FIELD OF THE INVENTION  
       [0001]     The present invention relates to reaction chambers used in the fabrication of integrated circuits on semiconductor wafer substrates. More particularly, the present invention relates to a system and-method for constraining temperatures of a substrate support in a reaction chamber within narrow limits to minimize thermal deviation of the substrate during reaction processes.  
       BACKGROUND OF THE INVENTION  
       [0002]     Integrated circuits are formed on a semiconductor substrate, which is typically composed of silicon. Such formation of integrated circuits involves sequentially forming or depositing multiple electrically conductive and insulative layers in or on the substrate. Etching processes may then be used to form geometric patterns in the layers or vias for electrical contact between the layers. Etching processes include “wet” etching, in which one or more chemical reagents are brought into direct contact with the substrate, and “dry” etching, such as plasma etching.  
         [0003]     Various types of plasma etching processes are known in the art, including plasma etching, reactive ion (RI) etching and reactive ion beam etching. In each of these plasma processes, a gas is first introducted into a reaction chamber and then plasma is generated from the gas. This is accomplished by dissociation of the gas into ions, free radicals and electrons by using an RF (radio frequency) generator, which includes one or more electrodes. The electrodes are accelerated in an electric field generated by the electrodes, and the energized electrons strike gas molecules to form additional ions, free radicals and electrons, which strike additional gas molecules, and the plasma eventually becomes self-sustaining. The ions, free radicals and electrons in the plasma react chemically with the layer material on the semiconductor wafer to form residual products which leave the wafer surface and thus, etch the material from the wafer.  
         [0004]     In the fabrication of semiconductor devices, particularly sub-micron scale semiconductor devices, profiles obtained in the etching process are very important. Careful control of a surface etch process is therefore necessary to ensure directional etching. In conducting an etching process, when an etch rate is considerably higher in one direction than in the other directions, the process is called anisotropic. A reactive ion etching (RIE) process assisted by plasma is frequently used in an anisotropic etching of various material layers on top of S semiconductor substrate. In plasma enhanced etching processes, the etch rate of a semiconductor material is frequently larger than the sum of the individual etch rates for ion sputtering and individual etching due to a synergy in which chemical etching is enhanced by ion bombardment.  
         [0005]     To avoid subjecting a semiconductor wafer to high-energy ion bombardment, the wafer may also be placed downstream from the plasma and outside the discharge area. Downstream plasma etches more in an isotropic manner since there are no ions to induce directional etching. The downstream reactors are frequently used for removing resist or other layers of material where patterning is not critical. In a downstream reactor, radio frequency may be used to generate long-lived radioactive species for transporting to a wafer surface located remote from the plasma. Temperature control problems and radiation damage are therefore significantly reduced in a downstream reactor. Furthermore, the wafer holder can be heated to a precise temperature to increase the chemical reaction rate, independent of the plasma.  
         [0006]     In a downstream reactor, an electrostatic wafer holding device known as an electrostatic chuck is frequently used. The electrostatic chuck attracts and holds a wafer positioned on top electrostatically. The electrostatic chuck method for holding a wafer is highly desirable in the vacuum handling and processing of wafers. An electrostatic chuck device can hold and move wafers with a force equivalent to several tens of Torr pressure, in contrast to a conventional method of holding wafers by a mechanical clamping method.  
         [0007]     Referring to the schematic of  FIG. 1 , a conventional plasma etching system is generally indicated by reference numeral  10 . The etching system  10  includes a reaction chamber  12  having a typically grounded chamber wall  14 . An electrode, such as a planar coil electrode  16 , is positioned adjacent to a dielectric plate  18  which separates the electrode  16  from the interior of the reaction chamber  12 . Plasma-generating source gases are introduced into the reaction chamber  12  by a gas supply (not shown). Volatile reaction products and unreacted plasma species are removed from the reaction chamber  12  by a gas removal mechanism, such as a vacuum pump (not shown).  
         [0008]     The dielectric plate  18  illustrated in  FIG. 1  may serve multiple purposes and have multiple structural features, as is well known in the art. For example, the dielectric plate  18  may include features for introducing the source gases into the reaction chamber  12 , as well as those structures associated with physically separating the electrode  16  from the interior of the chamber  12 .  
         [0009]     Electrode power such as a high voltage signal, provided by a power generator such as an RF (radio frequency) generator (not shown), is applied to the electrode  16  to ignite and sustain a plasma in the reaction chamber  12 . Ignition of a plasma in the reaction chamber  12  is accomplished primarily by electrostatic coupling of the electrode  16  with the source gases, due to the large-magnitude voltage applied to the electrode  16  and the resulting electric fields produced in the reaction chamber  12 . Once ignited, the plasma is sustained by electromagnetic induction effects associated with time-varying magnetic fields produced by the alternating currents applied to the electrode  16 . The plasma may become self-sustaining in the reaction chamber  12  due to the generation of energized electrons from the source gases and striking of the electrons with gas molecules to generate additional ions, free radicals and electrons. A semiconductor wafer  20  is positioned in the reaction chamber  12  and is supported by an ESC (electrostatic chuck)  22 . The ESC  22  is typically electrically-biased to provide ion energies that are independent of the RF voltage applied to the electrode  16  and that impact the wafer  20 .  
         [0010]     As further shown in  FIG. 1 , the plasma etching system  10  typically includes a temperature control system  23  which may include a chiller  24  that contains a supply of a coolant fluid  26 . The coolant fluid  26  is maintained at a desired set point temperature for the ESC  22  and the wafer  20 , typically about 60° C. A coolant delivery line  28  distributes the coolant fluid  26  to the ESC  22 , where the coolant is distributed throughout coolant channels (not shown) in the ESC  22  to maintain the ESC  22 , and thus, the wafer  20  supported thereon, at-the desired set point temperature. Typically, the set point temperature for the ESC  22  is 60° C., the same temperature as the coolant fluid  26 . After it is distributed through the ESC  22 , the coolant fluid  26  is returned to the chiller  24  through a coolant return line  30 . Accordingly, the coolant fluid  26  is continually circulated from the chiller  24 , through the ESC  22  and back to the chiller  24  to maintain the ESC  22 , and thus, the wafer  20 , at the desired set temperature.  
         [0011]     In the graph of  FIG. 2 , ESC temperature (progressing vertically along the Y-axis) is plotted as a function of reaction time (progressing rightward along the X-axis) which elapses during a typical plasma etch reaction. The horizontal line  32  represents the set point temperature for the ESC, typically about 60° C., whereas the angled line  34  represents a temporary elevation in ESC temperature during the plasma induction phase of the etching process. Accordingly, at t 1 , when the plasma induction phase begins, the temperature of the electrostatic chuck gradually rises by as many as 5 degrees Celsius or more, until the ESC temperature reaches a peak when the plasma induction phase ends, at t 2 . From t 2  to t 3 , the ESC temperature drops back to the set point temperature.  
         [0012]     For advanced semiconductor technology, precise temperature control is of utmost importance since unintended variations in process temperatures may result in excessive oxide growth on the substrate, among other considerations. Critical dimension (CD) shifts occur at a rate of over 1 nm (nanometer) per degree Celcius change in reaction temperature, and within-wafer CD shifts as great as 3 nm have been known due to process temperature variations. As device features become smaller and smaller, these unintended process temperature variations become increasingly problematic. Conventional temperature control methods and systems are capable of controlling unintended shifts in ESC temperatures to within about 5 degrees Celsius. Accordingly, a system and method is needed which is capable of controlling ESC temperature shifts to within 0.5 degrees Celsius.  
         [0013]     An object of the present invention is to provide a system and method for constraining temperatures of a substrate within desired limits.  
         [0014]     Another object of the present invention is to provide a system and method for preventing or minimizing unintended variations in temperature of a semiconductor wafer substrate during a plasma etch process.  
         [0015]     Still another object of the present invention is to provide a system and method which provides thermal compensation for elevated temperatures induced in an electrostatic chuck or other wafer holder during a semiconductor fabrication process.  
         [0016]     Yet another object of the present invention is to provide a system and method which eliminates or minimizes disparities in critical dimension (CD) of device features due to unintended temperature variations during a semiconductor fabrication process.  
         [0017]     A still further object of the present invention is to provide a system and method which provides compensation for elevated temperatures induced in an electrostatic chuck or other wafer holder as a result of plasma induction during a plasma etch process.  
