Abstract:
The embodiments described herein generally relate to methods of noise compensation for proper temperature detection in thermal processing chambers and devices for achieving the same. Methods can include determining noise produced by a lamp zone and extrapolating the noise from the detected photocurrent. Devices can include a processing chamber, a substrate support disposed in the processing chamber, the substrate support having a high thermal mass, a pyrometer below the substrate support and oriented to view radiation emitted by the substrate and a controller configured to subtract a time invariant noise component and a time variant noise component from the pyrometer signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims benefit of U.S. Provisional Patent Application Ser. No. 61/818,158, filed May 1, 2013 (APPM/20379USL), which is incorporated by reference herein. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    Embodiments disclosed herein generally relate to pyrometry in rapid thermal processing chambers. 
         [0004]    2. Description of the Related Art 
         [0005]    Rapid thermal processing (RTP) and epitaxial deposition (Epi) systems are employed in semiconductor chip fabrication to create, chemically alter or etch surface structures on semiconductor wafers. RTP and Epi typically depend upon an array of high-intensity incandescent lamps fit into a lamphead and directed at the substrate or wafer. The lamps are electrically powered and can be very quickly turned off and on and a substantial fraction of their radiation can be directed to the substrate. As a result, the wafer can be very quickly heated without substantially heating the chamber and can be nearly as quickly cooled once the power is removed from the lamps. 
         [0006]    A number of infrared lamps are located in the lamphead. During processing, radiation from the lamps radiates through an upper window, light passageways and a lower window onto a rotating semiconductor substrate in the processing chamber. In this manner, the wafer is heated to a required processing temperature. The lamphead may include a number of light pipes to deliver highly collimated radiation from tungsten-halogen lamps to the processing chamber. The lamps are divided into multiple zones, which are located in a radially symmetrical manner. Each zone is separately powered by a lamp driver that is, in turn, controlled by a multi-input, multi-output controller. The lamps are connected to the lamp drivers through a large wiring collar and heavy-duty electrical cabling. 
         [0007]    During a heating process, the heated substrate releases radiation which is received by one or more pyrometers. The radiation received is used to determine the temperature of the substrate. However, it is believed that other radiation sources, such as the lamps, produce noise which distorts the detected radiation at the pyrometer. This noise combines with the radiation from the substrate to create a combination photocurrent which represents the detection of both the heat and the noise at the pyrometer. One approach to address the problem is to mechanically or optically shield or block stray radiation to the pyrometers using shield rings or spectrally selective windows or lenses. However, mechanical shielding or blocking can be insufficient to prevent noise while allowing proper radiation to pass. Further, for some applications that require the pyrometer to be on the same side as the lamphead, such shielding approaches are not feasible. 
         [0008]    Thus, there is a need in the art for correction of noise detected at a pyrometer during thermal processing. 
       SUMMARY OF THE INVENTION 
       [0009]    The embodiments described herein generally relate to noise correction of radiation as detected by a pyrometer during thermal processing. 
         [0010]    In one embodiment, a method of noise correction can include measuring lamp noise detected by a pyrometer in a process chamber comprising one or more pyrometers, repeating the performance of the measurement using each of the one or more lamp zones and at each of the one or more pyrometers until each of the one or more pyrometers has been measured with relation to each of the one or more lamp zones, calculating a linear relation between the steady state values for each of the one or more lamp zones and each of the one or more pyrometers, subtracting the steady state values and the first order time-response values from a measured value detected by the one or more pyrometers to create an adjusted value and determining an actual temperature based on the adjusted value. The measurement can include setting the initial voltages to each of the one or more lamp zones at 0% of the maximum voltage, wherein the lamp zones comprise at least a first lamp zone, measuring the photocurrent at a first pyrometer, once the photocurrent is less than 1*10-7 Amps, supplying a first voltage which is no greater than 10% of the maximum voltage to the first lamp zone, maintaining the first voltage for a first period of time, wherein the photocurrent is measured by the first pyrometer during the first period of time, supplying a second voltage which is greater than the first voltage by no more than 60%, maintaining the second voltage for a second period of time, wherein the second period of time is less than the first period of time, and wherein the photocurrent is measured by the pyrometer during the second period of time, repeating the steps of supplying the first voltage, maintaining the first voltage, supplying the second voltage and maintaining the second voltage one or more times to collect a plurality of first order time-response values and averaging at least a portion of each of the first order time response values to create a steady state value for each of the first order time response values. 
         [0011]    In another embodiment, a device can include a processing chamber, a substrate support disposed in the processing chamber, the substrate support having a high thermal mass, a pyrometer positioned below the substrate support and oriented to view radiation emitted by the substrate and create a pyrometer signal using the viewed radiation and a controller configured to subtract a time invariant noise component and a time variant noise component from the pyrometer signal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
           [0013]      FIG. 1  illustrates a schematic sectional view of a backside heating process chamber according to one embodiment; 
           [0014]      FIG. 2  is a schematic depiction of the model-based noise estimation concept according to one embodiment; 
           [0015]      FIGS. 3A and 3B  are a flow diagram of a method for measuring lamp noise according to one embodiment; 
           [0016]      FIG. 4  is a flow diagram of a method for noise correction according to one embodiment; 
           [0017]      FIG. 5  is graph depicting the detection of the first order transient response at a pyrometer according to one embodiment; and 
           [0018]      FIG. 6  is a graph depicting the steady state value at a pyrometer according to one embodiment. 
       
