Abstract:
A method and apparatus that provides a dechucking voltage applied to an electrostatic chuck that facilitates removal of a workpiece or workpiece therefrom. The method incorporates residual chucking force information obtained from the preceding dechuck operation to modify and improve the dechucking algorithm for the subsequent wafer dechucking cycle. To avoid charge accumulation on the electrostatic chuck when processing a succession of workpieces, the chucking and dechucking voltages reverse polarity after each workpiece is dechucked.

Description:
BACKGROUND OF THE DISCLOSURE 
     1. Field of the Invention 
     The invention relates to electrostatic chucks for holding a workpiece and, more specifically, to a method and apparatus for dechucking a workpiece from an electrostatic chuck. 
     2. Background of the Invention 
     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 fabrication process system. Although electrostatic chucks vary in design, they all are based on the principle of applying a voltage to one or more electrodes in the chuck so as to induce opposite polarity charges in the workpiece and electrode(s), respectively. The electrostatic force between the opposite charges pulls the workpiece against the chuck, thereby retaining the workpiece. 
     A typical problem with electrostatic chucks is the difficulty of removing the electric charge from the workpiece and the chuck when it is desired to release the workpiece from the chuck. One conventional solution is to connect both the electrode and the workpiece to ground to drain the charge. Another conventional solution, which purportedly removes the charge more quickly, is to reverse the polarity of DC voltage applied to the electrodes. This technique is described in the context of a chuck having two electrodes (a bipolar chuck) in U.S. Pat. No. 5,117,121 issued May 26, 1992 to Watanabe, et al. 
     A shortcoming that has been observed with these conventional approaches to removing the electric charge is that they fail to completely remove the charge, so that some electrostatic force remains between the workpiece and the chuck. This residual electrostatic force typically necessitates the use of a mechanical force to separate the workpiece from the chuck. When the workpiece is a semiconductor wafer, the force required for removal sometimes cracks or otherwise damages the wafer. Even when the wafer is not damaged, the difficulty of mechanically overcoming the residual electrostatic force sometimes causes the wafer to pop off the chuck into an unpredictably position from which it is difficult to retrieve by a conventional wafer transport robot. 
     To more accurately reduce the residual electrostatic attractive force that remains between the workpiece and the chuck, attempts have been made to optimize the dechucking voltage by performing measurements upon the chucked wafer to determine an optimal dechucking voltage and dechucking period. Examples of dechucking arrangements are disclosed in commonly assigned U.S. Pat. No. 5,459,632, issued Oct. 17, 1995, to Birang, et al., and commonly assigned U.S. Pat. No. 5,818,682, issued Oct. 6, 1998, to Loo. 
     However, when successively processing a plurality of workpieces, these chucking/dechucking methods have not completely eliminated or compensated for chuck dielectric polarization and an incrementally increasing accumulation of residual charge on the chuck surface. The result of such charge accumulation is a progressive increase in the difficulty of dechucking each successive workpiece. 
     Therefore, there is a need in the art for a method that applies a dechucking signal that compensates for progressive charge accumulation upon the chuck surface when successively processing a plurality of workpieces. 
     SUMMARY OF THE INVENTION 
     The disadvantages heretofore associated with the prior art are overcome by the present invention of a method and apparatus that determines a dechucking signal applied to an electrostatic chuck to facilitate dechucking of a workpiece therefrom. Specifically, the method comprises the steps of determining a first dechucking signal based on a stored first residual attraction force metric; applying the dechucking signal to the electrostatic chuck; and, removing a first of the workpieces while measuring a second residual attraction force metric. 
     This method of dechucking the workpiece is accomplished after each workpiece is processed such that a residual charge does not accumulate upon the electrostatic chuck from workpiece to workpiece. Optionally, the polarity of the chucking and dechucking signals are reversed after each workpiece is dechucked to minimize charge accumulation. 
    
    
     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 is a sectional view, partially in schematic form, of a semiconductor wafer process system having an electrostatic chuck for retaining a semiconductor wafer during processing therein; 
     FIG. 2 is a sectional view, partially in schematic form, of the semiconductor wafer process system of FIG. 1 depicting the electrostatic chuck rotated to a vertical position; and, 
     FIG. 3 depicts a flow diagram of the method 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 is a schematic illustration of a cross-sectional view of a semiconductor wafer processing system  100 , for example, an ion implant system. The system  160  comprises a vacuum chamber  160 , an ion generator  162 , an electrostatic chuck  164 , a backside gas source  166 , a lift system  126  and control electronics  102 . The invention effectively provides a rapid, i.e., less than 200 msec, dechucking of a workpiece, e.g., a semiconductor wafer  108 , from the electrostatic chuck  164 . Although the invention is described in an exemplary ion implant system, the invention is also generally applicable to other semiconductor wafer processing systems wherever an electrostatic chuck is used to retain a wafer within a processing chamber. 
     The electrostatic chuck  164  is disposed in the chamber  160 . The electrostatic chuck  164  has a pair of coplanar electrodes  110  embedded within a dielectric  112  that forms a support surface  134  upon which the electrostatic chuck  164  retains the wafer  108 . The dielectric  112  typically is fabricated of cylindrical puck of ceramic material such as aluminum nitride, boron-nitride, alumina doped with a titanium oxide or a chromium oxide, or other dielectric material. The dielectric  112  includes a passage  168  that permits a heat transfer gas or gases, such as helium, to be supplied from the backside gas source  166  to an interstitial space between the support surface  134  and the wafer  108  to promote heat transfer. The electrostatic chuck  164  additionally comprises a plurality of lift pins  132  that pass through the dielectric  112  and selectively contact the wafer  108 . The electrodes  110  typically are a plate or layer of a conductive material such as molybdenum. Although the exemplary electrostatic chuck  164  is depicted in bipolar form, those skilled in the art will realize from the following discussion that the present invention may be used with any type of electrostatic chuck, including monopolar chucks, dielectric chucks, ceramic chucks, and the like. 
     The lift system  126  generally comprises a lift actuator  128  coupled to a contact pad  130 . To lift the wafer  108  from the support surface  134 , the lift actuator  128  elevates the contact pad  130  to move the lift pins  132  against the wafer  108 , raising the wafer  108  above the support surface  134 . 
     The control circuitry  102  comprises a DC power supply  104  that is controlled by a computer  106 . The DC power supply produces a variable positive and a variable negative voltage for each electrode of the bipolar chuck. In general, the computer  106  sets the output voltage value of the DC power supply  104 . The control circuitry  102  additionally comprises a metric measuring device  170 . The metric measuring device  170  is coupled to a sensor  172  that monitors flow conditions of the gas supplied by the backside gas source  166  to the support surface  134 . The metric measuring device  170  provides a signal to the computer  106  representative of a residual force metric as further discussed below. Alternatively, a sensor  174  or a sensor  176  may be coupled to the metric measuring device  170 . The sensor  174  is disposed on one of the lift pins  132  for measuring the force required to remove the wafer  108 . The sensor  176  measures the change in capacitance between the wafer  108  and the electrostatic chuck  164 . 
     The computer  106  is a general purpose, programmable computer system containing a central processing unit (CPU)  114  connected to conventional support circuits  116  such as a power supply, cache, memory, timing circuits, and the like. In addition, the CPU is connected to memory circuits  118  such as read-only memory (ROM) and random access memory (RAM). Furthermore, the RAM temporarily stores such values as a chucking voltage, a chucking period and a residual attraction force metric that are used during wafer processing. The present invention is implemented as a software program stored in memory  118  as a dechucking routine  122 . Upon execution of this dechucking routine  122 , the computer system becomes an apparatus for controlling the DC power supply to dechuck the workpiece in accordance with the operational steps of the dechucking routine  122  further detailed below. 
     Once a chucking voltage is applied by the power source  104  to the electrodes  110 , charges migrate from the electrodes  110  to the support surface  134  of the dielectric  112  and opposite charges are induced on the backside of the wafer  108  such that the wafer is held, i.e., chucked by the generated electrostatic attraction force. The attraction force is sufficient to permit the electrostatic chuck  164  to be rotated from a horizontal position to a vertical position without the wafer  108  moving across the support surface  134  of the electrostatic chuck  164 . While in the vertical position depicted in FIG. 2, the electrostatic chuck  164  is moved in the vertical plane (as indicated by arrow  122 ). An ion beam  124  or other source of ions for implantation generated by the ion generator  162  is scanned horizontally while the wafer  108  is being displaced vertically such that all locations on the wafer  108  may be exposed to the ion beam  124 . 
     FIG. 3 depicts a flow diagram of an illustrative dechucking routine  122 . For the best understanding of the dechucking routine  112 , the reader is encouraged to refer simultaneously to the apparatus depicted in FIG.  1  and the flow diagram of FIG.  3 . 
     The dechucking routine  122  comprises generally a dechucking subroutine step  150 , a residual attraction force feedback step  154 , and optionally includes a charge minimization step  152 . The dechucking routine  122  begins at step  200  and proceeds to the dechucking subroutine  150 . The dechucking subroutine  150  generally consists of an algorithm for determining a dechucking signal wherein the algorithm factors the residual force metric obtained through historical or actual measurements of the residual force metric taken during dechucking. As such, a number of conventional algorithms can be modified to use the residual force metric to obtain a precise dechucking signal. The increase precision of the dechucking signal results in a faster removal of the workpiece while minimizing the likelihood of workpiece damage. A number of methods are available that provide a method for dechucking wafer which can be executed by step  150 , an example of which is the commonly assigned U.S. Pat. No. 5,818,682, issued Oct. 6, 1998, to David Loo, and is hereby incorporated by reference in it&#39;s entirety. Other methods for determining and applying a dechucking force may also be effectively utilized. 
     The illustrative subroutine  150  begins with step  202 . At step  202 , the chucking voltage, chucking period and residual chucking force metric are retrieved from registers within the computer system. For the first wafer, the residual chucking force metric is typically initialized as zero. Alternately, the residual chucking force metric may be retrieved from memory  118  at a predetermined or historical value. The chucking voltage and chucking period are entered into the registers as a predetermined value. At step  204 , the routine computes the dechucking signal. 
     An illustrative algorithm for the dechucking signal is provided as: 
     
       
           T   PW   =T   NOM   +K   P ( F   DC   −F   NOM ) 
       
     
     where: 
     T PW  is the period of the dechucking signal for the current workpiece; 
     T NOM  is the period of the expected dechucking signal. T NOM  is generally a function of the dechucking voltage (although the dechucking voltage may be varied, in the illustrative example, it is held at a predetermined level); 
     K P  is a constant that is empirically determined; 
     F NOM  is the expected residual force required to remove the current wafer; and 
     F DC  is the measured residual force metric required to remove the preceding wafer (or other historical removal force metric). 
     The reader should note that other algorithms may be readily formulated to provide a dechucking signal utilizing the residual force metric as taught herein. As such, the algorithm presented above is but one dechucking method that utilizes dechucking information from previous dechuckings. 
     The dechucking voltage is typically −1.5 times the chucking voltage for a workpiece such as an oxide wafer. However, the dechucking voltage may be computed by any one of a number of available dechucking voltage optimization methods that are available in the art. One such method is described in commonly assigned U.S. Pat. No. 5,459,632 Oct. 17, 1995 to Birang, et al, and is hereby incorporated by reference in its entirety. 
     Once the dechucking signal has been computed, the routine  150  proceeds to step  208 . At step  208 , the computer system sends the control signal (i.e., the dechucking signal) to the DC voltage supply to apply the dechucking voltage for a period equal to the dechucking period. At this point, the dechucking process is complete and the workpiece may be physically removed from the electrostatic chuck. 
     During step  154 , the residual attraction force metric is obtained during the physically removal of the workpiece in step  208 , and fed back to the computer system register for retrieval during step  202 . The residual attraction force metric may be obtained through various methods known in the art, including mechanical force measurements, changes in capacitance between the wafer and the electrostatic chuck, and changes in the flow conditions of backside gases between the wafer and the electrostatic chuck. For example, the method described in commonly assigned U.S. Pat. No. 5,684,669, issued Nov. 4, 1997, to Collins et al., teaches that the residual attraction force metric may be obtained using a lift pin fitted with a force gauge or monitoring changes in the backside gas flow conditions. As such, any drift in residual attraction force metric caused, for example, by charge accumulation on the electrostatic chuck surface, can be detected during the dechucking of a first workpiece and used to more accurately calculate the dechucking parameters used to dechuck a subsequent workpiece. For example, if an increase in the residual force is found, then a commensurate change in the dechucking signal (i.e., an increase in the dechucking voltage and/or dechucking signal) is used to dechuck the next wafer. Thus, such changes in residual attracting resulting in changed dechucking requirements are compensated for during the determination of the dechucking voltage and period in steps  204  and  208 , respectively, for next wafer to be processed. 
     To minimize the potential charge accumulation upon the electrostatic chuck, the charge minimization routine of step  152  should be applied after each wafer is processed. Thus, at step  210 , the routine queries whether another workpiece is to be processed. If the query is negatively answered, the routine proceeds to step  212  that ends the dechucking routine  122 . However, if the query is affirmatively answered, the routine proceeds to step  214 . 
     To further improve the dechucking process the chucking voltage should be reversed in polarity after each wafer is processed. Reversing polarity assists the dielectric material of the chuck from becoming polarized and correspondingly, making dechucking progressively more difficult. Therefore, at step  214 , the polarity of chucking voltage is reversed and, at step  216 , the next workpiece is processed. By reversing the polarity, the residual attraction force is maintained in a limit cycle wherein the amplitude of the attraction force remains at lower levels than if the chuck only experienced dechucking voltages of the same polarity. This is due in part to the residual attraction force not being repeatedly subjected voltages of the same polarity that result in an accumulating charge. To dechuck the next workpiece, the routine returns to the dechucking subroutine  122  at step  202 . In one embodiment, the use of described method permits the subsequent workpieces to be dechucked in less than about 200 milliseconds. 
     The embodiment of the invention illustratively described above discloses a chucking signal comprising both a dechucking voltage and a dechucking period. Alternately, dechucking signals may comprise solely a dechucking voltage. Although a single embodiment incorporating the teachings of the present invention has been shown and described in detail, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings.