Abstract:
A system and method for controlling cooling of a thermal load having different cooling requirements in different sections based on direct thermal exchange using a two-phase refrigerant employs the pressure/temperature characteristics of the refrigerant to particular benefit for this multi-level cooling system. The two-phase refrigerant is first adjusted to have temperature/enthalpy characteristics chosen as the starting level for different cooling demands at related temperatures. After appropriate generation of a mixture of two-phase refrigerant initial reference temperature and pressure are established. Thereafter, incremental changes in the comprising hot gas and expanded cooled liquid/vapor, an temperature cooling medium area made by lowering the pressure by predetermined amounts, or alternatively by bypassing the pressure drop and proceeding to the next stage.

Description:
REFERENCE TO PRIOR ART 
     This application relies for priority on provisional application Ser. No. 61/161,091 filed Mar. 18, 2009 by K. W. Cowans et al and entitled “System and Method for Thermal Control from a Single Source of Different Heat Loads”. 
    
    
     BACKGROUND OF THE INVENTION 
     U.S. Pat. No. 7,178,353 and U.S. Pat. No. 7,415,835 disclose a novel temperature control concept which has been termed Transfer Direct of Saturated Fluids (TDSF) and which concept has material advantages for many temperature control systems used in modem high technology processes. In accordance with this concept, a two-phase fluid that is being used for temperature control is first compressed to a high pressure, high temperature gas, which is variably divided into two flows, the mass flow of one of which is directly controlled. The remaining second flow is condensed to a liquid state by cooling and then cooled further by being expanded to a vapor or liquid/vapor state as a saturated mist. The flow of hot compressed gas is varied under command signals derived from sensing the load temperature to a level such that the pressurized component, when mixed with the differential flow of saturated liquid/vapor mist provides a combined output of selectively controlled enthalpy, pressure and temperature. The load temperature is thus adjusted to the desired target level by direct control of the proportion of gas flow alone, accompanied by concurrent indirect control of the second or liquid/vapor state. After thermal exchange of the combined flow with the thermal load, the two-phase fluid is returned, using appropriate conditioning, for recompression in a closed loop operation. 
     Systems and method employing two-phase refrigerant in direct contact with a thermal load in this manner can provide precise but changeable control of load temperature over a wide dynamic range. The concept of uniting a variably pressurized two-phase refrigerant with a differential flow of saturated mist component to provide precise and stable temperature control of a thermal load has many advantages. The thermodynamic cycle is efficient, and also avoids the costs and thermal losses involved in using a separate thermal exchange medium. Additionally, because the process relies on pressure as well as enthalpy, rapid changes in a precisely defined target temperature can sometimes be introduced by pressure change alone. Where the temperature to be controlled and the heat load permit, temperature stabilization can be aided by use of the latent heat of evaporation or condensation in a two-phase mixture at thermal equilibrium. 
     The TDSF concept, therefore, not only has many current applications but also potential for more and different embodiments. Some operative non-uniformities which can be encountered, for example in some semiconductor manufacturing operations, have given rise to novel problems. When processing a semiconductor wafer, for example, the wafer may be mounted with its base side in contact with a thermally controlled (cooled) support platen. Then the upper side of the wafer, typically after having received a patterned protective layer, is exposed to a very high intensity energy source, such as a plasma, and a desired pattern is etched by bombardment into the wafer surface. 
     The TDSF system has proven to be superior for these, as well as other applications in general. Because of the historical trend in the semiconductor industry toward use of wafers of ever larger size, (wafers may now be greater than 300 mm diameter) the problems involved in laying down essentially microscopic high density patterns uniformly across the wafer area have been exacerbated. Moreover, significant differences in thermal exchange characteristics can exist between different areas of such large wafer surfaces. For example, the intensity of the heating source may vary materially across different areas of the wafer and thermal exchange efficiency may also vary somewhat with location on the wafer. Such variations can unacceptably reduce output yield in terms of the percentage of high quality micro images that can be formed within the patterns introduced into the wafer surface. 