       SUMMARY OF THE INVENTION  
       [0018]     In accordance with these and other objects and advantages, the present invention is generally directed to a system and method which is capable of compensating for unintended elevations in process temperatures induced in a substrate during a semiconductor fabrication process in order to reduce or eliminate disparities in critical dimensions of device features. The system may be a plasma etching system comprising a process chamber that contains an electrostatic chuck (ESC) for supporting a wafer substrate. A chiller outside the process chamber includes a main coolant chamber, which contains a main coolant fluid, as well as a compensation coolant chamber, which contains a compensation coolant fluid. A main circulation loop normally circulates the main coolant fluid from the main coolant chamber through the electrostatic chuck to maintain the chuck at a desired set point temperature during the etching process. When plasma induction begins in the process chamber, a compensation circulation loop circulates the compensation coolant fluid, which has a temperature less than that of the main coolant fluid, through the chuck, to cool the chuck and cancel the heating effects of the plasma. Consequently, the chuck; and thus, the wafer supported thereon, is substantially maintained at the set point temperature throughout the etching process. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     The invention will now be described, by way of example, with reference to the accompanying drawings, in which:  
         [0020]      FIG. 1  is a sectional schematic view of a typical conventional plasma etching system;  
         [0021]      FIG. 2  is a graph illustrating plasma-induced elevation of ESC temperatures during an etching process;  
         [0022]      FIG. 3  is a sectional schematic view of a plasma etching system of the present invention;  
         [0023]      FIG. 4  is a graph illustrating an actual temperature characteristic line achieved through use of the temperature control system of the present invention and a main temperature characteristic line and temperature compensation characteristic line shown as mirror images of each other on opposite sides of the actual temperature characteristic line  
         [0024]      FIG. 5  is a schematic view of another embodiment of a temperature control system of the present invention;  
         [0025]      FIG. 5A  is a cross-sectional view of a P/N junction module of the temperature control system of  FIG. 5 ;  
         [0026]      FIG. 6  is a graph illustrating closing of valves in the temperature control system plotted as a function of voltage applied to the valves; and  
         [0027]      FIG. 7  is a graph illustrating opening of valves in the temperature control system plotted as a function of voltage applied to the valves.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]     The present invention has particularly beneficial utility in preventing or minimizing plasma-induced elevations in process temperatures of a wafer substrate during a plasma dry etching process in the fabrication of semiconductor integrated circuits. However, the invention is not so limited in application, and while references may be made to such plasma etching processes, the invention is more generally applicable to maintaining process temperatures within desired limits in a variety of applications.  
         [0029]     Referring to  FIG. 3 , an illustrative embodiment of a plasma etching system in implementation of the present invention is generally indicated by reference numeral  40 . While the plasma etching system  40  is typically a dry etching system and may include the particular features hereinafter described, it is understood that the present invention may be equally applicable to process systems having features in addition to or other than those hereinafter described. Accordingly, the following description is not intended to limit the present, invention in any manner.  
         [0030]     The plasma etching system  40  includes a reaction chamber  42  having a typically grounded chamber wall  44 . An electrode, such as a planar coil electrode  46 , may be positioned adjacent to a dielectric plate  48  which separates the electrode  46  from the interior of the reaction chamber  42 . The dielectric plate  48  may serve multiple purposes and have multiple structural features, as is well known in the art. For example, the dielectric plate  48  may include features for introducing source gases into the reaction chamber  42 , as well as structures associated with physically separating the electrode  46  from the interior of the chamber  42 . An electrostatic chuck (ESC)  52  is included inside the reaction chamber  42  for supporting a semiconductor wafer  50  thereon during an etching process carried out on the wafer  50 , as hereinafter described. The ESC  52  is typically electrically-biased to provide ion energies that are independent of the RF voltage applied to the electrode  46  and that impact the wafer  50 .  
         [0031]     As further shown in  FIG. 3 , the plasma etching system  40  includes a temperature control system  54  in accordance with the present invention. The temperature control system  54  includes a chiller  56  that contains a main coolant chamber  58  which is separated from a compensation coolant chamber  60  by an internal partition  66  in the chiller  56 . In application, as hereinafter described, the main coolant chamber  58  contains a supply of main coolant fluid  59 , whereas the compensation coolant chamber  60  contains a supply of compensation coolant fluid  61 . In a typical embodiment, the main coolant chamber  58  has a volume of about 2-3 gallons, whereas the compensation coolant chamber  60  has a volume of about ¼ the volume of the main coolant chamber  58 , typically about ½ gal-¾ gal.  
         [0032]     A main circulation loop  67  of the temperature control system  54  includes a main coolant delivery line  62  that confluently connects the main coolant chamber  58  of the chiller  56  to the ESC  52  of the reaction chamber  42 , typically through a delivery line valve  70 , which may be a solenoid valve. The main coolant delivery line  62  is disposed in fluid communication with a network of main coolant channels  82  which are distributed throughout the ESC  52  for substantially uniformly imparting a temperature of the main coolant  59  to the ESC  52  as the main coolant  59  flows through the main coolant channels  82 , as hereinafter further described. The main circulation loop  67  further includes a main coolant return line  63  that confluently connects the main coolant channels  82  in the ESC  52  to the main coolant chamber  58  typically through a return line valve  71 , which may be a solenoid valve. The main coolant delivery line  62  may be confluently connected to the main coolant return line  63  through a line connecting valve  79 . A controller  89  for the plasma etching system  40  may be operably connected to the delivery line valve  70  and return line valve  71  for automatic operation of the valves  70  and  71 , respectively.  
         [0033]     A compensation circulation loop  68  of the temperature control system  54  includes a compensation coolant delivery line  64  that confluently connects the compensation coolant chamber  60  of the chiller  56  to the ESC  52  of the reaction chamber  42 , typically through a typically solenoid delivery line valve  73  which is typically operably connected to the controller  89  for automatic operation. The compensation coolant delivery line  64  is disposed in fluid communication with a network of compensation coolant channels  83  which are distributed throughout the ESC  52  for absorption of heat energy from the ESC  52  by the compensation coolant fluid  61  as the compensation coolant fluid  61  flows through the compensation coolant channels  83 , as hereinafter further described. The compensation circulation loop  68  further includes an compensation coolant return line  65  that confluently connects the ESC  52  back to the compensation coolant chamber  60  typically through a typically solenoid return line valve  74  which is typically operably connected to the controller  89  for automatic operation. The compensation coolant delivery line  64  may be confluently connected to the compensation coolant return line  65  through a line connecting valve  80 . An interchamber line  76 , typically fitted with an interchamber valve  77 , may confluently connect the main coolant chamber  58  directly to the compensation coolant chamber  60 .  
         [0034]     Referring again to  FIG. 3 , in application of the temperature control system  54 , the main coolant chamber  58  contains a supply of the main coolant fluid  59 , whereas the compensation coolant chamber  60  contains a supply of the compensation coolant fluid  61 . The main coolant fluid  59  and the compensation coolant fluid  61  may be any type of cooling fluid including but not limited to water. The main coolant fluid  59  is maintained at a desired set point temperature for the ESC  52  and the wafer  50  in a plasma etch process, typically about 60° C., whereas the compensation coolant fluid  61  is maintained at a temperature which is about 5° C. to about 10° C. lower than the main coolant fluid  59 , typically at about 50° C. The semiconductor wafer  50  placed on the ESC  52  for etching of a layer or layers on the wafer  50 .  
         [0035]     As the etching process commences, the reaction chamber  42  is heated to the predetermined set point temperature, such as 60° C., for optimal etching of the wafer  50 . Simultaneously, the main coolant fluid  59 , maintained at the set point temperature (60° C. in this case) in the main coolant chamber  58  of the chiller  56 , is continually circulated from the main coolant chamber  58 , through the main coolant delivery line  62  and open delivery line valve  70 , respectively, and distributed throughout the main coolant channels  82  of the ESC  52 , as the delivery line valve  70  and the return line valve  71  remain open typically by operation of the controller  89 . The main coolant fluid  59  is finally returned to the main coolant chamber  58  through the open return line valve  71  and the main coolant return line  63 . As it circulates through the main coolant channels  82 , the main coolant  59  maintains the ESC  52  and the wafer  50  supported thereon at the 60° C. set point temperature for optimum etching of the wafer  50 . While the main coolant fluid  59  is continually circulated through the main circulation loop  67 , the compensation coolant fluid  61  initially remains in the compensation coolant chamber  60 , as the delivery line valve  73  and the return line valve  74  of the compensation circulation loop  68  remain closed typically by the controller  89 .  
