Abstract:
An apparatus and method for actively controlling surface potential of an electrostatic chuck. The apparatus and method utilize a sensor and a control circuit. The sensor comprises an antenna on the chuck surface coupled to a field effect transistor (FET). The sensor produces a signal indicative of an electrical characteristic such as surface potential of the electrostatic chuck. The sensor signal provides feedback to the control circuit. The control circuit compares the sensor signal to a predetermined setpoint corresponding to a desired value of the surface potential. The control circuit provides a control signal to a power supply coupled to one or more chuck electrodes. The control signal causes the power supply to change the electrode voltage such that the resulting change in surface potential tends to null the difference between the sensor signal and the set point thus ensuring a constant chucking force.

Description:
BACKGROUND OF THE DISCLOSURE 
     1. Field of the Invention 
     The invention relates to electrostatic chucks and, more particularly, the invention relates to in-situ measurement and control of a potential on the surface of an electrostatic chuck. 
     2. Description of the Background Art 
     Electrostatic chucks are used for holding a workpiece in various applications ranging from holding a sheet of paper in a computer graphics plotter to holding a semiconductor wafer within a semiconductor wafer process chamber. In semiconductor wafer processing equipment, electrostatic chucks are used for clamping wafers to a pedestal during processing. These chucks find use in etching, chemical vapor deposition (CVD), and physical vapor deposition (PVD) and other applications. 
     Electrostatic chucks secure a workpiece by creating an electrostatic attractive force between the workpiece and the chuck. A voltage is applied to one or more electrodes that are embedded in the chuck so as to induce opposite polarity charges in the workpiece and electrodes, respectively. The opposite charges pull the workpiece against the chuck, thereby retaining the workpiece. For example, in a “monopolar” electrostatic chuck, voltage is applied to a single conductive chuck electrode that is embedded within a dielectric or semiconductive chuck body. The magnitude of the chucking voltage is relative to some ground reference. When the voltage is applied, the workpiece is referred back to the same ground reference as the voltage source by a conductive connection to the workpiece. Electrostatic force is established between the workpiece being clamped and the electrostatic chuck. A “bipolar” electrostatic chuck generally contains two electrodes embedded within a unitary dielectric or semiconductive chuck body. When a chucking voltage is applied between the two electrodes, a small current flows between the electrodes and through the workpiece such that oppositely polarized charges respectively accumulate on the backside of the wafer and on the surface of the chuck body. These charges establish an electrostatic force, between the chuck and the workpieces via the Johnsen-Rahbek effect. 
     In either type of electrostatic chuck, a surface potential appears on the dielectric above the electrodes when a voltage is applied to the electrodes. The surface potential is directly proportional to the chucking force. For an ideal dielectric, the surface potential is equal in magnitude to the voltage on the underlying chuck electrode. Charging effects, polarization and other material specific phenomena can cause the surface potential on top of the dielectric to be different from the voltage on the underlying electrode. As a result, the performance (i.e., chucking force) of the chuck is affected. For example, some chucks exhibit a degradation of the surface potential over time after a chucking voltage has been applied to the chuck electrodes. This is believed to be due to the existence of low mobility charge carriers in the bulk dielectric material of the chuck. Other chucks exhibit a transient repelling force when the chucking voltage is turned off. This is due to repulsion of residual charges on the wafer backside by charges of like polarity induced on the chuck electrodes by a transient overpotential on the chuck electrodes when the chuck is turned off. The chucking performance varies in an unpredictable fashion from chuck to chuck. Furthermore, the behavior of a given chuck varies in an unpredictable fashion over time. 
     Active control of the surface potential requires that surface potential measurement be made in situ during wafer processing. Furthermore, reliable measurement and active control of the surface potential require that the surface potential be measured at a fixed location on the chuck surface in order to compare measurements of the surface potential taken over time. In the prior art, electrostatic probes and meters have been used to measure the surface potential of electrostatic chucks. Unfortunately, the probes used for these measurements are not suitable for the often harsh environment that exists inside semiconductor wafer processing chambers when the chamber is in use. Consequently, such probe measurements are not performed while the chamber is operating and therefore must be made in air, when the chamber is open, or when the chuck is removed from the chamber. 
     Therefore, a need exists in the art for an apparatus and method for reliably measuring and actively controlling electrostatic chuck surface potential during wafer processing. 
     SUMMARY OF THE INVENTION 
     The disadvantages associated with the prior art are overcome by the present invention of an apparatus and method for actively controlling the surface potential of an electrostatic chuck. The apparatus and method utilize a surface potential sensor and a control circuit. The sensor produces a signal indicative of an electrical characteristic such as a surface potential of the electrostatic chuck. The sensor signal provides feedback to the control circuit. The control circuit controls a high voltage power supply connected to one or more electrodes in the chuck such that the chucking voltage can be dynamically altered in response to the measured surface potential. 
     The sensor comprises an antenna, mounted to the chuck surface and a field effect transistor (FET). The antenna is coupled to a gate of the FET such that changes in the surface potential control a current between the source and drain of the FET. The source-drain current provides the sensor signal that is coupled to the control circuit. The control circuit compares the sensor signal to a predetermined setpoint corresponding to a desired value of the surface potential. In response to the sensor signal, the control circuit provides a control signal to the power supply that causes the chuck power supply to change the electrode voltage such that the resulting change in surface potential tends to null the difference between the sensor signal and the set point. 
     Active control of the surface potential provides a more reliable and reproducible chucking force. The present invention can be used to control the surface potential on a chuck having any number of electrodes and/or any type of chuck body material. 
    
    
     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 schematic view of a semiconductor wafer processing system having a bipolar chuck that employs the apparatus of the present invention; and 
     FIG. 2 a detailed view of a surface potential sensor according to 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 schematic view of a semiconductor processing system  100  that includes the present invention. The system  100  generally comprises a processing chamber  110  that encloses a pedestal  116 , where the pedestal  116  comprises a pedestal base  117  and an electrostatic chuck  120 . The electrostatic chuck  120  is coupled to an apparatus  102  for actively controlling a surface potential of the electrostatic chuck  120 . The processing chamber  110  comprises a set of walls  111 , a floor  112  and a lid  113  that define a volume  114 . The pedestal  116  is centrally disposed within the chamber  110 . An exhaust system  115  evacuates the volume  114  to provide a suitable environment for semiconductor wafer processing. The chamber  110  can be any suitable chamber for any process requiring an electrostatic chuck. Such chambers include those used for etch, chemical vapor deposition (CVD), physical vapor deposition (PVD), ion implant, pre-clean and cool-down chambers. By way of example, the chamber  110  is a Preclean IIe chamber manufactured by Applied Materials of Santa Clara, Calif. 
     The electrostatic chuck  120  is supported in the chamber  100  by the pedestal base  116 . The electrostatic chuck  120  generally comprises a dielectric (or semiconductive) body  121  having a support surface  122 . The support surface  122  supports a semiconductor wafer  101  during processing. The electrostatic chuck  120  can be any type of chuck. The chuck  120  is, for example, a bipolar chuck that contains two chuck electrodes  124 A and  124 B embedded in a dielectric or semiconductive chuck body  121 . Although a bipolar chuck  120  having two electrodes  124 A and  124 B is depicted herein, the semiconductor processing system  100  can employ a chuck  120  containing any number of chucking electrodes and any type of chucking electrode structure including monopolar, bipolar, tripolar, interdigitated, zonal and the like. The chuck body  121  is typically fabricated from a polymer material such as polyimide or a ceramic material such as aluminum oxide or aluminum nitride. 
     An electrode power supply  150  provides voltage to each of the electrodes  124 A and  124 B. Preferably the electrode power supply  150  is a voltage driven bipolar power supply that applies DC voltages of opposite polarity to each of the chuck electrodes  124 A and  124 B. The electrode power supply  150  comprises a pair of voltage sources  152 A and  152 B. The voltage sources  152 A and  152 B are referenced to a common center tap  154 . Furthermore, the power supply  150  is a voltage controlled power supply wherein the voltage sources  152 A and  152 B are responsive to control signals C A  and C B  from the control circuits  140 A and  140 B. 
     The apparatus  102  for controlling the surface potential of the electrostatic chuck  120  comprises sensors  130 A and  130 B and control circuits  140 A and  140 B coupled to the electrodes  124 A and  124 B via the bipolar electrode power supply  150 . The sensors  130 A and  130 B measure an electrical property of the chuck surface and produce sensor signals S A  and S B . The sensor signals S A  and S B  provide feedback to the control circuits  140 A and  140 B. The control circuits  140 A and  140 B, in turn, control the voltage sources  152 A and  152 B of the power supply  150 . The sensors  130 A and  130 B can be any type of sensor that measures an electrical characteristic at the chuck surface  122  that is related to a chucking force exerted on the wafer  101 . Suitable electrical characteristics include surface potential, electric field strength and electric field density. By way of example, the sensors  130 A and  130 B are surface potential sensors that produce signal voltages S A  and S B  that are proportional to the surface potential of the chuck  120 . 
     Specific details of one of the sensors  130 A, is depicted in FIG.  2 . Each of the sensors  130 A and  130 B comprises a conductive antenna  131 , mounted to the surface  122  of the electrostatic chuck  120  above one of the chuck electrodes  124 . Preferably, the antenna  131  is made of a material such as aluminum or titanium, deposited onto the chuck surface  122  by sputtering through a mask or the like. The antenna  131 , being conductive, acts as an equipotential probe, i.e., the electric potential (voltage) on the antenna is the same as the surface potential on the chuck surface  122  underneath the antenna  131 . The antenna  131  is wired to an electronic device, such as a field effect transistor (FET)  134 , for amplifying the antenna signal. Such FETs include enhancement mode and depletion mode FETs. Any type of field effect transistor or similar device can be used to amplify the signal from the antenna  131 . For example, if the current from the antenna is sufficiently large, a bipolar transistor can be used in place of the FET  134 . The FET  134  can be embedded into a depression  123  formed in the surface  122  of the chuck  120  and secured by an adhesive, such as a putty or glue, or a mechanical fastener such as a screw. The FET  134  can be fabricated as an integrated circuit chip by means well known in the art. Alternatively, the FET  134  can be fabricated onto the chuck surface  122  using surface mount technology. 
     The FET  134  comprises a semiconducting bulk layer  135 , having doped source  136 A and drain  136 B regions connected by a channel  137  and a gate electrode  138  that is insulated from the channel  137  by an insulator layer  138 A. A transverse electric field in the channel  137  controls a current between the source  136 A and the drain  136 B. The antenna  131  is connected to the gate electrode  138  and the bulk layer  135  through a voltage divider circuit  139 . The voltage divider circuit  139  is embedded within the surface  122  of the chuck  120  and secured by means similar to those used to secure the FET  134 . For example, the voltage divider circuit may be secured within a depression (not shown) in the surface  122  by an adhesive. The bulk layer  135  is coupled to a convenient voltage reference such as the center tap  154  of the electrode power supply  150 . A sensor power supply  133  is coupled to the FET  134  to provide a voltage between the source  136 A and drain  136 B. If there is a constant voltage between the source  136 A and drain  136 B, the transverse electric field in the channel  137 , and hence the source-drain current, is dependent of the voltage between the gate  138  and the bulk layer  135 . The surface potential on the surface  122  of the electrostatic chuck  120 , induces a voltage between the gate  138  and the bulk layer  135  that modulates the source drain current. The voltage divider circuit  139  reduces this voltage to a level suitable for controlling the FET  134 , typically between 1 and 100 volts. Thus when the surface potential changes, the voltage between the gate  138  and the bulk layer  135  changes thereby controlling the current between the source  136 A and drain  136 B. The source-drain current flows though a resistor  141  to ground as shown in FIG.  1 . To properly function, either the source  136 A or the drain  136 B must be connected to ground. The additional voltage provides the sensor signal S A  that is coupled to the control circuit  140 A. 
     Referring to FIG. 1, the control circuits  140 A and  140 B include, for example, comparators  142 A and  142 B and setpoint power supplies  144 A and  144 B. The comparators  142 A and  142 B have first inputs  143 A and  143 B coupled to the setpoint power supplies  144 A and  144 B, and second inputs  145 A and  145 B coupled to the sensor signals S A  and S B  (e.g., coupled between the sensor power supply  133 A and the load resistor  141 A), and outputs  146 A and  146 B coupled to the one of the voltage sources  152 A or  152 B of the electrode power supply  150 . The setpoint power supplies  144 A and  144 B establish predetermined setpoint voltages V A  and V B , with respect to ground, that are related to desired surface potentials. The comparators  142 A and  142 B are, for example, operational amplifiers. The comparators  142 A and  142 B compare the sensor signals S A  and S B  to the predetermined setpoint voltages V A  and V B  and produce control signals C A  and C B  at the outputs  146 A and  146 B. The control signals C A  and C B  cause the voltage sources  152 A and  152 B to change the voltage applied to the corresponding electrode  124 A or  124 B thereby changing the surface potential in such a way as to null the difference between the sensor signals S A  and S B  and the setpoint voltages V A  and V B . More complex circuits, such as proportional integral differential (PID) circuits, can be utilized to control the voltage sources  152  of the power supply  150 . 
     The present invention can be implemented using any number of sensors. For example, multiple sensors can be distributed across the surface of a chuck with multiple electrodes that provide multiple chucking zones. Each sensor provides feedback to control a voltage source for the corresponding electrode to overcome material related weakening of the chucking force in localized chucking zone on the chuck surface. The embedded sensors are also protected from the harsh environment in the chamber. The apparatus of the present invention provides for active control of the chucking force in response to changes in the surface potential. As a result, wafers are chucked more reliably and repeatably. Wafer processing is therefore more uniform and fewer wafers are defective. Fewer defective wafers means lower cost per wafer and increased profitability. 
     Although various embodiments which incorporate the teachings of the present invention have been shown and described in detail herein, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings.