Passive and active calibration methods for resistive heaters

Abstract

A method of calibrating a heater, comprising powering the heater to a first temperature setpoint, the heater comprising a resistive heating element having a varying temperature coefficient of resistance, the method further including simultaneously obtaining a plurality of resistance measurements of the resistive heating element and a plurality of reference temperature measurements of a reference member as the heater cools from the first temperature setpoint to a second temperature setpoint that is lower than the first temperature setpoint, and generating a resistance-temperature calibration table that associates the plurality of resistance measurements with the plurality of reference temperature measurements.

Description

[Technical Field]

[0001] cross reference This application claims priority to and the benefit of U.S. Provisional Patent Application No. 63 / 027,285, filed May 19, 2020, the disclosure of which is incorporated herein by reference.

[0002] The present disclosure relates to calibrating resistive heaters. [Background technology]

[0003] The statements in this section merely provide background information related to the present disclosure and do not necessarily constitute prior art.

[0004] Pedestal heaters for semiconductor processing typically include a heating plate having a substrate and one or more resistive heating elements disposed thereon that define one or more heating zones. In some applications, the resistive heating element has only two leads (e.g., two for the heating element and two for a separate temperature sensor) to function as both a heater and a temperature sensor, rather than four leads. In such resistive heating elements, the resistive material defines a temperature coefficient of resistance (TCR), and the temperature of the resistive heating element can be determined based on the TCR and a resistance measurement of the heating element. Summary of the Invention [Problem to be solved by the invention]

[0005] Pedestal heaters, such as multi-zone heaters, can be controlled by a control system that determines the temperature of a resistive heating element based on its resistance. To control the multi-zone heater, the control system calculates a resistance based on voltage and / or current measurements and determines the temperature of each zone based on the calculated resistance. While predefined resistance-temperature data, such as a table relating resistance to temperature, can be used, even if the resistive heating elements are made from the same material, heaters may perform differently from one another. This can occur, for example, due to manufacturing variations, material batch variations, heater age, number of cycles, and / or other factors, which can cause inaccuracies in the calculated temperature. These and other problems associated with the use of two-wire resistive heaters, for example, in multi-zone applications, are addressed by the present disclosure. [Means for solving the problem]

[0006] This section provides a general summary of the invention and is not intended to be a comprehensive disclosure of its entire scope or all of its features.

[0007] In one aspect, the disclosure relates to a method that includes powering a heater in an isothermal environment, the heater including a resistive heating element having a varying temperature coefficient of resistance, to a first temperature setpoint, the method further including simultaneously obtaining a plurality of resistance measurements of the resistive heating element and a plurality of reference temperature measurements of a reference member while the heater is passively cooled from the first temperature setpoint to a second temperature setpoint that is lower than the first temperature setpoint, and generating a resistance-temperature calibration table that associates the plurality of resistance measurements with the plurality of reference temperature measurements.

[0008] In another form, the method further includes removing power to the heater when the heater is at the first temperature setpoint to passively cool the heater.

[0009] In yet another embodiment, the reference member is the outer surface of the heater.

[0010] In one form, the plurality of reference temperature measurements of the surface of the heater are obtained by an infrared camera.

[0011] In another aspect, the plurality of reference temperature measurements are obtained by a thermocoupled wafer, and the reference member is the thermocoupled wafer.

[0012] In yet another embodiment, the method further includes measuring at least one of a current and a voltage simultaneously with the plurality of reference temperatures to obtain one resistance measurement value of the plurality of resistance measurements, and determining the resistance measurement value based on at least one of the measured current and voltage.

[0013] In one aspect, the disclosure relates to a method that includes powering a heater in a specified environment, the heater including a resistive heating element having a varying temperature coefficient of resistance, to a first temperature setpoint, the method further including simultaneously obtaining a plurality of resistance measurements of the resistive heating element and a plurality of reference temperature measurements of a reference member while the heater is passively cooled from the first temperature setpoint to a second temperature setpoint that is lower than the first temperature setpoint, and generating a resistance-temperature calibration table that associates the plurality of resistance measurements with the plurality of reference temperature measurements.

[0014] In another aspect, the designated environment for the heater is an isothermal environment.

