Abstract:
An electrostatic clamp, which more effectively removes built up charge from a substrate prior to and during removal, is disclosed. Currently, the lift pins and ground pins are the only mechanisms used to remove charge from the substrate after implantation. The present discloses describes a clamp having one of more additional low resistance paths to ground. These additional conduits allow built up charge to be dissipated prior to and during the removal of the substrate from the clamp. By providing sufficient charge drainage from the backside surface of the substrate  114 , the problem whereby the substrate sticks to the clamp can be reduced. This results in a corresponding reduction in substrate breakage.

Description:
This application claims priority of U.S. Provisional Patent Application Ser. No. 61/169,950, filed Apr. 16, 2009, the disclosure of which is hereby incorporated by reference. 
    
    
     FIELD 
     This disclosure relates to a substrate handling, and more particularly to an apparatus and a method of handling a substrate. 
     BACKGROUND 
     An electronic device may be created from a substrate that has undergone various processes. One of these processes may include introducing impurities or dopants to alter the electrical properties of the original substrate. For example, charged ions, as impurities or dopants, may be introduced to a substrate, such as a silicon wafer, to alter electrical properties of the substrate. One of the processes that introduces impurities to the substrate may be an ion implantation process. 
     An ion implanter is used to perform ion implantation or other modification of a substrate. A block diagram of a conventional ion implanter is shown in  FIG. 1 . The conventional ion implanter may comprise an ion source  102  that may be biased by a power supply  101 . The system may be controller by controller  120 . The operator communicates with the controller  120  via user interface system  122 . The ion source  102  is typically contained in a vacuum chamber known as a source housing (not shown). The ion implanter system  100  may also comprise a series of beam-line components through which ions  10  pass. The series of beam-line components may include, for example, extraction electrodes  104 , a 90° magnet analyzer  106 , a first deceleration (D 1 ) stage  108 , a 70° magnet collimator  110 , and a second deceleration (D 2 ) stage  112 . Much like a series of optical lenses that manipulate a light beam, the beam-line components can manipulate and focus the ion beam  10  before steering it towards a substrate or wafer  114 , which is disposed on a substrate support  116 . 
     In operation, a substrate handling robot (not shown) disposes the substrate  114  on the substrate support  116  that can be moved in one or more dimensions (e.g., translate, rotate, and tilt) by an apparatus, sometimes referred to as a “roplat” (not shown). Meanwhile, ions are generated in the ion source  102  and extracted by the extraction electrodes  104 . The extracted ions  10  travel in a beam-like state along the beam-line components and implanted on the substrate  114 . After implanting ions is completed, the substrate handling robot may remove the substrate  114  from the substrate support  116  and from the ion implanter  100 . 
     Referring to  FIGS. 2A and 2B , there is shown a block diagram illustrating the workpiece support  116  supporting the substrate  114  during the ion implantation process. As illustrated in  FIG. 2A , the workpiece support  116  may comprise a sealing ring  202  and a plurality of embossments  204  that are in contact with the substrate  114 . The sealing ring may be an annular ring of about 0.25 inches in width, and having a height of 5 microns. The embossments  204  may be about 1 mil in diameter and 5 microns high. In addition, the workpiece support  116  may also include at least one cooling region  206 . During the implantation process, cooling gas may be provided to the cooling region  206  prevent the substrate  114  from overheating. The workpiece support  116  may have gas channels and conduits to allow this cooling gas to flow to the cooling region  206 . The workpiece support  116  may further include a plurality of lift pins  208  that may move so as to push the substrate  114  away from the workpiece support  116  in the direction indicated by the arrows. The lift pins  208  may be retracted within the workpiece support  116 , as illustrated in  FIG. 2B . The workpiece would also be normally in contact with a plurality of ground pins  205 . 
     The workpiece support  116  may be cylindrical in shape, such that its top surface is circular, so as to hold a disc-shaped substrate. Of course, other shapes are possible. To effectively hold the substrate  114  in place, most workpiece supports typically use electrostatic force. By creating a strong electrostatic force on the upper side of the workpiece support  116 , the support can serve as the electrostatic clamp or chuck, the substrate  114  can be held in place without any mechanical fastening devices. This minimizes contamination, avoids wafer damage from mechanical clamping and also improves cycle time, since the substrate does not need to be unfastened after it has been implanted. These clamps typically use one of two types of force to hold the substrate in place: coulombic or Johnson-Rahbeck force. 