    
    
       [0019]    To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation. 
       DETAILED DESCRIPTION 
       [0020]    Embodiments disclosed herein generally relate to semiconductor processing and more specifically to reduction of the effect of noise as detected in an Epi or a RTP chamber. During a heating process, the pyrometers receive both radiation from the components of the chamber, including the substrate, and radiation from the lamps. Embodiments described herein incorporate the measured raw photocurrent signal and the first order transient response during heat up and cool down to separate lamp noise radiation from radiation received from the heated substrate. Collected data regarding steady state value and first order time-response value can be determined at a single point in time, and adjusted at points in time corresponding to the life cycle of the lamps or at other points in time as desired by the user. In this way, the temperature of the substrate and surrounding structures can be separated from noise produced by the lamps. The embodiments of the inventions disclosed herein are more clearly described with reference to the figures below. 
         [0021]      FIG. 1  illustrates a schematic sectional view of a backside heating process chamber  100  according to one embodiment. One example of the process chamber that may be adapted to benefit from the invention is an Epi process chamber, which can be mounted on a Centura or Producer platform, all available from Applied Materials, Inc., located in Santa Clara, Calif. It is contemplated that other processing chambers, including those from other manufacturers, may be adapted to practice the present invention. 
         [0022]    The process chamber  100  may be used to process one or more substrates, including the deposition of a material on an upper surface of a substrate  108 . The process chamber  100  generally includes an array of radiant heating lamps  102  for heating, among other components, a back side  104  of a substrate support  106  disposed within the process chamber  100 . The substrate support  106  may be a disk-like substrate support  106 , or may be a ring-like substrate support (not shown), which supports the substrate from the edge of the substrate or may be a pin-type support which supports the substrate from the bottom by minimal contact posts or pins. 
         [0023]    The substrate support  106  is located within the process chamber  100  between an upper dome  128  and a lower dome  114 . The upper dome  128  and the lower dome  114 , along with a base ring  136  that is disposed between the upper dome  128  and lower dome  114 , generally define an internal region of the process chamber  100 . The substrate  108  (not to scale) can be brought into the process chamber  100  and positioned onto the substrate support  106  through a loading port (not shown), which is obscured by the substrate support  106 . 
         [0024]    The base ring  136  generally includes the loading port, a process gas inlet  174 , and a gas outlet  178 . The base ring  136  may have a generally oblong shape with the long side on the loading port and the short sides on the process gas inlet  174  and the gas outlet  178 , respectively. The base ring  136  may include one or more coolant flow channels (not shown) disposed in proximity to O-rings  182 ,  184  for cooling of the base ring. 
         [0025]    The substrate support  106  can divide the internal volume of the process chamber  100  into a process gas region  156  that is above the substrate, and a purge gas region  158  below the substrate support  106 . The substrate support  106  is rotated during processing by a central shaft  132  to minimize the effect of thermal and process gas flow spatial anomalies within the process chamber  100  and thus facilitate uniform processing of the substrate  108 . The substrate support  106  is supported by the central shaft  132 , which moves the substrate  108  in an up and down direction  134  during loading and unloading, and in some instances, processing of the substrate  108 . The substrate support  106  may be formed from silicon carbide or graphite coated with silicon carbide to absorb radiant energy from the lamps  102  and conduct the radiant energy to the substrate  108 . 
         [0026]    In general, the central window portion of the upper dome  128  and the bottom of the lower dome  114  are formed from an optically transparent material such as quartz. The thickness and the degree of curvature of the upper dome  128  may be configured to provide a flatter geometry for uniform flow uniformity in the process chamber. 
         [0027]    A reflector  122  may be optionally placed outside the upper dome  128  to reflect infrared light that is radiating off the substrate  108  back onto the substrate  108 . The reflector  122  may be secured to the upper dome  128  using a clamp ring  130 . The reflector  122  can have one or more machined channels  126  connected to a cooling source (not shown) for cooling the reflector  122 . 
         [0028]    Process gas supplied from a process gas supply source  172  can be introduced into the process gas region  156  through a process gas inlet  174 . The process gas can exit the process gas region  156  through a gas outlet  178  located on the opposite side of the process chamber  100  as the process gas inlet  174 . Removal of the process gas through the gas outlet  178  may be facilitated by a vacuum pump  180  coupled thereto. Purge gas supplied from a purge gas source  162  can be introduced to the purge gas region  158  through a purge gas inlet  164  formed in the sidewall of the base ring  136 . The purge gas can be exhausted out of the process chamber through the gas outlet  178  located on the opposite side of the process chamber  100  as the purge gas inlet  164 . 
         [0029]    One or more lamps, such as an array of lamps  102 , can be disposed adjacent to and beneath the lower dome  114  in a specified manner around the central shaft  132  to independently control the temperature at various regions of the substrate  108  as the process gas passes over, thereby facilitating the deposition of a material onto the upper surface of the substrate  108 . The lamps  102  may include a heat generating element, depicted here as a lamp bulb  141 , and be configured to heat the substrate  108  to a temperature within a range of about 200 degrees Celsius to about 1600 degrees Celsius. In a further embodiment, the heat generating element can include non-incandescing solid radiators, such as an LED, or further radiation producing devices. 
