Patent Publication Number: US-8991332-B2

Title: Apparatus to control semiconductor film deposition characteristics

Description:
This application is a divisional under 35 U.S.C. §121 of U.S. application Ser. No. 11/205,647, filed Aug. 17, 2005, the disclosure of which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     Embodiments of the present invention pertain to the formation of films on a substrate. These films, including but not limited to Si, SiGe, SiC and SiGeC, in both their doped and undoped forms, are used in the manufacture of advanced electronic components. Such films exhibit various properties, such as morphology and doping concentrations, which must be controlled to within certain tolerances. The advances exhibited in electronics over the past few decades are the direct result of the ability of semiconductor foundries to increase circuit pattern densities. As these pattern densities increase, the tolerances for the thin films required to make the circuits become increasingly strict. Therefore, careful control of the formation of thin films, and the resultant properties of such films, is essential for continued advances in electronics. These aforementioned films are typically made in an apparatus of the type shown and described in U.S. Pat. No. 6,083,323. 
     A substrate typically has a top face upon which a film can be formed and a bottom face. To grow the film, the substrate is placed into a reaction chamber. The top face of the substrate faces a top surface of the reaction chamber; similarly, the bottom face of the substrate faces a bottom surface of the reaction chamber. During the film formation process, the substrate is heated according to process parameters. 
     As noted above, it would be desirable to provide methods and apparatus for providing improved control of film characteristics, including but not limited to growth rate, morphology, faceting, doping distributions, etc. It is also desirable to provide methods and systems that provide a high level of process repeatability. 
     SUMMARY 
     Aspects of the present invention provide methods, apparatus and systems, for forming thin films on a substrate. During the film formation process, the substrate is heated according to process parameters. Also, during the formation process, the temperature of at least a portion of the surface of the reaction chamber is modulated so that the temperature of this surface varies with the process time in a predetermined manner. This temperature-modulated portion of the reaction chamber surface may be the top surface, the bottom surface, adjacent surfaces or the entire chamber surface. In one embodiment, the temperatures of a plurality of surfaces in the reaction chamber are individually modulated. In one embodiment, the top surface is modulated according to a first temperature parameter, and the bottom surface is modulated according to a second temperature parameter. 
     In one embodiment, a system or apparatus for forming films includes a cooling system that can be controlled during the film formation period to regulate the temperature of at least a portion of the reaction chamber surface. In one embodiment, the cooling system utilizes one or more setpoints to set the level of cooling power employed by the cooling system to adjust the temperature modulated portion of the reaction chamber surface during the film formation process. In another embodiment, the cooling system employs a temperature feedback loop to adjust the power employed by the cooling system so that the temperature of the cooled surface follows a predetermined, time-dependent trajectory across the film formation processing period. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a cross-sectional view of a first embodiment of a thermal reaction chamber; 
         FIG. 2  is a diagram of control logic that can be used in accordance with one or more embodiments; 
         FIG. 3  is a graph illustrating a hypothetical temperature trajectory; 
         FIG. 4  is a diagram of control logic that can be used according to one or more embodiments; 
         FIG. 5  shows a cross-sectional view of a second embodiment of a thermal reaction chamber; 
         FIG. 6  is a diagram of control logic that can be used according to one or more embodiments; 
         FIG. 7  is a graph illustrating hypothetical first and second temperature trajectories for the embodiment shown in  FIGS. 5 and 6 ; 
         FIG. 8  shows a cross-sectional view of a third embodiment of a thermal reaction chamber; and 
         FIG. 9  shows a cross-sectional view of a fourth embodiment of a thermal reaction chamber. 
     
    
    
     DETAILED DESCRIPTION 
     Before describing several exemplary embodiments of the invention, it is to be understood that the invention is not limited to the details of construction or process steps set forth in the following description. The invention is capable of other embodiments and of being practiced or being carried out in various ways. 