         [0036]     At the beginning of the plasma-induction phase of the etching process, plasma-generating source gases are introduced into the reaction chamber  42  by a gas supply (not shown), typically in conventional fashion. Volatile reaction products and unreacted plasma species are removed from the reaction chamber  42  by a gas removal mechanism, such as a conventional vacuum pump (not shown). Electrode power such as a high voltage signal, provided by a power generator such as an RF (radio frequency) generator (not shown), is applied to the electrode  46  to ignite and sustain a plasma in the reaction chamber  42 . Ignition of a plasma in the reaction chamber  42  is accomplished primarily by electrostatic coupling of the electrode  46  with the source gases, due to the large-magnitude voltage applied to the electrode  46  and the resulting electric fields produced in the reaction chamber  42 . Once ignited, the plasma is sustained by electromagnetic induction effects associated with time-varying magnetic fields produced by the alternating currents applied to the electrode  46 . The plasma may become self-sustaining in the reaction chamber  42  due to the generation of energized electrons from the source gases and striking of the electrons with gas molecules to generate additional ions, free radicals and electrons.  
         [0037]     Formation of the plasma causes an inherent temperature rise inside the reaction chamber  42 , and this increase in temperature in the reaction chamber  42  in turn tends to raise the temperature of the ESC  52  and the wafer  50  by convection and must be counteracted for optimum etching of the wafer  50 . Accordingly, at the same time the plasma induction phase of the etching process begins, the controller  89  autmatically opens the delivery line valve  73  and the return line valve  74  of the compensation circulation loop  68 . The compensation coolant fluid  61 , maintained at the cooling temperature (50° C. in this case) in the compensation coolant chamber  60  of the chiller  56  is continually circulated from the compensation coolant chamber  60 , through the compensation coolant delivery line  64  and open delivery line valve  73 , respectively, and distributed throughout the compensation coolant channels  83  in the ESC  52 . As it is continually distributed throughout the compensation coolant channels  83  in the ESC  52 , the compensation coolant fluid  61  absorbs excess heat imparted to the ESC  52  by the plasma and thus, maintains the ESC  52 , and thus, the wafer  50  supported thereon, substantially at the desired set point temperature. The compensation coolant fluid  61  is returned to the compensation coolant chamber  60  through the open return line valve  74  and the compensation coolant return line  65 , where it is cooled back to the cooling temperature (50° C. in this case) and re-circulated through the compensation circulation loop  68 . Coolant fluid may be distributed from the main coolant chamber  58 , through the interchamber line  76  and into the compensation coolant chamber  60 , as needed, by opening the interchamber valve  77 .  
         [0038]     In the graph  84  of  FIG. 4 , ESC temperature (progressing vertically along the Y-axis) is plotted as a function of reaction time (progressing rightward along the X-axis) which elapses during a plasma etch reaction in implementation of the temperature control system  54  of the present invention. The horizontal line  85  represents the set point temperature for the ESC  85  during the plasma etching process (60° C. in this case), whereas the downwardly-sloped temperature compensation characteristic curve  86  represents the temperature of the ESC  85  which would be caused by the cooling effects of the temperature control system  54  in the absence of a plasma-induction phase during the etching process. The upwardly-sloped main temperature characteristic curve  87  represents an elevation in ESC temperature which would otherwise occur during the plasma induction phase of the etching process without the cooling effects of the temperature control system  54 . When the plasma induction phase begins, as indicated at t 1 , thereby elevating process temperatures in the reaction chamber, the temperature of the electrostatic chuck remains substantially constant, typically at 60° C.,±0.5° C. This set point temperature is maintained through the end of the plasma etching phase, at t 2 , and through completion of the etching process at t 3 .  
         [0039]     According to a method of the present invention, a main temperature characteristic curve  87  on a graph  84 , having ESC temperature plotted vs. time, is first obtained by operating the plasma etching system  40  and cooling the ESC  52  using the main coolant fluid  59  without the compensation coolant fluid  61 . A temperature compensation characteristic curve  86  is then obtained by forming a mirror reflection of the main temperature characteristic curve  87  below the horizontal set point temperature line  85 . Accordingly, the main temperature characteristic curve  87  and the temperature compensation characteristic curve  86  are symmetrical with respect to each other above and below, respectively, the horizontal set point line  85 . The temperature control system  54  is then operated according to the temperature compensation characteristic curve  86  to maintain the ESC  52  at a substantially constant set point temperature as indicated by the horizontal line  85 .  
         [0040]     Referring next to  FIG. 5-9 , another embodiment of the temperature control system  120  of the present invention includes a main coolant tank  122  which contains a supply of main coolant  123  and a compensation coolant tank  124  which contains a supply of compensation coolant  125 . A main coolant delivery line  126  connects the main coolant tank  122  in fluid communication with coolant channels  111  extending through an electrostatic chuck (ESC)  110  of a plasma etch system  104  to be cooled in a process chamber  108 , for example, as heretofore described with respect to  FIG. 3 . A main coolant return line  128  further connects the ESC  110  in fluid communication with the main coolant tank  122 .  
         [0041]     A compensation coolant delivery line  132  connects the compensation coolant tank  124  to the main coolant delivery line  126 . A valve  131  may be provided in the compensation coolant delivery line  132 . A compensation coolant return line  130  extends from the main coolant return line  128  and is provided in fluid communication with the compensation coolant tank  124 . A valve  133  may be provided in the compensation coolant return line  130 . A circulation valve  134  may be provided between the compensation coolant delivery line  132  and the compensation coolant return line  130  to facilitate circulation of compensation coolant  124  through the compensation coolant delivery line  132 , valve  134 , compensation coolant return line  130  and back into the compensation coolant tank  124 , respectively.  
         [0042]     A P/N junction module  136  is provided in thermal contact with the ESC  110  and is operably connected to a power supply  114  through wiring  112 . The power supply  114  is connected to a controller  116 , which is electrically connected to the valve  131 , valve  133  and circulation valve  134  through wiring  118 . As hereinafter described, the P/N junction module  136  measures the temperature of the coolant flowing through the coolant channels  111  in the ESC  110  and opens or closes the valve  131 , the valve  133  and/or the circulation valve  134 , through the controller  116  as necessary to micro-adjust the temperature of the ESC  110 .  
         [0043]     As shown in  FIG. 5A , the P/N junction module  136  includes spaced-apart sheets of electrical insulation  137  and a typically copper, electrically-conductive sheet  138  provided on the inner surface of each electrical isulation sheet  137 . Multiple p-type semiconductors  139   a  and n-type semiconductors  139   b  are sandwiched between the electrically-conductive sheets  138 . The wiring  112  is connected to the respective electrically-conductive sheets  138 .  
         [0044]     Referring to  FIGS. 5, 8  and  9 , in application of the temperature control system  120 , the main coolant fluid  123  is maintained at a desired set point temperature for the ESC  110  in a plasma etch process, typically about 60° C., whereas the compensation coolant  125  is maintained at a temperature which is about 5° C. to about 10° C. lower than the main coolant fluid  123 , typically at about 50° C. A semiconductor wafer  106  is placed on the ESC  110  for etching of a layer or layers on the wafer  106  in the plasma etch system  104 . As the etching process commences, the reaction chamber  108  is heated to the predetermined set point temperature, such as 60° C., for optimal etching of the wafer  106 . The P/N junction module  136 , through the controller  116 , normally maintains a potential of zero voltage to the valves  131 ,  133  and  134 , respectively, such that the valves  131 ,  133  are closed, as shown in  FIG. 9 , and the valve  134  is open, as shown in  FIG. 8 . Accordingly, the main coolant fluid  123 , maintained at the set point temperature (60° C. in this case) in the main coolant chamber  122 , is continually circulated from the main coolant chamber  122 , through the main coolant delivery line  126  and distributed throughout the main coolant channel  111  of the ESC  110 , as the valve  131  and the valve  133  remain closed typically by operation of the controller  116 . The main coolant fluid  123  is finally returned to the main coolant chamber  122  through the main coolant return line  128 . As it circulates through the main coolant channels  111 , the main coolant  123  maintains the ESC  110  and the wafer  106  supported thereon at the 60° C. set point temperature for optimum etching of the wafer  106 . While the main coolant fluid  123  is continually circulated through the main circulation channel  111 , the compensation coolant fluid  115  initially remains in the compensation coolant chamber  124 , as the valve  131  of the compensation coolant delivery line  132  and the valve  133  of the compensation coolant return line  130  remain closed typically by the controller  116 .  
         [0045]     At the beginning of the plasma-induction phase of the etching process, plasma-generating source gases are introduced into the reaction chamber  108  by a gas supply (not shown), typically in conventional fashion. Formation of the plasma causes an inherent temperature rise inside the reaction chamber  108 , and this increase in temperature in the reaction chamber  108  in turn tends to raise the temperature of the ESC  110  and the wafer  106 . Accordingly, the P/N junction module  136  senses the temperature of the ESC  136  and causes the controller  116  to apply a positive voltage to the valves  131 ,  133  and  134 , respectively. As shown in  FIG. 8 , this causes the valve  134  to close to a degree which depends on the magnitude of the voltage applied to the valve  134 . Simultaneously, as shown in  FIG. 9 , the positive voltage applied to the valves  131 ,  133  causes these valves to open the compensation coolant delivery line  132  and the compensation coolant return line  130 , respectively, to a degree which depends on the magnitude of the voltage applied to the valves  131 ,  133 . The compensation coolant  125 , maintained at the cooling temperature (50° C. in this case) in the compensation coolant chamber  124 , is continually circulated from the compensation coolant chamber  124 , through the compensation coolant delivery line  132  and open valve  131 , respectively, and main coolant delivery line  126 , and distributed throughout the coolant channels  111  in the ESC  110 . As it is continually distributed throughout the coolant channel  111  in the ESC  110 , the compensation coolant fluid  125  absorbs excess heat imparted to the ESC  110  by the plasma and thus, maintains the ESC  110 , and thus, the wafer  106  supported thereon, substantially at the desired set point temperature. The compensation coolant fluid  125  is returned to the compensation coolant chamber  125  through the open valve  133  and the compensation coolant return line  130 , where it is cooled back to the cooling temperature (50° C. in this case) and re-circulated through the coolant channels  111 .  
         [0046]     As the compensation coolant  125  is circulated through the coolant channels  111 , the P/N junction module  136  continually senses the temperature of the ESC  110 . When the temperature of the ESC  110  rises above the set point temperature, the P/N junction module  136  applies a correspondingly higher voltage to the valves  131 ,  133 , thereby opening these valves to facilitate distribution of a correspondingly larger volume of compensation coolant  125  through the coolant channels  111 , as shown in  FIG. 9 . This maintains the ESC  110  at the set point temperature and facilitates micro-adjustment of the temperature of the ESC  110 .  
         [0047]     Referring again to  FIG. 4 , according to a method of the present invention, a main temperature characteristic curve  87  on a graph  84 , having ESC temperature plotted vs. time, is first obtained by operating the plasma etching system  104  and cooling the ESC  110  using the main coolant fluid  123  without the compensation coolant fluid  125 . A temperature compensation characteristic curve  86  is then obtained by forming a mirror reflection of the main temperature characteristic curve  87  below the horizontal set point temperature line  85 . The temperature control system  120  is then operated according to the temperature compensation characteristic curve  86  to maintain the ESC  110  at a substantially constant set point temperature as indicated by the horizontal line  85 .  
         [0048]     While the preferred embodiments of the invention have been described above, it will be recognized and understood that various modifications can be made in the invention and the appended claims are intended to cover all such modifications which may fall within the spirit and scope of the invention.
A system and method which is capable of compensating for unintended elevations in process temperatures induced in a substrate during a semiconductor fabrication process in order to reduce or eliminate disparities in critical dimensions of device features. The system may be a plasma etching system comprising a process chamber containing an electrostatic chuck (ESC) for supporting a wafer substrate. A chiller outside the process chamber includes a main coolant chamber, which contains a main coolant fluid, as well as an compensation coolant chamber, which contains an compensation coolant fluid. A main circulation loop normally circulates the main coolant fluid from the main coolant chamber through the electrostatic chuck to maintain the chuck at a desired set point temperature.