     Even though a thermal control system based upon the TDSF concept provides a stable and precise temperature source at a target level, areal dispersion of the flowing plasma or other medium across the wafer may thus not be uniform. Variations in heat loads and heat transfer across different areas may therefore have to be compensated for, if possible, to improve quality and yield. The present invention introduces expedients which equalize thermal exchange between the temperature control medium and different areas of a relatively large heat load such as a modern semiconductor wafer. Such improvements are particularly suitable for TDSF system but can be applicable to other thermal control systems as well. 
     It is known to vary the temperature of refrigerant gas within a thermal transfer fluid loop, as described in Cowans U.S. Pat. No. 6,775,996, issued Aug. 17, 2004. That system, however, is not a TDSF system, and the expedients described are not applicable to the problem of providing differential thermal energy exchange between different parts of a thermal load, particularly a semiconductor wafer. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, the interrelated dual flow control and mixing approach that characterizes TDSF systems is employed to generate a mixed input coolant stream which differentially controls separate regions of an areally varying thermal load. This approach may specifically be used, for example, where different thermal transfer requirements exist during concurrent processing of different areas of a semiconductor wafer. This may occur when the heat source varies with location on the wafer, or conversely where coolant distribution varies for some reason. 
     After appropriate flow control and mixing of the differentially related flows of variable temperatures and pressure in a TDSF system the combined flow of controlled pressure and enthalpy at selected levels is further diverted into two or more separate sub-flows of different thermal properties. This is feasible because the two-phase characteristic of the coolant media enables the coolant properties to be varied by pressure as well as flow rate for different zones of the load. Thus, by introducing pressure drops or flow rate differences in separate flow branches from a mixed flow of hot gas and cooled expanded fluid different thermal transfer requirements can be met by a TDSF-based system. 
     In a first example, where there is a known differential in cooling flow requirements between wafer areas, the flow paths of coolant form a common source directed toward two different areas of a wafer are serially connected by a pressure dropping device. The temperature of the saturated fluid in the flow path between adjacent zones is thus controllably lowered between the first and second zones. The temperature of the coolant in the first zone can thus be preselected to compensate for a first known thermal transfer requirement, where the cooling need is lower because the radiation intensity is relatively lower. In the second zone area, which receives exposure to higher intensity radiation, the temperature of the coolant is lowered by introducing a pressure drop, and thus the wafer is cooled in this zone to the same temperature as in the first zone. Thus the processing conditions are equalized, improving process uniformity throughout two or more zones. In addition, the pressure dropped device can be bypassed, affording greater operative flexibility in the system. 
     In an alternative version of a multi-zone temperature control system using the TDSF control approach, the temperature of flows into the separate zonal areas can be unidirectionally adjusted downward from an initial level in multiple steps in accordance with needed thermal exchange conditions. In a specific example, control in each separate zone is effected by flowing coolant serially through successive heat exchange regions via paired combinations of pressure dropping and bypass valves. Each pressure-dropping valve is opened or bypassed to establish the desired process temperature for that region of the wafer. This approach enables a wider range of variables to be encompassed in the temperature compensating control system and retains local control to the degree feasible. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A better understanding of the invention may be had by reference to the following description, taken in conjunction with the accompanying drawings, in which like numbers refer to like parts, and in which: 
         FIG. 1  is a block diagram representation of a temperature control system for differentially controlling zonal temperatures using a single temperature controlled two-phase refrigerant mixture as a source; 
         FIG. 2  is a simplified perspective representation of a portion of a semiconductor processing facility, providing one example of how zonally different thermal requirements arise in processing a wafer; 
         FIG. 3  is an idealized and simplified side-sectional fragmentary view of the relation between a wafer on a platen, which is associated with an underlying cooling system and in which the wafer is impacted by a plasma excitation medium, and 
         FIG. 4  is a block diagram depiction of an alternative multi-zone cooling system providing capability for thermal control of more than two zones using the Transfer Direct of Saturated Fluids approach. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIG. 1 , a TDSF system for temperature control of more than one zone is depicted in block diagram form and includes the principal features of a TDSF system as described in the above-referenced patents. Briefly, such a system  110  includes two principal flow paths from the output of a compressor  112 , these flow paths branching from a main output line  124  from the compressor  112 . One branch line  118  from the compressor output line  124  is directed through a condenser  114  that provides a cooled liquefied output to a thermo-expansion valve  119 , which expands and cools the flow to a liquid/vapor mix which is dependent in temperature on the pressure of said liquid/vapor mix after expansion. The flow rate of said mix is indirectly controlled, since it varies in intensity with respect to the mass flow in the main output line  124  as determined by a controller  131  which operates a proportional valve  125  in the output line  124 . The separate flow, which is cooled and expanded, as described hereafter, provides one input to a mixing mechanism  133 , specifically at a mixing tee  140 . The mixing tee  140  also receives the pressurized hot gas from the compressor  112 , after it has been directly varied at the externally controlled proportional valve  125  (followed by a check valve  123 ) governed by the controller  131  so that it is delivered at a variable mass flow rate to the mixing tee  140 . While the thermo-expansion valve  119  is shown as settable by the controller  131 , this expedient is not needed for control, but provides an added capability. The direct control of hot gas flow provides indirect control of the remainder flow, as described in the referenced patents. 
     Condensate output from the condenser  114 , which has a flow rate constituting the differential in flow remaining after the proportional valve  125  diversion, is thus variably expanded through the thermo-expansion valve  119  to provide the maximum refrigeration available, dependent on the pressure at the input to the compressor  112 . This expanded flow is lowered in pressure by a Δp valve  132  before reaching the input of the mixing tee  140  to which it is coupled. The controller  131  thus directly sets the proportional valve  125  to provide, ultimately, a precisely determined temperature and pressure from the mixing mechanism  133 . This controlled output from the mixing mechanism  133  is fed as an input to the thermal energy exchange site  130 , for controlled cooling of the thermal load  184  that is being processed, such as a semiconductor wafer  190  undergoing a fabrication step (such as etching by radiation from a high intensity source  191 ), on a platen  192 . 
     As described in more detail in the previously filed patents and applications, and therefore only briefly here, the return line  124  from the heat exchange site  130  is coupled through intermediate devices back to the input to the compressor  112  to complete the refrigeration control loop. For example, along the return line  124  a coupling  126  provides an equalizing pressure via a line  134  to the thermo-expansion valve (TXV)  119 , which is controlled by a pressure signal provided via a coupling  120  from a sensor bulb  122  in close thermal contact to the return line  124 . 
     The system  110  has other features which need only be referred to briefly herein, including a feedback line  164  intercepting the output of the proportional valve  126  and including a flow control orifice. The line  164  also includes a control valve  163  which couples the compressor  112  output back to the compressor  112  input from a point following the proportional valve  125 . This valve  163 , when opened by the controller  131 , provides flow from line  164  to the compressor  112  input. This is useful to increase control at the lower temperatures. Also, the compressor  112  output can be fed back conventionally, through a hot gas bypass valve  165 , to the compressor  112  input. This action adjusts the lower limit of pressure input to compressor  112 . In addition the compressor  112  input also may be guaranteed to be substantially entirely gaseous by the controller  131 , through an electrical heater  156  before its input. Said heater  156  boils off any liquid in the mix supplied to the compressor  112  input. A desuperheater valve (DSV)  152  in a line from the output of the condenser  114  is coupled to the input to the compressor  112 , before the heater  156 . A sensor bulb  154  proximate the compressor  112  input controls the desuperheater valve (DSV)  152  in conventional fashion. Further, a close-on-rise (COR) regulator  150  is included in the input line to the compressor  112  prior to the junction with the DSV  152  line, as described in the TDSF patents referred to above. 
     In this exemplification of the TDSF system, the two different heat exchange regions  184 ,  185  exist in the heat exchange site (load)  130 . These heat exchange regions, which may in practice be adjacent, concentric or in some other geometry, serially receive coolant input with intermediate flow control  180  operated by the controller  131 . If the requirements for cooling differentially are invariant then flow controls responsive to the controller  131  are not required. A first heat exchange section, designated  184  and “HEX area  1 ” is physically adjacent a first predetermined area of the wafer  190  via the platen  192 . A second heat exchange section, designated  185  and “HEX area  2 ” is spatially distinct from the first area  184  but physically adjacent a different area of the wafer  190 , and in heat exchange relationship thereto via the adjacent part of the platen  192  on which it resides. 
     In one exemplification, as seen more particularly in  FIGS. 2 and 3 , the wafer  190  is functionally divided into an inner radial zone (HEX area # 1 ) and an outer radial zone (HEX area # 2 ), both receiving impinging radiation from the plasma or other source (symbolically depicted in  FIGS. 2 and 3 ) so that a pattern is etched in accordance with a conventional protective overlaying pattern (not shown). The difference in heat exchange requirements arises from differential bombardment of areas of the wafer in process. As seen particularly in  FIG. 3 , the first cooled heated area  184  is subjected to lower power density bombardment than is the second heated area  185 , because here the second area  185  of the wafer  190  is closer to the geometric center of the wafer  190 . The wafer  190  is exposed to impinging plasma on its upper side, while its lower side is in contact and thermal exchange relation with the platen  192  ( FIGS. 2 and 3 ). The platen  192  and the wafer  190  are cooled by conductive heat exchange between the coolant flowing in encircling passageways defined by continuous tubing  194  ( FIG. 3 ) distributed at different radii throughout the area of the platen  192 . The wafer  190  is in close thermal contact with the upper surface of the platen  192  and thus with the coolant in the thermally connected tubing  194 , which is in essentially continuous contact, along its length, with underside of the platen  192 . Thus the refrigerant cools the differentially heated wafer  190  at the heat exchange site  130  ( FIG. 1 ) and in accordance with the invention this cooling is effected in an appropriate differential manner by using the particular properties of the two-phase refrigerant. 
     The coolant flow path here incorporates a separate flow control  180  ( FIG. 1 ) for limited adjustment of the temperature of thermal transfer coolant in thermal contact with the first and second heat exchange areas  184 ,  185 . The flow temperatures within these areas  184 ,  185  can be altered when needed, as is usually the case, by the pressure dropping (Δp) valve  182  in the serial flow path between the two areas  184 ,  185 , as seen in both  FIGS. 1 and 3 . This valve  182  introduces a pressure drop, therefore a temperature drop in the flow along segments of the heat exchanger tubing  194 . Thus the two radially adjacent areas  184  and  185  of the wafer  190  are cooled differentially, each as needed in accordance with the level of impinging plasma energy, to be held at a like selected level despite a difference in the thermal energy of impinging excitation. 
     The controller  131  may alternatively open the bypass valve  183 , effectively shunting all flow around the Δp valve  182 , to provide like temperatures in the first and second HEX areas  184 ,  185  under appropriate circumstances. 
     Consequently, significant temperature variations between different wafer areas that arise from non-uniformity in the incident radiant power can be compensated for by manipulation of pressure and temperature of a two-phase refrigerant in a manner to equalize wafer surface temperature, thus improving fabrication reliability and yield. Because the coolant in the TDSF system is a two-phase fluid of saturated mist, the expedient of introducing a fixed temperature drop is an advantageous way to introduce the desired temperature differential in coolant flow. 
     In the example of  FIGS. 1-3 , the coolant flows circumferentially about the tubing  194  at different radii as the coolant passes within it relative to the surface of the wafer  190  and the supporting platen  192 , as best depicted in  FIG. 3 . For simplicity and by way of example only it is assumed that, as seen in  FIG. 2 , the intensity distribution of plasma from the source  191  ( FIG. 1 ) has higher intensity radiation on one side (“area  2 ”) and lower intensity radiation on the other side (“area  1 ”) of radially adjacent zones, as viewed in  FIG. 2 . 