[0015] In yet another aspect, the designated environment is an operating environment in which the heater is operable to heat a workpiece.

[0016] In one form, the method further includes removing power to the heater when the heater is at the first temperature setpoint to passively cool the heater.

[0017] In another embodiment, the reference member is the outer surface of the heater.

[0018] In yet another aspect, the plurality of reference temperature measurements of the surface of the heater are obtained by an infrared camera.

[0019] In one embodiment, the plurality of reference temperature measurements are obtained by a thermocoupled wafer, and the reference member is the thermocoupled wafer.

[0020] In another form, the method further includes measuring at least one of a current and a voltage simultaneously with the plurality of reference temperatures to obtain one resistance measurement value of the plurality of resistance measurements, and determining the resistance measurement value based on at least one of the measured current and voltage.

[0021] In yet another aspect, the present disclosure relates to a control system for controlling a heater including a resistive heating element. The control system includes a power converter configured to provide an adjustable output voltage to the heater and a controller configured to determine the output voltage applied to the heater. The controller includes a memory configured to store a plurality of control programs for controlling the heater, the plurality of control programs including a calibration process. The controller further includes a processor configured to execute the plurality of control programs, the heater being placed in a specified environment. The calibration process includes instructions for: initiating a supply of power to the heater to heat the heater to a first temperature setpoint; simultaneously obtaining a plurality of resistance measurements of the resistive heating element and a plurality of reference temperature measurements of a reference member while the heater is passively cooled from the first temperature setpoint to a second temperature setpoint; and generating a resistance-temperature calibration table relating the plurality of resistance measurements to the plurality of reference temperature measurements.

[0022] In one form, the calibration process further includes instructions for removing power to the heater when the heater is at the first temperature setpoint to passively cool the heater.

[0023] In another embodiment, the reference member is the outer surface of the heater.

[0024] In yet another embodiment, the second temperature set point is lower than the first temperature set point.

[0025] In yet another aspect, the designated environment is an isothermal environment.

[0026] In another form, to obtain one resistance measurement of the plurality of resistance measurements, the calibration process further includes instructions for measuring at least one of a current and a voltage simultaneously with the plurality of reference temperature measurements, and determining the resistance measurement based on the at least one of the current and the voltage.

[0027] Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. [Brief explanation of the drawings]

[0028] In order that the present disclosure may be better understood, various aspects thereof will now be described, by way of example, with reference to the accompanying drawings, in which:

[0029] [Figure 1A] FIG. 1 is a functional block diagram of a thermal system according to the present disclosure.

[0030] [Figure 1B] FIG. 1B is a functional block diagram of a control system for the thermal system of FIG. 1A.

[0031] [Figure 2A] FIG. 1 is a top view of an exemplary heater having a resistive heating element.

[0032] [Figure 2B] 2B is a representative partial cross-sectional view of the heater of FIG. 2A.

[0033] [Figure 3] 10 is a graph illustrating resistance temperature offset for a two-zone pedestal heater according to the present disclosure.

[0034] [Figure 4] FIG. 1 illustrates a passive calibration setup according to the present disclosure.

[0035] [Figure 5A] FIG. 1 illustrates an active calibration test setup according to the present disclosure. [Figure 5B] FIG. 1 illustrates an active calibration test setup according to the present disclosure.

[0036] [Figure 6] 1 is a flow chart of a resistance-temperature calibration process according to the present disclosure.

[0037] The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. DETAILED DESCRIPTION OF THE INVENTION

[0038] The following description is merely exemplary in nature and is in no way intended to limit the present disclosure, application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features.

[0039] The present disclosure generally relates to a resistance-temperature (RT) calibration process for a heater, which may be a multi-zone heater having a resistive heating element operable as a heater and a sensor. The RT calibration process described herein generates RT offset data that correlates multiple resistance measurements with multiple reference temperature measurements. The RT offset data is used to determine the temperature of the resistive heating element based on the resistance measurements of the resistive heating element during normal operation of the multi-zone heater.

