Pyrometry error detection sensor for RTP temperature control systems.

Abstract

Embodiments disclosed herein include a method for determining a temperature error of a pyrometer. In one embodiment, the method includes measuring a first signal with a first sensor of the pyrometer and measuring a second signal with a second sensor of the pyrometer. In one embodiment, the method further includes determining a reflectivity of a reflector plate from the first signal and the second signal, and using the reflectivity to determine the temperature error.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Patent Application No. 17 / 403,693, filed August 16, 2021, the entire contents of which are incorporated herein by reference.

[0002] TECHNICAL FIELD Embodiments of the present disclosure relate to the field of semiconductor processing, and in particular to rapid thermal processing (RTP) temperature control systems. [Background technology]

[0003] A rapid thermal processing (RTP) system includes a chamber with an array of heating elements on the opposite side from the substrate. The heaters rapidly increase the temperature of the substrate to implement processing conditions on the substrate. The heating elements are controlled, at least in part, by pyrometers below the substrate. Pyrometers are a non-contact temperature measurement solution.

[0004] During processing in an RTP system, molecules from the substrate surface can sublimate and redeposit on chamber surfaces. The pyrometer viewports used to measure wafer radiation and determine wafer temperature gradually become coated during use of the RTP system. This creates temperature errors that can cause the control system to drive the process outside of specifications. Summary of the Invention

[0005] Embodiments disclosed herein include a method for determining a temperature error of a pyrometer. In one embodiment, the method includes measuring a first signal with a first sensor of the pyrometer and measuring a second signal with a second sensor of the pyrometer. In one embodiment, the method further includes determining a reflectivity of a reflector plate from the first signal and the second signal, and determining the temperature error using the reflectivity.

[0006] Embodiments disclosed herein may also include a processing tool with a pyrometer capable of detecting temperature errors. In one embodiment, the processing tool comprises a chamber, a substrate support in the chamber, and a reflector below the substrate support. In one embodiment, a heating unit is coupled to the chamber, the heating unit configured to heat a substrate supported by the substrate support. In one embodiment, the processing tool further comprises a pyrometer extending through the reflector, the pyrometer configured to measure a temperature of the substrate. In one embodiment, the pyrometer comprises a light pipe, focusing optics, a first sensor configured to detect light emitted from a relatively small spot on the substrate above the light pipe, and a second sensor configured to detect light from a wider angle than the first sensor.

[0007] Embodiments disclosed herein may also include a system with a pyrometer capable of detecting temperature errors. In one embodiment, a system for processing a substrate includes a chamber, a substrate holder for supporting the substrate, a heating element for heating the substrate, a reflector plate below the substrate, and a pyrometer configured to penetrate the reflector and measure the temperature of the substrate. In one embodiment, the pyrometer can be used to detect temperature errors resulting from deposition on the reflector plate. In one embodiment, a method for detecting temperature errors includes measuring a first signal with a first sensor of the pyrometer, measuring a second signal with a second sensor of the pyrometer, determining a reflectivity of the reflector plate from the first signal and the second signal, and determining the temperature error using the reflectivity. [Brief explanation of the drawings]

[0008] [Figure 1] 1 illustrates a cross-sectional view of a rapid thermal processing (RTP) system, according to one embodiment. [Figure 2] FIG. 1 is a schematic diagram of a dual channel pyrometer in an RTP system, according to one embodiment. [Figure 3]1 is a flow diagram of a process for determining pyrometer temperature error in an RTP system, according to one embodiment. [Figure 4A] 1 is a graph of effective emissivity versus reflector plate reflectance according to one embodiment. [Figure 4B] 1 is a graph of relative signal loss versus substrate emissivity, according to one embodiment. [Figure 4C] 1 is a graph of temperature error versus substrate temperature according to one embodiment. [Figure 5] 1 is a flow diagram of a process for comparing a temperature error provided by a pyrometer with a temperature error provided by a thermal model, according to one embodiment. [Figure 6] FIG. 1 is a block diagram of an exemplary computer system according to one embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0009] A rapid thermal processing (RTP) temperature control system is described herein. In the following description, numerous specific details are set forth to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to those skilled in the art that embodiments of the present disclosure may be practiced without these specific details. In other instances, well-known aspects will not be described in detail so as not to unnecessarily obscure embodiments of the present disclosure. Furthermore, it should be understood that the various embodiments shown in the figures are illustrative representations and are not necessarily drawn to scale.

[0010] As mentioned above, redeposition of material on internal surfaces of an RTP system (e.g., reflector plates or quartz windows) can cause errors in temperature measurements made by pyrometers. Additionally, other changes to the surface (e.g., chemical reactions or other damage) can also cause errors in temperature measurements made by pyrometers. Thus, the temperature of the substrate can potentially be shifted outside of specifications, causing damage to the substrate and, in some cases, potentially requiring the substrate to be scrapped. Accordingly, embodiments disclosed herein include systems and processes for detecting temperature errors in pyrometer readings. Thus, the true temperature of the substrate can be determined and controlled to fall within given specifications.

[0011] In one embodiment, temperature error calculations are performed using a dual-channel pyrometer. The first channel is an emissometer sensor that detects light emitted from a small spot on the substrate. Readings from the first channel can be denoted as IE. The second channel is a main sensor that detects a range of radiation from a wider angle. Most of the main channel radiation comes from multiple reflections in the area around the pyrometer location. Due to the many reflections, the substrate essentially becomes a virtual block body with a reflectance R essentially equal to 1. Readings from the second channel can be denoted as IM. The ratio of IE / IM can then be used to determine the reflectivity of a reflective plate below the substrate. The decrease in reflectivity is proportional to the temperature error. A series of charts are used to translate the proportional value of the temperature error to the actual value of the temperature error. In addition to finding the probability in reflectivity using a dual-channel pyrometer, changes in reflectivity can also be determined using a laser-based system. For example, a processing tool may include a laser source that is used to probe a surface (eg, a reflector plate) to detect changes in reflectivity.

[0012] In yet another embodiment, a similar process can be utilized to detect coatings on a quartz window that separates a heating element from the chamber. The substrate is removed and a pyrometer can detect the temperature of the quartz window. Dual channel interrogation of the quartz window can be used to determine the presence of a coating on the quartz window, which can alter the processing of the substrate in the tool.

[0013] In some embodiments, the temperature error may be used in the context of artificial intelligence (AI) or machine learning (ML). For example, a measured temperature error may be compared to a temperature error calculated in a physical model. If the measured temperature error and the calculated temperature error are relatively close to each other, the physical model may be accepted as true. If there is a larger difference between the measured temperature error and the calculated temperature error, the physical model may be retuned.

[0014] In some embodiments, changes in reflectance can also be used in the cleaning process to detect the cleaning endpoint. For example, a plasma etching process can be used to remove a coating on an interior surface. As the coating is removed, the reflectance will change. When the reflectance value returns to a known level (i.e., the known reflectance of the reflector in an uncoated state), the cleaning process can be determined to be complete.

[0015] 1 , a cross-sectional view of an RTP tool 100 is shown, according to one embodiment. In one embodiment, the RTP tool comprises a chamber 110. A heating element 105 is provided above the chamber 110. The heating element 105 may be separated from the chamber 110 by a window 106. Although shown as a single device, the heating element 105 may comprise multiple individual heating elements. For example, multiple heat lamps may be provided across the top of the chamber 110.

[0016] In one embodiment, the substrate 115 may be supported by a support 116 in the chamber 110. The substrate 115 may be any type of substrate specific to semiconductor manufacturing processes. For example, the substrate 115 may be a wafer, such as a silicon wafer. In other embodiments, the substrate 115 may be a glass substrate with a non-circular form factor. For example, the substrate 115 may have a rectangular shape in some embodiments. In one embodiment, the support 116 may raise the substrate 115 above the reflector plate 121. The support 116 may also be configured to rotate the substrate 115 in some embodiments.

[0017] In one embodiment, a reflector plate 121 may be provided on the base 122. Multiple pyrometers 125 may extend through the base 122 and the reflector plate 121. In one embodiment, the top portion of the pyrometer 125 is shown in FIG. 1. In particular, the light pipe portion of the pyrometer 125 is shown. Additional optics and sensors of the pyrometer 125 are not shown in FIG. 1 and are described in more detail below with respect to FIG. 2.

[0018] In one embodiment, the pyrometer 125 is used in a control system to maintain the substrate 115 at a desired temperature. For example, the pyrometer 125 may be coupled to a controller that takes temperature information from the pyrometer 125 as an input and provides a control effort to the heating element 105 as an output. As described in more detail below, the pyrometer 125 may be a dual-channel pyrometer. The use of the dual-channel pyrometer 125 allows a temperature error to be calculated, as described in more detail below.