         [0030]    Each of the lamps  102  can be coupled to a power distribution board, such as printed circuit board (PCB)  152 , through which power is supplied to each of the lamps  102 . The lamps  102  are positioned within a lamphead  145  which may be cooled during or after processing by, for example, a cooling fluid introduced into channels  149  located between the lamps  102 . The lamphead  145  conductively and radiatively cools the lower dome  104  due in part to the close proximity of the lamphead  145  to the lower dome  104 . The lamphead  145  may also cool the lamp walls and walls of the reflectors (not shown) around the lamps. Alternatively, the lower dome  104  may be cooled by a convective approach. The positioning of the lamps  102  in the lamphead  145  and on the PCB  152  form lamp zones  202 , which are described with reference to  FIG. 2  below. Each lamp zone  202  is separately powered by a lamp driver that is, in turn, controlled by a controller (not shown), such as a multi-input, multi-output controller. 
         [0031]    A circular shield  167  may be optionally disposed around the substrate support  106  and coupled to sidewall of the chamber body  101 . The circular shield  167  prevents or minimizes leakage of heat/light noise from the lamps  102  to the device side  116  of the substrate  108  in addition to providing a pre-heat zone for the process gases. The circular shield  167  may be made from CVD SiC, sintered graphite coated with SiC, grown SiC, opaque quartz, coated quartz, or any similar, suitable material that is resistant to chemical breakdown by process and purging gases. 
         [0032]    The temperature of the substrate  108  can be inferred from pyrometer measurements of the bottom of the substrate support  106  by back pyrometers  154 . The back pyrometers  154  can be in optical connection with ports  190  formed in the lamphead  145 . One or more pyrometers  154  can be used to detect radiation from the substrate and thereby determine the temperature of the substrate. In conjunction with or in lieu of the back pyrometers  154 , one or more front side pyrometers, such as front pyrometer  118 , can be used for temperature measurements on the substrate. The front pyrometer  118  can sense radiation from the hot substrate  108  with minimal background radiation from the lamps  102  directly reaching the optical pyrometer  118  due to the circular shield  167 . Though the front pyrometer  118  and the back pyrometers  154  are depicted as one and two pyrometers respectively, it is understood that the number of pyrometers depicted are not intended to be limiting of possible embodiments. Any number of pyrometers (including no pyrometers) may be used on either the front side, the back side or combinations thereof. In one embodiment, the process chamber  100  has four back pyrometers  154  configured in optical connection with the substrate  108  or the substrate support  106 . 
         [0033]    Without intending to be bound by theory, it is believed that the lamps  102  can create noise in the pyrometer measurements as received by either front side optical pyrometer (e.g. front pyrometer  118 ) or back side optical pyrometers (e.g. back pyrometers  154 ). It is believed that radiation from the lamps  102  may reflect to some degree either back to the back pyrometers  154  or between contact points between the shield  167  and the substrate support  106 . As such, either the front side pyrometer approach, the back side pyrometer approach or both may require corrections to be made for reflected radiation from the lamps  102  particularly at low substrate temperatures. 
         [0034]    The PCB  152  and the pyrometers are in connection with the controller. The controller determines a noise value at a time point, such as every control cycle. The controller then uses the noise value to adjust a set point for the temperature controller. 
         [0035]      FIG. 2  is a schematic depiction of the model-based noise estimation concept according to one embodiment. Only components that assist in the explanation are included and it is understood that certain components which may be utilized in one or more embodiments are deleted for sake of clarity. Depicted here, a plurality of lamp zones  202  are configured to deliver radiant energy  204  to the substrate support  106 . The radiant energy  204  is both received by the substrate support  106  and reflected by the substrate support  106 . Though the radiant energy  204  is depicted as being delivered at an angle, a specific directionality or orientation is not necessary for the functioning of one or more embodiments. In one or more embodiments, the substrate  108  is exposed directly to the radiant energy  204  and thus will both receive and reflect the radiant energy in place of the substrate support  106 . 
         [0036]    The lamp zones  202  are clusters of one or more lamps  145  which can receive voltage as a group through the PCB  152 . Thus, the lamp zones  202  can deliver radiant energy to the process chamber  100  as a group. In embodiments described herein, the process chamber can be configured with one or more lamp zones  202 . In this embodiment, the process chamber  100  is depicted as having six lamp zones  202 , though it is understood that more or fewer lamp zones  202  with various quantities of lamps can be used. 
         [0037]    The radiant energy  204  which is received by the substrate support  106  is absorbed and converted to heat. The heat can both heat the substrate support  106 , the substrate  108  or combinations thereof. A portion of the radiant energy  204  which is reflected from the substrate support  106  can then be received by the back pyrometer  154 . Simultaneously, the substrate support  108  can radiate thermal radiation  206 , some of which is received by the back pyrometer  154 . The portion of radiant energy  204  which is in the detected wavelength (referred to herein as “lamp noise”) as well as the thermal radiation  206  are detected by the back pyrometer  154  and used to calculate the temperature of the substrate  108 . 