[0040] To illustrate the RT calibration process according to the disclosed technique, an exemplary configuration of a thermal system having a multi-zone heater and a control system is first provided. As shown in Figures 1A and 1B, the thermal system 100 includes a multi-zone pedestal heater 102 and a control system 104 having a heater controller 106 and a power converter system 108. In one embodiment, the heater 102 includes a heating plate 110 and a support shaft 112 disposed on a bottom surface of the heating plate 110. The heating plate 110 includes a substrate 111 and a plurality of resistive heating elements (not shown) embedded in or disposed along the surface of the substrate 111. For example, such a heater is described in co-pending U.S. patent application Ser. No. 16 / 196,699, filed Nov. 20, 2018, and entitled "MULTI-ZONE PEDESTAL HEATER HAVING A ROUTING LAYER," which is commonly owned with this application and is hereby incorporated by reference in its entirety.

[0041] In one form, the substrate 111 is made from ceramic or aluminum. The resistive heating elements are independently controlled by a heater controller 106 and define multiple heating zones 114, as shown by the dashed lines in FIG. 1A. It will be readily understood that the heating zones may have different configurations and may include more than one heating zone while remaining within the scope of the present disclosure. For example, as shown in FIGS. 2A and 2B, the heater 102 may be a heater 200 including a dielectric layer 202, a resistive layer 204 defining one or more resistive heating wires (i.e., resistive heating elements), and a protective layer 206 disposed on a substrate 208.

[0042] In one form, the heater 102 is a "two-wire" heater in which the resistive heating element has only two leads operatively connected to it rather than four, functioning as both a heater and a temperature sensor. Such two-wire capabilities are disclosed, for example, in commonly assigned U.S. Patent No. 7,196,295, incorporated herein by reference in its entirety. Typically, in a two-wire system, the resistive heating element is defined by a material whose resistance changes with temperature, such that the average temperature of the resistive heating element can be determined based on the change in resistance of the resistive heating element. In one form, the resistance of the resistive heating element is calculated by first measuring the voltage across and current through the heating element and then utilizing Ohm's Law. The resistive heating element is defined by a material with a relatively high temperature coefficient of resistance (TCR), a negative TCR material, or in other words, a material with a nonlinear TCR. Although heater 102 is presented as a pedestal heater, the present disclosure is applicable to other types of heaters, such as electrostatic chuck (ESC) heaters, nozzle heaters, or fluid heaters, among others, and should not be limited to the pedestal heaters shown and described herein.

[0043] The control system 104 is adapted to control the operation of the heater 102, and more particularly to independently control the power to each of the zones 114. In one form, the control system 104 is electrically connected to the zones 114 via terminals 115 such that each zone 114 is connected to two terminals for supplying power and sensing temperature.

[0044] In one form, the control system 104 is communicatively coupled (e.g., wirelessly and / or via a wired connection) to a computing device 117 having one or more user interfaces such as a display, keyboard, mouse, speakers, touch screen, etc. A user can use the computing device 117 to provide inputs or commands such as temperature setpoints, power setpoints, commands to run tests, or processes stored by the control system.

[0045] The control system 104 is electrically connected to a power supply 118, which provides an input voltage (e.g., 240V, 208V) to the power converter system 108, via an associated interlock 120. The interlock 120 controls the power flow between the power supply 118 and the power converter system 108 and is operable by the heater controller 106 as a safety mechanism to disconnect power from the power supply 118. Although shown in FIG. 1A, the control system 104 need not include the interlock 120.

[0046] The power converter system 108 regulates the input voltage to produce an output voltage (V OUT) to the heater 102. In one form, the power converter system 108 comprises a plurality of power converters 122 (shown as 122-1 through 122-N) operable to apply adjustable power to the resistive heating elements of a given zone 114 (shown as 114-1 through 114-N). An example of such a power converter system is disclosed in commonly assigned U.S. Pat. No. 10,690,705, the entire contents of which are incorporated herein by reference. In this example, each power converter comprises a step-down converter operable by a heater controller to generate a desired output voltage less than or equal to the input voltage to one or more heating elements of a given zone 114. Thus, the power converter system is operable to provide a customizable amount of power (i.e., the desired power) to each zone of the heater.

[0047] Using two-wire heaters, the control system 104 includes sensor circuits 124 (i.e., 124-1 through 124-N in FIG. 1B ) to measure the electrical characteristics (i.e., voltage and / or current) of the resistive heating elements, which are used to determine zone performance characteristics such as resistance, temperature, and other suitable information. In one form, a given sensor circuit 124 includes an ammeter 126 and a voltmeter 128 to measure the current flowing through and the voltage applied to the heating element of a given zone 114, respectively. Each ammeter 126 includes a shunt 130 for measuring the current, and each voltmeter 128 includes a voltage divider 132, which is represented by resistors 132-1 through 132-2. Alternatively, the ammeters 126 can measure current using HAL sensors or current transformers instead of the shunts 130. In one form, the ammeter 126 and voltmeter 128 are provided as power measurement chips that simultaneously measure current and voltage regardless of the power applied to the heating element. In another form, voltage and / or current measurements are taken at zero crossings, as described in U.S. Patent No. 7,196,295.

[0048] The heater controller 106 includes one or more microprocessors and memory for storing computer-readable instructions executed by the microprocessors. The heater controller 106 is adapted to execute one or more control processes in which the heater controller 106 determines a desired power to be applied to a zone, such as 100% of the input voltage, 90% of the input voltage, etc. Exemplary control processes are described in commonly assigned U.S. Pat. Nos. 10,690,705 and 10,908,195, which are incorporated by reference in their entireties. In one form, the control process adjusts the power applied to the resistive heating element based on the temperature of the resistive heating element and / or the temperature of the workpiece.

[0049] To obtain accurate temperature measurements, the heater controller 106 is operable to execute the RT calibration process 150 of the present disclosure to generate a correlation between the resistance of the resistive heating element and the temperature of a reference area (i.e., reference temperature) around the heater 102. More specifically, during normal operation when the heater 102 is heating a workpiece, the heater controller 106 determines the surface temperature of the heater 102 upon which the workpiece is positioned based on the current resistance measurements and the RT offset data, thereby eliminating the need for a separate sensor.

[0050] Referring again to FIG. 1A , for the RT calibration process, the thermal system 100 is provided with one or more separate reference sensors 152 for measuring the temperature of a reference area. The reference sensor 152 can be an infrared camera, a thermocouple (TC) wafer, one or more thermocouples, a resistance-temperature detector, and / or other suitable sensors for measuring temperature. For example, in one embodiment, the reference sensor 152 is an infrared camera positioned on the heater 102 to measure the surface temperature of the heater 102, with the surface of the heater 102 being the reference area and the surface temperature being the reference temperature. In another example, the reference sensor can be a TC wafer having a wafer and multiple TCs distributed along the wafer for measuring the temperature. During calibration, the TC wafer is placed on the heater 102 and secured to the surface using various methods, including, but not limited to, pressurizing a chamber containing the heater 102 and TC wafer, gluing the TC wafer to the heater 102, or gravity. Each TC on the TC wafer measures a temperature that is provided to the control system 104. With the surface of the TC wafer in contact with the heater 102, the reference area is provided as the surface of the heater 102, and the reference temperature is the temperature along the surface of the heater.

[0051] For the RT calibration process, the control system 104 is configured to heat the heater 102, and more specifically, to heat the surface of the heater 102 to a first temperature setpoint (T_sp1). Once the surface has a consistent temperature profile, the control system 104 stops powering the heater and simultaneously measures the reference temperature and the resistance of the resistive heating element for each zone until the reference temperature equals a second temperature setpoint (T_sp2) that is less than the first temperature setpoint. For the resistance measurements, the control system 104 obtains voltage and current measurements from the sensor circuit to determine the resistance of the resistive heating element. In one embodiment, the reference temperature and resistance measurements are measured continuously based on the processing speed of the reference sensor and sensor circuit. In another embodiment, the reference temperature and resistance measurements are measured periodically (e.g., every 5 minutes, every 10 minutes, among other time intervals). It should be readily understood that any number of measurements may be obtained to determine the temperature offset data and is not limited to the examples described herein.