[0019] 2, a cross-sectional view of pyrometer 225 is shown, according to one embodiment. As shown, pyrometer 225 includes a light pipe 231. Light pipe 231 penetrates a base (not shown) and reflector plate 221. Light pipe 231 captures light from different areas. A first portion of the input light is referred to as IE. The IE light is from substrate 215 directly above light pipe 231. A second portion of the input light is referred to as IM. The IM light is reflected multiple times between substrate 215 and reflector 221 before reaching light pipe 231.

[0020] In one embodiment, pyrometer 225 further includes a first optical region 232. First optical region 232 collimates the incident light before it reaches filter 235. A second optical region 233 is provided after filter 235. Second optical region 233 is a concentrator. The concentrator in second optical region 233 focuses the light onto first sensor 236. First sensor 236 receives the IM light and may be referred to as an IM sensor 236. A reflector 234 may also be provided in second optical region 233. Reflector 234 is oriented at the center of pyrometer 225 and reflects the IE light to second sensor 237. Second sensor 237 may be referred to as an IE sensor 237. In one embodiment, IM sensor 236 and IE sensor 237 are photodetectors. That is, photons of the IM light and IE light induce a current that can be used to determine the intensity of either light source. The IM sensor 236 and the IE sensor 237 may be electrically coupled to pyrometer electronics 238. The pyrometer electronics 238 may be used to perform one or more processes for determining the temperature error, as described in more detail below.

[0021] 3, a flow diagram of a process 370 for determining pyrometer temperature error is shown, according to one embodiment. In one embodiment, process 370 may be executed on an RTP tool, such as the RTP tool described above with respect to FIGS. 1 and 2. In a particular embodiment, the RTP tool includes a dual-channel pyrometer that enables detection of IE and IM signals.

[0022] In one embodiment, process 370 may begin with operation 371, which includes measuring a first signal with a first sensor of a pyrometer. In one embodiment, the first signal may be an IE signal, and the first sensor may be an IE sensor. In one embodiment, the IE signal may be routed to the IE sensor using a reflector in the pyrometer. The reflector may be similar to reflector 234, described in more detail above. Thus, the IE signal may be light from directly above the light pipe. In one embodiment, the value of IE is provided by Equation 1. In Equation 1, ε is the emissivity of the substrate, σ is the Stefan-Boltzmann constant, and T is temperature. IE~ε·σ·T 4 formula 1

[0023] In one embodiment, process 370 may continue with operation 372, which includes measuring a second signal with a second sensor of the pyrometer. In one embodiment, the second signal may be an IM signal, and the second sensor may be an IM sensor. The IM signal may be obtained from a wide angle above the light pipe. In one embodiment, the value of IM is provided by Equation 2. In Equation 2, ε is the emissivity of the substrate, σ is the Stefan-Boltzmann constant, T is the temperature, and R is the reflectivity of the reflective plate. TIFF0007727829000001.tif11170

[0024] Although described as two separate operations, it should be appreciated that operations 371 and 372 may occur simultaneously, i.e., the IM and IE values ​​may be obtained at substantially the same time. However, it should be appreciated that the IM and IE values ​​may be obtained at different times in some embodiments.

[0025] In one embodiment, process 370 continues with operation 373, which includes determining the reflectivity of the reflector plate from the first signal and the second signal. In particular, the reflectivity R may be obtained by taking the ratio of IE to IM. The ratio of IE to IM is provided in Equation 3: TIFF0007727829000002.tif9170

[0026] Equation 3 can be rearranged to isolate the reflectance R into Equation 4: TIFF0007727829000003.tif13170

[0027] The reflectance R can be used to find the absorption coefficient α, as shown in Equation 5, where R is the original reflectance of the reflector plate. The absorption coefficient α is proportional to the loss due to changes in the reflector plate during processing of multiple substrates. The absorption coefficient α can be used to provide a relative temperature error. However, it should be appreciated that embodiments can also include the ability to calculate the actual temperature error. Thus, the true value of the temperature can be used to control the RTP tool. TIFF0007727829000004.tif9170

