Abstract:
An apparatus for improving the temperature uniformity of a workpiece during processing is disclosed. The apparatus includes a platen having a separately controlled edge heater capable to independently heating the outer edge of the platen. In this way, additional heat may be supplied near the outer edge of the platen, helping to maintain a constant temperature across the entirety of the platen. This edge heater may be disposed on an outer surface of the platen, or may, in certain embodiments, be embedded in the platen. In certain embodiments, the edge heater and the primary heating element are disposed in two different planes, where the edge heater is disposed closer to the top surface of the platen than the primary heating element.

Description:
FIELD 
     Embodiments of the present disclosure relate to apparatus for improving the temperature uniformity of a workpiece during processing, and more particularly, improving the temperature uniformity of a heated workpiece. 
     BACKGROUND 
     The fabrication of a semiconductor device involves a plurality of discrete and complex processes. To perform these processes, a workpiece is typically disposed on a platen. The platen may be an electrostatic chuck, designed to retain the workpiece through the application of electrostatic forces produced by electrodes within the platen. 
     Platens are typically designed to be slightly smaller in diameter than the workpieces that they support. This insures that the platen is not exposed to the incoming ion beam. Contact with the ion beam could cause the generation of contaminants, or may do damage to the platen. Additionally, some platens have sloped or tapered side walls to minimize the possibility that the sidewall is exposed to the incoming ion beam. 
     In addition to retaining the workpiece in place, the platen may also serve to heat or cool the workpiece. Specifically, the platen is typically a larger mass of material, capable to drawing heat from the workpiece in some embodiments, or supplying heat to the workpiece in other embodiments. In certain embodiments, the platen has conduits on its upper surface which supply a back side gas to the space between the upper surface of the platen and the back surface of the workpiece. 
     Because the platen is somewhat smaller than the workpiece, the outer edge of the workpiece may not be heated or cooled as effectively by the platen. In fact, in some embodiments, the temperature near the outer edge may be 4-10% less than the rest of the workpiece. Further, the outer edge of the platen radiates more heat to the environment than the rest of the platen, which serves to lower the temperature of the platen at its outer edge. Thus, in embodiments where the platen supplies heat to the workpiece, the outer edge of the workpiece may be cooler than the rest of the workpiece. Conversely, in embodiments where the platen is removing heat from the workpiece, the outer edge of the workpiece may be hotter than the rest of the workpiece. 
     This difference in temperature may impact the yield of the workpiece. Additionally, these temperature gradients along the outer edge of the platen may exert thermal stress on the platen, which may lead to platen failure. Therefore, it would be beneficial if there were an apparatus to achieve better temperature uniformity across a workpiece, especially in embodiments where the workpiece is heated by the platen. 
     SUMMARY 
     An apparatus for improving the temperature uniformity of a workpiece during processing is disclosed. The apparatus includes a platen having a separately controlled edge heater capable to independently heating the outer edge of the platen. In this way, additional heat may be supplied near the outer edge of the platen, helping to maintain a constant temperature across the entirety of the platen. This edge heater may be disposed on an outer surface of the platen, or may, in certain embodiments, be embedded in the platen. In certain embodiments, the edge heater and the primary heating element are disposed in two different planes, where the edge heater is disposed closer to the top surface of the platen than the primary heating element. 
     According to a first embodiment, a workpiece holding and heating apparatus is disclosed. The workpiece holding and heating apparatus comprises a platen, wherein the platen comprises a top surface, a bottom surface, and a sidewall extending between the top surface and the bottom surface; a primary heating element disposed in a first plane; and an edge heater disposed in a second plane, where the second plane is closer to the top surface than the first plane. In certain embodiments, the edge heater may be embedded in the platen. In other embodiments, the edge heater is affixed to an exterior surface of the platen. 
     According to a second embodiment, a workpiece holding and heating apparatus is disclosed. The workpiece holding and heating apparatus comprises a platen, wherein the platen comprises a top surface, a bottom surface, and a sidewall extending between the top surface and the bottom surface, where the sidewall comprises a horizontal annular ring portion, parallel to the top surface; a primary heating element; and an edge heater disposed on the horizontal annular ring portion. 
     According to a third embodiment, a workpiece holding and heating apparatus is disclosed. The workpiece holding and heating apparatus comprises a platen, wherein the platen comprises a top surface, a bottom surface, and a tapered sidewall extending between the top surface and the bottom surface; a primary heating element; and an edge heater disposed parallel to the tapered sidewall. In certain embodiments, the edge heater is affixed to the tapered sidewall. In other embodiments, the edge heater is embedded in the platen. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       For a better understanding of the present disclosure, reference is made to the accompanying drawings, which are incorporated herein by reference and in which: 
         FIG. 1  is a block diagram of the system having a platen with an edge heater according to one embodiment; 
         FIGS. 2A-2B  are side views of platens with an edge heater affixed to a tapered sidewall according to one embodiment; 
         FIGS. 3A-3B  are side views of platens with an edge heater according to another embodiment; 
         FIGS. 4A-4B  are side views of platens with an embedded edge heater according to another embodiment; 
         FIG. 5  is a side view of a platen having multiple edge heaters according to another embodiment; 
         FIG. 6  is a side view of a platen having multiple embedded edge heaters according to another embodiment; 
         FIG. 7  is a side view of a platen having a embedded edge heater disposed along the tapered sidewall; and 
         FIG. 8  shows a comparison of the temperature of the platen as a function of temperature with and without an edge heater. 
     
    
    
     DETAILED DESCRIPTION 
     As described above, the edges of workpieces disposed on traditional platens may overhang the platen, causing these edges to maintain a different temperature than the rest of the workpiece. Furthermore, since the outer edge of the platen is exposed to the environment, the outer edge tends to radiate more heat into the environment, effectively lowering the temperature along the outer edge of the platen. In other words, the heat generated in the interior of the platen is more likely to remain in the platen, while heat generated near the outer edge is more likely to dissipate into the environment. 
     One approach to address this issue is to provide a separately controlled edge heater near the outer edge of the platen. This edge heater may be used to compensate for the heat that is known to be lost near the outer edge of the platen. In certain embodiments, the edge heater is disposed closer to the workpiece than the primary heating element in the platen. 
       FIG. 1  shows a representative illustration showing the components of the system  10 . The platen  100  may be an electrostatic chuck (ESC), or any other type of platen. In some embodiments, the platen  100  comprises a plurality of conduits terminating on the upper surface of the platen  100 , which deliver back side gas to the volume between the upper surface of the platen  100  and the bottom surface of the workpiece  30 . The platen  100  may also have an outer seal ring (not shown) near its outer edge, which serves to confine the back side gas in this volume and minimize back side gas leakage. The outer seal ring extends upward from the upper surface of the platen  100  and contacts the workpiece  30 , forming a wall that contains the back side gas. This outer seal ring may be effective because the outer seal ring contacts the workpiece  30 . Further, the platen  100  may include an upper dielectric layer, under which a plurality of electrodes is disposed. Alternating voltage waveforms are applied to these electrodes, which create an electrostatic force that holds the workpiece  30  in place. This upper dielectric layer may be unable to withstand ion beam strike. Thus, because of the outer seal ring and the upper dielectric layer, the platen  100  is typically smaller than the workpiece  30  that is disposed thereon, to insure that the ion beam cannot strike the platen  100 . In some embodiments, the workpiece  30  may overhang the platen  100  by 2-3 mm, although other dimensions are also possible and within the scope of the disclosure. 
     The platen  100  may be disposed on a base  20 , used to support the platen  100 . The base  20  may be made of the different material than the platen  100 . Further, the temperature of the platen  100  may be uneven or non-uniform, as the outer surfaces that are exposed to the environment, and particularly the sidewalls of the platen  100 , tend to radiate heat into the environment, lowering the temperature at these outer surfaces. 
     The platen  100  may have a cylindrical shape so as to support a disk-shaped workpiece. The outer edge of the platen  100  may be considered the portion of the platen  100  nearest the outer diameter of the platen  100 . For example, an annular ring of several millimeters having an outer diameter that is the circumference of the platen  100  may be considered to be the outer edge. This outer edge may have a temperature different than the rest of the platen  100 . 
     As shown in  FIG. 1 , the platen  100  may have a primary heating element  110 , used to heat the platen  100 . This primary heating element  110  may be disposed in the platen  100 , as shown in  FIG. 1 , or may be disposed on an outer surface of the platen  100 , such as the bottom surface of the platen  100 . This primary heating element  110  may be a film comprising an electrically resistive material, so that the flow of current through the primary heating element  110  causes the generation of heat through the resistive material. The temperature of the platen  100  may be controlled by regulating the amount of current passing through the primary heating element  110 . In certain embodiments, a controller  40  may be used to regulate the temperature of the platen  100  by controlling the flow of current through the primary heating element  110 . 
     The platen  100  may also include a first temperature sensor  115 , which is used to monitor the temperature of the platen  100 . The controller  40  may regulate the temperature of the platen  100  by monitoring the temperature of the platen  100  using the first temperature sensor  115  and adjusting the current through the primary heating element  110  based on the monitored temperature. Thus, the first temperature sensor  115 , the primary heating element  110  and the controller  40  form a first control loop to monitor and control the temperature of the platen  100 . 
     The system  10  also comprises an edge heater  120 , which is disposed near the outer edge of the platen  100 . While  FIG. 1  shows the edge heater  120  as being separate from the platen  100 , it is understood that the edge heater  120  may be disposed on a surface of the platen  100  in certain embodiments. In other embodiments, the edge heater  120  may be disposed within the platen  100 . The edge heater  120  may be a thin film material having an electrically resistive material. The use of thin film may allow the edge heater  120  to be more readily applied to various surfaces, as will be described in more detail below. In certain embodiments, the thin film may be about 0.01 mm in thickness, although other thicknesses are also possible. A current is then passed through the electrically resistive material to generate heat. The thickness or/and width of the thin film material may be varied to control the amount of heat generated in the film to affect temperature uniformity. 
     A second temperature sensor  125  may be disposed near the outer edge of the platen  100  so as to measure the temperature at the outer edge. As explained above, the outer edge of the platen  100  may be cooler than the rest of the platen  100  due to the radiation of heat to the environment. While the second temperature sensor  125  is shown mounted on the edge heater  120 , other embodiments are possible. In certain embodiments, the second temperature sensor  125  is disposed on the platen  100 , near its outer edge, separate from the edge heater  120 . This second temperature sensor  125 , the edge heater  120  and the controller  40  may form a second control loop used to monitor and control the temperature of the platen  100  at its outer edge. This second control loop may operate independent of the first control loop. 
     The controller  40  may be any suitable device, which is capable of receiving inputs from one or more sources, such as the first temperature sensor  115  and the second temperature sensor  125 . The controller  40  is also capable of providing outputs, such as current control to the primary heating element  110  and the edge heater  120 . 
     Having defined the general architecture of the system  10 , various embodiments will be described. It should be noted that these embodiments are only illustrative and the disclosure is not limited to those embodiments presented. 
       FIG. 2A  shows a first embodiment of a platen  200  having an edge heater  220 .  FIG. 2A  shows a side view of the platen  200 , which may have a sidewall  201 , which may be sloped or tapered. In other words, the sidewall  201  of the platen  200  may form an acute angle, such as 45° with the top surface of the platen  200 . The angle that the sidewall  201  forms with the top surface is an implementation decision and is not limited by the present disclosure. The use of a tapered sidewall may reduce the amount of sputtering caused by the ion beam striking the sidewall  201 . 
     In this particular embodiment, the primary heating element  210  may be disposed on the bottom of the platen  200 . As such, the primary heating element  210  may be about 8-10 mm away from the top surface of the platen  200 . Since the sidewall  201  is tapered, the primary heating element  210  does not extend to the outermost portions of the top surface. Therefore, due to the location at which the heat is generated and the losses incurred at the sidewalls, the outer edge of the platen  200  may be at a lower temperature than the rest of the platen  200 . 
       FIG. 2B  shows a variation of  FIG. 2A , where the primary heating element  211  is disposed within or is embedded in the platen  250 . This may be achieved by manufacturing the platen  250  as two portions. For example, the platen  250  may comprise a ceramic material. The primary heating element  211  may then be placed between the two horizontal ceramic portions. The wires used to connect to the primary heating element  210  may extend through the outside of the platen  250 . The assembly, which includes the two ceramic portions with the primary heating element  211  interposed therebetween, may then be fired, encapsulating the primary heating element  211  in the platen  250 . In yet another embodiment, there may be multiple primary heating elements  211  disposed within the platen  250 . In this embodiment, the primary heating element  211  may be disposed about 1-5 mm from the top surface of the platen  250 . 
     Like the platen  200  of  FIG. 2A , the platen  250  may have a sidewall  201  that is tapered. Again, because of the taper of the sidewall  201 , the primary heating element  211  does not extend to the outer diameter of the top surface of the platen  250 . 
     In certain embodiments, the embodiments of  FIGS. 2A and 2B  may be combined, such that there is a primary heating element  210  disposed on the bottom surface of the platen and at least one primary heating element  211  embedded in the platen. In other words, the placement and number of primary heating elements  210 ,  211  are not limited by the disclosure. 
     In each of these embodiments, the edge heater  220  is disposed on the sidewall  201 . The edge heater  220  may be a thin film, as described above, which is affixed to the sidewall  201 . Since the sidewall  201  is tapered, the edge heater  220  may be frustoconical in shape to match the shape of the sidewall  201 . In certain embodiments, the edge heater  220  may be affixed to the sidewall  201 , such as by using an adhesive. To reduce the possibility that the edge heater  220  is exposed to the ion beam, a protective coating may be disposed over the edge heater  220 . For example, a glass layer may be used to cover the thin film that forms the edge heater  220 . Of course, other materials may be used to cover the edge heater  220 . 
     The placement of the edge heater  220  along the sidewall  201  serves several purposes. As described above, the heat in the platen radiates from those surfaces exposed to the environment. Consequently, the sidewall  201  is a source of heat loss. By disposing the edge heater  220  on the sidewall  201 , the edge heater  220  may be used to counteract the losses that are attributed to the sidewall  201 . This may help insure that the temperature across the entirety of the platen is more uniform. 
       FIGS. 3A-3B  shows another embodiment using an edge heater  320 , which may be flat. In the embodiment of  FIG. 3A , the sidewall  301  of the platen  300  has a tapered portion  302 , similar to that shown in  FIGS. 2A-2B . As before, the tapered portion  302  may form an acute angle with the top surface of the platen  300 , such as about 45°. However, in this embodiment, the sidewall  301  also has a notch  303 . This notch  303  creates a horizontal portion of the sidewall  301 , which is parallel to the top surface of the platen  300 . This horizontal portion extends around the entirety of the platen  300  and thus forms a horizontal annular ring portion  304 . A vertical portion  305  is also created by the notch  303 . In certain embodiments, the vertical portion  305  may not be perfectly vertical. Rather, the vertical portion  305  is simply the surface that joins the horizontal annular ring portion  304  to the bottom surface of the platen  300 . 
     As shown in  FIG. 3A , the horizontal annular ring portion  304  is a distance “t” from the top surface of the platen  300 . This distance “t” may be less than the distance “t 2 ” from the primary heating element  210  to the top surface of the platen  300 . In some embodiments, the distance “t” may be 2-3 mm although other distances are also possible. In contrast, the distance “t 2 ” may be 8-10 mm. 
     The distance “d” represents the inner diameter of the horizontal annular ring portion  304 . In certain embodiments, the inner diameter of the horizontal annular ring portion  304  may be about 280 mm, while the diameter of the platen  300  may be about 294 mm. The outer diameter of the horizontal annular ring portion  304  may be about 284-290 mm. Thus, the horizontal annular ring portion  304  may have a width of about 3-4 mm. 
     The creation of a horizontal annular ring portion  304  around the platen  300  allows the edge heater  320  to be a flat ring, as opposed to the frustoconical shape of  FIGS. 2A-2B . This flat ring may be advantageous from a manufacturing, cost and assembly perspective. Further, the use of a horizontal annular ring portion  304  also creates a surface, which is parallel to the top surface of the platen  300 , on which the edge heater  320  may be disposed. 
     As stated above, the edge heater  320  may be a flat ring, comprising a thin film. As described above, the edge heater  320  may be covered with a protective coating, such as a glass layer to protect the edge heater  320  from the ion beam. The edge heater  320  may be affixed to the horizontal annular ring portion  304 , using an adhesive. In other embodiments, the edge heater  320  may be bonded to the horizontal annular ring portion  304  by heat or other means. 
     Since the edge heater  320  is closer to the top surface of the platen  300  than the primary heating element  210 , the edge heater  320  may be able to provide more focused heat to the outer edge of the platen  300  and the workpiece  30 . Additionally, the heat generated by the edge heater  320  is independent of the heat generated by the primary heating element  210 . Therefore, it is possible to achieve a more uniform temperature profile across the top surface of the platen  300 . 
       FIG. 3B  shows another embodiment of a platen  350  having a horizontal annular ring portion  304 . Like  FIG. 3A , this embodiment comprises a sidewall  301  having a tapered portion  302 , a horizontal annular ring portion  304 , and a vertical portion  305 , where the edge heater  320  is disposed on the horizontal annular ring portion  304 . This embodiment differs from  FIG. 3A  in that the primary heating element  211  is embedded in the platen  350 . The primary heating element  211  may be embedded using the technique described with respect to  FIG. 2B . In this embodiment, the distance from the primary heating element  211  to the top surface of the platen  300 , labeled “t 2 ” may be 2-8 mm, while the distance from the edge heater  320  to the top surface of the platen  300 , labeled “t”, may be 1-7 mm. In certain embodiments, the primary heating element  211  may be 1 mm above the bottom surface, while the edge heater  320  is disposed at a position closer to the top surface. 
     The embodiments of  FIG. 3A-3B  modify the shape of the platen to create a horizontal annular ring portion  304  on the underside of the platen. This horizontal annular ring portion  304  may be parallel to the top surface. In both embodiments, the sidewall  301  of the platen may include a tapered portion  302  that extends from the top surface to the horizontal annular ring portion  304 , the horizontal annular ring portion  304 , and a vertical portion  305  that extends from the horizontal annular ring portion  304  to the bottom surface of the platen. As stated above, the vertical portion  305  may not be vertical, but rather is simply the surface that extends between the horizontal annular ring portion  304  and the bottom surface of the platen. 
       FIGS. 4A-4B  show another embodiment. In this embodiment, unlike the previous embodiments, the edge heater  420  is embedded in the platen  400 , rather than being affixed to the platen  400 . In  FIG. 4A , like the embodiments of  FIGS. 2A and 3A , the primary heating element  210  may be disposed on the bottom surface of the platen  400 . The platen  400  may have a thickness “t 2 ”, which may be about 8 mm. As above, the platen  400  may have sidewalls  401  that are tapered. 
     In the embodiment of  FIG. 4A , the edge heater  420  is embedded in the platen  400 . This may be achieved by manufacturing the platen  400  from two horizontal portions. For example, the platen  400  may comprise a ceramic material. The edge heater  420  may then be placed between the two horizontal ceramic portions. The assembly may then be fired, encapsulating the edge heater  420  in the platen  400 . The edge heater  420  may be a flat ring, similar to that shown in  FIG. 3A . The electrical connections to the edge heater  420  may pass through the platen  400  to the environment, where they are connected to the controller  40 . Like the embodiment of  FIG. 3A , the edge heater  420  may be disposed a distance “t” below the top surface of the platen  400 , where that distance “t” may be about 1-7 mm. Additionally, the distance “d” may be the inner diameter of the edge heater  420  and may be about 280 mm, while the diameter of the platen  400  may be about 294 mm. The edge heater  420  may have a width of about 1-4 mm. 
       FIG. 4B  shows a variation of the embodiment of  FIG. 4A  where the primary heating element  211  is also embedded in the platen  450 . The distance from the primary heating element  211  to the top surface of the platen  450 , labeled “t 2 ” may be 2-8 mm, while the distance from the edge heater  420  to the top surface of the platen  450 , labeled “t”, may be 1-7 mm. 
     The manufacturing of the platen  450  of  FIG. 4B  may be more complex than the previous embodiments. The platen may be made up of three horizontal portions; a top portion, a middle portion and a bottom portion. The edge heater  420  is disposed between the top portion and the middle portion, while the primary heating element  211  is disposed between the middle portion and the bottom portion. The entire assembly, which includes the three ceramic portions with the primary heating element  211  and edge heater  420  disposed therebetween, may then be fired, encapsulating the primary heating element  211  and the edge heater  420  in the platen  450 . 
       FIGS. 3A-3B and 4A-4B  show the use of an edge heater that is in a different horizontal plane than the primary heating element  210 . Specifically, the primary heating element, the edge heater and the top surface of the platen may all form parallel planes. The distance between the plane in which the edge heater is disposed and the top surface may be less than the distance between the plane in which the primary heating element is disposed and the top surface. In other words, the edge heater in these embodiments may be disposed closer to the top surface of the platen than the primary heating element. By disposing the edge heater closer to the top surface, the edge heater may better regulate the temperature of the platen along the outer edge. The configuration described here where the edge heater and the primary heating element are disposed in different planes can be achieved to several ways. For example, the primary heating element  210  may be disposed on the bottom surface of the platen (See  FIGS. 3A and 4A ) or the primary heating element  211  may be disposed within the platen (see  FIGS. 3B and 4B ). The edge heater  320  may be disposed on an exterior surface of the platen, such as on a horizontal annular ring portion  304  (See  FIGS. 3A and 3B ), or may be disposed within the platen (see  FIGS. 4A and 4B ). 
     While  FIGS. 3A-3B and 4A-4B  show a single edge heater that is ring shaped, other embodiments are possible. For example, a plurality of edge heaters may be utilized. In certain embodiments, the edge heater closest to the outer circumference of the platen may be the closest to the top surface, with each successive edge heater being disposed further from that top surface. 
       FIG. 5  shows a variation of the embodiment of  FIG. 3B , which includes a primary heating element  211  disposed within the platen  500 . In this embodiment, the platen  500  has a plurality of notches, each of which creates a horizontal annular ring portion. The sidewall  501  of the platen  500  may have a tapered portion  502 . The sidewall  501  may have a first notch  503 , which creates a first horizontal annular ring portion  504  and a first vertical wall  505 . Further, the sidewall  501  may have a second notch  506 , which creates a second horizontal annular ring portion  507  and a second vertical wall  508 . The second vertical wall  508  extends between the second horizontal annular ring portion  507  and the bottom surface of the platen  500 . Of course, additional notches, which create additional horizontal annular ring portions, may also be included. Disposed on the first horizontal annular ring portion  504  may be a first edge heater  520 . Disposed on the second horizontal annular ring portion  507  may be a second edge heater  525 . As noted above, the first horizontal annular ring portion  504 , which is closer to the outer edge of the platen  500  is disposed closer to the top surface than the second horizontal annular ring portion  507 , which is disposed further from the outer edge of the platen  500 . If additional horizontal annular ring portions were present, each would be slightly further from the top surface than its adjacent neighbor. For example, first horizontal annular ring portion  504  may be disposed 1-2 mm below the top surface, while second horizontal annular ring portion  507  may be disposed 3-4 mm below the top surface of the platen  500 . Each edge heater  520 ,  525  may be between 3-10 mm wide. 
     Further, although not shown, a similar variation of  FIG. 3A , which has multiple notches and the primary heating element  210  disposed on the bottom surface of the platen  500  may also be employed. As described above, this embodiment may have two or more horizontal annular ring portions, where an edge heater is disposed on each of these horizontal annular ring portions. 
       FIG. 6  shows a variation of  FIG. 4B , which has multiple embedded edge heaters. In this embodiment, edge heaters  620 ,  621 ,  622  are all embedded in the platen  600 . As described above, the edge heater  620  that is disposed closest to the outer edge of the platen  600  may be disposed closest to the top surface of the platen. As the edge heaters get further from the outer edge, the distance between that edge heater and the top surface increases, as shown in  FIG. 6 , where edge heater  620  is disposed closest to the top surface of the platen  600 , and edge heater  622 , which is the furthest from the outer edge, is disposed furthest from the top surface of the platen  600 . For example, edge heater  620  may be 1-2 mm away from the top surface, while edge heater  622  may be 5-6 mm away from the top surface of the platen  600 . Additionally, the primary heating element  211  may also be disposed in the platen  600 . This platen  600  may be manufactured in the manner described in the embodiment of  FIG. 4B , where the platen  600  has a greater number of horizontal portions. Although three edge heaters  620 ,  621 ,  622  are shown, any number of edge heaters may be embedded in the platen  600 . 
     Further, a variation of  FIG. 4A , which has multiple embedded edge heaters and where the primary heating element  210  is disposed on the bottom surface of the platen may be used as well. 
       FIG. 7  shows another embodiment, which is similar to that shown in  FIG. 2B . In this embodiment, the edge heater  720  is disposed at an angle, which may be parallel to the tapered sidewall  701 . Thus, like the embodiments shown in  FIG. 2A-2B , the edge heater  720  may be frustoconical in shape. However, unlike the embodiment of  FIG. 2B , the edge heater  720  is embedded in the platen  700 . In this embodiment, the primary heating element  211  is disposed in the platen  700 . However, in another embodiment, the primary heating element may be affixed to the bottom surface of the platen  700 , as shown in  FIG. 2A . To manufacture this configuration, three or more pieces of ceramic may be used. The heater materials are sandwiched between those ceramic pieces. A co-fire process then follows which will result in a single platen with embedded heating elements. 
     Further, in certain embodiments, additional heating elements may be used to bridge between the edge heater and the primary heating element  210  to reduce thermal stresses in the platen. 
       FIG. 8  shows the improvement in temperature uniformity that may be achieved using the present system. Specifically, this figure shows the temperature profile of a traditional platen as compared to the system shown in  FIG. 3A . As can be seen, the temperature roll-off at the outer edge is greatly reduced by the system of  FIG. 3A . For example, the temperature gradient of a traditional platen may be as much as 8%. The system of  FIG. 3A  exhibited a temperature gradient of less than 2%. 
     The embodiments described above in the present application may have many advantages. For example, by introducing an edge heater, the temperature uniformity throughout the entirety of the platen may be improved. This improved temperature uniformity may reduce thermal stress within the platen, extending its life and reducing failures. Further, improved temperature uniformity may have a positive impact of semiconductor device yield, as the workpiece  30  may be more consistently heated. Further, the use of a separate edge heater allows independent current control of the outer edge of the platen. Thus, the amount of current used to heat the main portion of the platen does not affect the amount of current that may be used to heat the outer edge. This may also allow more uniform heating of the workpiece  30 . 
     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. Furthermore, 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. Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the present disclosure as described herein.