Abstract:
An apparatus and method for improving the temperature uniformity of a workpiece during processing is disclosed. The apparatus includes a ring heater assembly disposed along the outer circumference of the platen. The ring heater assembly includes heating elements disposed therein or thereon, where these heating elements create heat, which serves to warm the outer edge of the workpiece. In some embodiments, the ring heater assembly extends beyond the edge of the workpiece and may be exposed to the ion beam.

Description:
FIELD 
       [0001]    Embodiments of the present disclosure relate to apparatus and methods for improving the temperature uniformity of a workpiece during processing, and more particularly, improving the temperature uniformity of a heated workpiece. 
       BACKGROUND 
       [0002]    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. 
         [0003]    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. 
         [0004]    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 that supply a back side gas to the space between the upper surface of the platen and the back surface of the workpiece. 
         [0005]    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. 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. 
         [0006]    This difference in temperature may impact the yield of the workpiece. Therefore, it would be beneficial if there were an apparatus and method to achieve better temperature uniformity across a workpiece, especially in embodiments where the workpiece is heat by the platen. 
       SUMMARY 
       [0007]    An apparatus and method for improving the temperature uniformity of a workpiece during processing is disclosed. The apparatus includes a ring heater assembly disposed along the outer circumference of the platen. The ring heater assembly includes heating elements disposed therein or thereon, where these heating elements create heat, which serves to warm the outer edge of the workpiece. In some embodiments, the ring heater assembly extends beyond the edge of the workpiece and may be exposed to the ion beam. 
         [0008]    According to one embodiment, a workpiece holding and heating apparatus is disclosed. The apparatus comprises a platen; and a ring heater assembly surrounding an outer circumference of the platen, the ring heater assembly comprising a protective shield on a top surface and a heating element disposed beneath the protective shield. In a further embodiment, the heating element may be encased in the protective shield. In certain embodiments, the protective shield is a ceramic material. In certain embodiments, a mounting system holds the ring heater assembly in place and includes a plurality of flexures. In some embodiments, the mounting system connects to a based used to hold the platen. In certain embodiments, the ring heater assembly also includes a temperature sensor disposed near the top surface. In some embodiments, a power supply is in communication with the ring heater assembly. 
         [0009]    According to another embodiment, a method of heating a workpiece is disclosed. The method comprises using conductive heating through the use of a back side gas to provide heat to a first portion of the workpiece; and using radiative heating to provide heat to a second portion of the workpiece. In certain embodiments, the second portion may be an outer edge of the workpiece. 
         [0010]    According to another embodiment, a ring heater assembly is disclosed. The ring heater assembly comprises a ring shaped protective shield; a heating element disposed beneath the ring shaped protective shield; and a mounting system to place the ring shaped protective shield in place. In certain embodiments, the mounting system comprises a mounting frame in communication with a based and a plurality of flexures extending from the mounting frame toward the heating element. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0011]    For a better understanding of the present disclosure, reference is made to the accompanying drawings, which are incorporated herein by reference and in which: 
           [0012]      FIG. 1  is a view of a ring heater assembly according to one embodiment; 
           [0013]      FIG. 2  is a view of a ring heater assembly according to another embodiment; 
           [0014]      FIGS. 3A-B  show different embodiments of the heating elements; 
           [0015]      FIG. 4  is a view of the heating elements and the mounting frame according to one embodiment; 
           [0016]      FIGS. 5A-B  show the connection between the ring heater and the platen base according to one embodiment; and 
           [0017]      FIGS. 6A-C  show various embodiments having different overhang portions. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    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. 
         [0019]      FIG. 1  shows a first embodiment of a ring heater assembly  100 . In this embodiment, the ring heater assembly  100  is disposed proximate and surrounding the outer circumference of the platen  10 . The platen  10  may be an electrostatic chuck (ESC), or any other type of platen. In some embodiments, the platen  10  comprises a plurality of conduits terminating on the upper surface of the platen  10 , which deliver back side gas to the volume between the upper surface of the platen  10  and the bottom surface of the workpiece  30 . The platen  10  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  10  and contacts the workpiece  30 , forming a wall that contains the back side gas. This outer seal ring is effective because it contacts the workpiece  30 . Further, the platen  10  may include an upper dielectric layer, under which a plurality of electrodes is disposed. Alternating voltage waveforms may be applied to these electrodes, which create an electrostatic force that holds the workpiece  30  in place on the platen  10 . This upper dielectric layer may be unable to withstand ion beam strike. Thus, because the outer seal ring contacts the workpiece  30 , the platen  10  is typically smaller than the workpiece  30  which is disposed thereon, to insure that the ion beam cannot strike the platen  10 . In some embodiments, the workpiece  30  may overhang the platen  10  by 2-3 mm, although other dimensions are also possible and within the scope of the disclosure. 
         [0020]    The platen  10  may be disposed on a base  20 , used to support the platen  10 . The base  20  may be made of the different material than the platen  10 . Extending outward from the base  20  is a mounting system, which may comprise one or more flexures  110 . These flexures  110  are coupled to the base  20  on one end, and extend outward and upward past the edge of the platen  10 . These flexures  110  may be any suitable material. In some embodiments, the flexures  110  may be tubes, in which electrical wires are disposed, as described in more detail below. In other embodiments, the flexures  110  may be solid, and the electrical wires  120  may be disposed outside of the flexures  110 . These flexures  110  may connect to a mounting frame  160 , which is used to hold the ring heater assembly  100  in place. The mounting frame  160  may also hold one or more heating elements  130  in place. The mounting frame  160  may be a block that may hold one end of the flexure  110  and connect it to the base  20 . The connection between the mounting frame  160  and the flexure  110  may be a mechanical connection, such as a screw or clamp. Of course, other mounting systems may be used to hold the ring heater assembly  100  in place. 
         [0021]    While  FIG. 1  shows two layers of heating elements  130 , the disclosure is not limited to any particular number of heating elements. These heating elements  130  may be protected by a protective shield  140 . This protective shield  140  may be a ceramic material, although other materials may be used. The material for the protective shield  140  may be selected to be relatively impervious to the ion beam, such that it resists sputtering. Further, the material may be selected so that any sputtered material minimally contaminates the workpiece. Silicon carbide and graphite are such materials, although other materials may be used. 
         [0022]    As shown in  FIG. 1 , the protective shield  140  may be disposed so that, when a workpiece  30  is disposed on the platen  10 , the workpiece  30  does not contact the protective shield  140 . 
         [0023]    Additionally, a heat shield  150  may be disposed beneath the heating elements  130 . This heat shield  150  serves to direct the heat generated by the heating elements  130  upward toward the protective shield  140  to maximize the heat transfer from the heating elements  130  to the protective shield  140 . Since the heat shield  150  is not exposed to the ion beam, it may be constructed of a metal, although other material may also be used. 
         [0024]    In operation, power is supplied through electrical wires  120  to the heating elements  130 , which convert the electrical energy to heat. This heat is reflected upward by the heat shield  150  toward the protective shield  140 . The protective shield  140  absorbs this heat, raising its temperature. The protective shield  140  then heats the workpiece  30  disposed above it via radiative heating. 
         [0025]    To supply power, the ring heater assembly  100  is in communication with a power supply (not shown). In some embodiments, the power supply may be capable of supplying a variable output, such that the amount of heat produced by the heating elements  130  can be controlled. In certain embodiments, the outputs of the power supply are controlled using closed loop control, where a temperature sensor (not shown) is disposed on or near the ring heater assembly  100 . In other embodiments, the temperature sensor may be embedded in the ring heater assembly  100 . The power supply regulates the output based on the desired temperature of the ring heater assembly  100  and the actual temperature as measured by the temperature sensor. In another embodiment, the output of the power supply is calibrated so that a table that equates desired temperature to output level is created. This table is then used to determine the appropriate output of the power supply, based on the desired temperature. 
         [0026]      FIG. 1  shows a ring heater assembly  100  having two layers of heating elements  130 . However, other embodiments are also possible. For example, the heating elements  130  may be disposed within a ceramic or impregnated graphite material. 
         [0027]      FIG. 2  shows another embodiment of a ring heater assembly  200 . In this embodiment, the heating elements are encased in one or more ring heaters  210 . The ring heater  210  may be a ceramic or impregnated graphite material, where the heating elements are disposed therein. In some embodiments, a separate protective shield is not used, as the material used to construct the ring heater  210  functions as the protective shield as well. In this embodiment, a heat shield  220  may be disposed on the underside of the ring heater  210 . Note that, in this embodiment, the platen  10  has a tapered edge. In this embodiment, the ring heater  210  may be constructed to be flush, or nearly flush against the platen  10 . If the platen  10  does not have a taper, the ring heater  210  may be constructed without this taper as well. 
         [0028]    Though not shown, the heating elements within the ring heater  210  are in communication with a power supply, as described above. 
         [0029]    Furthermore, the ring heater  210  may include a temperature sensor  230  disposed therein. The temperature sensor may be a thermocouple or a resistance temperature detector (RTD). In one embodiment, as shown in  FIG. 2 , a cavity  231  may drilled into the ring heater  210  so that the temperature sensor  230  may be inserted in the cavity  231 . In some embodiments, the temperature sensor  230  is disposed close to the upper surface of the ring heater  210  to better determine the temperature near the workpiece  30 . 
         [0030]    Note that the workpiece  30  extends beyond the edge of the platen  10  and is disposed above a portion of the ring heater  210 . This portion, referred to as the overhang portion, may vary. In some embodiments, only 2-3 mm of the workpiece comprise the overhang portion. However, in other embodiments, the platen  10  may be made smaller, relative to the workpiece, so that the overhang portion may be larger. For example, in some embodiments, the overhang portion may be as large as 15-20 mm. For example,  FIG. 2  shows an overhang portion of about 2-3 mm. However, the diameter of the platen  10  may be decreased, such as by 5 mm, 10 mm, 15 mm, 20 mm, or 25 mm, while the width of the ring heater  210  is increased correspondingly. The overhang portion is not limited by the present disclosure. 
         [0031]      FIG. 3A  shows an embodiment of a ring heater  300 , which may be used in the embodiment shown in  FIG. 2 . In this embodiment, one or more heating elements  310  are disposed within an outer material  320 . These heating elements  310  may be resistive wire heaters, designed to dissipate energy by the generation of heat. The outer material  320  may be ceramic, impregnated graphite or another material. In some embodiments, the heating elements  310  may be encapsulated in a metal tube  315  to physically separate them from the outer material  320 . In some embodiments, the ring heater  300  may include a temperature sensor. As shown in  FIG. 2 , in some embodiments, a cavity may be drilled in the outer material  320  and a temperature sensor may be placed in the drilled cavity. In another embodiment, the temperature sensor  380  may be formed in the outer material  320 , near the upper surface. Alternatively, a cavity could be formed in the top surface of the outer material  320 , and the temperature sensor  380  may be placed in that cavity. 
         [0032]      FIG. 3B  shows another embodiment of a ring heater  350 , which may be used in the embodiment shown in  FIG. 2 . In this embodiment, the heating elements  360  may be flat and may be encapsulated between a first layer  370  and a second layer  371 . To produce this ring heater  350 , the heating elements  360  may be silk screened, direct written or otherwise deposited on the second layer  371 , which may be an unfired ceramic material. The first layer  370 , which may also be an unfired ceramic material, is then placed on top of the second layer  371 . The assembly then gets fired, or sintered, and becomes a monolithic piece of ceramic with an embedded heater. Again, a temperature sensor  380  may be embedded in the first layer  370 , using any of the methods described above. 
         [0033]      FIG. 4  shows an embodiment of the ring heater assembly  400 , separated from the platen  10 . In this embodiment, the ring heater assembly  400  is made up of three ring heaters  410 , each being about ⅓ of the total circumference. If more ring heaters are used, each would be smaller than those shown in  FIG. 4 . The use of separate ring heaters  410  may allow for thermal expansion while minimizing thermal stress. Disposed between each adjacent pair of ring heaters  410  is the mounting frame  420 . The mounting frame  420  holds a flexure (see  FIG. 1 ) to support the ring heaters  410 , and also provides electrical conduits, which supply power to the heating elements disposed within the ring heaters  410 . Although not shown, the ring heaters  410  may be covered by a protective shield. The ring heaters  410  may be of the types shown in  FIGS. 3A-3B , or any other suitable type. 
         [0034]      FIG. 5A  illustrates a bottom view of one embodiment of the connection between the ring heaters  410  shown in  FIG. 4  and the base  20 .  FIG. 5B  illustrates a top view of this embodiment. In this embodiment, the base  20  comprises one or more receptacles  500  to mate with mounting frame  420 . In some embodiments, three or more receptacles  500  are equally disposed about the outer perimeter of the base  20 . The mounting frame  420  may hold a flexure  510  to support the ring heaters  410 . As stated above, the flexure  510  may be secured to the mounting frame  420  using a mechanical connection, such as a screw or clamp. In certain embodiments, the mounting frame  420  also comprises one or more electrical conduits  520 , which supply power to the ring heaters  410 . In certain embodiments, the electrical conduits  520  may be disposed in the flexure  510 . 
         [0035]    The flexure  510  may be connected to the ring heaters  410  in a variety of ways. In one embodiment, there may be a flange joined to the distal end of the flexure  510 . The flange may be joined using high temperature brazing. A block  522  having a threaded feature may be attached to the underside of the ring heaters  410 . The threaded feature accepts the flange. The connection between the flange and the threaded feature may be a threaded joint. The attachment of the block  522  to the ring heater  410  may be done in a variety of ways. In one embodiment, the ring heater  410  may be ceramic. In this embodiment, the block  522  may be metal and may be brazed to the ring heater  410 . In another embodiment, the ring heater  410  may be graphite. In this embodiment, traditional threaded hardware may be used, as graphite can accept threads with inserts. In an alternate embodiment, the block  522  may be clamped around a machined feature on the bottom of the graphite ring heater. In another embodiment, a heat shield (see  FIG. 2 ) is disposed under the ring heater  410 . If this heat shield is metal, the flexure  510  may connect to the heat shield. 
         [0036]    While  FIGS. 4, 5A and 5B  show one embodiment of a mounting frame  420  that can be used to attach the base  20  to the ring heater assembly, the disclosure is not limited to this embodiment. Other support mechanisms may be used to hold the ring heater assembly in place. 
         [0037]    In all of these embodiments, the ring heater assembly is an annular ring that surrounds the outer circumference of the platen  10 . The top of the ring heater assembly may be positioned so that it is slightly below the level of the top surface of the platen  10  to insure that the workpiece  30  does not contact the ring heater assembly. 
         [0038]    In each embodiment, the ring heater assembly comprises a protective shield, which is disposed on the upper surface of the ring heater assembly. Because it is exposed to the ion beam, the protective shield may be made of ceramic or impregnated graphite or any suitable material that is resistant to sputtering. 
         [0039]    Beneath the protective shield are one or more heating elements. These heating elements are typically resistive elements that are supplied with power from a power supply. The electrical energy passing through the heating elements is transformed into heat, which serves to heat the heating elements and the surrounding elements, such as the protective shield. In some embodiments, the heating elements are encased or encapsulated in a ceramic or graphite shell, which serves as the protective shield. In some embodiments, a temperature sensor is embedded or other affixed to the shell or outer material. 
         [0040]    Disposed beneath the heating elements may be a heat shield, which reflects the heat toward the upper surface of the ring heater assembly. This heat shield may be metal, as it is not exposed to the ion beam. 
         [0041]    Additionally, the ring heater assembly may be held in place through the use of a mounting system. In some embodiments, the mounting system comprises a plurality of flexures extending from the base. However, other mounting systems may also be used. 
         [0042]    Finally, the ring heater assembly may be in communication with a power supply, which powers the heating elements. As described above, the power supply may be capable of supplying a variable output, such that the amount of heat produced by the heating elements can be controlled. In certain embodiments, the outputs of the power supply are controlled using closed loop control, where a temperature sensor is disposed on or near the top surface of the ring heater assembly. In this embodiment, a controller (not shown) may be in communication with the temperate sensor and the power supply. The controller may also receive inputs regarding the desired temperature of the ring heater assembly. The controller then uses the input from the temperature sensor to adjust the output of the power supply. This may be done so that the temperature of the ring heater assembly matches that of the platen. In another embodiment, the output of the power supply is controlled by a controller using a table that equates desired temperature to output level. In this embodiment, the controller receives an input regarding the desired temperature of the ring heater assembly. The controller then indexes into the table to determine the appropriate output from the power supply. 
         [0043]    In some embodiments, the ring heater assembly is used in situations where the workpiece is to be heated above the ambient temperature. As explained above, in traditional platens, the workpiece overhangs the edge of the platen by about 2-3 mm. Additionally, the outer seal ring may be disposed several millimeters before the edge of the platen. As described above, the outer seal ring provides a barrier that contains the back side gas between the workpiece  30  and the platen  10 . This back side gas provides a heating mechanism between the platen and the workpiece, as the back side gas is heated by the platen and transfers that heat to the workpiece. In other words, the platen  10  provides conductive heating through the use of back side gas. Thus, the portion of the workpiece  30  that extends beyond the outer seal ring is not effectively heated by the platen  10  and the back side gas. In fact, in one example, the platen  10  was heated so as to maintain the workpiece  30  at 600° C. While most of the workpiece  30  was maintained at this temperature, the outer portion of the workpiece  30  was not. In fact, in one example, the outer edge of the workpiece  30  was 50° C. cooler than the rest of the workpiece  30 . 
         [0044]    The scenario above was repeated with the ring heater assembly mounted around the platen. In this example, the temperature of the outer edge of the workpiece  30  was within 5° C. of the rest of the workpiece  30 , demonstrating the effectiveness of this approach. 
         [0045]    The ring heater assembly heats the workpiece through radiative heating, as there is no contact between the ring heater assembly and the workpiece  30 . Further, there is no back side gas in the space between the ring heater assembly and the workpiece  30 . 
         [0046]    In certain embodiments, the diameter of the platen  10  relative to the workpiece may be reduced due to the presence of a ring heater assembly. As described above, in some embodiments, the overhang portion of the workpiece  30  may be 2-3 mm. However, in certain embodiments, the overhang portion may be increased to 10 mm, 15 mm, 20 mm, or even greater values. This approach may add structural rigidity to the ring heater assembly and have no effect on the overall temperature profile on the workpiece  30 . 
         [0047]      FIGS. 6A-C  shows various embodiments having different overhang portions. Note that in these embodiments, the edge of the platen is not tapered. Thus, in these embodiments, the inner circumference of the ring heater is straight and aligned to the platen. 
         [0048]    In  FIG. 6A , the overhang portion is about 2-3 mm. In other words, about 2-3 mm of the workpiece  30  are disposed above the ring heater  600 . In one embodiment, the workpiece  30  may have a diameter of 300 mm, while the platen  10  has a diameter of about 295 mm. In this embodiment, the ring heater  600  may have a width of between about 7 and 22 mm. 
         [0049]    In  FIG. 6B , the overhang portion is about 17 mm. In this embodiment, the workpiece  30  may have a diameter of 300 mm, while the platen  610  has a diameter of about 266 mm. In this embodiment, the ring heater  611  may have a width of between about 22 and 35 mm. 
         [0050]    In  FIG. 6C , the overhang portion is about 30 mm. In this embodiment, the workpiece  30  may have a diameter of 300 mm, while the platen  620  has a diameter of about 240 mm. In this embodiment, the ring heater  621  may have a width between about 35 and 50 mm. 
         [0051]    In each of these embodiments, the outer diameter of the ring heater is unchanged and is related to the outer diameter of the workpiece  30 . The inner diameter of the ring heater changes, based on the outer diameter of the platen. The outer diameter of the ring heater may extend beyond the outer diameter of the workpiece  30  by any amount. In some embodiment, increasing the outer diameter of the ring heater past that of the workpiece  30  reduces the heat lost at the outer edge of the workpiece  30 . In some embodiments, the outer diameter of the ring heater is selected to as to extend at least 5 mm past the outer edge of the workpiece  30 . In other embodiments, the outer diameter of the ring heater may extend more than 20 mm past the outer edge of the workpiece. Of course, other relationships between the diameter of the workpiece  30  and the outer diameter of the ring heater may be used. For example, the outer diameter of the ring heater may be selected so as to optimize the radiative heat lost at the edge of the workpiece  30  and the overall size of the ring heater. 
         [0052]    Thus, in one embodiment, the ring heater assembly is part of a workpiece holding and heating apparatus. The workpiece holding and heating apparatus includes a platen and a ring heater assembly, which surrounds the outer circumference of the platen. The platen heats the workpiece with conductive heating using back side gas. The platen also may hold the workpiece in place through the use of electrostatic forces. The ring heater assembly, which is an annular ring surrounding the platen, heats the workpiece using radiative heating. This combination is effective is creating a more uniform temperature profile across the entirety of the workpiece than can otherwise be created. 
         [0053]    While the disclosure describes the use of the ring heater with a heated platen, the disclosure is not limited to this embodiment. For example, in some embodiments, an ion implant may be performed at room temperature. However, even at room temperature, there may be some change in the temperature of the workpiece near its outer circumference, where the outer edge of the workpiece may be cooler than the rest of the workpiece. The use of the ring heater assembly may minimize this temperature roll off at the outer edge. 
         [0054]    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.