         [0038]    The true temperature of the substrate  108  is determined by the thermal radiation  206 . As the lamp noise received by the back pyrometer  154  is not related to the temperature of the substrate, any signal received in the absence of heating the chamber is believed to be related to lamp noise alone. Therefore, embodiments described herein differentiate between lamp noise by performing short, non-heating cycles. The detected signal can be used to determine both the steady state value and first order time-response model of the noise. 
         [0039]      FIGS. 3A and 3B  are a flow diagram of a method  300  for measuring lamp noise according to one embodiment. The method  300  begins at step  302  with the initial voltages to each of the one or more lamp zones being set to 0% of the maximum voltage. As described with reference to  FIGS. 1 and 2 , the process chamber can have one or more lamp zones. The one or more lamp zones comprise at least a first lamp zone. The lamp zones can comprise a plurality of lamps. The lamps can be divided approximately equally among the zones. In one embodiment, the process chamber has a total of 400 lamps in the lamphead with six lamp zones of approximately 66 lamps per zone. 
         [0040]    The photocurrent is then measured at a first pyrometer, as in step  304 . The photocurrent received by the pyrometer can vary based on whether the process chamber has been recently heated. The photocurrent measured at this point for a process chamber should include only radiance based on the ambient temperature from the environment. The photocurrent at this temperature is equal to the dark current offset of the pyrometer. A substrate can be present in the chamber during the steps of method  300 . It is understood that the first pyrometer can be either a back pyrometer or a front pyrometer, as described with reference to  FIG. 1 . 
         [0041]    The first voltage is supplied once the photocurrent is less than the dark current offset of the pyrometer. The dark current offset of the pyrometer is the detectable current by the pyrometer at ambient temperatures. In one embodiment, the dark current offset for a long-wave 3.3 um pyrometer is less than 1*10 −7  Amps. However, dark current offset may vary with different pyrometers. 
         [0042]    Once the detected photocurrent is less than the dark current offset of the pyrometer, a first voltage which is no greater than 10% of the maximum voltage can be supplied to the first lamp zone, as in step  306 . The voltage is applied to the lamps through an electrical connection, such as the PCB described with reference to  FIG. 1 . The voltage can be less than or equal to 5% of the maximum voltage in some embodiments. In one embodiment, the voltage supplied to the first lamp zone is 5% of the maximum voltage. Once a voltage is applied, the lamps will deliver radiant energy to the chamber, which can be absorbed by the chamber components or the substrate. 
         [0043]    The first voltage can then be maintained for a first period of time, as in step  308 . The first voltage causes the lamps to produce radiant energy which will be absorbed by components in the chamber and lamp noise which will be received by the first pyrometer. Thermal radiation from the chamber and lamp noise from the lamps produce a photocurrent at the first pyrometer. The photocurrent is then measured by the first pyrometer during the first period of time. The first period of time is a short period which does not significantly heat up the chamber. The first period of time can be between 120 and 250 seconds, such as from 150 and 180 seconds. In one embodiment, the first time period is 180 seconds. The photocurrent can reach an approximate peak level during the first period of time which sets a base line for further noise measurement. 
         [0044]    Without intending to be bound by theory, it is believed that maintaining a low voltage at the lamps prevents variability in noise measurement. The lamps are not believed to provide a constant signal from a cold state (when the lamps are at equilibrium with the environment). When detecting very small photocurrent changes, such as those produced by lamp noise, small differences between the lamps, the connections, or other components can create variability in the initial behavior of the lamps in each lamp zone. This variability is believed to be muted once the lamps have been minimally active for a short period of time. Thus, by applying a small voltage to the lamps, the early variability in measured photocurrent during heat up can be avoided. 
         [0045]    A subsequent voltage which can be greater than the first voltage, such as by no more than 60%, can then be supplied, as in step  310 . The subsequent voltage can be any voltage between the first voltage and about 70%, such as from about 1% to about 60%, from about 2% to about 60%, from about 3% to about 60%, from about 4% to about 60%, from about 5% to about 60%, from about 6% to about 60%, from about 7% to about 60%, from about 8% to about 60%, from about 9% to about 60%, and from about 10% to about 60%. In another embodiment, the subsequent voltage can be any voltage between the first voltage and about 50%. In one embodiment, the first voltage is about 5% and the subsequent voltage is about 15%. In another embodiment, the first voltage is about 5% and the subsequent voltage is about 25%. In another embodiment, the first voltage is about 5% and the subsequent voltage is about 35%. In another embodiment, the first voltage is about 5% and the subsequent voltage is about 45%. 
         [0046]    The subsequent voltage can then be maintained at the first lamp zone for a subsequent period of time, as in step  312 . The subsequent period of time is less than the first period of time. The subsequent period of time is a period of time which is not expected to significantly raise the temperature of the chamber. The subsequent period of time can be a relatively short period of time, such as no more than 10% of the first period of time, such as from about 3 seconds to about 15 seconds, for example from about 3 seconds to about 10 seconds. In one embodiment, the subsequent period of time is about 5 seconds. The photocurrent is measured by the pyrometer during the subsequent period of time. The photocurrent can be measured in a variety of ways, such as by continuous measurement during the subsequent period of time. The measurement during the subsequent period of time produces a first order response such that the initial lamp noise increases more dramatically at early time points and levels off at later time points. 