     Novel apparatus and techniques in chamber wall temperature regulation to improve the manufacturing of advanced electronics, such as transistor applications in the 65 nm and 45 nm technology nodes are disclosed. Improvements in growth rate and reduced faceting have been observed, and based upon these observations, improvements in yield, film quality and process repeatability are expected to be obtained. Improvements are particularly expected in low temperature film formation processes where the process temperatures are less than 850° C., or where films are formed on patterned wafers, or where high dopant concentration films (in the range of 1%) are formed. These improvements can be achieved without affecting other important film parameters and morphology, and which may be practiced in any device adapted for the growth or deposition of films, such as the Epi Centura® 300 mm CVD system of Applied Materials, Inc., providing an additional control parameter for controlling growth kinetics. 
     For purposes of the following disclosure, a film formation process spans the entire period of time, termed the processing period, between the insertion of a substrate into a processing chamber, and the extraction of the substrate from the processing chamber. A single process may incorporate one or more steps; for example, a process may include a pre-clean/surface conditioning step, a film deposition step, and a cool-down step. The invention may be applied, for example, to epitaxial and polycrystalline or amorphous film deposition processes, such as Si, SiGe, SiC (carbon doped silicon), SiGeC (in doped or undoped forms), silicon nitride and other compound semiconductor films (which may include III-V or II-IV materials), and may be broadly applied to other processes. 
       FIG. 1  shows a cross-sectional view of a thermal reaction chamber  103  used in a first embodiment chemical film formation system  100 . The chamber  103  has chamber walls  102  that define the inner volume of the chamber  103  in which reactive film formation processes are accomplished, such as suitable deposition processes, including but not limited to CVD. A housing  118  envelopes and supports the chamber walls  102 . A substrate support structure  104  is used to support a substrate  106  within the chamber  103  during CVD processing. The substrate  106  has a top face upon which a film is to be deposited or grown, and this top face typically faces away from the substrate support structure  104 , although this is not a requirement. Hence, the bottom face of the substrate  106  typically faces, and contacts, the substrate support structure  104 . 
     During processing, gases enter the chamber  103  through an entry port  110  and are removed through an exhaust port  112 . Also during processing, heat is provided by infrared radiation bulbs  114 . The infrared radiation bulbs  114  are mounted proximate the chamber walls  102 , on a support assembly  116  connected to the housing  118 . The chamber walls  102  of the chamber  103  are transparent, typically made of quartz, and allow infrared radiation from the radiation bulbs  114  to freely enter the reaction chamber  103  to heat the substrate  106 . The chamber walls  102  have a top surface  105  that faces the top face of the substrate  106 , and a bottom surface  107  that faces the bottom face of the substrate  106  and the substrate support structure  104 . 
     A more complete description of thermal reactors and their operation is disclosed in commonly assigned U.S. Pat. No. 5,258,824 entitled “In-Situ Measurement Of A Thin Film Deposited On A Wafer” and U.S. Pat. No. 5,108,792 entitled “Double-dome Reactor for Semiconductor Processing”, the entire contents of each of which is incorporated herein by reference. 
     During processing, the chamber walls  102 , although substantially transparent, still become heated. A coolant flow for cooling the chamber walls  102  is supplied to the housing  118  from a blower  140  via inlet conduit  120 , directed past the chamber walls  102  and exhausted through outlet conduit  122 . More specifically, the coolant flow is supplied via conduit  120  to housing  118  through upper inlet port  124  and lower inlet port  126 . The coolant flow exits the housing  118  through upper exhaust port  128  and lower exhaust port  130 . Coolant entering through upper inlet port  124  passes across the top surface  105  of the chamber walls  102  and exits through upper exhaust port  128 . Similarly, coolant entering through lower inlet port  126  passes across the bottom surface  107  of the chamber walls  102  and exits through lower exhaust port  130 . The housing  118  forms a shroud that channels the coolant past the chamber walls  102 . This constant flow of coolant along the chamber walls  102  cools the chamber walls  102  of the reaction chamber  103 . Typically, the coolant is air. 