     However, if the energy distribution of the impinging media is geometrically different, i.e. rectilinear, the pattern of tubing  194  in areas # 1  and # 2  of the wafer  190  and platen  192  can be shaped as rectilinear grids in adjacent zones, and interconnected by rectilinearly disposed tubing. At the region of juncture of these two zones, the coolant temperature can be dropped by the Δp valve  182  so the region exposed to lower power excitation will be cooled to a lesser degree. 
     In the example of  FIG. 1 , the bypass valve  183  provides an alternative path if no coolant differential needs to be used. Such a feature may be particularly expeditious during rapid cooling of the entire platen from one processing temperature to a lower one. Both the Δp valve  182  and the shunt bypass  183  can be operated by the controller  131 . 
     Accordingly, the thermal properties and two-phase character of the coolant in the TDSF system are used to introduce a precise temperature differential between the cooling power levels applied to the two adjacent but differently heated regions of the wafer  190 . Clearly, other appropriate flow path geometries and suitably placed pressure drops can be employed where the power distribution of incident radiation is different. 
     In a further-reaching application of this technology, shown in  FIG. 4 , the output from a thermal control unit, such as the system  110  shown in  FIG. 1 , can be fed to a heat exchange site  130 ′ in which the cooling needs require more than two levels of temperature, i.e. three or more levels.  FIG. 4  illustrates how this can be accomplished. As seen in  FIG. 4 , a wafer  190  is heated by a high intensity radiation source  191  as it rests on the platen  192  on the cooling system  185 ′, etc. Heat exchange areas  1 ,  2  and  3  are located in different parts of the heat exchanger  185 ′, etc. which receives coolant from the mixing tee  140  (not specifically shown in  FIG. 4 ). The controller  131  of  FIG. 1  operates the flow control  180 ′ which is commanded to control separate bypass valves  183 ′, each of which shunts a different Δp valve  182 ′ or  189 ′. 
     The flow paths illustrate how differential cooling of the different areas ( 1 ,  2  and  3 ) of the heat exchanger portions  185 ′ can be effected, using a single source of coolant from them mixing specifically, a two-phase coolant which varies in temperature dependent upon the pressure. Accordingly, coolant passing through heat exchange area  1  is directed either through a Δp valve  182 ′ or in parallel through a bypass valve  183 ′ before return to the heat exchanger for subsequent cooling of the next area. In other words, the coolant passed through area # 1  at an initial temperature set, at the mixing tee  140  (of  FIG. 1 ), is then fed at the same temperature to area # 2  (if the associated bypass valve  183 ′ is open by the flow control  180 ′). If the bypass valve  183 ′ is not opened, the output flow from area # 1  is through the Δp valve  182 ′. In this case, coolant at an incrementally lower temperature is supplied to area  2  in the next section of the heat exchanger  185 ′. 
     The areas are shown in  FIG. 4  as being sequentially disposed, but this is merely for clarity and idealized purposes. The areas of the wafer that are to be differentially cooled can be in an entirely different geometry, so that the only requirement is that the successive heat exchange areas related to areas on the wafer  190 ′ that are geometrically disposed in some known configuration. As in the transfer of coolant from area # 1  to area # 2 , the output of area # 2  is fed to the paired valve elements for area # 3 , namely the Δp valve  189 ′ and a parallel bypass valve  183 ′. These are again controlled (opened and closed) by the flow control  180 ′, and the output from area # 2 , either lowered in temperature or not, is then fed to area # 3  of the heat exchanger complex  184 ,  185 , etc. and the output from that area is returned to the compressor  112 . 
     The serial arrangement with successively equivalent temperature outputs or incrementally lowered temperatures can thus be extended for a greater number of areas, dependent only on the heat exchange requirements in the thermal load. It will be appreciated that the number of pairs of Δp valves and bypass valves will be required to be correspondingly increased if the number of stages are increased. 
     Although applicants have shown and described different configurations of systems for providing a range of cooling functions from a given source of two-phase refrigerant, the invention is not limited thereto, but encompasses all forms and variations within the scope of the appended claims.