         [0047]    Without intending to be bound by theory, it is believed to be beneficial to measure the lamp zones individually. The lamp noise detected at the pyrometer, such as the first pyrometer, is increased based on the proximity of the lamp zone to the optical connection for the pyrometer, such as the ports  109  described with reference to  FIG. 1 . Stated another way, the closer the lamp zone is to the optical connection for the pyrometer, the more lamp noise is received by the pyrometer. Further, during operation, the one lamp zone of the process chamber may be activated (receiving voltage) while another lamp zone is not receiving voltage. Therefore, it is believed to be helpful to determine the lamp noise for each zone at each pyrometer, such that it can be removed when the lamp zone is activated. 
         [0048]    The steps of supplying the first voltage (step  306 ), maintaining the first voltage (step  308 ), supplying the subsequent voltage (step  310 ) and maintaining the subsequent voltage (step  312 ) can be repeated one or more times to collect a plurality of first order response values, as in step  314 . The steps described above can be repeated to create a range of measurements based on the subsequent voltage. In one embodiment, the steps are repeated five times to produce measurements for 5%, 15%, 25%, 35% and 45% of the maximum voltage at the first lamp zone. In this embodiment, five groups of first order response values are collected at the first lamp zone. 
         [0049]    Averaging at least a portion of each of the first order time response values to create a steady state value for each of the first order time response values, as in step  316 . First order response values will “level off”, or reduce the rate of increase compared to the preceding value, later in the subsequent period of time. For example, in an embodiment where voltage is delivered over a period of 5 seconds, the rate of increase for the lamp noise values detected at the first pyrometer will be less at 4.5 seconds than lamp noise values detected at the first pyrometer at 1 second. As such, a steady state value can be approximated by averaging a portion of the first order time response values which correspond to later time points, such as the first order time response values which correspond to the time between 4.5 second and 5 seconds as measured at the first pyrometer. The average can be the steady state value. In further embodiments, different corresponding time ranges can be used, specific points in time can be averaged (such as the average of the value at 4.75 seconds and the value at 5 seconds) or a single point may be used as an approximate (such as the value at 5 seconds). 
         [0050]    Without being bound by theory, the transition between voltages produce noise that is not properly incorporated by the first order time response alone. The lamp noise produced is proportional to the voltage applied. Thus the first order time response values based on the lamp noise detected at a pyrometer for a specific voltage can provide a good approximation of the lamp noise during operation. However, changing voltages can momentarily change the lamp noise with relation to the new voltage. Thus, by calculating and subtracting the steady state value for the original voltage at the transition point 
         [0051]    Though the embodiments described herein discuss the measurement of lamp noise at a single pyrometer as received from the lamp zones, it is understood that multiple pyrometers can be used to measure the lamp noise at the same time. In one embodiment, two pyrometers are used to measure the lamp noise received from each of the lamp zones. 
         [0052]      FIG. 4  is a flow diagram of a method  400  for noise correction according to one embodiment. The method  400  begins at step  402  with lamp noise detected by a pyrometer in a process chamber comprising one or more pyrometers being measured as described above with reference to  FIG. 3 . 
         [0053]    Next, the performance of the measurement can be repeated using each of the one or more lamp zones and at each of the one or more pyrometers until each of the one or more pyrometers has been measured with relation to each of the one or more lamp zones, as in step  404 . Voltage at varying levels, as described with reference to  FIG. 3 , can be delivered to each of the lamp zones. Noise from each of the lamp zones can be measured at each of the pyrometers. 
         [0054]    After the measurements are complete, the linear relation between the steady state values can be calculated for each of the one or more lamp zones and each of the one or more pyrometers, as in step  406 . At this point, the steady state measured values from the can be used to produce a line graph. The line graph can be used to calculate the intervening steady state values based on the measured values. 
         [0055]    Then the steady state values and the first-order time response values can be subtracted from a measured value detected by the pyrometers to create an adjusted value, as in step  408 . Once the steady state values have been determined, the steady state values and the first-order time response values are subtracted from a measured value. The measured value can be defined as photocurrent value measured at the pyrometer to determine the temperature of the substrate. 
         [0056]    The background subtraction algorithm involves estimating lamp noise for each pyrometer based on the real-time instantaneous zone voltages and subtracting the lamp noise estimate from the total measured pyrometer signal to obtain estimated thermal photocurrent due to measured, real temperature from the substrate. 
         [0057]    To remove the lamp noise the following formula can be applied: 
         [0000]    
       
      
       I=I 
       t 
       +I 
       n  
      
     
         [0000]    wherein I=total measured pyrometer photocurrent, such as the photocurrent measured by the pyrometer during the measurement of a heated substrate, I t =photocurrent thermal signal from real temperature (the thermal radiance described with reference to  FIG. 2 ) and I n =photocurrent due to lamp/background noise (the lamp noise detected at the pyrometer from all lamp zones) 
         [0058]    To calculate the estimated portion of the photocurrent which is related to the thermal radiation alone, the formula can be rewritten as follows: 
         [0000]    
       
      
       I 
       t(est) 
       =I−I 
       n  
      
     
         [0059]    As described above, the lamp noise is a combination of all noise from the lamp zones detected at all pyrometers. Thus, the noise estimate for a pyrometer, pi, at any given sample time, t, with lamp zone voltage level (z1,z2,z3,z4,z5, and z6) can be computed as a linear additive composition of the independent zone levels using the following formula: 
         [0000]        I   n ( pi,t )( z 1, z 2, z 3, z 4, z 5, z 6)= I   n ( pi,z 1, t )+ I   n ( pi,z 2, t )+ I   n ( pi,z 3, t )+ I   n ( pi,z 4, t )+ I   n ( pi,z 5, t )+ I   n ( pi,z 6, t ) 
         [0060]    Various methods can be used to subtract the lamp noise estimates from the measured value at the pyrometer. In the Steady-State Noise Subtraction method, the estimated background noise for subtraction is the steady-state noise levels at a given lamp zone power level. These can be subtracted using the following formula: 
         [0000]        I   n ( pi,t ) ( z 1 , z 2, z 3, z 4, z 5, z 6)=[ I   n ( pi,z 1)+ I   n ( pi,z 2)+ I   n ( pi,z 3)+ I   n ( pi,z 4)+ I   n ( pi,z 5)+ I   n ( pi,z 6)][ ss]   
         [0000]    wherein I n (pi,zi) is the lamp noise (I n ) at a specific pyrometer (zi) based on a specific zone (zi) can be estimated per as described above either using piece-wise linear interpolation or using a 3rd order polynomial fit. 
         [0061]    In the real-time Model-Based Noise Subtraction method, the estimated background noise for subtraction is the estimated real-time noise levels at a given lamp zone power level. Since the noise response is an exponential first-order time-response of the lamp-filament heat-up response, this is captured in the formula below: 
         [0000]      Voltage in ( V   in )−voltage out ( V   out )=resistance ( R )*current ( C )* dV   out   /dt  
 
         [0062]    The phrase real-time reflects the determination of noise during operation. In one embodiment, the real-time noise subtraction is done at a specific frequency over a period of time during the operation of the chamber, such as during substrate processing. Frequency can relate to the periodicity of the measurements of the noise, the periodicity of the measurements of the temperature during processing or combinations thereof. 
         [0063]    Since the photocurrent noise is proportional to the voltage applied, the photocurrent noise can be directly modeled using a first-order time response and the time constant can be computed empirically, as depicted by the following formula: 
         [0000]        I   n ( pi,t )/ Z ( t )=[ K /(1+ τs )] 
         [0000]    wherein I n (pi,t) is the photocurrent noise (I n ) of pyrometer, pi, detected at time, t, Z(t) is the real-time voltage applied to all lamps, K is the gain, τ is the lamp time-constant and s is the Laplace transform variable. Since Z(t) is the same as the real time voltage applied to all lamps at steady state, Z(ss), values, the formula can be re-arranged as follows: 
         [0000]        I   n ( pi,t )/ I   n ( pi )( ss )=1/(1+ τs ) 
         [0000]    wherein I n (pi)(ss) is the steady-state photocurrent noise of pyrometer, pi, as determined in based on the method of  FIG. 3 . Expanding the Laplace transform variable and writing the discrete-time implementation of the above, the formula can be further rearranged and reduced as follows: 
         [0000]        I   n ( pi,t )+τ( dln ( pi,t )/ dt )= I   n ( pi )( ss )
 
         [0000]        I   n ( pi,n )= I   n ( pi,n− 1)+Δ T/τ ( I   n ( pi )( ss )− I   n ( pi,n− 1))
 
         [0064]    In the converted formula, n is the current sample time, n−1 is the previous sample time and ΔT is the sampling rate. 
         [0065]    The above formula can be applied to determine the approximate lamp noise component for a measured photocurrent value at a pyrometer and at a specific time point. For the final software implementation, the following algorithm can be used: 
         [0000]        I   n ( pi,n )=α I   n ( pi )( ss )+(1−α) I   n ( pi,n− 1)
 
         [0000]    wherein α=ΔT/τ, with τ being the time-constant. The time constant can be a user-defined attribute. ΔT is the set sampling rate measured in seconds. The sampling rate for the correction algorithm should be the same as the sampling rate of the pyrometer being corrected for. 
         [0066]    In the above formula, the steady state value for the lamp noise at a particular voltage as detected by a pyrometer, pi, is multiplied by the value of α which is the time constant divided by the sampling rate. The multiplied value is then added to the adjusted previous noise value, which is the previous value, I n (pi,n−1), multiplied by 1−α. In short, the current noise value at a particular voltage is a function of the previous lamp noise value and the steady state lamp noise value. The formula above approximates the first order 
         [0067]    As with either method of determining I n , the background subtracted signal or the estimated true temperature signal can be computed using the following formula: 
         [0000]        I   t ( pi,n )= I ( pi,n )− I   n ( pi,n )
 
         [0000]    wherein I(pi,n) is the detected photocurrent value at a specific pyrometer, I n (pi,n) is the calculated approximate lamp noise and I t (pi,n) is the approximate thermal radiance component of the detected photocurrent value, also described herein as the adjusted value. 
         [0068]    Finally, an actual temperature can be determined based on the adjusted value, as in step  410 . Once the adjusted values have been determined, the actual temperature of the substrate at each time point can be determined. This embodiment is described with reference to a long-wave 3.3 um pyrometer. However, the pyrometer described here is not intended to be limiting and a variety of pyrometers can be used with one or more embodiments. 
         [0069]    The long-wave 3.3 um Pyrometer described herein has a serial output which streams the total measured photocurrent and the calculated temperature. The output photocurrent reading is the total measured photocurrent (I(pi)) by the pyrometer used for the noise-background subtraction. The look-up photocurrent is calculated using I t  and factored for emissivity and pyrometer calibration coefficients. 
         [0070]    Tables 1-4 depict photocurrents detected at pyrometers 1-4. The photocurrents are separated by both the lamp zone where the voltage was delivered and the voltage delivered, described as a percentage of the maximum voltage for each lamp zone. The data is depicted in the tables below: 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Pyrometer 1 - Photocurrent 
               
             
          
           
               
                 V % 
                 Zone1 
                 Zone2 
                 Zone3 
                 Zone4 
                 Zone5 
                 Zone6 
               
               
                   
               
             
          
           
               
                 5 
                 4.31E−08 
                 5.01E−08 
                 4.75E−08 
                 3.50E−08 
                 2.72E−08 
                 2.18E−08 
               
               
                 15 
                 2.77E−07 
                 3.19E−07 
                 2.85E−07 
                 1.88E−07 
                 1.26E−07 
                 9.56E−08 
               
               
                 25 
                 5.57E−07 
                 6.53E−07 
                 5.65E−07 
                 3.66E−07 
                 2.43E−07 
                 1.80E−07 
               
               
                 35 
                 8.17E−07 
                 9.46E−07 
                 8.20E−07 
                 5.34E−07 
                 3.53E−07 
                 2.61E−07 
               
               
                 45 
                 1.02E−06 
                 1.17E−06 
                 1.01E−06 
                 6.55E−07 
                 4.36E−07 
                 3.21E−07 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Pyrometer 2 - Photocurrent 
               
             
          
           
               
                 V % 
                 Zone1 
                 Zone2 
                 Zone3 
                 Zone4 
                 Zone5 
                 Zone6 
               
               
                   
               
             
          
           
               
                 5 
                 2.93E−08 
                 3.55E−08 
                 4.80E−08 
                 4.71E−08 
                 4.08E−08 
                 3.09E−08 
               
               
                 15 
                 1.61E−07 
                 2.21E−07 
                 3.06E−07 
                 2.92E−07 
                 2.31E−07 
                 1.56E−07 
               