[0028] In one embodiment, process 370 continues with operation 374, which includes using reflectance to determine the temperature error. In one embodiment, a series of charts may be generated to allow the actual temperature error to be determined. For example, charts similar to those provided in FIGS. 4A-4C may be used. Referring now to FIG. 4A, a graph of reflector plate reflectance versus effective emissivity is shown for a range of wafer emissivities, according to one embodiment. In one embodiment, the effective emissivity E effis given by Equation 6. As shown by the graph, as the value of R approaches 1, the effective emissivity E eff also proceeds to 1. In such cases, the system is modeled as a true blackbody. TIFF0007727829000005.tif10170IM~E eff formula 7

[0029] The signal is calculated using the effective emissivity E as expressed in Equation 7. eff is proportional to. In particular, the derivative of the relative signal loss is provided by Equation 8. Equation 8 is provided graphically in FIG. 4B, which shows a graph of wafer emissivity versus relative signal loss per reflectance change for a range of reflector plate reflectivities. For example, for a substrate with an emissivity E of 0.7, the relative signal loss is 0.4. Since the signal loss is per reflectance change, a 1% change in reflectivity R results in a 0.4% radiation loss. However, it should be appreciated that other losses may also be included. For example, loss due to absorption from the light pipe surface may also add to the radiation loss. For example, a light pipe coating may result in an additional 1.0% radiation loss, resulting in a total radiation change of -1.4%. TIFF0007727829000006.tif8170

[0030] To translate from relative signal loss to an actual value of temperature error, the graph in Figure 4C is used. Figure 4C is a chart showing temperature error per percentage pyrometer current error versus substrate temperature. For example, at a substrate temperature of 1100°C, the temperature error is 1.2K / %. In the example described above, the total loss is -1.4%. This results in a temperature error equal to 1.2K / % x -1.4% = -1.7K.

[0031] Use of such a process can be used to determine a temperature error, which is useful in controlling a heating unit of an RTP tool to keep processing conditions within a predetermined threshold. In addition to control applications, the temperature offset can be used to validate one or more models running in conjunction with the RTP tool. For example, the accuracy of machine learning and / or artificial intelligence models can be confirmed using a process for determining a temperature offset. An example of such a process flow is provided in FIG. 5.

[0032] 5, a flow diagram of a process 580 for validating a model is shown, according to one embodiment. In one embodiment, the model being validated is a physics-based model. Physics-based models use physical parameters and physics-based phenomena to chart results for various processing conditions. The goal of a physics-based model is to be as close as possible to the actual processing in a chamber. Physics-based models can be used as part of machine learning or artificial intelligence algorithms to control a processing tool, such as an RTP tool.

[0033] In one embodiment, process 580 may begin with operation 581, which includes running a recipe. The recipe may be any recipe suitable for processing a substrate or for implementing planned maintenance (PM). In some embodiments, the recipe may be run at the end of PM to revalidate the physics-based model.

[0034] In one embodiment, process 580 branches into two parallel processes. In one embodiment, the first branch includes obtaining sensor information at block 583, and the second branch includes obtaining a physics model prediction at block 582. The sensor information block 583 may include a process similar to that described in more detail above to determine the pyrometer temperature error. For example, a dual-channel pyrometer may be used to find the IM and IE values ​​and the corresponding reflectance of the reflector plate. The reflectance may then be used to generate the actual temperature error of the pyrometer. On the second branch, a physics model is queried to determine what the physics model thinks the true temperature of the substrate is, given the recipe parameters, tool settings, and other appropriate tool information (e.g., historical data, etc.).

[0035] Process 580 then proceeds to operation 584, which includes comparing the sensor information to the model prediction. The sensor information will provide the true temperature of the substrate during the recipe (i.e., measured temperature + temperature error), and the model prediction provides the modeled temperature of the substrate during the recipe. In one embodiment, the true temperature and the modeled temperature may be a single temperature point during the process. In other embodiments, the true temperature and the modeled temperature may be a temperature map across the surface of the substrate. The temperature map may be at a single point in time during the recipe, or may include temperature maps at multiple times during the recipe.

[0036] In one embodiment, decision block 584 determines whether the true temperature and the modeled temperature match or do not match. Matched temperatures may refer to two temperatures that are within a certain threshold of each other. For example, a true temperature may be considered to match a modeled temperature if the modeled temperature is between 90% and 110% of the true temperature. In another embodiment, a modeled temperature is considered to match a true temperature if the modeled temperature is between 99% and 101% of the true temperature.

[0037] In one embodiment, if the temperatures match, then at block 585, the corrections to the temperatures are accepted. That is, the physical model is accepted, and the process may loop back to running a recipe. The same recipe may be run, or a different recipe may be run. For example, the subsequent recipe may be a recipe for a production substrate.

[0038] In one embodiment, when the temperatures do not match, process 580 proceeds to block 586, which includes stopping the process and requesting a monitor run. In one embodiment, the monitor run may be a method for recalibrating the physics-based model used in block 582. In one embodiment, process 580 then continues to operation 587, which includes monitoring metrology data. The metrology data may be physical metrology performed on a substrate processed with the initial recipe in block 581. In other embodiments, virtual metrology may be performed. Virtual metrology may refer to metrology derived from one or more sensor outputs in a processing tool. For example, a temperature error calculation may be an example of a virtual metrology that may be used to refine the physics-based model.

[0039] In one embodiment, process 580 may further continue at block 588 with tuning the physics model. Tuning the physics model may include applying various measurement inputs to the physics model to more accurately map a given process in the RTP tool. In one embodiment, tuning may be performed offline. In other embodiments, machine learning or artificial intelligence algorithms may be used to automatically tune the physics model.

[0040] In one embodiment, process 580 may further continue with operation 589, which includes updating the adjusted offsets and resetting the reference for the physical model. After the model has been reset according to the adjustment process, the process may continue by repeating the recipe at block 581. The process may then continue according to the process loop any number of times.

[0041] FIG. 6 shows a diagrammatic representation of a machine in the exemplary form of a computer system 600 within which a set of instructions may be executed to cause the machine to perform any one or more of the methodologies described herein. In alternative embodiments, the machine may be connected to (e.g., networked with) other machines in a local area network (LAN), an intranet, an extranet, or the Internet. The machine may operate as a server or a client machine in a client-server network environment, or as a peer machine in a peer-to-peer (or distributed) network environment. The machine may be a personal computer (PC), a tablet PC, a set-top box (STB), a personal digital assistant (PDA), a cellular telephone, a web appliance, a server, a network router, a switch, or a bridge, or any machine capable of executing a set of instructions (sequential or otherwise) that specify actions to be taken by the machine. Furthermore, although only a single machine is shown, the term “machine” shall also be taken to include any collection of machines (e.g., computers) that individually or together execute a set of instructions (or multiple sets) to perform any one or more of the methodologies described herein.

[0042] The exemplary computer system 600 includes a processor 602, a main memory 604 (e.g., read-only memory (ROM), flash memory, dynamic random access memory (DRAM) such as synchronous DRAM (SDRAM) or Rambus DRAM (RDRAM)), a static memory 606 (e.g., flash memory, static random access memory (SRAM), MRAM, etc.), and a secondary memory 618 (e.g., a data storage device), which communicate with each other via a bus 630.

[0043] Processor 602 represents one or more general-purpose processing devices, such as a microprocessor, a central processing unit, or the like. More specifically, processor 602 may be a complex instruction set computing (CISC) microprocessor, a reduced instruction set computing (RISC) microprocessor, a very long instruction word (VLIW) microprocessor, a processor implementing other instruction sets, or a processor implementing a combination of instruction sets. Processor 602 may also be one or more special-purpose processing devices, such as an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA), a digital signal processor (DSP), a network processor, or the like. Processor 602 is configured to execute processing logic 626 for performing the operations described herein.

[0044] Computer system 600 may further include a network interface device 608. Computer system 600 may also include a video display unit 610 (e.g., a liquid crystal display (LCD), a light emitting diode display (LED), or a cathode ray tube (CRT)), an alphanumeric input device 612 (e.g., a keyboard), a cursor control device 614 (e.g., a mouse), and a signal generation device 616 (e.g., a speaker).

[0045] The secondary memory 618 may include a machine-accessible storage medium (or more specifically, a computer-readable storage medium) 631 having stored thereon one or more sets of instructions (e.g., software 622) that embody any one or more of the methodologies or functions described herein. The software 622 may also reside completely or at least partially within the main memory 604 and / or within the processor 602 during execution of the software 622 by the computer system 600, with the main memory 604 and the processor 602 also constituting machine-readable storage media. The software 622 may further be transmitted or received over a network 661 via the network interface device 608.