[0052] The control system 104 correlates the reference temperature measurements with the resistance measurements of the resistive heating elements to obtain RT offset data. Based on the type and / or number of reference sensors, the control system 104 processes the raw measurements from the reference sensors to obtain reference temperature measurements. For example, with respect to an IR camera, the thermal image provided by the IR camera provides the surface temperature across the surface of the heater heated by multiple heating zones defined by one or more resistive heating elements. Thus, for a given heating element, the control system 104 correlates the resistance value of the given resistive heating element with the reference temperature measurement for each area heated by the given resistive heating element. A similar correlation is completed for TC wafers, such that temperature measurements from TCs provided to a particular area of ​​the wafer are associated with the resistive heating element heating that area.

[0053] The control system 104 generates and stores RT offset data and uses the RT offset data to determine a reference temperature based on resistance measurements of the resistive heating elements. In one form, the RT offset data can be provided as a table, chart, and / or algorithm, among other formats. The RT offset data can be provided simply as resistance and temperature measurements, or can be a parameter dependent on resistance and / or temperature, such as TCR vs. temperature. For example, FIG. 3 depicts a graph capturing RT offset for a two-zone pedestal heater. Specifically, the graph provides data (TCR vs. temperature) for pedestals A through D, each having a zone 1 (Z1) and a zone 2 (Z2).

[0054] The RT calibration process of the present disclosure is performed under various conditions to obtain material properties of the resistive heating element and correlate the material properties to, for example, the surface temperature of the heater or other reference area. In particular, the RT calibration process can be performed as a passive calibration with the heater thermally isolated or in an isothermal environment, and / or as an active calibration with the heater provided in its operating environment, such as a semiconductor processing chamber.

[0055] Instead of or in addition to a standard RT curve for the particular material that defines the resistive heating element, passive calibration generates a custom RT curve for the resistive heating element within the heater. To obtain the custom RT curve, the heater 102 is thermally insulated to minimize heat loss from the resistive heating element so that the surface temperature of the heater is equal to or substantially the same as the temperature of the resistive heating element.

[0056] In an exemplary configuration, FIG. 4 illustrates a setup 500 for passive calibration, in which a multi-zone heater is provided within an isothermal environment. Specifically, the setup 500 for passive calibration includes an isothermal chamber 502 that houses a multi-zone heater 504 having multiple resistive heating elements. The multi-zone heater 504 is similar to the heater 102. Here, the isothermal chamber 502 includes insulation that encases the heater 504 to thermally insulate the heater and reduce heat loss between the resistive heating elements and the surface of the heater 504. It should be understood that the isothermal environment for the multi-zone heater 504 can adopt other suitable configurations and is not limited to the isothermal chamber 502.

[0057] The setup 500 for passive calibration further comprises a control system 506 similar to the control system 104 for controlling the supply of power to the heater 504. Here, reference sensors are provided as multiple TCs 508 arranged to measure the surface temperature of the heater 504 at various locations along the surface, with at least one temperature measurement being taken for each heating zone.

[0058] In this configuration, the control system 506 performs the RT calibration process of the present disclosure to measure the resistance of the resistive heating element and the surface temperature in each of the zones. An operator can set the frequency of measurements, for example, to measure the resistance and temperature continuously or to periodically take measurements. Based on the received data, the control system 506 generates an RT curve relating the resistance of the resistive heating element to the surface temperature of the heater 504, which in turn indicates the temperature of the resistive heating element. In one form, the control system 506 provides an RT curve for each heating zone using the resistance measurements for the resistive heating element in a given zone and the temperature measurements taken in the heating zone. For example, FIG. 3 shows RT curves generated during passive calibration for various two-zone heaters, each having an inner zone and an outer zone.

[0059] For an active calibration process, the RT calibration process is performed to obtain RT offset data under the same operating conditions as when the heater 102 is heating a workpiece. That is, the active calibration process captures the effect that the operating conditions have on the heater 102, and therefore on the resistive heating element. Specifically, this RT offset data may differ from the RT offset data during a passive calibration process due to, for example, heat losses between the resistive heating element and the surface of the heater 102, and between the surface of the heater 102 and the external environment.