               
                 25 
                 3.18E−07 
                 4.56E−07 
                 6.16E−07 
                 5.82E−07 
                 4.47E−07 
                 3.00E−07 
               
               
                 35 
                 4.63E−07 
                 6.63E−07 
                 8.95E−07 
                 8.50E−07 
                 6.56E−07 
                 4.38E−07 
               
               
                 45 
                 5.78E−07 
                 8.20E−07 
                 1.10E−06 
                 1.05E−06 
                 8.05E−07 
                 5.42E−07 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Pyrometer 3 - Photocurrent 
               
             
          
           
               
                 V % 
                 Zone1 
                 Zone2 
                 Zone3 
                 Zone4 
                 Zone5 
                 Zone6 
               
               
                   
               
             
          
           
               
                 5 
                 1.17E−08 
                 1.79E−08 
                 3.19E−08 
                 3.65E−08 
                 4.73E−08 
                 3.62E−08 
               
               
                 15 
                 7.58E−08 
                 1.26E−07 
                 2.22E−07 
                 2.65E−07 
                 3.29E−07 
                 2.32E−07 
               
               
                 25 
                 1.50E−07 
                 2.63E−07 
                 4.52E−07 
                 5.39E−07 
                 6.48E−07 
                 4.61E−07 
               
               
                 35 
                 2.19E−07 
                 3.84E−07 
                 6.61E−07 
                 7.97E−07 
                 9.59E−07 
                 6.75E−07 
               
               
                 45 
                 2.73E−07 
                 4.75E−07 
                 8.17E−07 
                 9.90E−07 
                 1.17E−06 
                 8.31E−07 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Pyrometer 4 - Photocurrent 
               
             
          
           
               
                 V % 
                 Zone1 
                 Zone2 
                 Zone3 
                 Zone4 
                 Zone5 
                 Zone6 
               
               
                   
               
             
          
           
               
                 5 
                 3.47E−08 
                 3.84E−08 
                 5.20E−08 
                 6.08E−08 
                 6.89E−08 
                 8.03E−08 
               
               
                 15 
                 6.47E−08 
                 9.44E−08 
                 2.00E−07 
                 2.73E−07 
                 3.30E−07 
                 3.93E−07 
               
               
                 25 
                 1.01E−07 
                 1.64E−07 
                 3.77E−07 
                 5.29E−07 
                 6.32E−07 
                 7.61E−07 
               
               
                 35 
                 1.35E−07 
                 2.27E−07 
                 5.39E−07 
                 7.63E−07 
                 9.14E−07 
                 1.10E−06 
               
               
                 45 
                 1.61E−07 
                 2.74E−07 
                 6.67E−07 
                 9.32E−07 
                 1.13E−06 
                 1.34E−06 
               
               
                   
               
             
          
         
       
     
         [0071]      FIG. 5  is a depiction of a line graph  500  for the measured steady state values, according to one embodiment. In this embodiment, a thermal processing chamber was equipped with four pyrometers and a total of six lamp zones. The photocurrent (measured in Amps) was detected as described above using voltages of 5%, 15%, 25%, 35% and 45% of the maximum voltage at each of the six zones. The data disclosed in table 1 was converted to a line graph and a best fit line was fit to the graphical data. The best fit line or the numerical representation thereof can then be used to extrapolate the approximate photocurrent for lamp noise at a specific voltage. 
         [0072]    Though only the first pyrometer is graphically represented here, it is understood that the measured photocurrents from all pyrometers can be used to extrapolate the linear relation between the measured values. 
         [0073]      FIG. 6  depicts a line graph  600  showing the pyrometer signal adjusted using the background subtraction technique according to one embodiment. The graph  600  shows the measured photocurrent output, the actual temperature of the substrate as measured by a thermocouple, the DC background subtracted output and the frequency background subtracted output. and the frequency background subtracted output correlates to the Real-time Model-Based Noise Subtraction method. 
         [0074]    The measured photocurrent output line is the temperature based on the detected photocurrent output at the first pyrometer with heating from all lamp zones. As shown by comparison to the thermocouple, the temperature measured by the first pyrometer fails to correlate with the thermocouple temperature. The DC background subtracted signal (DC signal) correlates to the measured photocurrent output as adjusted using the Steady-State Noise Subtraction method, with the adjusted photocurrent value translated to the temperature in degrees Celsius. The DC signal shows much better correlation to the thermocouple, with the exception of overcompensation at the voltage transition points (shown by spikes in the DC signal). The frequency background subtracted signal (Freq. signal) correlates to the measured photocurrent output as adjusted using the Real-time Model-Based Noise Subtraction method, with the adjusted photocurrent value translated to the temperature in degrees Celsius. The Freq. signal shows much better correlation to the thermocouple than either of the previous lines with comparatively less overcompensation. 
         [0075]    While the foregoing is directed to embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof.