METHOD AND APPARATUS FOR REDUCING THERMAL GRADIENTS WITHIN A CERAMIC WAFER SUPPORT PEDESTAL

Abstract: A method and apparatus for reducing the thermal gradients within a ceramic wafer support pedestal. Specifically, the present invention is a heater controller that limits the amount of power that is applied to a resistive heater embedded within a ceramic pedestal. In a first embodiment of the invention limits the current that is delivered to the heater. In a second embodiment of the invention, the power applied to the heater coil is clamped to a maximum level.

Description: METHOD AND APPARATUS FOR REDUCING THERMAL GRADIENTS WITHIN A CERAMIC WAFER SUPPORT PEDESTAL BACKGROUND OF THE DISCLOSURE 1. Field of the Invention The present invention relates to semiconductor wafer processing equipment and, more particularly, to a method and apparatus for reducing the thermal gradients in a ceramic wafer support pedestal by controlling the power or current applied to a resistive wafer heater within the pedestal. 2. Description of the Background Art Within a process chamber of a semiconductor wafer processing system, a semiconductor wafer is typically supported by a pedestal while being processed. In many such systems to facilitate processing of the wafer, the pedestal is heated to raise the temperature of the wafer during one or more of the process steps. To facilitate heat transfer to the wafer, the pedestal is fabricated from a ceramic material and the heater is a resistive heater element embedded in the ceramic. The heater element is generally a coil of resistive wire or a metallized layer fabricated from a material such as tungsten. When current is applied to this wire or layer, the element becomes hot and the rising temperature of the heater is conductively transferred through the ceramic to the wafer. The resistance of the heater element varies substantially as current is applied to the element and the temperature of the element rises. The resistance of the heater element may change by as much as a factor of three. Consequently, the power applied to by the heater varies from, e.g., 2400 watts, when the element is at room temperature (relatively low resistivity of approximately 6 ohms at room temperature), to, e.g., 800 watts, when the element is at operating temperature of approximately 550" C (relatively high resistivity of approximately 18 ohms at 550 β C) . Such a large amount of power applied to the heater element when it is cold produces substantial thermal gradients within the ceramic of the pedestal. Such thermal gradients tend to cause cracks in the ceramic which ultimately renders the pedestal useless. Therefore, there is a need in the art for a method and apparatus for reducing the thermal gradients in a ceramic pedestal by controlling the amount of power or current applied to a resistive heater within a ceramic pedestal. SUMMARY OF THE INVENTION The disadvantages heretofore associated with the prior art are overcome by the present invention of a method and apparatus for reducing the thermal gradients in a ceramic pedestal. Specifically, the present invention is a heater controller that limits the amount of power or current that is applied to a resistive heater embedded within a ceramic pedestal to a predefined value. In a first embodiment of the invention, the thermal gradient produced in the pedestal is controlled by limiting the current that is delivered to a resistive heater. This is accomplished by using a current transformer coupled to the wires that deliver current from an AC power supply to the heater. The voltage at the output of the current transformer is indicative of the current flowing to the heater. This voltage is supplied to a clamping circuit which clamps its output to a particular threshold voltage level. Illustratively, the threshold voltage level represents an output current limit of 10 amperes. As, such, the output of the clamp will rise linearly with the measured current value until attaining a value that exceeds the threshold voltage level. The output voltage of the clamp is supplied to a phase angle controller that generates a firing angle control signal. The phase angle controller is coupled to a pair of silicon-controlled rectifiers (SCRs) that perform the switching function within the AC power supply that powers the resistive heater. The AC output of the SCRs is coupled through the current transformer to the resistive heater. In a second embodiment of the invention, the output power level of the AC power supply is limited to a maximum level, e.g., 1000 watts. As such, the power delivered to the heater is limited such that the thermal gradients are maintained within a limited range. Consequently, the ceramic of the pedestal will not crack as the pedestal is heated. Although the current and/or power levels in each of the embodiments of the invention are described as being "hard" limits that are preset at a particular level, these limits may be varied with time to provide a dynamic power control circuit. Such dynamic control can be facilitated using a predefined control algorithm or an adaptive feedback circuit that responds to various input parameters, e.g., temperature, output power and the like. BRIEF DESCRIPTION OF THE DRAWINGS The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: FIG. 1 depicts a cross-sectional view of a ceramic pedestal containing a resistive heater that is driven by a heater controller in accordance with the present invention; FIG. 2 depicts a sectional view of the resistive heater taken along line 2-2 of FIG. 1; FIG. 3 depicts a block diagram of the heater controller in accordance with a first embodiment of the present invention; FIG. 4 depicts a graph of power versus temperature for a resistive heater that is controlled in accordance with the present invention; and FIG. 5 depicts a block diagram of