EP1766103A2

EP1766103A2 - Method and system for supplying carbon dioxide

Info

Publication number: EP1766103A2
Application number: EP05755008A
Authority: EP
Inventors: Michael Clinton Johnson; Martin L. Timm; John Fredric Billingham
Original assignee: Praxair Technology Inc
Current assignee: Praxair Technology Inc
Priority date: 2004-06-07
Filing date: 2005-05-31
Publication date: 2007-03-28
Also published as: CN101137451A; TW200610049A; KR20070027689A; WO2005121407A2; JP2008505297A; US20050268938A1; WO2005121407A3

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 (80) and determining the amount of carbon dioxide to be delivered to the application tool (100). The pressure in the buffer volume (80) is maintained at a pressure that exceeds the pressure required by the application tool (100). The temperature in the buffer volume (80) is further adjusted to modify the density of the carbon dioxide and based thereon the size of the buffer volume (80). Thereafter, the carbon dioxide from the buffer volume (80) is delivered at a variable flow rate as required by the application tool (100).

Description

METHOD AND SYSTEM FOR SUPPLYING CARBON DIOXIDE

BACKGROUND OF THE INVENTION

Field of the Invention [0001] 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.

Description of Related Art [0002] 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. Patent 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. Patent 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. [0003] U.S. Patent 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 .

[0004] 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. Patent No. 6,085,762 to Baton and U.S. Patent No. 6,403,544 Bl 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. [0005] U.S. Patent 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 ^λbuffer 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. [0006] 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. [0007] 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 . [0008] It is yet another object of the present invention to adjust the buffer volume capacity without modifying the physical dimension of the buffer volume. [0009] 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. [0010] 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 . [0011] 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. [0012] 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 [0013] The foregoing objectives are met by the process of the present invention. [0014] 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 . [0015] 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. [0016] 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. [0017] 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 [0018] The invention will be better understood by reference to the figures wherein like numbers denote same features throughout and wherein: [0019] Fig. 1 illustrates a schematic diagram of the overall system for delivery of carbon dioxide to a semiconductor application tool having variable flow requirement ; [0020] Fig. 2 depicts an exemplary operational five minute cycle for delivery of carbon dioxide, in accordance with the invention; [0021] Fig. 3 illustrates a schematic diagram of the overall system having a single core heat exchanger; [0022] 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; [0023] 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 [0024] 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 [0025] 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) . [00261 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.

[0027] 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 . [0028] By way of explanation, the application tool 100 requires a maximum instantaneous flow rate (S_raax) of carbon dioxide at a certain set pressure level (P) . The pressure in buffer volume 80 (Pbv) must always be at least equal to the set pressure (i.e., P_b = or > 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_mi_n) which is greater than the pressure required by the application tool (i.e., P_bv = or > P_mi_n = or > P) . [0029] The average flow rate (F_avg) of high-pressure carbon dioxide feed stream 75 (Fi) 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 (Si) delivered to application tool 100. Thus, when Fi is greater than Si, the pressure in buffer volume 80 increases. Conversely, when Si is greater than Fi, 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. [0030] 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 .

[0031] 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, Fi, 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 Fi and Si for the entire cycle, or alternatively, by taking note of the visual representation on the graph. [0032] 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. [0033] 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 volume¹ 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_mi_n. If the temperature in buffer volume 80 is known, the carbon dioxide density can be determined. [0034] 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_mi_n) 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. [0035] 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.

[0036] 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/ft³. The density of carbon dioxide at 2900 psig and 110 °F is 51.418 lb/ft³. Thus, dividing the required buffer volume capacity of 1.35 lbs by the difference in density (i.e., 52.238 - 51.418) lb/ft³ shows that the buffer volume is 1.65 ft³. Modifying this temperature modifies the amount of carbon dioxide delivered to the application tool . [0037] 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 calculation's were carried out in the manner explained above .

[0038] 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. [0039] Increasing the buffer volume temperature results in an increased ability of the buffer volume to I 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 . [0040] 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.

[0041] Returning to Fig. 1, the entire carbon dioxide supply system in accordance with one embodiment is discussed in detail. The carbon dioxide (C0₂) source for the system is C0₂ bulk tank 10. The tank can range in size from a two hundred liter Dewar to a tank holding three hundred tons of C0₂. Typically, the capacity of the bulk tanks range from six tons to one hundred tons. For example, the C0₂ bulk tank 10 can be refilled from a C0₂ delivery trailer without interrupting the operation of the C0₂ supply system. [0042] The C0₂ bulk tank 10 normally contains liquid C0₂ 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 C0₂ booster pump 15. The pump raises the pressure of the C0₂ by approximately 50 to 120 psi, but the temperature is increased only incrementally. The subcooled liquid C0₂ 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 . [0043] The C0₂ is pressurized in C0₂ pressurization means 20 to a pressure as high as the pressure at maximum loading in buffer volume 80 plus the line pressure drop between C0₂ pressurization means 20 and buffer volume 80. The C0₂ normally increases in temperature by approximately 20°F across C0₂ pressurization means 20, and exits at a temperature ranging from about 10 to 30°F.

[0044] Part of the subcooled C0₂ liquid exiting booster pump 15, and which exceeds the maximum capacity of C0₂ pressurization means 20 is circulated through backpressure regulator 16 and returned to C0₂ bulk tank «

10. A small portion of the subcooled liquid C0₂ flashes to vapor as it passes through backpressure regulator 16. Over time, this vapor would increase the pressure in C0₂ bulk tank 10. To prevent this phenomenon, refrigeration system 12 conveys a refrigerant through cooling coil 11 in C0₂ bulk tank 10 to recondense the C0₂ vapor that forms from flashing across backpressure regulator 16. This cooling also recondenses C0₂ vapor that forms due to heat leak into C0₂ bulk tank 10 from ambient. [0045] The pressurized C0₂ exiting pressurization means 20 is conveyed through heat exchangers 29 and 30 where it exchanges heat indirectly with some or all of the C0₂ returned from C0₂ purification unit 40. Heat exchangers 29 and 30 allow for heat recovery from C0₂ purification unit 40 that takes place at a temperature greater than that of the C0₂ exiting C0₂ pressurizati n means 20. The C0₂ flows through heater unit 35, which raises the temperature of the C0₂ to the temperature required for C0₂ purification unit 40. Heater 35 is necessary because the heat recovery in heat exchangers 29 and 30 is not totally efficient. [0046] C0₂ purification unit 40 can be selected from any purification device capable of operating at a temperature greater than the temperature of the C0₂ 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.

[0047] Filtration at elevated temperature may be part of the C0₂ purification unit'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. [0048] When the C0₂ purification unit 40 is operated at a temperature higher than the buffer volume 80 temperature, the C0₂ becomes more gas-like. Therefore, filtration will be more effective. When a filter is included as part of C0₂ purification unit 40, it should be placed downstream of any particle generating means of purification such as adsorption and catalytic oxidation. [0049] C0₂ removed from the C0₂ purification unit 40 is re-routed through heat exchanger 30, where it exchanges heat indirectly with C0 directed from C0₂ pressurization means 20 thereto. C0₂ 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, C0₂ exceeding a predetermined level is recirculated to the bulk tank 10. C0₂ that is not needed in buffer volume 80 is routed through heat exchanger 29 where additional heat is recovered. C0₂ exiting heat exchanger 29 flows as recirculation stream 59 through valve 60 which releases the pressure to approximate the pressure of C0₂ bulk tank 10. Some of the C0₂ will flash to vapor across valve 60, but this vapor is recondensed in C0₂ bulk tank 10 by refrigerant supplied by refrigeration system 12 to cooling coil 11. [0050] Recirculation of C0₂ to bulk tank 10 is optional, as there are times when it is desirable to have an average flow through C0₂ pressurization means 20 and C0₂ 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 C0₂ pressurization means 20 and provide energy/heat to C0₂ purification unit 40. [0051] In this embodiment, heat recovery from C0₂ purification 40 is performed utilizing two heat exchangers 29 and 30 rather than one heat exchanger. Otherwise, if all the C0₂ from purification unit 40 exchanges energy/heat with the C0₂ 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 C0 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.

[0052] The temperature of the C0₂ routed through high pressure carbon dioxide feed stream heating or cooling means 74 is adjusted depending on the inlet C0₂ 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. [0053] The C0₂ 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. [0054] When application tool 100 requires C0₂, 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 C0₂ 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 C0₂ is then routed through filter 92 on its way to ' application tool 100, where it is used in semiconductor wafer processing. [0055] 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 C0₂ 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.

[0056] Fig. 4 exhibits another embodiment of the supply system. The C0₂ 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 C0₂ pressurization means 20. The C0₂ routed from C0₂ bulk tank 10 is subcooled by 20°F or more in subcooler heat exchanger 14, prior to C0₂ pressurization means 20. [0057] 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 C0₂ vapor that forms from the pressure drop across valve 60 is recondensed in condenser heat exchanger 65 prior to returning it to C0₂ bulk tank 10. [0058] Although not shown, alternatively, the supply system could be disposed to include C0₂ 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. [0059] 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. [0060] 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 C0₂ pressurization means and heat exchanger 33. [0061] In addition to using a pump as C0₂ pressurization means 20, a compressor could also be employed. In the event that a compressor is employed as C0₂ pressurization means 20, the C0₂ from bulk tank 10 would need to be conveyed through a vaporizer before it is routed to C0₂ pressurization means 20. Therefore, C0₂ booster pump 15 and backpressure regulator 16 would not be utilized in such arrangement. [0062] 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. [0063] The purification portion of the supply system is likewise, optional. In some cases, the C0₂ from C0₂ 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 C0₂ purification 40 are removed. The C0₂ is routed directly from C0₂ pressurization means 20 to high pressure carbon dioxide feed stream heating or cooling means 74. [0064] 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.

Claims

What is claimed is: 1. A method for supplying high-pressure carbon dioxide to an application tool having a variable carbon dioxide flow requirement, comprising: providing a high-pressure carbon dioxide feed stream to a buffer volume; maintaining the pressure in said buffer volume between a minimum pressure that exceeds the pressure required by said application tool and a maximum pressure such that the average density of carbon dioxide contained in said buffer volume at said minimum pressure is different from the average density of carbon dioxide contained in said buffer volume at said maximum pressure; adjusting the average temperature in said buffer volume to modify the difference between the average density of carbon dioxide contained in said buffer volume at said minimum pressure and the average density of carbon dioxide contained in said buffer volume at said maximum pressure such that the flow requirement associated with said application tool is satisfied; and delivering a high-pressure carbon dioxide supply stream from said buffer volume at a variable flow rate as required by the application tool .

2. The method according to claim 1, further comprising: reducing the buffer volume requisite to deliver a predetermined amount of carbon dioxide to the application tool based on the temperature adjustments to the buffer volume.

3. The method according to claim 1, wherein the flow requirement to the application tool is discontinuous .

4. The method according to claim 1, wherein the application tool is a semiconductor application tool.

5. The method according to claim 1, wherein the carbon dioxide requisite for delivery to the application tool is predetermined.

6. The method according to claim 1, wherein said buffer volume is at least one pressure vessel.

7. The method according to claim 1, further comprising: heating or cooling the carbon dioxide contained in said buffer volume via a heating or cooling device.

8. A system for supplying high-pressure carbon dioxide to an application having a variable carbon dioxide flow requirement, comprising: a buffer volume to receive a high-pressure carbon dioxide feed stream, wherein the pressure in said buffer volume is maintained between a minimum pressure that exceeds the pressure required by said application tool and a maximum pressure such that the density of said carbon dioxide at said minimum pressure is different from the density of said carbon dioxide at said maximum pressure, and the temperature in said buffer volume is adjusted; a carbon dioxide purification unit disposed upstream of said buffer volume to deliver a high- pressure carbon dioxide feed stream; and an application tool disposed downstream of said buffer volume to receive carbon dioxide from said buffer volume at a variable flow rate as required by the application tool.

9. The system according to claim 8 , further comprising : a high-pressure carbon dioxide feed stream heating or cooling device disposed downstream of said carbon dioxide purification unit.

10. The system according to claim 8, further comprising: a carbon dioxide pressurization device disposed upstream of the purification unit to provide carbon dioxide supplied from a bulk storage tank.