     As seen in  FIG. 2A , the clamp  116  traditionally consists of several layers. The first, or top, layer  210 , which contacts the substrate  114 , is made of an electrically insulating or semiconducting material, such as alumina, since it must produce the electrostatic field without creating a short circuit. In some embodiments, this layer is about 4 mils thick. For those embodiments using coulombic force, the resistivity of the top layer  210 , which is typically formed using crystalline and amorphous dielectric materials, is typically greater than 10 14  Ω-cm. For those embodiments utilizing Johnsen-Rahbeck force, the volume resistivity of the top layer, which is formed from a semiconducting material, is typically in the range of 10 10  to 10 12  Ω-cm. The term “non-conductive” is used to describe materials in either of these ranges, and suitable for creating either type of force. The coulombic force can be generated by an alternating voltage (AC) or by a constant voltage (DC) supply. 
     Directly below this layer is a conductive layer  212 , which contains the electrodes that create the electrostatic field. This conductive layer  212  is made using electrically conductive materials, such as silver. Patterns are created in this layer, much like are done in a printed circuit board to create the desired electrode shapes and sizes. Below this conductive layer  212  is a second insulating layer  214 , which is used to separate the conductive layer  212  from the lower portion  220 . 
     The lower portion  220  is preferably made from metal or metal alloy with high thermal conductivity to maintain the overall temperature of the workpiece support  116  within an acceptable range. In many applications, aluminum is used for this lower portion  220 . 
     Initially, the lift pins  208  are in a lowered position. The substrate handling robot  250  then moves a substrate  114  to a position above the workpiece support  116 . The lift pins  208  may then be actuated to an elevated position (as shown in  FIG. 2A ) and may receive the substrate  114  from the substrate handling robot  250 . Thereafter, the substrate handling robot  250  moves away from the workpiece support  116  and the lift pins  208  may recede into the workpiece support  116  such that the sealing ring  202  and the embossments  204  of the workpiece support  116  may be in contact with the substrate  114 , as shown in  FIG. 2B . The ground pins  205  are also normally in contact with the substrate  114 . The implantation process may then be performed with the lift pins  208  in this recessed position. After the implantation process, the substrate  114  is unclamped from the workpiece support  116 , having been held in place by electrostatic force. The lift pins  208  may then be extended into the elevated position, thereby elevating the substrate  114  and separating the substrate  114  from the edge  202  and the embossments  204  of the workpiece support  116 , as shown in  FIG. 2A . The lift pins  208  are either insulating or conductive and therefore may not remove any remaining charge from the substrate  114 . The substrate handling robot  250  may then be disposed under the substrate  114 , where it can retrieve the implanted substrate  114  at the elevated position. The lift pins  208  may then be lowered, and the robot  250  may then be actuated so as to remove the substrate  114  from the implanter. 
     A condition that can occur with a conventional ion implanter  100  may be found in the process of removing the substrate  114  from the workpiece support  116 . After multiple cycles of clamping and unclamping a substrate  114  to a workpiece support  116 , the side of the substrate  114  clamped to the workpiece support  116  may exhibit damage. This damage may be due to electrical discharge caused by electrostatic charge buildup on the substrate  114  and the top layer  210  of the workpiece support  116 . The electrostatic charge may discharge (arc) to a ground pin  205  or directly to the surface of the workpiece support  116 . 
     Previously, substrates  114  have been grounded via contact with metal lift pins  208  or ground pins  205 . Substrates  114  also have been grounded previously using a plasma flood gun (PFG). Due to the brief contact time and small contact area between the lift pins  208  or ground pins  205  and the substrate  114  area containing the electrostatic charge, a condition can exist wherein the lift pins  208  and ground pins  205  do not effectively drain the electrostatic charge from the substrate  114 . Accordingly, there is a need in the art for an improved electrostatic clamp that can remove charge. 
     SUMMARY 
     The problems of the prior art are overcome by the apparatus and method of this disclosure. An electrostatic clamp which more effectively removes built up charge from a substrate prior to removal is disclosed. Currently, the lift pins and the ground pins are the only mechanism used to remove charge from the substrate after implantation. The present discloses describes a clamp having one of more additional low resistance conduits to ground. These additional conduits allow built up charge to be dissipated during the removal of the substrate from the clamp. By providing sufficient charge drainage from the backside surface of the substrate, the problem whereby the substrate sticks to the clamp can be reduced. This results in a corresponding reduction in substrate breakage. In some embodiments, these ground paths are intermittent, so as not to be present when the electrostatic forces are being generated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to facilitate a fuller understanding of the present disclosure, reference is now made to the accompanying drawings, in which like elements are referenced with like numerals. These drawings should not be construed as limiting the present disclosure, but are intended to be exemplary only. 
         FIG. 1  represents a traditional ion implantation system; 
         FIG. 2A  represents a block diagram showing a workpiece support supporting a substrate with the lift pins extended; 
         FIG. 2B  represents a block diagram showing a workpiece support supporting a substrate with the lift pins recessed; 
         FIG. 3  represents a top view of an embodiment of an electrostatic clamp; 
         FIG. 4  represents a cross-sectional view of the embodiment of  FIG. 3 ; 
         FIG. 5  represents a top view of a second embodiment of an electrostatic clamp; 
         FIG. 6  represents a cross-sectional view of the embodiment of  FIG. 5 ; and 
         FIG. 7  represents a cross-sectional view of the embodiment of  FIG. 6  where the substrate sticks to the clamp. 
     
    
    
     DETAILED DESCRIPTION 
     In the present disclosure, several embodiments of an apparatus and a method for handling a processed substrate are introduced. For purpose of clarity and simplicity, the present disclosure will focus on an apparatus and a method for handling a substrate that is processed by a beam-line ion implanter. Those skilled in the art, however, may recognize that the present disclosure is equally applicable to other types of processing systems including, for example, a plasma immersion ion implantation (“PIII”) system, a plasma doping (“PLAD”) system, an etching system, an optical based processing system, and a chemical vapor deposition (CVD) system. As such, the present disclosure is not to be limited in scope by the specific embodiments described herein. 
     The embodiments disclosed herein provide a more reliable and lower resistance path to ground for a substrate and the top layer of an electrostatic clamp. Some portion of the substrate will be contacted to ground regardless of how or in what direction the substrate is released from the electrostatic clamp. By providing sufficient charge drainage from the backside surface of the substrate, substrate “sticking” to the electrostatic clamp and substrate breakage can be reduced. 
       FIG. 3  is a top perspective view of an embodiment of an electrostatic clamp (or “clamp”)  300 . The electrostatic clamp  300  is one example of a workpiece support. The clamp  300  has an outer annulus or sealing ring  301 . In one instance, the ring  301  may be approximately 0.25 inches in width. Although not shown, the upper surface of the clamp  300  may also have embossments. Lift pins  302  are used to lift the substrate from the clamp  300  after processing of the substrate has been completed. As seen in  FIG. 4 , the sealing ring  301  is connected to ground. Since the sealing ring  301  is made of high resistivity material, there may need to be a plurality of connections  310 , spatially separated around the sealing ring  301 . This connection  310 , which can be sealed, may be very low resistance, such as through a conductive material, such as copper. In other embodiments, the connection  310  to ground may be through a semiconducting material, such as carbon film. In some embodiments, the resistivity of the material used to connect the sealing ring  301  to ground may be about 10 7  Ω-cm. 
       FIG. 4  shows a cross-section view of the clamp  300  of  FIG. 3 . As described above, the lower portion  320  of the electrostatic clamp  300  is typically made of a metal, and is also connected to ground. Therefore, in some embodiments, the sealing ring  301  is connected to ground by connecting the sealing ring  301  to the lower portion  320  of the clamp  300 . This connection  310  can be made by applying a conductive or semiconductive coating around the entire circumference of the sealing ring  301 , so that the sealing ring  301  is in continuous contact with the lower portion  320 . In other embodiments, the connection  310  between the ring  301  and the lower portion  302  is not around the entire circumference. Rather, a number of discrete connections  310  are made between the lower portion  320  and the sealing ring  301 . In some embodiments, a carbon film is applied around the circumference of the sealing ring  301 , connecting it to the lower portion  320 . Other materials, such as Aguadag® paint, which is a water-based colloidal graphite suspension, or other carbon-based materials may also be used. 
     While  FIG. 4  shows the sealing ring  301  connected to the lower portion  320  of the electrostatic clamp  300  via connection  310 , other ground connections are possible and within the scope of the disclosure.  FIG. 4  describes just one possible embodiment. For example, the sealing ring  301  may be connected to a ground other than through the lower portion  320  of the electrostatic clamp  300 . 
     As described above, the top layer  304  of the electrostatic clamp  300  is constructed using non-conductive materials, where the resistivity of the material can be in the range between 10 8  Ω-cm and 10 15  Ω-cm. At a resistivity near the lower end of this range, the connection  310  of the sealing ring  301  to ground may be sufficient to eliminate the built up charge on the top layer  304  of the electrostatic clamp  300  and the substrate  114 . In other words, the resistivity of the top layer  304  is sufficiently low to allow the charge built up on the top layer  304  and the substrate  114  to flow to the sealing ring  301 , which is connected to ground. 
     Furthermore, tests have shown that the grounding of the sealing ring  301  (i.e. passive connection to ground) has minimal impact on the clamping force of the electrostatic clamp  300 . This is due to the generally high resistivity of the top surface  304 , which limits the effect of the grounded sealing ring  301 . However, in some embodiments, the sealing ring  301  may be intermittently connected to ground (i.e. active ground connection). For example, using a switch or other device, the ground connection  310  may be interrupted while the electrodes  306  are actively generating an electrostatic field. In other words, the switch is in series between the sealing ring  301  and ground, such that actuation of the switch either enables or disables the connection to ground. When the electrodes  306  are inactive, the grounding connection  310  may be restored. This modification insures that the grounding of the top surface  304  of the clamp  300  has minimal or no impact on the electrostatic clamp force. 
     In other embodiments, the resistivity of the top layer  304  may be great, such as more than 10 12  Ω-cm. In such embodiments, the grounding of the sealing ring  301  may be insufficient to drain the built up charge on the substrate  114  and the top layer  304 . In other words, the resistivity of the top layer  304  is too high to allow the built up charge to freely flow to the sealing ring  301 . In such an embodiment, conductive, or semiconductive conduits may be deposited on (or in) the top layer  304 . These conduits are intended to allow built up charge to flow more readily to the sealing ring  301 . 
       FIG. 5  shows a top perspective view of a second embodiment of an electrostatic clamp (or “clamp”)  400 . In some embodiments, the clamp  400  may have a cross section similar to that shown in  FIG. 4  where there is a non-conductive top layer, an electrically conductive layer, an insulating layer and a lower portion. As described above, the electrostatic clamp  400  has an outer annulus or sealing ring  401 , which may be approximately 0.25 inches in width. In one embodiment, the sealing ring  401  may correspond to the sealing ring  301  in  FIG. 3 . As before, the sealing ring  401  is connected to ground using a ground connection  403 . The electrostatic clamp  400  also includes lift pins  430  and ground pins  405 . The electrostatic clamp  400  also includes a number of conduits  402  from various locations on the top surface of the electrostatic clamp  400  to the sealing ring  401 . While six conduits  402  are illustrated in  FIG. 5 , more or fewer conduits  402  may be used and this embodiment is not solely limited to six conduits  402 . Furthermore, different patterns of conduits  402  than that illustrated in  FIG. 5  are possible. The conduits  402  allow charge to flow to the sealing ring  401 . 
     Furthermore, the conduits  402  in  FIG. 5  are shown as radial spokes. However, other conduit  402  patterns are possible. The conduits  402  may be arranged to allow a path of lower resistance (than currently exists) between points on the top surface of the electrostatic clamp  400  and ground. 
     The conduits  402  may be fabricated of, for example, carbon or SiC. The conduits  402  also may be fabricated of some other conductive deposited material known to those skilled in the art. In some embodiments, the conduits  402  are applied to the top surface of the electrostatic clamp  400  using chemical vapor deposition (CVD). These conduits  402  are intended to reduce the resistance to ground. However, these conduits  402  may still exhibit some resistivity. For example, in some embodiments, the conduits  402  have a resistivity of between 10 4  and 10 8  Ω-cm. 
       FIG. 6  is a cross-sectional view of an embodiment of an electrostatic clamp  400 . The substrate  114  is disposed on the electrostatic clamp  400 . In this position, ground pins  405  may normally be in contact with the substrate  114 . If the substrate  114  is not in direct contact with the sealing ring  401 , the conduit  402  (represented by the shaded portion in  FIG. 6 ) can carry charge from the substrate  114  to the sealing ring  401 . If the substrate  114  is in contact with the ring  401  and conduits  402 , charge flow may increase. While a single conduit  402  is illustrated in  FIG. 6 , other numbers of conduits  402  may be present. Furthermore, while the conduit  402  is illustrated as protruding from the top surface of the clamp  400  in  FIG. 6 , the conduit  402  may be recessed in the top surface of the clamp  400 . 
       FIG. 7  is a cross-sectional view of the embodiment of  FIG. 6  with the substrate sticking to the sealing ring. The substrate  114  is being lifted using lift pin  430 . As the lift pins  430  elevate, the connection between the ground pins  405  and the substrate  114  is broken. Because of the connection to the grounded sealing ring  401 , electrical discharge may not occur. The conduits  402  remove charge from the top surface, thereby minimizing the amount of charge that can accumulate on the substrate prior to unclamping. 
     If the substrate  114  sticks to the clamp  400  due to electrostatic charge, the charge can pass to ground using the conduit  402 . Using, for example, the pattern of conduits  402  illustrated in  FIG. 5 , regardless of where the substrate  114  sticks to the clamp  400 , the charge can pass to ground. This will prevent sticking and damage to the substrate  114 . 
     The presence of lower resistance conduits on the top of the electrostatic clamp  400  may reduce the electrostatic force that holds the substrate  114  to the electrostatic clamp  400 . As described above, in some instances, it may be advantageous to have the ground connection for the conduits  402  interrupted through the use of a switch while the electrostatic force is being generated. This can be done by interrupting the connection  403  between the sealing ring  401  and ground. In other embodiments, the switch is located between the conduits  402  and the sealing ring  401  such that the connection between the conduits  402  and the sealing ring  401  may be interrupted when the electrostatic force is being generated. 
     The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments of and modifications to the present disclosure, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Further, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes.