Abstract:
A workpiece support, which more effectively cools a textured workpiece is disclosed. A layer is added on top of a workpiece support. This layer is sufficiently soft so as to conform to the textured workpiece. Furthermore, the layer has a dielectric constant such that it does not alter the normal operation of the underlying electrostatic clamp. In some embodiments, the locations of the ground and lift pins are moved to further reduce the leakage of backside gas.

Description:
FIELD 
     This disclosure relates to workpiece cooling, and more particularly to an apparatus and a method of cooling a textured workpiece. 
     BACKGROUND 
     An electronic device may be created from a workpieces that has undergone various processes. One of these processes may include introducing impurities or dopants to alter the electrical properties of the original workpiece. For example, charged ions, as impurities or dopants, may be introduced to a workpiece, such as a silicon wafer, to alter electrical properties of the workpiece. One process that introduces impurities to the workpiece may be an ion implantation process. 
     An ion implanter is used to perform ion implantation or other modifications of a workpiece. A block diagram of a conventional ion implanter is shown in  FIG. 1 . Of course, many different ion implanters may be used. The conventional ion implanter may comprise an ion source  102  that may be biased by a power supply  101 . The system may be controlled 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 workpiece or wafer  114 , which is disposed on a workpiece support  116 . 
     In operation, a workpiece handling robot (not shown) disposes the workpiece  114  on the workpiece 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 workpiece  114 . After implanting ions is completed, the workpiece handling robot may remove the workpiece  114  from the workpiece 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 workpiece  114  during the ion implantation process. As illustrated in  FIG. 2A , the workpiece support  116  may comprise a top layer  210  that is in contact with the workpiece  114 . 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 workpiece  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 workpiece  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 support  116  may be cylindrical in shape, such that its top surface is circular, so as to hold a disc-shaped workpiece. Of course, other shapes, such as squares, are possible. To effectively hold the workpiece  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 workpiece  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 workpiece 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 Johnsen-Rahbek force. 
     As seen in  FIG. 2A , the workpiece support  116  traditionally consists of several layers. The first, or top, layer  210 , which contacts the workpiece  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 top layer  210  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-Rahbek force, the volume resistivity of the top layer  210 , 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 workpiece handling robot  250  then moves a workpiece  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 workpiece  114  from the workpiece handling robot  250 . Thereafter, the workpiece 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 top layer  210  may be in contact with the workpiece  114 , as shown in  FIG. 2B . The implantation process may then be performed with the lift pins  208  in this recessed position. After the implantation process, the workpiece  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 workpiece  114  and separating the workpiece  114  from the top layer  210  of the workpiece support  116 , as shown in  FIG. 2A . The workpiece handling robot  250  may then be disposed under the workpiece  114 , where it can retrieve the implanted workpiece  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 workpiece  114  from the implanter. 
     This technique is effective, especially when the workpiece  114  and the workpiece support  116  are both substantially planar. This allows the workpiece  114  and workpiece support  116  to couple together closely when clamped. This tight coupling serves to confine the cooling gas to the cooling regions  206 . 
     However, in some embodiments, the workpiece may not be planar. For example, it is advantageous for the surface of a solar cell to be textured, to minimize reflection of photons and thus maximize cell efficiency. One common method to achieve this textured surface is to bathe the workpiece in acid or alkaline solutions. While such baths are less expensive than other processes, they will texture both sides, not just the surface exposed to the photons. However, since manufacturing costs are critical for the solar cell industry, this may be an accepted consequence. Also ion implantation into the rear surface of the cell is beneficial in producing a back surface field, so even were only the front of the cell textured it would still be necessary to clamp the textured surface for this application. 
     One consequence of textured workpiece surfaces is that the workpiece support  116  and the workpiece  114  no longer form a tight coupling as described earlier.  FIG. 3  shows an exaggerated view of the interface between a textured workpiece  200  and a workpiece support  116 . This interface presents several issues related to the cooling of the workpiece  200 . First, the textured surface of workpiece  200  implies that a lower percentage of the surface of the workpiece  200  is in physical contact with the workpiece support  116 . This reduces the ability of the workpiece support  116  to pull heat away from the workpiece  200  via conduction. A second issue is related to the cooling gas. The workpiece  116  may have cooling conduits  210 , as shown in  FIG. 3 . Gas is injected into the area between the workpiece  200  and the workpiece support  116 , as described above, through the cooling conduits  210 . However, since there is less contact between the textured workpiece  200  and the workpiece support  116 , the gas is not confined to cooling regions (as described in connection to  FIG. 2A ). As result, the gas escapes from the edges between the textured workpiece  200  and the workpiece support  116 . This increases the pressure within the chamber, which is preferably held as close to vacuum as possible, and decreases the pressure between workpiece and clamp. This is detrimental to the ion implantation process, and is detrimental in cooling the workpiece  200 . A third issue is the lower available electrostatic clamp force due to the higher average gap. 
     Accordingly, there is a need in the art for an improved workpiece support that can effectively cool textured workpieces. 
     SUMMARY 
     The problems of the prior art are overcome by the apparatus and method of this disclosure. A layer is added on top of a workpiece support. This layer is sufficiently soft so as to conform to the textured workpiece. Furthermore, the layer has a dielectric constant such that it does not alter the normal operation of the underlying electrostatic clamp. In some embodiments, the locations of the ground and lift pins are moved to further reduce the leakage of backside gas. 
    
    
     
       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 workpiece with the lift pins extended; 
         FIG. 2B  represents a block diagram showing a workpiece support supporting a workpiece with the lift pins recessed; 
         FIG. 3  represents an exaggerated view of the interface between a textured workpiece and a workpiece support in the prior art; 
         FIG. 4  represents an exaggerated view of the interface between a textured workpiece and a workpiece support according to one embodiment; and 
         FIG. 5  is a top view of a workpiece support according to one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the present disclosure, several embodiments of an apparatus and a method for cooling a textured workpiece are introduced. For purpose of clarity and simplicity, the present disclosure will focus on an apparatus and a method for cooling a textured workpiece 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, other implantation systems, 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. 
     As described above in  FIG. 3 , a tight coupling may not be possible when a textured workpiece  200  is placed atop a conventional workpiece support  116 .  FIG. 4  shows a first embodiment, where a layer  220  is applied to the top surface of the workpiece support  116 . The layer  220  is sufficiently soft so as to conform to the shape of the textured workpiece  200 . Typically hardness is measured by durometer, and a Shore D scale of about 40 to 90 is appropriate. In addition, it is desirable that the dielectric constant of the layer is relatively low, but much greater than air. The dielectric constant of air is approximately 1 and the dielectric constant of typical electrostatic clamp hard dielectrics is 5 to 10. Desirable dielectric constants for the compliant dielectric are in the range of 2 to 5. Furthermore, the breakdown voltage of the material should be fairly high, so as to function properly when the electrostatic fields are applied. High quality SiO 2  has a breakdown voltage of 10E6 volts/cm, and the breakdown voltage of the dielectric is typically above 5E6/cm. 
     Materials satisfying these requirements are referred to as “compliant dielectrics”, and include materials such as silicone rubber, polymers such as polyurethane, fluorocarbons like Teflon and certain epoxies. 
     The layer of compliant dielectric does not need to be thick to perform its intended function. In fact, the thickness of layer  220  can be between 10 μm and 50 μm. This layer  220  can be applied in several ways. For example, many compliant dielectrics are available as thin sheets and can be applied by bonding to the platen surface using some amount of heat. In another embodiment, the compliant dielectric is deposited on the workpiece support. In this embodiment, the dielectric is deposited from a vapor with a subsequent phase change, but without a chemical change from the precursor). In other embodiments, chemical vapor deposition (CVD) from a mixture of precursor gasses (with a chemical change as the film deposits onto the surface) is performed. In other embodiments, physical deposition, such as sputtering from a target made of the dielectric, is performed. For each of these deposition approaches, the apertures for lift and ground pins, and any gas distribution holes are typically masked to prevent deposition in these regions. 
     The ability of the layer  220  to conform to the shape of the textured workpiece  200  allows a tighter coupling between the textured workpiece  200  and the workpiece support  116 . As stated above, this will improve heat transfer from the workpiece  200  to the workpiece support  116 . Furthermore, this tighter coupling provides closed cooling regions  230  into which the gas can be injected via conduits  210  between the workpiece  200  and the workpiece support  116  (also known as backside gas). Because the layer  220  conforms to the shape of the textured workpiece  200 , the backside gas does not escape from the edges between these components and remains within the closed cooling regions  230 . 
     In some embodiments, the locations of the gas conduits  210 , relative to the lift pins and ground pins are also altered. Because the textured workpiece  200  may allow gas to escape from the edges, the gas conduits  210  are moved closer to the middle of the workpiece support.  FIG. 5  shows a top view of a workpiece support  300 , with gas conduits  210  located near the center of the support  300 . This creates a gas distribution region  240  that is distanced from the edge of the workpiece support  300 . Located outside of the gas distribution region  240  are the ground and lifting pins  260 . 
     Note that in some embodiments, the compliant dielectric creates an adequate seal such that the gas distribution region  240  can be larger and include a greater portion of the workpiece. In further embodiments, the gas conduits  210  are located outside of the lifting and ground pins  260 . 
     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.