     Alternate ways of controlling the temperature of the chamber walls  102  may also include water cooling surfaces in contact with the chamber walls  102 , or the use of nitrogen, helium, argon or other inert gases flowing across the chamber walls  102 . A coolant regulator  131 , such as an air vane or other coolant flow control device, located in the inlet conduit  120 , controls the amount of coolant flow to the housing  118  and, in turn, controls the temperature of the chamber walls  102 . Alternatively, other devices for controlling the coolant flow may be used, such as an adjustable iris, a valve, blower speed control circuitry for the blower  140  and the like. Together, the blower  140  and coolant regulator  131 , or other suitable ways as outlined above, provide a cooling system for cooling the chamber walls  102 , in which the cooling power provided by the cooling system for the chamber walls  102  is controllable, i.e., the rate of heat removal from the chamber walls  102  may be controlled by the cooling system so as to regulate the temperature of chamber walls  102  in a controlled manner. 
     The temperature of the chamber walls  102  may be monitored using conventional temperature measuring devices familiar to those in the art, such as optical pyrometers, thermocouples or the like. For example, the temperature of the top surface  105  of the chamber walls  102  may be monitored using optical pyrometer  132 ; optical pyrometer  134  may be used to measure the temperature of the substrate  106 ; optical pyrometer  136  may be used to measure the temperature of the substrate support structure  104 , and optical pyrometer  138  may be used to monitor the temperature of the bottom surface  107  of the chamber walls  102 . 
     A first signal, encoding the measured temperature of the top surface  105 , is output from the top surface temperature measuring device  132  and received for processing by control logic  200 , an embodiment of which is represented in  FIG. 2 . Similarly, a second signal, encoding the measured temperature of the bottom surface  107 , is output from the bottom surface temperature measuring device  138  and received for processing by the control logic  200 . The control logic  200  utilizes the first signal, the second signal or a function of the two to control the cooling power of the cooling system so as to modulate the temperature of the chamber walls  102  over the processing period according to a predetermined temperature trajectory. 
     In one embodiment, the control logic  200  comprises a processor  210  in electrical communications with a memory  220 . The memory  220  comprises control code  221 , which is executed by the processor  210  and which controls the operations of the processor  210 ; the control code  221  serves as the operating system for the control logic  200 . In the following, when the processor  210  is described as performing an act, it should be understood that it is the control code  221  that causes the processor  210  to perform the act described; providing the control code  221  program should be well within the means of one reasonably skilled in the art. 
     In the embodiment disclosed in  FIG. 2 , the processor  210  obtains the temperature of the top surface  105  via a top surface temperature input  231  that receives the first signal from the top surface temperature measuring device  132 . Similarly, the processor  210  obtains the temperature of the bottom surface  107  via a bottom surface temperature input  232  that receives the second signal from the bottom surface temperature measuring device  138 . The control logic  200  is used, amongst other things, to control the cooling system used to cool the chamber walls  102  so as to modulate the chamber wall  102  temperature over the process period in a predetermined manner, thereby providing an independent parameter for controlling the kinetics of the film formation process. It should be clear, however, that the control logic  200  may contain many other additional inputs that are not indicated in  FIG. 2 , such as inputs for measuring gas flow rates, substrate  106  and substrate support structure  104  temperatures, etc., as known in the art. 
     The control logic  200  may be provided a display  238  to present process-relevant information to a user, and an input device  239  to permit the user to enter information into the control logic  200 . The processor  210  can control the display  238  to present, for example, the temperatures of the top surface  105 , bottom surface  107 , substrate  106  and substrate support structure  104 , the current process step, the current process time, gas flow rates, etc. Likewise, the processor  210  may change parameters stored within the memory  220  according to data received from the input device  239 , with such changes potentially resulting in changes to the process steps executed by the processor  210 , and hence changes in the way the processor  210  controls the CVD system  100 . The display  238 , input device  239 , control code  221  and processor  210  together form a user input/output (I/O) interface, in a manner familiar to those in the art, which permits the user to both monitor and control the CVD system  100 . 
     In the embodiment shown, the memory  220  of the control logic  200  also contains a temperature parameter  222  that is used to control and modulate the temperature of at least a portion of the chamber wall  102  over the processing period. The temperature parameter  222  comprises at least one setpoint  223 , and typically will have two or more setpoints  223 . Each setpoint  223  contains a respective time value  224  and temperature value  225 . The time value  224  indicates a time within the processing period, and may be in any format suitable to encode such information, such as a 24-hour time, a process-relative time (i.e., the amount of time elapsed since the beginning of the process, or to the end of the process), a step-relative time (i.e., the amount of time elapsed since the beginning of a current step within the process, or to the end of the step) or the like. The temperature value  225  indicates a temperature that is desired for the temperature-modulated portion of the chamber wall  102  at the related time value  224  in the setpoint  223 , and may be in any form suitable to indicate such temperature information; examples include an absolute temperature, as in degrees Celsius or Kelvin, or a relative temperature, as in an offset from a process temperature. 
     Together, the setpoints  223  provide temperature trajectory information for the temperature-modulated portion of the chamber wall  102  over the processing period. At predetermined intervals during the processing period, such as 0.01 second intervals, the control logic  200  obtains chamber wall  102  temperature information from the temperature inputs  231 ,  232 , and utilizes this information to generate a current measured temperature  229 . Any method may be used to generate the current measured temperature  229 , such as by averaging, weighted averaging, using only one of the temperature inputs  231 ,  232 , etc. This may be selectable by the user via the user I/O interface. The processor  210  then uses the current time (as obtained from timer  240 ) and the time values  224  to index into the temperature parameter information  222  to find the closest setpoints  223  between which the current time lies. 
     Still referring to  FIG. 2 , next, the processor  210  performs linear interpolation (or any other suitable interpolation), using the associated temperature values  225  of the closest setpoints  223 , to determine the current target temperature  228  of the temperature-modulated portion of the chamber wall  102 . Typically two setpoints  223  (or one setpoint if before or beyond the minimum and maximum time values  224 ) are used as the closest setpoints  223 , but three or more may be used depending upon the type of interpolation performed. The processor  210  utilizes the current measured temperature  229  and current target temperature  228  as inputs into a standard feed-back loop to control the power level of the cooling system for the temperature-modulated portion of the chamber walls  102  so that the current measured temperature  229  reaches the current target temperature  228 . By way of continuous feedback, the current measured temperature  229  as a function of time should substantially track the temperature parameter  222 , within measurement errors and the mechanical limitations of the cooling system. 
     In the embodiments depicted in  FIGS. 1 and 2 , for example, the processor  210  sends signals to a cooling power control output  233  to control the coolant regulator  131  based upon the measured and target temperatures  229  and  228 . If the difference between the current measured temperature  229  and the current target temperature  228  is positive (i.e., the temperature-modulated portion of the chamber wall  102  is currently hotter than desired), then the processor  210  sends a signal to the cooling power control output  233  to open the coolant regulator  131  more to increase the rate of coolant flowing over the chamber walls  102 , i.e., to increase the cooling power of the cooling system. Conversely, if the difference is negative, the processor  210  would instruct the coolant regulator  131  to further restrict the flow of coolant, so as to decrease the cooling power of the cooling system. The processor  210  may utilize the magnitude of the difference between the current measured and target temperatures  229 ,  228  to determine how restrictive or permissive of air flow the coolant regulator  131  should be, i.e., by how much the cooling power should be increased or decreased. 
     By way of example,  FIG. 3  illustrates a hypothetical desired temperature trajectory of the average temperature of the chamber walls  102  over a portion of the processing period. The graph in  FIG. 3  is normalized to show temperature differentials with respect to a predefined process temperature, which may be the starting temperature of the film formation step. The user may desire that the chamber walls  102  cool down to the process temperature from a pre-bake step, spend about ten seconds at the process temperature to stabilize, and then, upon the start of the film formation step, begin an asymptotic-like slope down to a temperature that is about 65° C. below the process temperature at the completion of the film formation step. This steadily decreasing temperature of the chamber walls  102  over the film formation step helps to reduce faceting. The user may decide to use six points  252 - 257  to approximate an asymptotic curve  260 , and two points  251 ,  252  to provide for the ten second temperature stabilization period prior to the film formation step. 
     If the film formation step begins at a process time of 1340 seconds, the I/O system of the control logic  200  may then be utilized to enter seven corresponding setpoints  223  for the temperature parameter  222 : a first setpoint  223  with a time  224  of 1330 and a temperature  225  of 0° C. for a first point  251 ; a second setpoint  223  with a time  224  of 1340 and a temperature  225  of 0° C. for a second point  252 ; a third setpoint  223  with a time  224  of 1370 and a temperature  225  of −25° C. for a third point  253 ; a fourth setpoint  223  with a time  224  of 1405 and a temperature  225  of −35° C. for a fourth point  254 ; a fifth setpoint  223  with a time  224  of 1440 and a temperature  225  of −45° C. for a fifth point  255 ; a sixth setpoint  223  with a time  224  of 1510 and a temperature  225  of −60° C. for a sixth point  256 , and finally a seventh setpoint  223  with a time  224  of 1560 and a temperature  225  of −65° C. for a seventh point  257 . The I/O system might then be utilized to instruct the control logic  200  to use an average value obtained from the top surface temperature input  231  and the bottom surface temperature input  232  to generate the current measured temperature  229 . During the film formation process, the control logic  200  would then use the seven setpoints  223  of the temperature parameter  222  to control the coolant regulator  131  so that the current measured temperature  229  tracks the current target temperature  228 . Of course, it should be clear that the target temperature  228  in this case is not an actual temperature, but a temperature differential based upon a predefined process temperature. That is, when generating the current measured temperature  229 , the control logic  200  may subtract the known, constant process temperature to yield a temperature differential for the current measured temperature  229 . For example, at a process time of 1470, as shown in  FIG. 3 , the control logic  200  would extrapolate between the fifth and sixth setpoints  223  for the fifth point  255  and sixth point  256  to find a current target temperature  228  of 53° C. below the process temperature. The control logic  200  would then send signals to the cooling power control output  233 , based upon the current target temperature  228  of −53° C. and the value of the current measured temperature  229 , to regulate the cooling system so that the average temperature of the chamber walls  102  tracks the temperature trajectory defined by the setpoints  223 . The coolant regulator  131  could also be manually adjusted to control the temperature trajectory of the chamber walls  102 . 
     In the above exemplary embodiment, the setpoints  223  utilize a temperature value  225  to construct the temperature parameter  222  that defines a desired temperature trajectory of the modulated surface of the chamber walls  102  over the processing period. However, with reference to  FIG. 4 , because there is a tight correlation between the power of the cooling system (i.e., the speed of the blower  140  and/or the setting of the coolant regulator  131 ) and the radiance of the heating elements  114 , as another embodiment it is equally possible to define the temperature parameter  322  as having one or more setpoints  323 , each with an associated time value  324  and cooling power level value  325 . In this case, the processor  310  of the second embodiment control logic  300  generates a current target cooling power level  328  in a manner analogous to that used above to find the current target temperature  228 , and then sends signals to the cooling power control output  333  to set the power of the cooling system (i.e., blower  140  and/or coolant regulator  131 ) to match the current target cooling power level  328 . 
     Because of the inherent consistency and reproducibility of processing runs, the temperature parameter  322  defined as a series of cooling power levels  325  at respective time values  324  is functionally similar to the temperature parameter  222  of the above embodiment. However, variations between the desired and actual temperatures of the modulated region of the chamber walls  102  may be greater than in the first embodiment. 
     Embodiments of the present invention provide for controlled modulation of the entire chamber wall  102 , or a portion of the chamber wall  102 , over the processing period. In particular, as indicated in  FIG. 3 , the controlled modulation of the chamber wall  102  within individual steps of the film formation process can be achieved. For example, a higher overall growth rate with reduced faceting can be achieved by providing a temperature parameter  222 ,  322  that initially increases the temperature of the top surface  105 , and then slowly decreases the top surface  105  temperature as the deposition or growth of the film on the substrate progresses. Selection of the temperature parameter  222 ,  322  will depend on the property that is to be optimized. For example, increasing the wall  102  temperature causes gas to crack or decompose better, thereby enhancing the growth rate. Film composition can be varied through this mechanism as well, since some dopant species absorb or incorporate better when decomposed. All of this may be done within a process step, as the film formation procedure goes through various stages. It will thus be understood that the skilled artisan can empirically determine and select the temperature parameter  222 ,  322  to achieve the desired film properties. 
     With reference to  FIG. 5  and  FIG. 6 , a third embodiment system  400  provides additional independent parameters for the film deposition process by allowing independent control of the temperatures of multiple portions of the chamber wall  102 . For ease of presentation, components in  FIG. 5  that are essentially identical to those in the prior embodiments have been provided the same reference numbers. As indicated in the discussion of  FIG. 1 , coolant entering inlet port  124  passes across the top surface  105  of the chamber walls  102 , thus cooling the top surface  105 . Similarly, coolant entering the bottom inlet port  126  cools the bottom surface  107 . Hence, by independently controlling the amount of coolant entering the top inlet port  124  and the bottom inlet port  126  it is possible to respectively independently control the temperature of the top surface  105  and the bottom surface  107  of the chamber walls  102 . To effectuate this, this embodiment provides a first coolant regulator  431  for controlling the rate of coolant flow into the upper inlet port  124 , and a second coolant regulator  439  for controlling the rate of coolant flow into the lower inlet port  126 . 
     The coolant regulators  431 ,  439  may be air vanes, adjustable irises, valves, liquid-cooled surfaces in contact with their respective chamber wall  102  surfaces, or the like. Alternatively, one of the coolant regulators  431 ,  439  may be an air vane, adjustable iris, valve, cooled surface or the like, and the other may utilize blower speed control circuitry to control the speed of the blower  140 . 
     Control logic  500  for the embodiment shown in  FIG. 5  is analogous to that of the previous embodiments, but provides for independent control of the first coolant regulator  431  and the second coolant regulator  439  according to a first temperature parameter  560  and a second temperature parameter  570 , respectively, stored in the memory  520 . The first temperature parameter  560  defines a desired temperature trajectory of the top surface  105  over the processing period. The second temperature parameter  570  defines a desired temperature trajectory of the bottom surface  107  over the processing period. 
     For example, with reference to  FIG. 7 , the first temperature parameter  560  may have fifteen setpoints  563  defining a first temperature trajectory  601 , analogous to that depicted in  FIG. 3 , for the top surface  105  relative to the process temperature. For example, seven points  611 - 617  may comprise a pre-bake temperature trajectory  610  for the top surface  105 . Four points  621 - 624  may comprise a film deposition temperature trajectory  620  for the top surface  105  that increases during the film deposition step. This increase may be substantially asymptotic from the process temperature to a higher target temperature over the time period of the film deposition step. Such an increase in temperature of the top surface  105  during the film deposition step yields higher deposition rates. Four points  631 - 634  may comprise a cool-down temperature trajectory  630  for the top surface  105 . 
     The second temperature parameter  570  may also have, for example, eight setpoints  573  defining a second temperature trajectory  602  for the bottom surface  107  across the entire film formation process. The processor  510  may utilize the first temperature parameter  560  to generate a first current target temperature  523 , and utilize the second temperature parameter  570  to generate a second current target temperature  524 . Monitoring of inputs, such as the top surface temperature input  531 , which receives first signals from the top surface temperature measuring device  132 , enables the processor  510  to generate a current top surface temperature  521 . Similarly, by monitoring the bottom surface temperature input  532 , which receives second signals from the bottom surface temperature measuring device  138 , the processor  510  may generate a current bottom surface temperature  522 . Of course, the current top surface temperature  521 , as well as the current bottom surface temperature  522 , may be a function of a plurality of inputs, as desired by the user. 
     Analogous to the previous embodiments, the processor  510  utilizes the first current target temperature  523  and the current top surface temperature  521  to send signals to the first cooling power control output  533  to control the first coolant regulator  431 , and hence to modulate the top surface  105  temperature according to the first temperature parameter  560 . Similarly, the processor  510  utilizes the second current target temperature  524  and the current bottom surface temperature  522  to send signals to the second cooling power control output  534  to control the second coolant regulator  439 , and hence to modulate the bottom surface  107  temperature according to the first temperature parameter  560 . Of course, the first temperature parameter  560  and the second temperature parameter  570  may be defined by respective cooling power levels rather than temperatures, as is done in the second embodiment, in which case it may not be necessary to monitor the current top surface temperature  521  or the current bottom surface temperature  522  to control the first coolant regulator  431  and the second coolant regulator  439 . 
     As shown in  FIG. 8 , it is possible to use a first variable speed blower  701 , and a second variable speed blower  702 , to respectively control the temperature of the top surface  105  and bottom surface  107  of the chamber walls  102 . With further reference to  FIG. 6 , the control logic  500  is equally suited to control the embodiment depicted in  FIG. 8 . First cooling power control output  533  may control the speed of first blower  701 , while second cooling power control output  534  may control the speed of second blower  702 . 
     With reference to  FIG. 9 , it is possible to independently control the temperature of the top surface  105  and the bottom surface  107  by changing the irradiancy bias between top lamps  802  and bottom lamps  804 . The top lamps  802  are disposed above the top surface  105  of the chamber walls  102 , and thus heat the top surface  105  while heating the substrate  106 . The bottom lamps  804  are disposed below the bottom surface  107  of the chamber walls  102 , and thus heat the bottom surface  107  while heating the substrate  106 . The combined irradiancy of the top lamps  802  and bottom lamps  804  determines the final temperature of the substrate  106 . If the irradiancy of the top lamps  802  is increased while the irradiancy of the bottom lamps  804  is decreased, it is possible to increase the temperature of the top surface  105  and decrease the temperature of the bottom surface  107 , while keeping the substrate  106  at the same temperature. Reversing this irradiancy bias will lead to heating of the bottom surface  107  and cooling of the top surface  105 , while maintaining the temperature of the substrate  106 . The irradiancy of the top lamps  802  may therefore be controlled independent of the irradiancy of the bottom lamps  804 . With reference to  FIG. 6 , independent control of the top lamps  802  and bottom lamps  804  permits the control circuit  500  to control the irradiancy bias between the top lamps  802  and bottom lamps  804 . First cooling power control output  533  may thus be used to control the speed of variable speed blower  140 , while second cooling power control output  534  may be used to control the irradiancy bias between the top lamps  802  and the bottom lamps  804 . It will be appreciated that second cooling power control output  534  may actually be two independent outputs that respectively control the irradiancy of the top lamps  802  and the bottom lamps  804 , and the difference between these two irradiancy outputs yields the irradiancy bias that preferentially heats and cools one of the surfaces  105 ,  107  over the other surface  107 ,  105 . 
     For example, to cool the bottom surface  107 , the processor  510  may control the second cooling power control output  534  so that the irradiancy of the top lamps  802  increases, while the irradiancy of the bottom lamps  804  decreases. From the temperature perspective of the substrate  106 , little has changed. However, from the point of view of the bottom surface  107 , as less radiant energy impinges upon the bottom surface  107 , the bottom surface  107  will begin to cool. It will be appreciated that, since more radiant energy will impinge upon the top surface  105 , the top surface  105  may begin to heat beyond its first temperature parameter  560 . In response to this, the processor  510  may control the first cooling power control output  533  to increase the speed of the variable speed blower  140  to cool the top surface  105 , which will incidentally lead to even more cooling of the bottom surface  107 . Similarly, reversing the bias can lead to heating of the bottom surface  107 . Hence, by using the second cooling power control output  534  to modulate the irradiancy bias between the lamps  802 ,  804 , the processor can selectively raise or lower the temperature of the bottom surface  107  with respect to the top surface  105 . 
     It is possible not only to temperature-modulate the top and bottom surfaces  105 ,  107  of the chamber walls  102 , but also to modulate side portions of the chamber walls  102 . For example, with reference to  FIG. 1 , with suitable control of inlet and outlet ducting, it is possible to control the respective temperatures of left adjacent top surface  151  and right adjacent top surface  152 . The adjacent top surface  151 ,  152  are adjacent to the top surface  105 , and hence adjacent to the top surface of the substrate  106 . Similarly, it is possible to temperature-modulate left adjacent bottom surface  153  and right adjacent bottom surface  154 , which are adjacent to the bottom surface  107 , and hence adjacent to the bottom surface of the substrate  106 . The control logic may be easily expanded to accommodate as many temperature parameters as there are individual chamber wall surface portions to temperature-modulate, and the method of doing so should be clear in light of this disclosure. 
     Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It will be apparent to those skilled in the art that various modifications and variations can be made to the method of the present invention without departing from the spirit and scope of the invention. Thus, it is intended that the present invention include modifications and variations that are within the scope of the appended claims and their equivalents.