Abstract:
A substrate support and method of fabricating the same are provided. Generally, one method of fabrication includes assembling a subassembly comprising a first reinforcing member and a heating element, supporting the subassembly at least 40 mm from a bottom of a mold, encapsulating the supported subassembly with molten aluminum, and applying pressure to the molten aluminum. Alternatively, a method of fabrication includes assembling a subassembly comprising a stud disposed through a heating element sandwiched between a first reinforcing member and a second reinforcing member, supporting the subassembly above a bottom of a mold, encapsulating the subassembly disposed in the mold with molten aluminum to form a casting, forming a hole in the casting by removing at least a portion of the stud, and disposing a plug in at least a portion of the hole.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a divisional of co-pending U.S. patent application Ser. No. 09/921,104, filed Aug. 1, 2001, which is hereby incorporated by reference herein in its entirety. 
     
    
     
       BACKGROUND OF THE DISCLOSURE  
         [0002]    1. Field of the Invention  
           [0003]    Embodiments of the invention generally provide a substrate support utilized in semiconductor processing and a method of fabricating the same.  
           [0004]    2. Description of the Background Art  
           [0005]    Liquid crystal displays or flat panels are commonly used for active matrix displays such as computer and television monitors. Generally, flat panels comprise two glass plates having a layer of liquid crystal material sandwiched therebetween. At least one of the glass plates includes at least one conductive film disposed thereon that is coupled to a power supply. Power supplied to the conductive film from the power supply changes the orientation of the crystal material, creating a pattern such as text or graphics seen on the display. One fabrication process frequently used to produce flat panels is plasma enhanced chemical vapor deposition (PECVD).  
           [0006]    Plasma enhanced chemical vapor deposition is generally employed to deposit thin films on a substrate such as a flat panel or semiconductor wafer. Plasma enhanced chemical vapor deposition is generally accomplished by introducing a precursor gas into a vacuum chamber that contains a substrate. The precursor gas is typically directed through a distribution plate situated near the top of the chamber. The precursor gas in the chamber is energized (e.g., excited) into a plasma by applying RF power to the chamber from one or more RF sources coupled to the chamber. The excited gas reacts to form a layer of material on a surface of the substrate that is positioned on a temperature controlled substrate support. In applications where the substrate receives a layer of low temperature polysilicon, the substrate support may be heated in excess of 400 degrees Celsius. Volatile by-products produced during the reaction are pumped from the chamber through an exhaust system.  
           [0007]    Generally, the substrate support utilized to process flat panel displays are large, often exceeding 550 mm×650 mm. The substrate supports for high temperature use typically are casted, encapsulating one or more heating elements and thermocouples in an aluminum body. Due to the size of the substrate support, one or more reinforcing members are generally disposed within the substrate support to improve the substrate support&#39;s stiffness and performance at elevated operating temperatures (i.e., in excess of 350 degrees Celsius and approaching 500 degrees Celsius). Although substrate supports configured in this manner have demonstrated good processing performance, manufacturing supports has proven difficult.  
           [0008]    One problem in providing a robust substrate support is that the reinforcing member may occasionally displace, deform and sometimes break during the casting process. The reinforcing member typically includes portions that are unsupported in the pre-cast state of the substrate support. After assembling the reinforcing member, the heating elements and thermocouples into a subassembly, the subassembly is supported in a mold and encapsulated with molten aluminum. Conventional presses used in the casting process typically have one or twin rams that provide up to about 500 tons of pressure that works not whole area of cast surface but local area flowing the molten aluminum around the subassembly disposed in the substrate support mold. In this case, there is always nonuniformity of pressure working on the molten aluminum. Occasionally, this nonuniformity of the weight and pressure of the aluminum flowing in the mold during the casting process causes the reinforcing member to displacement, deformation and sometimes fracture. Additionally, this casting process results in undesirable heterogeneous grain size of aluminum cast. Furthermore, such pressures stress the substrate support up to about 28 MPa, which is not enough to get a desired uniform micro-grain size within the aluminum cast.  
           [0009]    Another problem with substrate support formed using this molding process is the lack of integrity of the aluminum where the flow of molten aluminum comes back together on the side of the substrate support furthest from the molten aluminum source. As a substantial amount of aluminum and time is needed to encapsulate the heating elements, thermocouples and reinforcing members, the flow of aluminum may cool causing a seam to be created where the leading edges of the aluminum flow merges under the subassembly at less than acceptable temperatures.  
           [0010]    Depending on the temperature of the aluminum when the seam is formed, the seam may become a source of a variety of defects. For example, vacuum leaks may propagate through the seam between the interior of the chamber and the environment surrounding the chamber. Vacuum leakage may degrade process performance and may lead to poor heater performance that contributes to pre-mature heater failure. Moreover, thermal cycling of the substrate support may cause the substrate support to fracture along the seam, thereby causing failure and possible release of particulates into the chamber environment.  
           [0011]    As the cost of materials and manufacturing the substrate support is great, failure of the substrate support is highly undesirable. Additionally, if the substrate support fails during processing, a substrate supported thereon may be damaged. This can occur after a substantial number of processing steps have been preformed thereon, thus resulting in the expensive loss of the substrate support. Moreover, replacing a damaged support in the process chamber creates a costly loss of substrate throughput while the process chamber is idled during replacement or repair of the substrate support. Moreover, as the size of the next generation substrate supports are increased to accommodate substrates in excess of 1.44 square meters at operating temperatures approaching 500 degrees Celsius, the aforementioned problems become increasingly important to resolve.  
           [0012]    Therefore, there is a need for an improved substrate support.  
         SUMMARY OF THE INVENTION  
         [0013]    Generally, a substrate support and method of fabricating the same are provided. In one embodiment, a method of fabricating a substrate support includes the steps of assembling a subassembly comprising a first reinforcing member and a heating element, supporting the subassembly at least 40 mm from a bottom of a mold, encapsulating the supported subassembly with molten aluminum, and applying pressure to the molten aluminum.  
           [0014]    In another embodiment, a method of fabricating a substrate support includes the steps of a method of fabrication includes assembling a subassembly comprising a stud disposed through a heating element sandwiched between a first reinforcing member and a second reinforcing member, supporting the subassembly above a bottom of a mold, encapsulating the subassembly disposed in the mold with molten aluminum to form a casting, forming a hole in the casting by removing at least a portion of the stud, and disposing a plug in at least a portion of the hole.  
           [0015]    In another aspect of the invention, a substrate support is provided. In one embodiment, the substrate support includes at least a first reinforcing member and a heating element disposed within a cast aluminum body. At least one hole is formed in the aluminum body between an outer surface and at least the heating element or the reinforcing member. A plug is disposed in the hole between the outer surface and the heating element or the reinforcing member. In another embodiment, the hole houses a stud during casting that maintains the heating element and the reinforcing member in a spaced-apart relation and is at least partially removed from the hole before insertion of the plug. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which:  
         [0017]    [0017]FIG. 1 depicts a schematic sectional view of one embodiment of a processing chamber having a substrate support of the present invention;  
         [0018]    [0018]FIG. 2 is one embodiment of a method of fabricating a substrate support;  
         [0019]    [0019]FIG. 3A is a sectional view of one embodiment of a subassembly;  
         [0020]    [0020]FIG. 3B is a plan view of the subassembly of FIG. 3A;  
         [0021]    [0021]FIG. 4 is a schematic of the subassembly of FIG. 3A disposed in a press; and  
         [0022]    [0022]FIG. 5 is a sectional view of an embodiment of a substrate support. 
     
    
       [0023]    To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures.  
       DETAILED DESCRIPTION  
       [0024]    The invention generally provides a substrate support and methods of fabricating a substrate support. The invention is illustratively described below in reference to a plasma enhanced chemical vapor deposition system, such as a plasma enhanced chemical vapor deposition (PECVD) system, available from AKT, a division of Applied Materials, Inc., Santa Clara, Calif. However, it should be understood that the invention has utility in other system configurations such as physical vapor deposition systems, ion implant systems, etch systems, other chemical vapor deposition systems and any other system in which processing a substrate on a substrate support is desired.  
         [0025]    [0025]FIG. 1 is a cross sectional view of one embodiment of a plasma enhanced chemical vapor deposition system  100 . The system  100  generally includes a chamber  102  coupled to a gas source  104 . The chamber  102  has walls  106 , a bottom  108  and a lid assembly  110  that define a process volume  112 . The process volume  112  is typically accessed through a port (not shown) in the walls  106  that facilitates movement of the substrate  140  into and out of the chamber  102 . The walls  106  and bottom  108  are typically fabricated from a unitary block of aluminum or other material compatible for processing. The lid assembly  110  contains a pumping plenum  114  that couples the process volume  112  to an exhaust port (that includes various pumping components, not shown).  
         [0026]    The lid assembly  110  is supported by the walls  106  and can be removed to service the chamber  102 . The lid assembly  110  is generally comprised of aluminum. A distribution plate  118  is coupled to an interior side  120  of the lid assembly  110 . The distribution plate  118  is typically fabricated from aluminum. The center section includes a perforated area through which process and other gases supplied from the gas source  104  are delivered to the process volume  112 . The perforated area of the distribution plate  118  is configured to provide uniform distribution of gases passing through the distribution plate  118  into the chamber  102 .  
         [0027]    A heated substrate support assembly  138  is centrally disposed within the chamber  102 . The support assembly  138  supports a substrate  140  during processing. In one embodiment, the substrate support assembly  138  comprises an aluminum body  124  that encapsulates at least one embedded heating element  132  and a thermocouple  190 . At least a first reinforcing member  116  is generally embedded in the body  124  proximate the heating element  132 . A second reinforcing member  166  may be disposed within the body  124  on the side of the heating element  132  opposite the first reinforcing member  116 . The reinforcing members  116  and  166  may be comprised of metal, ceramic or other stiffening materials. In one embodiment, the reinforcing members  116  and  166  are comprised of aluminum oxide fibers. Alternatively, the reinforcing members  116  and  166  may be comprised of aluminum oxide fiber combined with aluminum oxide particles, silicon carbide fiber, silicon oxide fiber or similar materials. The reinforcing members  116  and  166  may include loose material or may be a pre-fabricated shape such as a plate. Alternatively, the reinforcing members  116  and  166  may comprise other shapes and geometry. Generally, the reinforcing members  116  and  166  have some porosity that allows aluminum to impregnate the members  116 ,  166  during a casting process described below.  
         [0028]    The heating element  132 , such as an electrode disposed in the support assembly  138 , is coupled to a power source  130  and controllably heats the support assembly  138  and substrate  140  positioned thereon to a predetermined temperature. Typically, the heating element  132  maintains the substrate  140  at a uniform temperature of about  150  to at least about 460 degrees Celsius.  
         [0029]    Generally, the support assembly  138  has a lower side  126  and an upper side  134  that supports the substrate. The lower side  126  has a stem cover  144  coupled thereto. The stem cover  144  generally is an aluminum ring coupled to the support assembly  138  that provides a mounting surface for the attachment of a stem  142  thereto.  
         [0030]    Generally, the stem  142  extends from the stem cover  144  and couples the support assembly  138  to a lift system (not shown) that moves the support assembly  138  between an elevated position (as shown) and a lowered position. A bellows  146  provides a vacuum seal between the chamber volume  112  and the atmosphere outside the chamber  102  while facilitating the movement of the support assembly  138 . The stem  142  additionally provides a conduit for electrical and thermocouple leads between the support assembly  138  and other components of the system  100 .  
         [0031]    The support assembly  138  generally is grounded such that RF power supplied by a power source  122  to the distribution plate  118  (or other electrode positioned within or near the lid assembly of the chamber) may excite the gases disposed in the process volume  112  between the support assembly  138  and the distribution plate  118 . The RF power from the power source  122  is generally selected commensurate with the size of the substrate to drive the chemical vapor deposition process.  
         [0032]    The support assembly  138  additionally supports a circumscribing shadow frame  148 . Generally, the shadow frame  148  prevents deposition at the edge of the substrate  140  and support assembly  138  so that the substrate does not stick to the support assembly  138 .  
         [0033]    The support assembly  138  has a plurality of holes  128  disposed therethrough that accept a plurality of lift pins  150 . The lift pins  150  are typically comprised of ceramic or anodized aluminum. Generally, the lift pins  150  have first ends  160  that are substantially flush with or slightly recessed from a upper side  134  of the support assembly  138  when the lift pins  150  are in a normal position (i.e., retracted relative to the support assembly  138 ). The first ends  160  are generally flared to prevent the lift pins  150  from falling through the holes  128 . Additionally, the lift pins  150  have a second end  164  that extends beyond the lower side  126  of the support assembly  138 . The lift pins  150  may be actuated relative to the support assembly  138  by a lift plate  154  to project from the support surface  130 , thereby placing the substrate in a spaced-apart relation to the support assembly  138 .  
         [0034]    The lift plate  154  is disposed proximate the lower side  126  of the support surface. The lift plate  154  is connected to the actuator by a collar  156  that circumscribes a portion of the stem  142 . The bellows  146  includes an upper portion  168  and a lower portion  170  that allow the stem  142  and collar  156  to move independently while maintaining the isolation of the process volume  112  from the environment exterior to the chamber  102 . Generally, the lift plate  154  is actuated to cause the lift pins  150  to extend from the upper side  134  as the support assembly  138  and the lift plate  154  move closer together relative to one another.  
         [0035]    [0035]FIG. 2 depicts a flow chart of one embodiment of a method  200  for fabricating the support assembly  138 . Generally, the method  200  begins at step  202  of assembling a subassembly that includes the reinforcing members  116 ,  166 , the heating element  132  and the thermocouple  190 . At step  204  and step  206 , the subassembly  300  is supported in a mold that is disposed in a press and respectively encapsulated with aluminum to form a casting. At step  208 , the casting is processed to form an unfinished substrate support. At step  210 , the unfinished substrate support is finished by anodizing the substrate support assembly  138  and coupling the heating elements  132  to the appropriate electrical connections, for example, soldering lead wires to the heating elements  132 .  
         [0036]    [0036]FIG. 3A depicts one embodiment of a subassembly  300  assembled at step  202 . The subassembly  300  generally includes the first reinforcing member  116 , the second reinforcing member  166 , the heating element  132  and the thermocouple  190 . A plurality of studs  302 , for example, fasteners, pins, rods, bolts and the like comprised of a ceramic or metallic material such as stainless steel, are utilized to support and maintain a predetermined spacing between the reinforcing members  116 ,  166 , the heating element  132  and the thermocouple  190 . The studs  302  vary in number and be arranged in different patterns, for example, a grid comprising  12  equally spaced studs  302  (see FIG. 3B). The studs  302  generally are passed through the first reinforcing member  116  and configured to support the first reinforcing member  116  at least 40 mm from an end  304  of the stud  302 . In one embodiment, the position of the first reinforcing member  116  relative to the end  304  of the studs  302  is maintained by providing a first ledge  306  in the stud  302  on which the first reinforcing member  116  rests. Optionally, the stud  302  may incorporate other features or devices such as standoffs, threads, tapers and the like to maintain the relative positions of the studs  302  and the first reinforcing member  116 .  
         [0037]    The heating elements  132  and the thermocouples  190  are disposed on the studs  302  proximate the first reinforcing member  116  from the side of the stud  302  opposite the end  304 . The heating elements  132  and the thermocouple  190  are generally disposed against the first reinforcing member  116  but may be maintained in a spaced-apart relation to the first reinforcing member  116 . In one embodiment, a spaced-apart relation is maintained by resting the heating elements  132  and the thermocouple  190  on a second ledge  308  of the stud  302 . Alternatively, threads, standoffs, spacers or geometry such as bosses incorporated into one or both of the heating elements  132 , the thermocouple  190  and first reinforcing member  116  may be used to maintain the relative spacing therebetween.  
         [0038]    The second reinforcing member  166  is disposed on the stud  302  proximate the heating element  132 . Generally, the second reinforcing member  166  is disposed against the heating element  132  but may optionally be maintained in a spaced-apart relation by providing a third ledge  310  on which the second reinforcing member  166  rests. The spacing between the heating elements  132  and the second reinforcing member  166  may alternatively be maintained as described above.  
         [0039]    The subassembly  300  may optionally be secured to prevent movement between the first reinforcing member  116 , the second reinforcing member  166 , the heating element  132  and the thermocouple  190  during casting. In one embodiment, the first reinforcing member  116  is retained against the first ledge  306  by a metallic collar  312  pressed on at least some of the studs  302 . The second reinforcing member  166  is retained against the third ledge by another collar  312  while the heating element  132  and the thermocouple  190  are respectively retained against the second ledge  308  by another collar  312 . The collars  312  are preferably fabricated from stainless steel. Alternatively, the subassembly  300  may be secured on the studs  302  by other devices such as nuts (with threaded studs), adhesives, friction on the studs (i.e., press or snap fit), wire, ceramic string, twine and the like. Optionally, the first reinforcing member  116 , the second reinforcing member  166 , the heating element  132  and the thermocouple  190  may include interlocking geometry integral to the subassembly such as mating pins and bosses, standoffs, press and snap fits and the like.  
         [0040]    Optionally, the studs  302  may be coupled at their end  304  to a base plate  314 . The base plate  314  is typically comprised of a metallic material and is utilized to position the subassembly  300  in a predetermined position in the mold  400 . In one embodiment, the base plate  314  is a perforated steel plate having a plurality of threaded holes to accept the studs  302 . The thickness of base plate  314  is at least 40 mm to prevent a deformation during the casting.  
         [0041]    [0041]FIG. 4 depicts a schematic of one embodiment of the subassembly  300  disposed in the mold  400  which is disposed in the press  404 . Generally, the subassembly  300  is positioned within the mold  400  such that the subassembly is supported from a bottom  402  of the mold  400  by at least  40  mm at step  204 . The back plate  314  that is coupled to the subassembly  300  typically rests in a predetermined bottom  402  of the mold  400 . The back plate  314  may be located relative the mold  400  in the predetermined position by dowel pins, geometric interfacing and the like. By maintaining the subassembly  300  in this position, adequate encapsulation around all sides of the subassembly  300  is ensured.  
         [0042]    Alternatively, the subassembly  300  may be supported in the mold  400  in other ways. For example, mold pins (not shown) may project from the bottom  402  of the mold  400  and support the subassembly  300 . In another configuration, one or more members (not shown) may extend between other portions of the mold  400  to support the subassembly  300 . The studs  302  may be directly disposed on or in locating holes in mold bottom  402  while maintaining at least  40  mm between the first reinforcing plate  116  and the mold bottom  402  on subassemblies  300  that do not include the back plate  314 .  
         [0043]    The mold  400  is generally heated to minimize the cooling of the molten aluminum used to encapsulate the subassembly. The mold  400  may be heated through any conventional means including circulated fluids, resistance heaters and burners. Generally, the mold  400  is heated to a temperature between about 300 and about 350 degrees Celsius.  
         [0044]    The molten aluminum at about 800 to about 900 degrees Celsius is generally dispensed into the mold in a single shot at step  206 . The single shot minimizes seam formation at the interface between shots due to cooling of the aluminum that occurs during utilizing conventional processes. The aluminum may be dispensed manually or automatically through an opening in the top of the mold or one or more other passages (not shown). Generally, aluminum alloy  6061  is utilized but other alloys may be substituted. The molten aluminum at about 800 to about 900 degrees Celsius is generally dispensed into the mold in a single shot at step  206 . The single shot minimizes seam formation at the interface between shots due to cooling of the aluminum that occurs during utilizing conventional processes. The aluminum may be dispensed manually or automatically through an opening in the top of the mold or one or more other passages (not shown). Generally, aluminum alloy 6061 is utilized but other alloys may be substituted.  
         [0045]    Once the molten aluminum is in the mold, pressure is applied to the aluminum to assist the aluminum in flowing around and in between the components of the subassembly  300 . The applied pressure additionally impregnates the reinforcing members  116  and  166  with aluminum. In one embodiment, a single ram  406  of the press  404  applies pressure to an area  408  of the molten aluminum above the subassembly  300 . Generally, the area  408  is at least as large as the area of the subassembly  300  and may include the entire width of the mold  400 . The pressure applied by the ram  406  is generally less than about 3,000 tons. The space between the support assembly  138  and the bottom  402  of the mold  400  or the base plate  314  enhances the flow the aluminum therebetween. Optionally, the mold  400  may include a vacuum applied to the mold&#39;s vents (not shown) to assist the flow of aluminum. The use of a single ram  406  over a large area  408  results in uniformity application of stress, preferably in excess of about 40 MPa, to the entire area of the support assembly  138 , which eliminates the displacement, deformation and fracture of the reinforcing members  116 ,  166 . The high stress correspondingly increases the homogeneity of grain size of aluminum cast and decreases the integrity of any seams or flow lines that may form during casting. The molten aluminum at about 800 to about 900 degrees Celsius is generally dispensed into the mold in a single shot at step  206 . The single shot minimizes seam formation at the interface between shots due to cooling of the aluminum that occurs during utilizing conventional processes. The aluminum may be dispensed manually or automatically through an opening in the top of the mold or one or more other passages (not shown). Generally, aluminum alloy  6061  is utilized but other alloys may be substituted.  
         [0046]    [0046]FIG. 5 depicts one embodiment of the substrate support assembly  138  in the form of a post-molding casting  500 . Generally, the casting  500  is processed at steps  206  to form an unfinished processing support. In one embodiment, the processing step  208  generally includes annealing the casting  500  to relieve residual stresses in the casting  500 . In one embodiment, the casting  500  is annealed at about 510 to about 520 degrees Celsius for about 2 to about 3 hours.  
         [0047]    Next, the casting is machined to roughly the dimensions of the finished substrate support assembly  138 . The studs  302  are at least partially removed from the bottom side and replaced with an aluminum plug  502  that is welded to the substrate support assembly  138 . The stem cover  144  is then welded to the substrate support assembly  138 . The support assembly  138  is annealed once more before a final machining step that brings the substrate support  138  to its final dimensions. Electrical leads are then attached to the heating element  132  and fed through the stem  142  which is then welded to the stem cover  144 .  
         [0048]    The surface of the support assembly  138  is then treated to remove tool marks left by the machining operations. The step of removing the tool marks may optionally be completely or partially performed before the second anneal step. The surface treatments may include grinding, electropolishing, abrasive or bead blasting, chemical etching and the like. In one embodiment, the substrate support is treated by blasting the substrate support with aluminum oxide balls and exposing the support to an alkaline or acid etchant. At step  210 , the substrate support  138  is anodized to provide a protective finish to the substrate support.  
         [0049]    Although several preferred embodiments which incorporate the teachings of the present invention have been shown and described in detail, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings.