CO 2  pressurization means  20 , the CO 2  from bulk tank  10  would need to be conveyed through a vaporizer before it is routed to CO 2  pressurization means  20 . Therefore, CO 2  booster pump  15  and backpressure regulator  16  would not be utilized in such arrangement.  
         [0064]     High pressure carbon dioxide feed stream heating or cooling means  74  and high pressure carbon dioxide supply stream heating or cooling means  86  are optional, as are filters  88 ,  89  and  92 .  
         [0065]     The purification portion of the supply system is likewise, optional. In some cases, the CO 2  from CO 2  bulk tank  10  is pure enough for use in application tool  100 . If this is the case, heat exchangers  29  and  30 , heater  35  and CO 2  purification  40  are removed. The CO 2  is routed directly from CO 2  pressurization means  20  to high pressure carbon dioxide feed stream heating or cooling means  74 .  
         [0066]     While the invention has been described in detail with reference to specific embodiments thereof, it will become apparent to one skilled in the art that various changes and modifications can be make, and equivalents employed, without departing from the scope of the appended claims.