[0060] 5A and 5B show a setup 600 for an active calibration test in which a heater 602 is located within a semiconductor processing chamber 604 designed to heat semiconductor wafers. Heater 602 is a multi-zone heater similar to heater 102. In this example, semiconductor processing chamber 604 is for a test process and mimics an actual semiconductor processing chamber. In one variation, the active calibration process may be performed in an actual semiconductor chamber manufacturing facility.

[0061] The setup 600 for active calibration testing further comprises a control system 606, similar to control system 104, to control the power supply to the heater 602. Here, a reference sensor is provided as a TC wafer 608 that measures the surface temperature of the heater 602, which is the reference area that is measured. Instead of the TC wafer 608, one or more TC or IR cameras may be used to measure the surface temperature of the heater 102. The control system 606 performs the RT calibration process of the present disclosure to measure the resistance of the resistive heating element and the surface temperature at each of the zones and generate RT offset data as described above.

[0062] Although not shown in the calibration setups of FIGS. 4, 5A, and 5B, each control system is communicatively coupled to other components, such as a reference sensor and / or a heater.

[0063] In one form, a heater (such as, for example, heater 102) undergoes passive and active calibration to obtain RT offset data relating controlled resistance measurements of the resistive heating element in the passive calibration to uncontrolled resistance measurements in the active calibration. In another form, the heater can undergo active calibration but not passive calibration.

[0064] As shown in FIG. 6 , an RT calibration process 700 is provided that can be performed by a control system of the present disclosure. With a reference sensor in place, the control system applies power to a heating zone to generate heat at 702 and obtains a reference temperature measurement from the reference sensor at 704. At 706, the control system determines whether the obtained reference temperature measurement is equal to a first temperature setpoint (T_sp1). That is, the control system receives temperature measurements for each heating zone of the heater and determines whether the heater surface temperature is constant (i.e., T_sp1). If so, the control system stops power to the heater at 708 and simultaneously measures the resistance and reference temperature. At 710, the control system determines whether the reference temperature is equal to a second temperature setpoint (T_sp2). If so, the control system stops the measurement at 712 and correlates the reference temperature to the resistance measurement to obtain RT offset data.

[0065] It should be understood that the RT calibration process 700 is just one example of an RT calibration process and that other suitable routines may be used.

[0066] Unless expressly stated otherwise, all numerical values ​​indicating mechanical / thermal properties, compositional proportions, dimensions and / or tolerances, or other characteristics will be understood as being modified by the word "about" or "approximately" when describing the scope of this disclosure. This modification may be desirable for various reasons, including industrial practices, material, manufacturing, and assembly tolerances, and performance testing.

[0067] As used herein, the phrase at least one of A, B, and C should be construed to mean a logical non-exclusive OR (A OR B OR C), and not to mean "at least one of A, at least one of B, and at least one of C."

[0068] In the drawings, the direction of the arrows indicated generally indicates the flow of information (data or instructions) that is of interest to the illustration. For example, if element A and element B exchange various information, but the information sent from element A to element B is relevant to the illustration, the arrow points from element A to element B. This one-way arrow does not imply that other information is not being sent from element B to element A. Also, in response to information sent from element A to element B, element B can send element A a request for that information or an acknowledgment of receipt of that information.

[0069] As used herein, the term "controller" is interchangeable with the term "circuitry." Controller means, is a part of, or comprises: an application-specific integrated circuit (ASIC); digital, analog, or mixed analog / digital discrete circuitry; digital, analog, or mixed analog / digital integrated circuitry; combinatorial logic circuitry; field-programmable gate array (FPGA); processor circuitry (shared, dedicated, or group) that executes code; memory circuitry (shared, dedicated, or group) that stores code to be executed by the processor circuitry; other suitable hardware components that provide the above functionality; or a combination of some or all of the above, such as a system-on-chip.

[0070] The term "code" includes software, firmware, and / or microcode, and may also be referred to as programs, routines, functions, classes, data structures, and / or objects. The term "memory" is part of the term "computer-readable medium." The term "computer-readable medium," as used herein, does not include transitory electrical or electromagnetic signals propagating through a medium (such as a carrier wave). As such, the term "computer-readable medium" may be considered tangible and non-transitory.

[0071] The description of the present disclosure is merely exemplary in nature and, thus, variations that do not depart from the gist of the disclosure are intended to be within the scope of the disclosure. Such variations are not to be regarded as a departure from the spirit and scope of the disclosure.

Claims

1. 1. A method of calibrating a heater, comprising: applying power to a heater in an isothermal environment, the heater comprising a resistive heating element having a temperature coefficient of resistance, to a first temperature setpoint; removing power to the heater when the heater is at the first temperature setpoint to passively cool the heater; simultaneously obtaining a plurality of resistance measurements of the resistive heating element and a plurality of reference temperature measurements of a reference member while the heater is passively cooled from the first temperature setpoint to a second temperature setpoint; generating a resistance-temperature calibration table relating the plurality of resistance measurements to the plurality of reference temperature measurements; A method comprising:

2. The method of claim 1 , wherein the reference member is an outer surface of the heater.

3. The method of claim 2 , wherein the plurality of reference temperature measurements of the exterior surface of the heater are obtained by an infrared camera.

4. The method of claim 1 , wherein the plurality of reference temperature measurements are taken by a thermocoupled wafer, and the reference member is the thermocoupled wafer.

5. 2. The method of claim 1, further comprising: measuring at least one of a current and a voltage simultaneously with acquiring the plurality of reference temperature measurements to obtain one resistance measurement of the plurality of resistance measurements; and determining the resistance measurement based on the measured at least one of the current and voltage.

6. 1. A method of calibrating a heater, comprising: applying power to a heater in a designated environment, the heater comprising a resistive heating element having a varying temperature coefficient of resistance, to a first temperature setpoint; removing power to the heater when the heater is at the first temperature setpoint to passively cool the heater; simultaneously obtaining a plurality of resistance measurements of the resistive heating element and a plurality of reference temperature measurements of a reference member while the heater is passively cooled from the first temperature setpoint to a second temperature setpoint that is lower than the first temperature setpoint; generating a resistance-temperature calibration table relating the plurality of resistance measurements to the plurality of reference temperature measurements; A method comprising:

7. The method of claim 6 , wherein the designated environment is an isothermal environment.

8. 7. The method of claim 6, wherein the designated environment is an operating environment within which the heater is operable to heat a workpiece.

9. The method of claim 5 , wherein the reference member is the outer surface of the heater.

10. The method of claim 9 , wherein the plurality of reference temperature measurements of the exterior surface of the heater are obtained by an infrared camera.

11. 6. The method of claim 5, wherein the plurality of reference temperature measurements are taken by a thermocoupled wafer, and the reference member is the thermocoupled wafer.

12. 6. The method of claim 5, further comprising: measuring at least one of a current and a voltage simultaneously with obtaining the plurality of reference temperature measurements to obtain one resistance measurement of the plurality of resistance measurements; and determining the resistance measurement based on the measured at least one of the current and voltage.

13. 1. A control system for controlling a heater comprising a resistive heating element, comprising: a power converter adapted to provide an output voltage to the heater that is adjustable; a controller configured to determine the output voltage applied to the heater; The control unit a memory adapted to store a plurality of control programs for controlling the heater, the plurality of control programs including a calibration process; a processor configured to execute the plurality of control programs; The calibration process includes, with the heater in a designated environment: commencing power to the heater to heat the heater to a first temperature setpoint; removing power to the heater when the heater is at the first temperature setpoint to passively cool the heater; simultaneously obtaining a plurality of resistance measurements of the resistive heating element and a plurality of reference temperature measurements of a reference member while the heater is passively cooled from the first temperature setpoint to a second temperature setpoint; generating a resistance-temperature calibration table relating the plurality of resistance measurements to the plurality of reference temperature measurements; a control system including instructions for performing the steps of:

14. The control system of claim 13 , wherein the reference member is an outer surface of the heater.

15. 14. The control system of claim 13, wherein the second temperature set point is lower than the first temperature set point.

16. The control system of claim 13 , wherein the designated environment is an isothermal environment.

17. 14. The control system of claim 13, wherein the calibration process further includes instructions for measuring at least one of a current and a voltage simultaneously with obtaining the reference temperature measurement to obtain one resistance measurement of the plurality of resistance measurements, and determining the resistance measurement based on the at least one of the current and the voltage.

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