the heater controller in accordance with a second embodiment of the present invention. To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. DETAILED DESCRIPTION FIG. 1 depicts a cross-sectional view of a ceramic pedestal assembly 100 which contains a ceramic pedestal 102, a heater controller 106, a proportional integral-derivative (P1D) controller 116 and an optional electrostatic chuck controller 108. The pedestal 102 has a top surface 118 upon which a wafer 104- is supported. FIG. 2 depicts a cross- section view of the ceramic pedestal 100 taken along line 2-2 of FIG. 1. To best understand the invention the reader should simultaneously refer to FIGS. 1 and 2. The resistive heater 112 contains a heater element, for example, a coil or a metallized layer of resistive material such as tungsten. When a current is applied to the heater 112 the resistance of the heater element generates heat which, in turn, heats the ceramic surrounding the heater element and ultimately the wafer 104 supported by the ceramic pedestal 100. The heater element should be interpreted as any element whose temperature rises as an electrical signal is applied to the element. The power supplied to the heater 112 is coupled through wires 114 from a heater controller 106. A control signal SI is generated by a conventional PID controller 116. The PID controller depicted in FIG. 1, monitors the temperature of the pedestal using a thermocouple 118. The signal SI is generally a voltage in the range of 0 to 10 volts. The specific magnitude of signal SI indicates the amount of current necessary to achieve a desired temperature for the pedestal. This value is dynamically adjusted using a conventional PID algorithm to avoid temperature ringing and/or overshoot as the temperature of the pedestal approaches the desired value. In accordance with the present invention, the heater controller 106 processes signal SI and limits the amount of power or current coupled through wires 114 to the resistive heater 112. Although the wafer 104 may be clamped to the surface 118 of the pedestal 102 in a variety of ways including a peripheral mechanical clamp, a vacuum clamp, gravity and the like, preferably, an electrostatic chuck facilitates clamping the wafer 104 to the surface 118 of the pedestal 102. Use of an electrostatic chuck facilitates uniform thermal transfer from the wafer to the pedestal as well as improves wafer stability and, reduces particle generation, and the like. This optional electrostatic chuck is drawn in phantom as having coplanar electrodes 110. and 110 2 coupled to an electrostatic chuck controller 108. The electrostatic chuck operates as a bipolar chuck having two coplanar electrodes 110. and 110.. The electrostatic chuck controller 108 applies to each electrode an equal and opposite polarity voltage which generates an electric field between the electrodes. This electric field is coupled through the semiconductor wafer 104 such that charges accumulate on the underside of the wafer 104 and opposing charges accumulate on the electrodes 110. and 110 2 such that an electrostatic force between the charges retains the semiconductor wafer 104 upon the surface 118 of the pedestal 102. Although a bipolar electrostatic chuck is shown, any form of electrostatic chuck could be used with the present invention including a monopolar structure having a single electrode, an interdigital structure having a plurality of electrodes and the like. The chuck can be powered by AC or DC voltage. FIG. 3 depicts a detailed block diagram of the heater controller 106 in accordance with the first embodiment of the invention. Specifically, the controller 106 contains a current clamp 300, a phase angle controller 302, AC voltage switching electronics 304, and a current transformer 310. The current clamp 300 limits its output voltage S2 to a maximum value established by the clamp circuit. The circuitry generally limits the output current coupled to the heater at, for example, 10 amperes. The current clamp 300 contains an RMS converter 320, a signal sealer 322, an inverting limit amplifier 324 and a summing amplifier 326. The transformer 310 provides an AC sample of the current then flowing to the heater 112. The RMS converter 320 converts the AC signal to a DC value having a magnitude that represents the magnitude of the current flowing to the heater 112. The RMS converter 320 is coupled to the sealer 322. The sealer adjusts the magnitude of signal S4 to a value comparable in magnitude to signal SI. The scaled signal S4 is applied to the inverting limit amplifier 324, where signal S4 is compared to a predetermined voltage signal corresponding to the desired current limit. If the voltage difference between S4 and the limit voltage signal is positive, this difference is inverted, amplified and output as signal S5. If the voltage difference between S4 and the limit voltage signal is negative or zero signal S5 remains zero volts. Both signals SI and S5 are coupled to a summing amplifer 326 whose output signal S2 is the sum of signals SI and S5. The current clamp 300, in this manner, clamps the output voltage S2 to a predefined level such that the output current of the heater does not exceed a certain value, e.g., 10 amperes. As such, the output signal S2 is proportional to the signal SI from the PID controller until the magnitude of signal S4 attains a level equal to or greater than the clamping voltage. At that point, the magnitude of signal S2 is clamped to a maximum voltage value. The polarity of signal S5 is opposite the polarity of the PID controller signal SI. As such, signal S2=S1-S5. The phase angle controller 302 conventionally develops a firing angle control signal on path 316 in response, to the signal S2. The firing angle control signal on path 316 is coupled to the switching electronics 304. The switching electronics 304 comprises a pair of silicon-controlled rectifiers (SCRs) 306 and 308. This SCR circuit is conventional in the sense that, given a particular firing angle control signal, a specific percentage of each cycle of the AC input voltage (e.g., 120 volts) is applied to the output of each of the SCRs and ultimately to the resistive heater 112. As such, changing the duration of the firing angle control signal changes the amount of voltage, and therefore the current that is applied to the resistive heater 112. FIG. 4 depicts a graph 400 of power (axis 402) versus temperature (axis 404) as produced by the present invention. Consequently, using the present invention with a limit of 10 amperes causes the power level to rise (along path 406) to a maximum of, e.g., 1200 watts, relatively linearly as the resistance of the header element increases with temperature and the current is limited at 10 amperes. During this portion of the curve, the PID controller signal SI is at a maximum value and the signal S5 is at the threshold value. The apex 408 of the curve 400 occurs at about 300" C. Once the power attains the 1200 watt level, the resistance of the heater element continues to increase and the current falls below the 10 ampere limit such that the curve along portion 412 linearly declines from point 408 to point 414 where a nominal temperature of 550" C. is attained and the heater consumes about 800 watts. During this portion of the curve, the PID controller signal SI is at or near the maximum value and signal S5 is zero, resulting in no current limit signal to summing amplifier 326. Consequently, maximum or near maximum voltage is applied to the heater 121. The graph 400 also depicts the power to the heater when no current limiting is applied (curve portion 416). At room temperature (without limiting), the power applied to the heater is 2400 watts as compared to 600 watts with current limiting. As such, using current limiting avoids applying excessive power to the heater element and reduces the thermal gradients within the pedestal. In a second embodiment of the invention depicted in FIG. 5, rather than limit the current applied to the heater, the total power delivered to the heater is limited at, e.g., 1000 watts. As such, as the resistance of the heater changes, with respect to the heater resistance the current control input is varied in an inverse relationship such that the power is maintained at a constant value (e.g., 1000 watts) . To perform power limiting, the heater controller 106 of FIG. 3 is modified to become a heater controller 500 of FIG. 5. To determine the power being coupled to the heater, the invention samples both current and voltage at the output of the SCR pair 306 and 308. The current is monitored with a current transformer 310 and the voltage is monitored with voltage transformer 514. Both the voltage and current signals (S10 and Sll, .respectively) are respectively processed by an RMS converter 504 and 508 and signal sealers 506 and 510. As such, signal S12 is a DC value that represents the magnitude of the output voltage and signal S13 is a DC value that represents the magnitude of the output current. A multiplier 512 multiplies signal S12 and signal S13 to produce signal S14 which represents the RMS power delivered to the heater 112. As with the previous embodiments, an inverting limit amplifier 324 outputs an amplified inverted difference signal S15 when S14 exceeds a predefined voltage corresponding to the desired power limit. The summing amplifer 326 adds the measured signal (S15 in this embodiment) to signal SI from the PID controller to produce signal S2. As with the previous embodiment, signal S2=S1- S15. .Signal S2 is a voltage limited signal that is coupled to the phase angle controller 302. Consequently, the power clamp 502 limits the output power delivered to the heater 112. As depicted in FIG. 4, such a power limit provides a flat portion 410 of the curve 400. As such, the power curve is flat along portion 410 for all temperatures below approximately 400° C. and declines along portion 412 before reaching the nominal power consumption of approximately 800 watts at point 414. Such a power limitation insures that the thermal gradients within the ceramic pedestal are insufficient to cause any physical damage to the pedestal. The two embodiments of the invention can be modified to dynamically control the current or power applied to a resistive heater. As depicted in FIG. 3, the current clamp 300 can be supplemented or replaced with an adaptive controller 318. The adaptive controller 318 responds to various operational parameters 320, e.g., pedestal temperature, power, current, time and the like, to maintain the power and/or current applied to the heater within a nominal range. The adaptive controller may be implemented as hardware, software executed on a microcontroller or a combination of both. Such a controller could establish particular power or cμrrent plateaus to be maintained for certain periods of time, the firing angle control signal could be non-linear, or the firing angle control signal could be adapted in response to an external operating parameter or condition. There has thus been shown and described a novel method and apparatus for controlling the thermal gradients within a ceramic pedestal that is heated by a resistive heater. Many changes, modifications, variations and other uses and applications of this subject invention will however become apparent to those skilled in the art after considering this specification and accompanying drawings which disclose the embodiments thereof. All such changes, modifications, variations and other uses and applications which do not depart from the spirit and scope of the invention are deemed to be covered by the invention, which is to be limited only by the claims which follow.

Claims: What is claimed is: 1. Apparatus for controlling thermal gradients generated within a ceramic wafer support pedestal comprising: a heater controller that limits the maximum amount of current that is supplied to the resistive heater to a predefined value. 2. The apparatus of claim 1 wherein the heater controller further comprises an AC power supply. 3. The apparatus of claim 2 wherein the AC power supply further comprises: a pair of silicon-controlled rectifiers (SCRs) for controlling application of an AC voltage to the resistive heater; and a phase angle control circuit, coupled to said SCRs, for generating a firing angle control signal that is responsive to a current control signal generated by said heater controller. 4. The apparatus of claim 1, wherein said heater controller further comprises: a clamp for producing a current control signal that limits the current supplied to the resistive heater to said predefined value; a pair of silicon-controlled rectifiers (SCRs) for controlling application of an AC voltage to the resistive heater; and a phase angle control circuit, coupled to said SCRs, for generating a firing angle control signal that is responsive to the current control signal generated by said clamp. 5. The apparatus of claim 1 , wherein said resistive heater is a resistive material embedded in the ceramic wafer support pedestal. 6. The apparatus of claim 1 wherein said heater controller further comprises : an adaptive controller for adaptively controlling the current coupled to the resistive heater in response to a plurality of parameters. 7. Apparatus for controlling the thermal gradients generated within a resistive heater embedded within said ceramic wafer support pedestal comprising; a heater controller that limits the maximum amount of power that is supplied to the resistive heater to a predefined value. 8. The apparatus of claim 7 wherein the heater controller further comprises an AC power supply. 9. The apparatus of claim 8 wherein the AC power supply further comprises: a pair of silicon-controlled rectifiers (SCRs) for controlling application of an AC voltage to the resistive heater; and a phase angle control circuit, coupled to said SCRs, for generating a firing angle control signal that is responsive to a power control signal generated by said heater controller. 10. The apparatus of claim 7 wherein said heater controller further comprises: a clamp for producing a power control signal that limits the power supplied to the resistive heater to said predefined value; a pair of silicon-controlled rectifiers (SCRs) for controlling application of an AC voltage to the resistive heater; and a phase angle control circuit, coupled to said SCRs, for generating a firing angle control signal that is responsive to the power control signal generated by said clamp. 11. The apparatus of claim 10 wherein said clamp further comprises a multiplier that derives a power value by multiplying a measured RMS current value with a measured RMS voltage value, where said power value is used to produce said power control signal. 12. The apparatus of claim 7 wherein said resistive heater is a resistive material embedded in the ceramic wafer support pedestal. 13. The apparatus of claim 7 wherein said heater controller further comprises: an adaptive controller for adaptively controlling the power coupled to the resistive heater in response to a plurality of parameters. 14. A method of reducing thermal gradients within a ceramic wafer support pedestal, comprising the steps of: limiting, while the temperature of the ceramic wafer support pedestal is relatively low, the current generated by a heater controller; and applying the limited current to a resistive heater embedded in the ceramic wafer support pedestal. 15. A method of reducing thermal gradients within a ceramic wafer support pedestal comprising the steps of: limiting, while the temperature of the ceramic wafer support pedestal is relatively low, the power generated by a heater controller; and applying said limited power to a resistive heater embedded in the ceramic wafer support pedestal. 16. The method of claim 15 wherein said limiting step further comprises: measuring a voltage and a current produced by said heater controller; multiplying said current and said voltage to form a power signal; limiting the magnitude of said power signal; and using said limited power signal to limit the power generated by said heater controller.

Inventors: BURKHART VINCENT E, BURKHART, VINCENT, E.
Applicants: APPLIED MATERIALS INC, APPLIED MATERIALS, INC.
Application year: 1998
Main classification: H01L21
IPC codes: H01L21, H05B3