Abstract:
The invention disclosed herein generally relates to a system and a method for supplying a carbon dioxide fluid feed to a plurality of applications. The method of the invention comprises the steps of directing a fluid feed, that includes a carbon dioxide component, from a carbon dioxide purifying means to a plurality of applications including at least two distinct applications, whereby contaminants are combined with the fluid at said applications, thereby forming an effluent that includes at least a portion of the carbon dioxide component and at least a portion of said contaminants; directing said effluent from at least one of the applications to said carbon dioxide purifying means; and purifying the carbon dioxide of the effluent at the carbon dioxide purifying means, thereby producing the carbon dioxide component of the fluid feed. The system of the invention is an apparatus for conducting the method of the invention.

Description:
RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/330,203, filed on Oct. 17, 2001, the entire teachings of which are incorporated herein by reference. This application also claims the benefit of U.S. Provisional Application No. 60/330,150, filed Oct. 17, 2001, No. 60/350,688, filed Jan. 22, 2002 and No. 60/358,065, filed Feb. 19, 2002, the entire teachings of all these applications are incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The manufacture of integrated circuits generally involves a number of discrete steps that are performed on a wafer. Typical steps include depositing or growing a film, patterning the wafer using photolithography, and etching. These steps are performed multiple times to build the desired circuit. Additional process steps may include ion implantation, chemical or mechanical planarization, and diffusion. A wide variety of organic and inorganic chemicals are used to conduct or to remove waste from these applications. Aqueous-based cleaning systems have been devised to eliminate some of the organic solvent requirements, but they generate large quantities of waste streams that must be treated prior to discharge or reclamation. The need for large quantities of water is often a major factor in choosing a location for a semiconductor fabrication facility. In addition, the high surface tension of water reduces its effectiveness in applications requiring the cleaning of fine structures, and drying steps must be included in the process to remove all traces of moisture.  
           [0003]    In recent years, supercritical carbon dioxide has been investigated as a potential replacement for some of the organic solvents and aqueous-based chemistries currently in use. Supercritical carbon dioxide systems have been used for decades in simple extraction applications, such as the decaffeination of coffee. The term supercritical fluid refers to a fluid that is above a critical temperature and pressure (e.g., at or above 31° C. and 1070 pounds per square inch absolute (psia) respectively, for carbon dioxide). Supercritical fluids have both gas- and liquid-like properties. The density of supercritical fluids can be varied as a function of temperature and pressure. Because solvating ability is a strong function of density this also means that the solvating properties can be varied. Pure supercritical carbon dioxide has solvent capabilities similar to a non-polar organic solvent such as hexane. Modifying agents such as cosolvents, surfactants, and chelating agents can be added to the carbon dioxide to improve its cleaning ability.  
           [0004]    Semiconductor-applications can generally produce a range of contaminants with vapor pressure either above or below that of carbon dioxide. The lighter, higher vapor pressure components may be some combination of fluorine, light fluorinated hydrocarbons and atmospheric gases such as nitrogen and oxygen. Carbon dioxide can also be contaminated with non-volatile resist residue compounds and co-solvents, which are difficult to transfer because they can exist as a solid/liquid mixture in combination with vapor phase carbon dioxide. Also, carbon dioxide purity requirements for many semiconductor manufacturing applications exceed those of currently available delivered bulk carbon dioxide. Furthermore, if supercritical carbon dioxide is to be widely used in the semiconductor industry, the quantities consumed will likely preclude the economic viability of total dependence on delivered carbon dioxide. Finally, a semiconductor manufacturing facility can have a number of different applications with distinct requirements.  
           [0005]    The prior art, however, does not teach a system or method by which these problems may be overcome. There is therefore a need for a method and apparatus for using carbon dioxide in a semiconductor manufacturing process that minimizes or eliminates these problems.  
         SUMMARY OF THE INVENTION  
         [0006]    The invention generally relates to a method and a system for supplying carbon dioxide to a plurality of applications.  
           [0007]    The method of the invention includes the steps of directing a fluid feed, that includes a carbon dioxide component, from a first carbon dioxide purifying means to a plurality of applications including at least two distinct applications. At the applications, one or more contaminants are combined with the fluid, thereby forming an effluent at each application, wherein each effluent includes at least a portion of the carbon dioxide component and at least a portion of the contaminants. At least a portion of at least one effluent is directed to the first purifying means, whereby the carbon dioxide component of the effluent is purified, thereby forming the fluid feed.  
           [0008]    The system of the invention includes a first carbon dioxide purifying means, which purifies a carbon dioxide component of an effluent to form a fluid feed that includes the carbon dioxide as a component of the fluid feed. The first purifying means includes at least one member of the group consisting of a catalytic oxidizer, a distillation column, a phase separator, and an adsorption bed. A supply conduit is included for directing the fluid feed from the first purifying means to a plurality of applications comprising at least two distinct applications. At the applications, one or more contaminants are combined with the fluid, thereby forming an effluent at each application, wherein each effluent includes at least a portion of the carbon dioxide component and at least a portion of the contaminants. A return conduit directs the effluent from at least one application to the first purifying means.  
           [0009]    The advantages of the invention disclosed herein are significant. Practicing the invention can significantly reduce the cost and complexity of supplying high-purity carbon dioxide to the multiple distinct applications in a semiconductor manufacturing facility. By recycling carbon dioxide, the amount, and therefore the cost of externally supplied carbon dioxide is reduced. By purifying bulk make-up carbon dioxide prior to the applications, the cost is reduced because the bulk carbon dioxide supplied to the manufacturing facility can be purchased at a lower purity level. By providing a central purifier, economies of scale are realized over individual purification and delivery units. The cost of serving multiple applications is reduced, and the cost of treating the effluent of multiple applications having different contaminant compositions is also reduced. Additionally, effluent stream combination, either from a time-staggered operation of multiple tools of the same type, or from different tools, provides a more uniform effluent stream, which is more readily purified in a central purifier. Another key advantage of a central purifier is consolidation of the analytical requirements. Yet another advantage of a central purifier is that by using a bypass circuit, the central purifier can be operated continuously, avoiding stagnant legs that can accumulate contaminants, and allowing the applications to be operated in a batch mode. A further advantage is that by combining a central purifier with distributed local purifiers, effluent streams that are chemically incompatible can be pre-purified so that they can be combined and sent to the central purifier.  
           [0010]    The combination of these advantages is expected to make supercritical carbon dioxide a viable replacement for existing organic solvent and aqueous chemistry applications, resulting in lower production costs for semiconductors. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 depicts an apparatus that is an embodiment of the invention.  
         [0012]    [0012]FIG. 2 depicts an apparatus that is an alternative embodiment of the invention with a carbon dioxide source and multiple semiconductor manufacturing applications with multiple tools.  
         [0013]    [0013]FIG. 3 depicts an apparatus that is a portion of an alternative embodiment of the invention, detailing the components of the first purifying means.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]    The foregoing and other objects, features, and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.  
         [0015]    The invention generally is related to a method and system for supplying carbon dioxide to a plurality, i.e., two or more, applications. As used herein, an application employs a fluid feed that includes a carbon dioxide component.  
         [0016]    In a semiconductor fabrication facility, for instance, carbon dioxide can be employed during wafer cleaning, photoresist deposition, chemical fluid deposition, photoresist developing, photoresist removal, photoresist developing, and other applications known to the art where solvents or aqueous solutions are used. Each application can require different operational conditions with respect to the carbon dioxide-containing fluid feed.  
         [0017]    The equipment used to perform an application is generally is referred to as a tool. Often, the same application is conducted using multiple tools, each tool operated independently of the others. A tool can include one or more chambers and each chamber can independently process its own wafer, or other workpiece.  
         [0018]    Applications that are distinct are applications that differ in at least one parameter of the fluid feed being delivered to the application, or the effluent leaving the application. Parameters can be chemical or physical conditions or can be related to volume and timing at which a fluid feed that includes a carbon dioxide component is employed at the application. Examples of parameters include flow rate, flow cycle (continuous or batch mode), cycle time, amount and kind of additives in the second component, temperature, pressure, contaminants, and other variables. As used herein, tools or chambers within the tool are distinct applications if they employ feed streams or produce effluents that differ in at least one parameter.  
         [0019]    [0019]FIG. 1 shows apparatus  10  of the invention, which can also be used to conduct the method of the invention. The system includes a first carbon dioxide purifying means  11 , which can purify a carbon dioxide component of an effluent, thereby forming a fluid feed containing a carbon dioxide component. The fluid feed can be directed from the first purifying means  11  via supply conduit  12  to a plurality of applications, including at least two distinct applications  14  and  16 . Preferably, first purifying means  11  includes pressurization means such that the pressure in supply conduit  12  is greater than the pressure in return conduit  20 . As discussed above, applications that are distinct employ fluid feeds that differ in at least one parameter, e.g. temperature, pressure, flow rate, timing of delivery of the fluid feed, amount or kind of additives present in the fluid feed, etc. At the applications, one or more contaminants, e.g. from a wafer being cleaned or processed, are combined with the fluid, thereby forming an effluent at each application. Return conduit  20  can direct at least a portion at least one effluent back to the purifying means to purify the carbon dioxide component of the effluent.  
         [0020]    [0020]FIG. 2 shows apparatus  22  of the invention, which can also be used to conduct the method of the invention. Carbon dioxide from source  24  can be added to the system via conduit  25  to make up for losses in normal processing or to increase the amount of carbon dioxide in the system as additional applications are brought on line. Examples of carbon dioxide sources are a liquid carbon dioxide tank, a carbon dioxide generating plant, a railroad tank car, and a truck trailer. The carbon dioxide that is added can be purified by one of several means before it reaches the application. There can be a second carbon dioxide purifying means included in source  24 , which contains at least a distillation column, a catalytic oxidizer, or an adsorption bed. When the carbon dioxide from the source is sufficiently pre-purified in this manner, it can be added to any point in the system. Preferably, however, carbon dioxide from the source is added to a point in the system, such as return conduit  20  or first purifying means  11 , that allows the existing first purifying means  1  to be used, thus obviating the need for an additional, external purification unit.  
         [0021]    As before, first purifying means  11  directs a fluid feed containing a carbon dioxide component to a plurality of applications. As used herein, a purifier can include one or more components such as phase separators, distillation columns, filters, adsorption beds, catalytic reactors, scrubbers, and other components known to the art. The resulting carbon dioxide fluid feed can contain less than 100 parts per million (ppm) of any impurity. Typically, the stream will contain less than 10 ppm of any impurity, and preferably, less than 1 ppm of any impurity. Another important element of means  11  is a purity analyzer. Analyzers for high purity gases include mass spectrometers of various kinds, and other detectors that are well-known to the art. Many such devices are commercially available and can be integrated into any of the systems or methods described herein.  
         [0022]    Prior to the applications, customizing units  26 ,  28 , and  30  modify the physical properties of the fluid feed of supply conduit  12 . The customizing units can have a heat exchanger, a pressure controller, or both. As used herein, a heat exchanger is any device that can raise or lower the temperature of a feed, such as an electric heater, a refrigeration unit, a heat pump, a water bath, and other devices know to the art. As used herein, a pressure controller can be any device that changes the pressure of a feed, including a pump, a compressor, a pressure reducing valve, and other devices known to the art. The temperature and pressure can then be modified to values that are appropriate for each application. Preferably, the fluid feed will be a high pressure liquid or supercritical fluid, with pressure in the range of between about 650 to about 5000 pounds per square inch gauge (psig), more preferably in the range of between about 800 to about 3500 psig, and most preferably in the range of between about 950 to about 3000 psig. In a preferred embodiment, the customization unit forms the carbon dioxide component of the fluid feed into a supercritical fluid, i.e., temperature greater than about 31° C. and pressure greater than about 1070 psig.  
         [0023]    The customization units can also incorporate a means to add a second component to the fluid feed for each application, where the second component is one or more co-solvents, surfactants, chelators, or other additives that enhance the performance of the fluid feed in each application. Alternatively, one or more of the heat exchanger, the pressure controller, or the means to add the second component may be incorporated directly into an application or tool.  
         [0024]    Following the customization units, three distinct applications are shown,  32 ,  34 , and  36 . For example, application  36  could be a wafer cleaner that uses carbon dioxide snow to clean the wafer surface, application  32  could be a photoresist developer and application  34  could be a photoresist stripper. Applications  32  and  34  as shown have multiple tools, with four tools a, b, c, and d for application  32 , and two tools e and f for application  34 . Application  36  is shown with only one tool. As before, one or more contaminants are combined with the fluid feed at each application, forming an effluent for each tool that contains carbon dioxide, one or more contaminants, and any second component that was added. Effluent from applications with multiple tools can be combined, as shown for  32 , or kept separate, as shown for  34 .  
         [0025]    In a preferred embodiment, each effluent can be sent to a third carbon dioxide purifying means  38 ,  40 , or  42 , which by reducing the pressure separates each effluent into a plurality of phases. Each third purification means  38 ,  40 , or  42  can be a phase separator such as a simple disengagement drum, a multi-stage contactor, or other devices known in the art. Optionally  38 ,  40 , or  42  can be combined with a heat exchanger to vaporize carbon dioxide in the effluent as a liquid and/or to heat the gas to counteract the cooling it experiences by being depressurized during phase separation. Alternatively, the third purifying means can include a distillation column, a catalytic oxidizer, or an adsorption bed.  
         [0026]    Usually there will be a liquid phase enriched in, for, example, co-solvents and contaminants from the application, and depending on the contaminants and the composition of the second component, there may be more than one liquid phase. Also depending on the contaminants and second component composition, there can be a solid phase or a solid phase suspended in a liquid phase, which can be removed directly at each third purification means as waste streams  44 ,  46 , and  48  by means such as a knockout pot, to allow droplets and particles to settle out by gravity. Optionally, further phase separation devices, such as coalescers and filters, can be used downstream of a gravity device to perform a more complete phase separation.  
         [0027]    All phases can contain carbon dioxide, but generally the phase most enriched in carbon dioxide will be a gas stream, of which at least a portion is then directed to the first purifying means  11  via return conduit  20 . The decision of whether, or how much of the effluent can be directed to first purifying means  11  or to waste stream  50  depends on several factors, the most important of which are pressure and composition. Effluent in return conduit  20  will typically operate at elevated pressure compared to first purifying means  11 . If the effluent stream pressure from a particular application is above that of the combined effluent in return conduit  20 , no compression of the effluent is required. However, if the effluent pressure is below that in return conduit  20 , it can be more cost effective for a particular application to send the effluent to the waste stream  50 . The decision to direct a portion of effluent to waste stream  50  can also be a composition based decision. For example, the first heavily contaminated cycle of a cleaning application can be directed to waste stream  50 , while subsequent cycles can be directed to the first purifying means  11 .  
         [0028]    The composition of the effluent directed by return conduit  20  to first purifying means  11  will be on average greater than about 50% carbon dioxide. Preferably, the average composition will more preferably be in excess of about 80% carbon dioxide, and more preferably in excess of about 90% carbon dioxide. The pressure of the combined effluent stream in return conduit  20  in this invention can be based on an optimization between the amount of carbon dioxide recovered and the purification costs. In general, the lower the pressure in return conduit  20 , the greater the proportion of the effluent and carbon dioxide enriched phases that return conduit  20  can accept. The operating pressure for conduit  20  is preferably in the range of between about 90 to about 900 psia, more preferably in the range of between about 100 to about 400 psia and most preferably in the range of between about 150 to about 350 psia.  
         [0029]    In another embodiment, a pressure-reducing bypass valve  51  connects supply conduit  12  and return conduit  20 . This allows continuous operation of the first purifying means and its supply and return conduits, while the various applications and third purification means can be operated in batch mode.  
         [0030]    In addition, the use of hold-up tanks (not shown) in the supply and return conduits can buffer the purification system from wide fluctuations in demand or supply. Hold-up in the return conduit can also smooth composition fluctuations.  
         [0031]    Waste streams  44 ,  46 , and  48  can be directed to appropriate disposal means or facilities that can recycle components for reuse.  
         [0032]    [0032]FIG. 3 shows apparatus  52  of the invention, which can also be used to conduct the method of the invention. Distinct applications  32  and  34  are supplied with a fluid feed from conduit  12 . The fluid feed can be further customized by pressurization and heating, for example, in customization units  26  and  28  to meet the conditions required for each application. In FIG. 3, the second components are added directly to the applications via  27  and  29 , rather than in  26  and  28 .  
         [0033]    Each application discharges a carbon dioxide/second component/contaminant effluent to third purification means  38  and  40 . The portion of the carbon dioxide enriched phases produced by  38  and  40  that is above the pressure in return conduit  20  is directed to conduit  20 . Gaseous exhaust to lower pressures can be vented to waste stream  50 , or alternatively, can be compressed and also combined with the effluent in return conduit  20 . Liquid and solid waste streams  44  and  46  can be sent to disposal or reclamation. Third purification means  38  and  40  can be heated to drive off carbon dioxide contained in a liquid phase to improve carbon dioxide recovery. Preferably, the performance of third purification means  38  and  40  is sufficient to avoid requiring return conduit  20  to be able to pass a multiphase mixture. Again, note that third purification means  38  and  40  are represented schematically and can in principle consist of one or more phase separators, distillation columns, adsorption beds and other purification devices tailored to the application.  
         [0034]    Pressure control device  54  may be used to further reduce or increase pressure of the carbon dioxide in return conduit  20 . The stream can be partially heated or cooled in exchanger  56 . It then passes to phase separation device  58  to remove any particulates or droplets that may be present as a result of heating or cooling in exchanger  56  or due to inefficiencies in third purifying means  38  and  40 . The stream is then directed via  60  into heavy contaminant removal distillation column  62 . Liquid collected in separator  58  can be sent to waste stream  59 . A portion of the high purity carbon dioxide can be taken via side stream  13  and directed through control valve  64  into the top of column  62 . In addition, carbon dioxide from source  24  can also be introduced at an upper location of column  62 . These streams serve to both cool the feed stream and to absorb heavy contaminants. The carbon dioxide from  24  can be required to overcome losses of carbon dioxide in the recycle system both at the application and with the impure streams leaving the purification system. Waste containing heavy impurities leaves the bottom of column  62  and can be directed to a liquid waste stream  59 . Examples of heavy contaminants that can be removed here are organic solvents, such as acetone, hexane and water, among many others. A reboiler  65  provides stripping vapor in the column, if necessary, depending on the temperature of the gas stream entering column  62  from  58 .  
         [0035]    Stream  68  from column  62  can then be substantially condensed in exchanger  70  along with vapor overhead from light contaminant removal distillation column  72 . The carbon dioxide liquid stream from the condenser flows into column  72 . Light contaminants include methane, nitrogen, fluorine, and oxygen, among many others. The light contaminants are concentrated in the vapor overhead leaving the system as stream  74 . Column  72  can be a vessel filled with suitable packing or trays to facilitate liquid and vapor contact. Exchanger  76  provides stripping vapor. Product liquid carbon dioxide can be taken from column  72  and compressed to an elevated pressure in pump  78  for conduits  13  and  12 . The temperature of the fluid in conduit  12  can be adjusted by passage through exchanger  56 .  
         [0036]    Refrigeration system  80  can be used to perform the condensing duty for column  72 . Optionally, the refrigeration system can be further heat integrated into the purification system by cooling the high-pressure refrigerant while providing the energy required in the reboilers  65  and  76 . For example, reboil exchanger  65  may provide sub-cooling duty to a liquid refrigerant stream in system  80 . Additionally, exchanger  56  may serve to reboil column  72  as well as cool the feed gas.  
         [0037]    The operating pressure of the purification train is preferably in the range of between about 150 to about 1000 psia, more preferably in the range of between about 250 to about 800 psia, and most preferably in the range of between about 250 to about 350 psia. The pressure downstream of the pump in conduits  13  and  12  is preferably in the range of between about 775 to about 5000 psia, more preferably in the range of between about 800 to about 4000 psia, and most preferably in the range of between about 800 to about 3000 psia. The final purity of the carbon dioxide can be dictated by each application&#39;s requirements. Typical purity requirements are expected to be similar to those for ingredient-grade, bulk liquid carbon dioxide but with more stringent requirements for low vapor pressure contaminants. These can potentially leave a residue on the wafer surface. For example, non-volatile residue specifications are typically about 10 ppm for bulk liquid used in semiconductor manufacturing. The purity requirements for semiconductor applications can be below about 1 ppm. The preferred purification route can utilize distillation and phase separation to accomplish purification. However, if contaminants have vapor pressures that are close to carbon dioxide, then additional purification means can be provided. Examples of contaminants that fall into this category include some hydrocarbons (e.g. ethane), oxygenated hydrocarbons, halogens and halogenated hydrocarbons. The additional purification means may include catalytic oxidation, water scrubbing, caustic scrubbing and dryers.  
         [0038]    The techniques used in semiconductor manufacturing are also being applied to other arenas where precision features are desired, such as the emerging field of micro electromechanical systems and micro fluidic systems, where a supercritical carbon dioxide process would also be useful.  
         [0039]    While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.