Abstract:
A heat exchanger is used to transfer heat from water to liquid nitrogen. As heat is transferred from the water to the liquid nitrogen, the temperature of the water becomes lower and the liquid nitrogen converts to gaseous nitrogen. The cooled water and gaseous nitrogen are used by one or more semiconductor fabrication equipment in the semiconductor fabrication process. Thus, overall power consumption of the semiconductor fabrication process is lowered because water is cooled by passing the water by liquid nitrogen to convert the liquid nitrogen to gaseous nitrogen for use in the semiconductor fabrication process.

Description:
BACKGROUND 
     1. Field of the Invention 
     This invention relates to a method and a system to produce coolant and gases for a semiconductor fabrication facility, and more particularly to the use of a eat exchanger to transfer heat from water to liquid nitrogen in order to lower the temperature of the water and convert the liquid nitrogen to gaseous nitrogen for use in the semiconductor fabrication process. 
     2. Description of Related Art 
     FIG. 1 illustrates a typical layout of a semiconductor fabrication facility (cleanroom). The cleanroom includes areas for film growth, deposition, photolithography, etching, ion implantation, and photo-resist stripping. Processing equipment in the cleanroom includes CVD (chemical vapor deposition) systems, PVD (physical vapor deposition) systems, implanters, furnaces, RTP (rapid thermal processing (such as to anneal)) systems, etchers, plasma CVD systems, steppers, and SEMs (scanning electron microscopes). Typically, a substantial portion (for example, on the order of one-third) of the heat generated by process equipment is carried away by process-cooling water. Chillers are used to produce process-cooling water and they constitute about 18% of the total power consumption of a typical cleanroom according to “ULSI Technology”, edited by C. Y. Chang and S. M. Sze, published by McGraw-Hill Companies, Inc, 1996. 
     Process equipment also uses gaseous nitrogen as carrier gas, reactant gas, dopant, purge gas, and dilution gas in various semiconductor fabrication processes. Nitrogen can even be used to actuate pneumatics in semiconductor fabrication equipment. Typically, nitrogen is stored in tanks in its liquid state to conserve space. To convert the liquid nitrogen to gaseous nitrogen, the liquid nitrogen travels through a pipe with fins exposed to the atmosphere (e.g., a heat exchanger) so that the liquid nitrogen can absorb heat from the ambient conditions. Other gases used in semiconductor fabrication processes include oxygen and argon. 
     One problem with the prior art is that a separate chiller is provided to cool the coolant. This takes space and consumes power, thus adding to the cost of wafer fabrication. In addition, liquefied gas must be heated to become a gas, again, consuming power and thus adding cost to the fabrication process. 
     SUMMARY 
     In accordance with one aspect of the invention, a cooling system includes a heat exchanger that supplies a coolant and a gas to one or more pieces of equipment used in a semiconductor manufacturing process. The heat exchanger is coupled to a source of coolant (e.g., water) and a source of liquid gas (e.g., liquid nitrogen) to transfer heat from the coolant to the liquid gas, thereby cooling the coolant and gasifying the liquid gas. The heat exchanger is coupled to supply the coolant and the gas to one or more units of semiconductor manufacturing equipment. Depending on the embodiment, the heat exchanger can supply the coolant and the gas to the same or different equipment. 
     In accordance with another aspect of the invention, a method for lowering the temperature of coolant (e.g., water) comprises supplying the coolant from a coolant supply to a heat exchanger, supplying a liquid gas (e.g., liquid nitrogen) from a liquid gas supply to the heat exchanger, and supplying the coolant from the heat exchanger to semiconductor fabrication equipment. The method may further include supplying the liquid gas in its gas state from the heat exchanger to the same semiconductor fabrication equipment or to different semiconductor fabrication equipment. 
     The present invention offers a substantial saving in the costs of semiconductor manufacturing. The coolant used in a semiconductor fabrication process is cooled with little or no use of chillers. Instead, the coolant is cooled using a liquid gas that must be converted to its gas state for use in the semiconductor fabrication process. Thus, energy consumption by the chillers is minimized and the cost of semiconductor manufacturing is reduced. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a conventional cleanroom layout. 
     FIGS. 2 and 6 illustrate block diagrams of cooling systems in accordance with aspects of the invention. 
     FIGS. 3A and 3B illustrate schematics of a heat exchanger of FIGS. 2 and 6 in accordance with one embodiment of the invention. 
     FIGS. 4,  5 , and  7  illustrate flow charts of methods for operating the cooling systems of FIGS. 2 and 6. 
     FIGS. 8 and 9 illustrate block diagrams of air conditioning units that can be used with the cooling systems of FIGS.  2  and  6 . 
    
    
     DETAILED DESCRIPTION 
     FIG. 2 illustrates a cooling system  100  in accordance with one aspect of the invention. A valve  102  selectively couples a liquid gas supply  104  to an inlet  106  of a heat exchanger  108  or an inlet  110  of a heat exchanger  112 . Valve  102  is, for example, a conventional valve available from Omega Engineering, Inc. of Stamford, Conn. As an example, liquid gas supply  104  stores liquid nitrogen and is a conventional tank that holds liquid gases. 
     Inlet  106  is coupled to a pipe  114  that travels through a chamber  113  defined by heat exchanger  108 . Pipe  114  is coupled to an outlet  118  of heat exchanger  108 . A valve  120  selectively couples outlet  118  of heat exchanger  108  or an outlet  122  of heat exchanger  112  to a gas supply pipe  124 . Valve  120  is the same type of valve as valve  102 . Pipe  124  is coupled to conventional semiconductor fabrication equipment  126 A to supply gaseous nitrogen. Equipment  126 A might be any one of CVD (chemical vapor deposition) systems, PVD (physical vapor deposition) systems, implanters, furnaces, RTP (rapid thermal processing (such as to anneal)) systems, etchers, plasma CVD systems, steppers, SEMs (scanning electron microscopes), and air conditioning units. 
     Heat exchanger  112  comprises a pipe  128  with its outer surface attached with fins  130 . Heat exchanger  112  is exposed to the open atmosphere to transfer heat from the ambient air to the liquid nitrogen that travels through heat exchanger  112 . 
     A valve  132  selectively couples a coolant supply  134  to an inlet  136  of heat exchanger  108  or an inlet  138  of a conventional chiller  140 . Valve  132  is the same type of valve as valve  102 . The coolant stored in coolant supply  134  is, for example, water supplied straight from the municipal water lines or in some situations, deionized water from a deionized water source. Coolant supply  134  may also include a water filtration device that removes contaminants in the water which adversely affect the ability of the water to transfer heat or which corrode or cause deposits to form on the inside of the coolant passages in the equipment being cooled. 
     Although liquid nitrogen and water are described in conjunction with the system  100  of FIG. 2, system  100  can be used to heat or cool other liquid gases and/or coolants needed in the semiconductor fabrication process. For example, other suitable liquid gases include liquid oxygen and liquid argon, and other suitable coolants include glycol. 
     Inlet  136  is coupled to heat exchanger  108  so that water flows into chamber  113  defined by heat exchanger  108 . While in chamber  113 , the water contacts pipe  114  and looses heat to the nitrogen that travels within pipe  114 . The water, cooled by the liquid nitrogen, exits heat exchanger  108  through an outlet  142 . 
     Chiller  140  is a conventional chiller used to lower the temperature of the water. Water travels into chiller  140  from inlet  138  and exits through an outlet  146 . A valve  144  selectively couples outlet  142  of heat exchanger  108  or outlet  146  of chiller  140  to a coolant supply pipe  148 . Pipe  148  is coupled to equipment  126 A to supply coolant water. In one embodiment, equipment  126 A further may consist of an air condition unit that uses the coolant water (or another suitable air conditioning coolant) to generate cooled air for the semiconductor manufacturing facility. Although pipes  124  and  148  are illustrated as respectively supplying gaseous nitrogen and cooled water to the same equipment  126 A, pipes  124  and  148  may respectively supply gaseous nitrogen and cooled water to one or more of the same or different semiconductor fabrication equipment (e.g., equipment  126 B and  126 C). 
     A control unit  150  controls valves  102 ,  120 ,  132 , and  144 . Control unit  150  is, for example, a conventional temperature control unit available from Omega Engineering, Inc. In a normal operation, control unit  150  causes (1) valve  102  to couple supply  104  to inlet  106 , (2) valve  120  to couple outlet  118  to pipe  124 , (3) valve  132  to couple supply  134  to inlet  136 , and (4) valve  144  to couple outlet  142  to pipe  148 . Within chamber  113  of heat exchanger  108 , the water contacts pipe  114  and heat is transferred from the water to pipe  114  and from pipe  114  to the nitrogen. The nitrogen changes from liquid to gas prior to exiting heat exchanger  108  through outlet  118 . The water becomes colder as it loses heat to the pipe  114  and reaches a desired temperature prior to exiting heat exchanger  108  through outlet  142 . The gaseous nitrogen and the cooled water are received by equipment  126 A for use in the semiconductor fabrication process. 
     If the water exiting heat exchanger  108  is too high a temperature, control unit  150  can bypass heat exchanger  108  and use chiller  140 . If the water exiting heat exchanger  108  is too cold, control unit  150  can divert liquid nitrogen from heat exchanger  108  to heat exchanger  112  until the water exiting heat exchanger  108  returns to the desired temperature. These operations are explained later in reference to FIG.  4 . 
     FIG.  3 A and FIG. 3B illustrate a schematic of the top, side, and front view of heat exchanger  108 . As shown in FIG. 3A, chamber  103  has width W, length L, and height H. As one skilled in the art understands, the actual dimensions of W, L, and H depend on the demand for cooled water and/or gaseous nitrogen of the cleanroom. 
     Pipe  114  comprises a combination of junctions  154 - i , straight pipes  156 - j , junctions  158 - k , straight pipes  160 - l , and U-shaped pipes  162 - m . Note that i,j, k, l, m are integers representing the number of pipes  154 ,  156 ,  158 ,  160 , and  162 , and their values vary according to the number of levels of pipe  114 . Referring to the top view (top of FIG.  3 A), pipe  114  starts with a junction  154 - 1  that couples inlet  106  to a pipe  156 - 1 . A junction  158 - 1  next couples pipe  156 - 1  with a pipe  160 - l . Another junction  158 - 2  then couples pipe  160 - l  with a pipe  162 - 1 . Pipe  162 - 1  is used to make a 180 degree turn of pipe  114  on a plane (first plane) where the previously described pipes are located. 
     A junction  158 - 3  next couples pipe  162 - 1  with straight pipe  160 - 2 . A junction  158 - 4  then couples straight pipe  160 - 2  with a pipe  162 - 2 . Pipe  162 - 2  is used to make a 180 degree turn of pipe  114  in a plane perpendicular to the first plane and aligned with pipe  160 - 2 . Referring to the side view (bottom of FIG.  3 A), the previously described combination of pipes is repeated for additional levels of pipe  114 . At the last level, junction  154 - 2  couples pipe  156 - 2  (the end of pipe  114 ) to outlet  118 . 
     As one skilled in the art understands, pipe  114  can be alternatively made of other combinations of pipes or a single pipe bent to form the structure shown in FIG.  3 A. Note that the use of a single pipe reduces leakage over time under the cyclical heating and cooling conditions inside heat exchanger  108 . 
     Referring to the front view (FIG.  3 B), the outer surface of pipe  114  is attached with fins  164  to increase the heat transfer between the water that fills chamber  103  and the nitrogen within pipe  114 . For example, eight fins  164  are equal spaced around the outer surface of pipe  114 . The front view also shows that adjacent pipes  160 - l  on the same level are spaced apart from their centers by distance S. Adjacent pipes  160 - l  on adjacent levels are also spaced apart from their centers by distance S. As one skilled in the art understands, the actual dimensions of S, the sizes and the lengths of the pipes depend on the demand for cooled water and/or gaseous nitrogen of the cleanroom. Furthermore, although pipes  160 -l are shown equally spaced apart by distance S, pipes  160 - l  can be spaced apart by different distances. 
     FIG. 4 illustrates a method  200  by which the system  100  maintains the temperature of the water exiting heat exchanger  108  between a temperature range of T 1  to T 2 , where T 1  is greater than T 2 . Method  200  starts in action  202 . In action  202 , an operator initializes control unit  150  with the desired temperatures T 1  and T 2 . In action  204 , control unit  150  starts to fill chamber  113  with water and allows the liquid nitrogen to flow through pipe  114 . Thus, control unit  150  causes (1) valve  102  to couple nitrogen supply  104  to inlet  106 , (2) valve  120  to couple outlet  118  to pipe  124 , (3) valve  132  to couple water supply  134  to inlet  136 , and (4) valve  144  to remain in a closed position that does not allow the water to leave chamber  113  via outlet  142 . 
     In action  206  (once chamber  113  is filled with water), control unit  150  uses temperature sensor  152  to determine if the temperature of the water near outlet  142  is greater than a temperature T 1 , i.e., if the water temperature is too high. If the water temperature near outlet  142  is too high, action  206  is followed by action  208 . Otherwise, action  206  is followed by action  210 . 
     Note that temperature of water near outlet  142  may vary in time depending on the amount of contact the water makes with pipe  114  within chamber  113 . One skilled in the art understands that to ensure consistent water temperature near outlet  142 , the water in chamber  113  must be evenly distributed so the water uniformly contacts pipe  114  when the water travels down chamber  113  toward outlet  142 . One skilled in the art will also understand that typical semiconductor fabrication processes have high tolerance (e.g., ±10 to 20° C.) for coolant water temperature (e.g., 30° C.) and that water is a good thermal conductor so that the water temperature is generally evenly distributed. Alternatively, a mechanical mixer can be employed to mix the water within chamber  113  to ensure uniform water temperature throughout chamber  113 . 
     In action  208 , control unit  150  bypasses heat exchanger  108  and uses chiller  140  to cool the water to the desired temperature. Thus, control unit  150  causes (1) valve  102  to couple nitrogen supply  104  to inlet  106 , (2) valve  120  to couple outlet  118  to pipe  124 , (3) valve  132  to couple water supply  134  to inlet  138 , and (4) valve  144  to couple outlet  146  to equipment  126 A. Note that heat exchanger  108  can still be used to supply gaseous nitrogen to equipment  126 A. Action  208  is followed by action  206  and method  200  cycles until the temperature of the water near outlet  142  is less than temperature T 1 . 
     In action  210 , control unit  150  uses heat exchanger  108  to supply cooled water and/or gaseous nitrogen to equipment  126 A. Thus, control unit  150  causes (1) valve  102  to couple nitrogen supply  104  to inlet  106 , (2) valve  120  to couple outlet  118  to pipe  124 , (3) valve  132  to couple water supply  134  to inlet  136 , and (4) valve  144  to couple outlet  142  to equipment  126 A. Action  208  is followed by action  210 . 
     In action  212 , control unit  150  uses temperature sensor  152  to determine if the temperature of the water near outlet  142  is less than temperature T 2 , i.e., if the water temperature is too low (e.g., if the water starts to freeze). If the water temperature exiting heat exchanger  108  is too low, action  212  is followed by action  214 . Otherwise, action  212  is followed by action  216 . 
     In action  214 , control unit  150  bypasses heat exchanger  108  and uses heat exchanger  112  to convert liquid nitrogen to gaseous nitrogen, thereby allowing the water in heat exchanger  108  to warm up a temperature greater than T 2 . Thus, control unit  150  causes (1) valve  102  to couple nitrogen supply  104  to inlet  110 , (2) valve  120  to couple outlet  122  to pipe  124 , (3) valve  132  to couple water supply  134  to inlet  136 , and (4) valve  144  to couple outlet  142  to equipment  126 A. Note that heat exchanger  108  can still be used to supply cooled water to equipment  126 A. Action  214  is followed by action  212  and method  200  cycles until the temperature of the water near outlet  142  is greater than temperature T 2 . 
     In action  216 , control unit  150  uses heat exchanger  108  to supply cooled water and/or gaseous nitrogen to equipment  126 A. Thus, control unit  150  causes (1) valve  102  to couple nitrogen supply  104  to inlet  106 , (2) valve  120  to couple outlet  118  to pipe  124 , (3) valve  132  to couple water supply  134  to inlet  136 , and (4) valve  144  to couple outlet  142  to equipment  126 A. Action  216  is followed by action  206 , and the previously described steps cycle to maintain the temperature of the water exiting heat exchanger  108 . 
     FIG. 5 illustrates a method  500  by which system  100  maintains the temperature of the water exiting heat exchanger  108  between a temperature range of T 1  to T 2 , where T 1  is greater than T 2 . Method  500  is the same as method  200  except that action  508  replaces action  208  and action  514  replaces action  214 . 
     Action  508  is the same as action  208  except that control unit  150  sets valve  132  in a position that supplies water to both heat exchanger  108  and chiller  140 . In one embodiment, valve  132  has a setting that concurrently supplies water to two destinations. In action  508 , heat exchanger  108  is used in conjunction with chiller  140  when the temperature of the water exiting heat exchanger  108  becomes too high. Diverting some water away from heat exchanger  108  allows a smaller amount of water to lose heat to the liquid nitrogen, thereby reducing the temperature of the water exiting heat exchanger  108 . 
     Action  514  is the same as action  214  except control unit  150  sets valve  102  in a position that supplies liquid nitrogen to both heat exchangers  108  and  112 . In one embodiment, valve  102  has a setting that concurrently supplies water to two destinations. In action  514 , heater exchanger  108  is used in conjunction with heat exchanger  112  when temperature of the water exiting heat exchanger  108  becomes too low. Diverting some liquid nitrogen away from heat exchanger  108  allows a smaller amount of liquid nitrogen to absorb heat from the water in chamber  113 , thereby increasing the temperature of the water exiting heat exchanger  108 . 
     FIG. 6 illustrates a cooling system  600  in accordance with another aspect of the invention. System  600  is the same as system  100  except that a valve  602  has been added downstream of valve  132  and upstream from inlet  136 , and a valve  604  has been added downstream of valve  102  and upstream from inlet  106 . Control unit  150  uses valves  602  and  604  to control the respective water and nitrogen flow rates into heat exchanger  108  as one of the means to control the temperature of the water exiting heat changer  108  (to be described with respect to method  700 ). Chiller  140 , heat exchanger  112 , and their associated pipes and valves are optional in one embodiment of system  600  where valves  602  and  604  are the only means by which control unit  150  controls the temperature of the water exiting heat exchanger  108 . 
     FIG. 7 illustrates a method  700  by which system  600  maintains the temperature of the water exiting heat exchanger  108  between a temperature range of T 1  to T 2 . Method  700  is the same as method  200  except that action  708  replaces action  208 , action  714  replaces action  214 , and control unit  150  opens valves  602  and  604  to a first position (e.g., 50% of open) during normal operation of heat exchanger  108  (action  210  and  216 ). 
     Action  708  is the same as action  210  except that control unit  150  reduces the water flow rate and/or increases the nitrogen flow rate of heat exchanger  108  instead of bypassing heat exchanger  108  with chiller  140 . Reducing the water flow rate allows a smaller amount of water to lose heat to the liquid nitrogen, thereby reducing the temperature of the water. Increasing the nitrogen flow rate allows a greater amount of liquid nitrogen to absorb heat from the water in chamber  113 , thereby reducing the temperature of the water. To reduce the water flow rate, control unit  150  closes valve  602  to a second position (e.g., 25% of open). To increase the nitrogen flow rate, control unit  150  opens valve  604  to a third position (e.g., 75% of open). 
     Action  714  is the same as action  216  except that control unit  150  increases the water flow rate and/or decreases the nitrogen flow rate of heat exchanger  108  instead of bypassing heat exchanger  108  with heat exchanger  112 . Increasing the water flow rate allows a greater amount of water to lose heat to the liquid nitrogen, thereby increasing the temperature of the water. Decreasing the nitrogen flow rate allows a smaller amount of liquid nitrogen to absorb heat from the water in chamber  113 , thereby increasing the temperature of the water exiting the heat exchanger. To increase the water flow rate, control unit  150  opens valve  602  to the third position (e.g., 75% of open). To decrease the nitrogen flow rate, control unit  150  closes valve  604  to the second position (e.g., 25% of open). 
     One of the uses of the cooled water exiting heat exchanger  108  is to cool refrigerants (coolant) of air conditioning units used to, for example, maintain the temperature within the tolerances of the many semiconductor fabrication equipment located within the semiconductor fabrication facility. FIG. 8 illustrates one embodiment of an air conditioning system  800  used in conjunction with systems  100  and  600 . Air conditioning system  800  includes a heat exchanger (coolant bath)  808  used to cool the refrigerant of air conditioning unit  850 . The refrigerant exits air conditioning unit and enters a chamber  813  of coolant bath  808  via an inlet pipe  806 . Inlet pipe  806  is coupled to one end of pipes  814  that travel within chamber  813 . Another end of pipes  814  is coupled to an outlet pipe  818  that returns the refrigerant to air conditioning unit  850 . Coolant bath  808  and pipes  814  are, for example, of similarly construction as heat exchanger  108  and pipes  114 , respectively. Cooled water from heat exchanger  108  enters chamber  813  via pipe  148  (which acts as inlet to heat exchanger  108 ), and exits chamber  813  via an outlet pipe  842 . Within chamber  813 , the cooled water contacts pipes  814  and cools the refrigerants therein. 
     In another embodiment, heat exchanger  108  also acts as a coolant bath. As illustrated in FIG. 9, refrigerant enters chamber  113  of heat exchanger  108  via pipe  806 . Pipe  806  is coupled to one end of pipes  814  that travel within chamber  113 . Another end of pipes  814  is coupled to a pipe  818  that returns the refrigerant to air conditioning unit  850 . 
     Although the invention has been described with reference to particular embodiments, the description is only an example of the invention&#39;s application and should not be taken as a limitation. For example, other types of heat exchangers can be used in place of heat exchanger  108  depending on the actual application. Also, if the capacity of heat exchanger  108  is insufficient, heat exchanger  108  can be used simultaneously with heat exchanger  112  and chiller  140  to produce the necessary amount of cooled water and/or gaseous nitrogen. Furthermore, in any of the above operations where the flow of liquid nitrogen or water is decreased, conventional equipment may be used in conjunction with systems  100  and  600  to meet the demand of the semiconductor fabrication equipment. Various other adaptations and combinations of features of the embodiments disclosed are within the scope of the invention as defined by the following claims.