[0046] While machine-accessible storage medium 631 is shown to be a single medium in the exemplary embodiment, the term "machine-readable storage medium" shall be interpreted to include a single medium or multiple media (e.g., centralized or distributed databases, and / or associated caches and servers) that store one or more sets of instructions. The term "machine-readable storage medium" shall also be interpreted to include any medium capable of storing or encoding a set of instructions for execution by a machine, causing the machine to perform any one or more of the methodologies of the present disclosure. The term "machine-readable storage medium" shall therefore be interpreted to include, but not limited to, solid-state memories, and optical and magnetic media.

[0047] According to one embodiment of the present disclosure, a machine-accessible storage medium has stored thereon instructions that cause a data processing system to perform a method for detecting a temperature error in a dual-channel pyrometer.

[0048] Thus, a method for measuring the temperature error of a dual channel pyrometer has been disclosed.

Claims

1. 1. A method for determining a temperature error of a pyrometer, comprising: measuring an IE signal with an emitometer sensor of the pyrometer; measuring an IM signal with a main sensor of the pyrometer; determining a reflectance of a reflector plate from the IE signal and the IM signal, wherein the reflectance of the reflector plate is determined using a ratio of the IE signal to the IM signal; determining the temperature error using the reflectance; and comparing the temperature error with an estimated temperature error provided by a physics-based model, wherein the temperature error is stored for future use when the absolute value of the difference between the temperature error and the estimated temperature error is less than a predetermined threshold; and A method comprising:

2. The method of claim 1 , wherein the temperature error is proportional to a change in the reflectivity of the reflector plate compared to an original state of the reflector plate.

3. The method of claim 2 , wherein the change in reflectivity is the result of material deposition on the reflector plate.

4. The method of claim 1 , wherein the reflectance is used to determine effective emissivity.

5. The method of claim 4 , wherein the effective emissivity is used to determine relative signal loss.

6. The method of claim 1 , wherein the predetermined threshold is 10% of the temperature error.

7. 1. A processing tool comprising: a chamber; a substrate support in the chamber; a reflector below the substrate support; a heating unit coupled to the chamber, the heating unit configured to heat a substrate supported by the substrate support; and a pyrometer penetrating the reflector, the pyrometer configured to measure a temperature of the substrate, the pyrometer for determining a temperature error using reflectance, the temperature error for comparing with an estimated temperature error provided by a physics-based model, the temperature error being stored for future use if an absolute value of a difference between the temperature error and the estimated temperature error is less than a predetermined threshold, the pyrometer comprising: A light pipe, a focusing optics; a first sensor configured to detect light emitted from a relatively small spot on the substrate above the light pipe; a second sensor configured to detect light from a wider angle than the first sensor; and a pyrometer comprising: A processing tool comprising:

8. The processing tool of claim 7 , wherein the first sensor is oriented vertically and the second sensor is oriented horizontally.

9. The processing tool of claim 8 , wherein the focusing optics includes a mirror for directing the light emitted from the relatively small spot on the substrate to the vertically oriented first sensor.

10. The processing tool of claim 7 , wherein the first sensor and the second sensor are configured to detect a change in reflectivity of the reflector.

11. The processing tool of claim 7 , wherein the processing tool is a rapid thermal processing (RTP) tool.

12. Multiple pyrometers The processing tool of claim 7 further comprising:

13. 1. A system for processing a substrate, comprising: a chamber; a substrate holder for supporting a substrate; a heating element for heating the substrate; a reflector plate below the substrate; a pyrometer configured to penetrate the reflector plate and measure the temperature of the substrate, the pyrometer being capable of being used to detect temperature errors resulting from deposition on the reflector plate; and 4. The method for detecting a temperature error, comprising: measuring a first signal with a first sensor of the pyrometer; measuring a second signal with a second sensor of the pyrometer; determining a reflectivity of the reflector plate from the first signal and the second signal; determining the temperature error using the reflectance; and comparing the temperature error with an estimated temperature error provided by a physics-based model, wherein the temperature error is stored for future use when the absolute value of the difference between the temperature error and the estimated temperature error is less than a predetermined threshold; and Including, the system.

14. 14. The system of claim 13, wherein the first sensor is an emisometer sensor, the second sensor is a main sensor, the first signal from the emisometer sensor is an IE signal, and the second signal from the main sensor is an IM signal.

15. The system of claim 14 , wherein the reflectivity of the reflector plate is determined using a ratio of the IE signal to the IM signal.

16. 16. The system of claim 15, wherein the temperature error is proportional to a change in the reflectivity of the reflector plate compared to an original state of the reflector plate.

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