Patent Publication Number: US-2006005771-A1

Title: Apparatus and method of shaping profiles of large-area PECVD electrodes

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
      This application claims priority to U.S. Provisional Patent Application No. 60/587,173, filed Jul. 12, 2004. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      Embodiments of the present invention generally relate to substrate processing methods, such as methods for processing flat-panel displays. Embodiments of the present invention also generally relate to a processing apparatus for processing flat panel displays. In addition, the invention relates to a plasma-enhanced CVD processing chamber.  
      2. Description of the Related Art  
      Flat panel displays are commonly used for computer screens, television monitors, cell phone displays, personal digital assistants, and other electronic equipment. Flat panel displays employ an active matrix of electronic devices, such as thin film transistors, or TFT&#39;s. The electronic devices are conventionally made on large flat substrates referred to as flat panel substrates. Generally, flat panel substrates are made of two thin plates of glass or, in some instances, a polymeric material. A layer of liquid crystal material is sandwiched between the thin plates. At least one of the plates includes a conductive film that is adapted to couple to a power source. Power supplied to the conductive film from the power source selectively changes the orientation of the crystal material, thereby creating a pattern display.  
      In order to manufacture these displays, a substrate is subjected to a plurality of sequential processes to create electronic devices on the substrate. Such devices may be conductors, insulators or thin film transistors (TFT&#39;s). Each of the processes is generally conducted in a process chamber adapted to perform a single step of the production process. In order to efficiently complete the entire sequence of processing steps, a number of process chambers are typically coupled to a central transfer chamber that houses a robot to facilitate transfer of the substrate between the process chambers. A processing platform having this configuration is generally known as a cluster tool, examples of which are the families of AKT plasma enhanced chemical vapor deposing (PECVD) processing platforms available from AKT America, Inc., which is a wholly owned division of Applied Materials, Inc., located in Santa Clara, Calif.  
      Various deposition techniques are known for placing a film onto large-area flat substrates. Chemical vapor deposition is commonly used to deposit thin films. In some instances, plasma-enhanced chemical vapor deposition, or “plasma enhanced CVD,” is employed. Plasma-enhanced CVD techniques promote excitation and/or dissociation of reactant gases by the application of radio-frequency (RF) energy. RF energy is directed to a reaction zone near the substrate surface, thereby creating a plasma. The high reactivity of the species in the plasma reduces the energy required for a chemical reaction to take place, and thus lowers the temperature required for CVD processes as compared to conventional thermal CVD processes.  
      For plasma-enhanced CVD, a parallel plate plasma reactor may be used. Parallel plate plasma reactors utilize opposing electrodes to form the plasma in a reaction zone between the two electrodes. In this respect, an RF bias voltage power level is applied to the electrodes in the processing chamber. One of the electrodes may be a substrate support and the other may be a gas diffusion plate. Parallel plate plasma reactors are available from AKT which manufactures various processing platforms for processing large-area flat substrates. Such substrates may be used to make thin film transistor liquid crystal diode (TFT-LCD) displays for flat panel televisions and for other TFT devices.  
      With the marketplace&#39;s acceptance of flat panel technology, the demand for larger displays, increased production and lower manufacturing costs has driven equipment manufacturers to develop new systems that accommodate larger size flat substrates. These larger flat substrates may also be used to form a greater number of smaller area displays on one substrate that may lower production costs per display. Previous generation large-area substrates were processed in sizes of about 550 mm×650 mm. However, current large-area substrates may be as large as 1800 mm×2200 mm or larger.  
      As flat substrate size increases, the electrodes that are used to produce the plasma are scaled to greater dimensions. Larger sizes can produce non-uniformities in the deposition properties of the plasma which may degrade display quality. For example, when a large-area flat substrate is placed on a heated substrate support, which may also function as one electrode in the chamber, the flat substrate may tend to deform during heating and deposition. Deformation of the flat substrate may generally include failure of the substrate to maintain a planar, flat profile on the substrate support, such as bowing. This deformation of the substrate may cause small amounts of gas to become trapped between the substrate and the substrate support, which can adversely affect the uniformity of the plasma and the deposition on the substrate. In addition, deformation of the substrate may cause a lack of uniform contact between the substrate support and the supported substrate. In these instances, good physical contact between the substrate and the substrate support may be lost, or may never even be obtained. Lack of good physical contact with the substrate support may affect the uniformity of the deposition process.  
      In some instances, fatiguing of the substrate support and supported substrate may cause the substrate to lose a desired orientation to the orientation of a gas diffusion plate, upper electrode, or a gas diffusion plate that also functions as a lower electrode. The upper electrode or the gas diffusion plate that forms the upper border of the reaction zone may be substantially planar or nonplanar. In such instances, the operator may desire to be able to control the profile of the substrate relative the upper electrode or gas diffusion plate.  
      A substrate support that resists deflection during high temperature processing was disclosed in commonly assigned U.S. Pat. No. 6,554,907. However, it is further desirable to provide a plasma-enhanced CVD chamber wherein the substrate support is selectively shaped before processing begins. Pre-shaping of the substrate support, in turn, allows the supported glass to be shaped into or out of planar orientation, as desired.  
     SUMMARY OF THE INVENTION  
      The present invention generally relates to a semiconductor processing apparatus. More specifically, the invention relates to a plasma-enhanced CVD chamber for processing large-area flat substrates made of glass, polymers, or other suitable substrate material capable of having electronic devices formed thereon.  
      A plasma-enhanced chemical vapor deposition (PECVD) chamber for processing a large-area flat substrate is first provided. The chamber includes an upper electrode and a lower electrode. The lower electrode supports the flat substrate. The lower electrode comprises a substrate support and a base structure. In one embodiment, the substrate support is fabricated from a material that has insufficient strength to rigidly support itself in a desired orientation under typical operating conditions such as low pressure and high temperature. The base structure is fabricated from a material that has sufficient strength to rigidly support itself and the substrate support during operating conditions. The base structure is pre-shaped to reinforce the substrate support in a desired orientation within the chamber. The substrate support may be fabricated from a thermally conductive metal such as aluminum and may have at least one heating element disposed therein.  
      In one embodiment, the substrate support is reinforced by a lattice-type base structure which may include at least one base plate oriented in a first direction, and at least two lateral support plates disposed on the at least one base plate. The lateral support plates are preferably oriented generally transverse to the at least one base plate. The base structure is preferably fabricated from a material that has sufficient strength to rigidly support itself under typical operating temperature and pressure conditions, for example, a ceramic material.  
      In one embodiment, the base structure is preshaped in a nonplanar shape. The foundational base structure may be preshaped to reinforce the substrate support in a parallel orientation relative to a nonplanar upper electrode. Alternatively, the base structure may be preshaped to reinforce the substrate support in a parallel orientation relative to a nonplanar gas diffusion plate, or showerhead, within the chamber.  
      In yet another embodiment, a desired shaping of the electrode is created by varying the thickness of the substrate support itself. For example, the upper electrode may be concave in shape. It is therefore desirable to shape the substrate by using a supporting substrate support that is convex so as to provide a more parallel orientation between the upper electrode and the flat substrate. Similarly, a gas diffusion plate may be provided in the chamber that is concave in shape. It is therefore desirable to shape the substrate by using a substrate support that is convex so as to provide a parallel orientation between the showerhead and the flat substrate.  
      In one arrangement, the upper electrode and the showerhead may both be planar. However, the substrate support and supported flat substrate may tend to bow into the base structure, forming a convex shape. To provide a planar shape to the substrate support and supported flat substrate under typical operating conditions, the thickness of the substrate support may be appropriately varied. Alternatively, the thickness of the base structure may be appropriately varied. In one embodiment, shims may be selectively placed on a top surface of the lattice-type base structure to compensate for “bowing” of the substrate support and the flat substrate thereon.  
      In another embodiment, a plasma-enhanced chemical vapor deposition (PECVD) chamber for processing a large-area flat substrate is provided. The chamber has an upper electrode, a substrate support assembly disposed below the upper electrode and supporting the flat substrate, a lower electrode within the substrate support assembly, a processing region formed between the upper and lower electrodes, a gas inlet, and a diffusion plate for delivering gases into the processing region. The lower electrode is pre-shaped in accordance with the various descriptions provided above to selectively conform the supported flat substrate in a nonplanar manner under operating temperature conditions.  
      A substrate support assembly for supporting a large-area glass substrate in a plasma-enhanced chemical vapor deposition (PECVD) chamber is also provided. In one arrangement, the substrate support assembly first includes a substrate support fabricated from a thermally conductive metal and serving as a lower electrode. The substrate support is fabricated from a material having insufficient strength to support itself under operating conditions. In one embodiment, the substrate support has an appropriately varied thickness to offset anticipated thermally induced planarity changes during substrate processing. In addition, the substrate support assembly includes a base structure for supporting the substrate support. Preferably, the base structure is a lattice-type structure that includes at least one ceramic base plate oriented in a first direction, and at least two ceramic support plates disposed on the at least one base plate and oriented generally transverse to the at least one base plate. Each of the ceramic support plates may have at least one shim disposed on a top surface to offset the nonplanar response of the substrate support under operating conditions. Preferably, the base structure has sufficient strength to rigidly support itself under operating conditions.  
      A method for shaping an electrode in a plasma-enhanced chemical vapor deposition (PECVD) chamber is also provided. The method includes the step of providing an upper electrode in the chamber, and also providing a substrate support fabricated from a thermally conductive metal. The substrate support is configured to receive a large-area flat substrate and to serve as a lower electrode in the chamber. The substrate support is fabricated from a thermally conductive metal of insufficient strength to rigidly support itself under operating conditions. The method also includes the step of providing a base structure for reinforcing the substrate support. In one aspect, shims are provided on top of the base structure to overcome anticipated nonplanar response of the substrate support under operating conditions.  
      In one embodiment of the method, the substrate support has a variable thickness to provide a nonplanar shape to the substrate support before being exposed to operating temperature conditions. For example, the substrate support may be concave before being placed on the base structure and substantially planar after being placed on the base structure under operating temperature conditions. In one aspect, the substrate support bows into a substantially planar shape when supported by the base structure under operating temperature conditions. In another aspect, the substrate support bows into a convex shape when supported by the base structure under operating temperature conditions. This step is beneficial where, for example, the upper electrode and/or the upper gas diffusion plate in the chamber is concave. Thus, a parallel orientation is provided between (1) the substrate support and supported large-area substrate, and (2) the upper electrode and/or the upper gas diffusion plate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only selected embodiments of this invention and are not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.  
       FIG. 1  is a side, cross-sectional view of a substrate processing chamber.  
       FIG. 2  is an exploded perspective view of a substrate support assembly, in one embodiment.  
       FIG. 3  is a plan view of the substrate support of  FIG. 2 .  
       FIG. 4  is a front view of the substrate support assembly of  FIG. 2 .  
       FIG. 5  is a side view of the substrate support assembly of  FIG. 2 .  
       FIG. 6  is a bottom view of the substrate support assembly of  FIG. 2 .  
       FIG. 7  is an alternate embodiment of a substrate support for the substrate support assembly.  
       FIG. 8  is an alternate embodiment of a lattice-type support structure for the substrate support assembly.  
       FIG. 9  is a side, cross-sectional view of a substrate processing chamber in an alternate embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       FIG. 1  is a side, cross-sectional view of one embodiment of a substrate processing chamber  100 . The processing chamber  100  is configured to receive a large-area flat substrate  140 , and to provide plasma-enhanced chemical vapor deposition on the substrate  140 . For purposes of this disclosure, the term “large-area substrate” refers to a substrate having a cross-sectional area of about 1.0 meters 2  or larger. In addition, for purposes of this disclosure, the term “plasma-enhanced chemical vapor deposition” (PECVD) refers to any chamber used in the processing of a large area substrate including a plasma etching chamber, a chemical vapor deposition (“CVD”) chamber, a rinse chamber, or other known chamber. In addition, the chamber  100  may be a stand-alone chamber, an in-line chamber, a cluster tool chamber, or some combination or variation thereof.  
      The chamber  100  includes a grounded chamber body  102  coupled to a gas source  104  and a power source  122 . The chamber body  102  has sidewalls  106 , a bottom  108 , and a lid assembly  110  that define a processing region  112 . The processing region  112  is typically accessed through a gate or port (not shown) in the sidewall  106  that facilitates movement of a large area substrate  140  into and out of the chamber body  102 . The sidewalls  106  and bottom  108  of the chamber body  102  are typically fabricated from aluminum or other material compatible with process chemistries. The lid assembly  110  contains a pumping plenum  114  that couples the processing region  112  to an exhaust port that is coupled to various pumping components (not shown).  
      The lid assembly  110  is supported by the sidewalls  106  and can be removed to service the chamber body  102 . The lid assembly  110  is generally comprised of aluminum. A gas distribution plate  118  is coupled to an interior side  120  of the lid assembly  110 , or to the sidewalls  106 . The distribution plate  118  is typically fabricated from aluminum and includes a center portion having a perforated area through which process and other gases supplied from the gas source  104  are delivered to the processing region  112 . The perforated area of the gas distribution plate  118  is configured to provide a uniform distribution of gases passing through the distribution plate  118  into the chamber body  102 . The power source  122  is coupled to the distribution plate  118  to provide an electrical bias that energizes the process gas to ignite and sustain a plasma formed from process gas in the processing region  112  below the gas distribution plate  118  during processing.  
      A substrate support assembly  210  is centrally disposed within the chamber body  102 . The substrate support assembly  210  supports the substrate  140  during processing and may include a support body  124  supported by a shaft  142  that extends through the chamber bottom  108 . The support body  124  is generally polygonal in shape and covered with an electrically insulative coating (not shown) over at least the portion of the support body  124  that supports the substrate  140 . The coating may also cover other portions of the support body  124 . Optionally, the substrate support assembly  210  may be coupled to ground at least during processing by one or more RF ground return paths  184  that provide a low-impedance RF return path between the substrate support assembly  210  and ground. In one embodiment, the RF ground return path  184  is a plurality of flexible straps (one of which is shown in  FIG. 1 ) coupled between a perimeter of the support body  124  and the chamber bottom  108 .  
      The support body  124  may be fabricated from metals or other comparably electrically conductive materials. The insulative coating may be a dielectric material such as an oxide, silicon nitride, silicon dioxide, aluminum dioxide, tantalum pentoxide, silicon carbide or polyimide, among others, which may be applied by various deposition or coating processes, including, but not limited to, flame spraying, plasma spraying, high energy coating, chemical vapor deposition, spraying, adhesive film, sputtering and encapsulating.  
      In one embodiment, the substrate support assembly  210  includes a support body  124  made of aluminum and has at least one embedded heating element  132  and a thermocouple (not shown). The heating element  132  may be an electrode or resistive element and is coupled to a power source  130  to controllably heat the substrate support assembly  210  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° Celsius to at least about 460° Celsius during processing. The support body  124  may include one or more stiffening members (not shown) comprised of metal, ceramic or other stiffening materials embedded therein.  
      Generally, the substrate support assembly  210  has a lower side  126  and an upper surface  134  that supports the substrate  140  thereon. The lower side  126  has a stem cover  144  coupled thereto. The stem cover  144  generally is an aluminum ring coupled to the substrate support assembly  210  that provides a mounting surface for the attachment of the shaft  142  thereto.  
      Generally, the shaft  142  extends from the stem cover  144  through the chamber bottom  108  and couples the substrate support assembly  210  to a lift system  136  that moves the substrate support assembly  210  between an elevated process position (as shown) and a lowered position that facilitates substrate transfer. A bellows  146  provides a vacuum seal between the chamber body  102  and the lift system  136  while facilitating the vertical movement of the substrate support assembly  210 . The shaft  142  additionally provides a conduit for electrical and thermocouple leads between the substrate support assembly  210  and other components of the chamber  100 .  
      The shaft  142  may be electrically isolated from the chamber body  102  by a dielectric isolator  128  disposed between the shaft  142  and chamber body  102 . The isolator  128  may additionally support or be configured to function as a bearing for the shaft  142 .  
      The substrate support assembly  210  optionally supports a shadow frame  148  configured to avoid deposition on the portion of the substrate support assembly  210  not covered by the substrate  140 . Alternatively or additionally, the shadow frame  148  may be configured to avoid deposition on the edge of the substrate  140  and the substrate support assembly  210 . Both configurations are contemplated to reduce sticking of the substrate  140  to the substrate support assembly  210 .  
      The substrate support assembly  210  has a plurality of holes disposed therethrough that accept corresponding lift pins  150 . The lift pins  150  are typically fabricated from ceramic or anodized aluminum, and have first ends that are substantially flush with or slightly recessed from the upper surface  134  of the substrate support assembly  210  when the lift pins  150  are retracted relative to the substrate support assembly  210 . As the substrate support assembly  210  is lowered to a transfer position, the lift pins  150  come into contact with the bottom  108  of the chamber body  102  and are displaced through the substrate support assembly  210  to project from the upper surface  134  of the substrate support assembly  210 , thereby placing the substrate  140  in a spaced-apart relation to the substrate support assembly  210 .  
      In one embodiment, lift pins  150  of a uniform length may be utilized in cooperation with bumps or plateaus  182  positioned beneath the outer lift pins  150 , so that the outer lift pins  150  are actuated before and displace the substrate  140  a greater distance from the upper surface  134  than the inner lift pins  150 . In another embodiment, the lift pins  150  may be of varying lengths and are utilized so that they come into contact with the bottom  108  and are actuated through the substrate support assembly  210  at different times. For example, the lift pins  150  that are spaced around the outer edges of the substrate  140 , combined with relatively shorter lift pins  150  spaced inwardly from the outer edges toward the center of the substrate  140 , allow the substrate  140  to be first lifted from its outer edges relative to its center. Alternatively, the chamber bottom  108  may comprise grooves or trenches positioned beneath the inner lift pins  150 , so that the inner lift pins  150  are actuated after and displaced a shorter distance than the outer lift pins  150 . Embodiments of a system having lift pins configured to lift a substrate in an edge to center manner from a substrate support assembly that may be used with the invention are described in U.S. Pat. No. 6,676,761, filed Dec. 2, 2002, entitled “Method and Apparatus for Dechucking a Substrate,” and described in U.S. patent application Ser. No. 10/460,916, filed Jun. 12, 2003, entitled “RF Current Return Path for a Large Area Substrate Plasma Reactor,” both of which are hereby incorporated by reference insofar as they teach the coordinated use of lift pins.  
      The substrate  140  may be moved in and out of the chamber  100  by means of a large handler blade (not shown) which may transfer the substrate  140  between a separate transfer chamber (not shown) and various processing chambers. The substrate  140  enters and exits the chamber  100  through a port (not shown) that also isolates the chamber  100  environment during substrate processing. It is to be understood that the substrate fabrication process involves multiple steps, and that different steps are typically conducted in different chambers that mechanically cooperate with the substrate handling blade. It is also to be understood that the substrate support assembly  210  disclosed herein is not limited in application to any particular type of CVD chamber.  
       FIG. 2  presents an exploded perspective view of one embodiment of a substrate support assembly  210  configured to support a large-area flat substrate. The substrate support assembly  210  comprises a substrate support  212 , such as a susceptor, which defines a generally rectangular surface that exceeds the dimensions of a substrate to be processed, although other geometries and dimensions may be used. Regardless of its shape, it may be desirable that the substrate support  212  and supported substrate be disposed parallel to the gas distribution plate  118  at during processing, with a processing region  112  being defined therebetween.  
      The substrate support  212  is fabricated from a thermally conductive material such as aluminum and may be coated with a layer of aluminum oxide, and in one embodiment functions as a lower electrode in the chamber  100 . The substrate support  212  typically has heating elements  221  within its structure to aid in maintaining the substrate at a desirable processing temperature during plasma enhanced CVD. Electrically conductive wires that provide power for heating the element may be provided through the shaft  142 . When the substrate support  212  is fabricated from a material of insufficient strength to support itself during operating conditions such as low pressure and high temperature, heating the substrate support  212  may cause the substrate support  212  to fatigue and deform.  
      The substrate may be heated during processing to a temperature up to about 460° C. The inventors have noted that under these operating temperature conditions, the substrate support  212  is not rigid, but tends to deform. This deformation of the substrate support was recognized and termed “deflection” in U.S. Pat. No. 6,149,365, issued to Applied Komatsu Technologies, Inc. in 2000. The inventors have also noted that the gas distribution plate  118 , which may function as the upper electrode, may also tend to deform due to operating temperature and pressure conditions. The gas distribution plate  118  may be supported along its perimeter, and may have a tendency over time to bow into a convex shape relative to the processing region  112  due to the operating temperature and pressure conditions.  
      For these reasons, it may be desirable to pre-shape the substrate support  212  to offset thermal and pressure induced deformation in the substrate support  212 . This pre-shaping may also be in anticipation of any deformation of the gas distribution plate  118 . Pre-shaping may be done either by appropriately varying the thickness of the substrate support  212  at manufacturing, by adjusting the profile of a base structure  214  underneath the substrate support  212 , or by pre-shaping the gas distribution plate  118 . Ideally, the pre-shape results in the upper electrode or showerhead being parallel relative to the supported substrate at operating conditions.  
      The substrate support  212  of  FIG. 2  includes a base structure  214  under the substrate support  212 . The illustrative base structure  214  is non-planar in order to impart a non-planar profile to the substrate support  212  and supported substrate  140 . In this embodiment, the base structure  214  is a lattice-type structure having an elongated base support plate  215 , and a plurality of lateral support plates  217  disposed generally transverse to the base support plate  215 . Although one base support plate  215  and four separate lateral support plates  217  are shown, it is to be under stood that any number of support plates  215 ,  217  may be used. The one or more base support plates  215  and two or more lateral support plates  217  are preferably fabricated from a material having minimal thermal and electrical conductivity properties. In addition, it is preferred that the plates  215 ,  217  be fabricated from a material of sufficient strength to retain rigidity under operating temperature and pressure conditions. An example of such a material is ceramic.  
      It is understood that the substrate support  212  rests immediately on the support plates  215 ,  217  although the view of  FIG. 2  shows the substrate support assembly  210  exploded for purposes of explanation. It is contemplated that the substrate support  212  and support plates  215 ,  217  do not move relative each other during processing. However, in one chamber  100  arrangement, the shaft  142  is movable vertically so as to permit movement of the substrate support  212  vertically toward and away from the gas distribution plate  118 . The shaft  142  also provides a passageway for electrical connectivity to the heating elements  221  in the substrate support  212 .  
      In another embodiment of the substrate support assembly  210 , shims  218 , such as spacers, are provided along the respective upper surfaces of each of the lateral support plates  217 . Preferably, the thickness of the shims  218  is from about 0.4 mm to about 3.5 mm. In this embodiment, the shims  218  are positioned at ends of the lateral support plates  217  however; the shims  218  may be located on other portions of the lateral support plates  217 . It is contemplated that the shape of the support plates  217  and/or the use of shims  218  will allow pre-shaping of the substrate support  212  that will translate a desired planar orientation to a substrate during processing as the heated substrate will conform to the planar orientation of the substrate support  212  during processing.  
       FIG. 3  is a plan view of the substrate support  212  of  FIG. 2 . The substrate support  212  has a plurality of openings  224  configured to receive the lift pins  150  for lifting the substrate off of the substrate support  212 .  
       FIG. 4  is a front view of the substrate support assembly  210  of  FIG. 2  with the substrate support  212  supported by the base structure  214 . A substrate  140  is shown supported by the substrate support  212 .  
       FIG. 5  shows a side view of the substrate support assembly  210  of  FIG. 2 , with the substrate support  212  supported by the base structure  214 . A substrate  140  is shown supported by the substrate support  212 .  
       FIG. 6  provides a bottom view of the substrate support assembly  210  of  FIG. 2  showing various connectors. First connectors  235  mechanically fasten the base support plate  215  to the respective lateral support plates  217 . Connectors  237  optionally mechanically fasten the lateral support plates  217  to the substrate support  212 . Connectors, such as pins  238 , connect the lateral support plates  217  to respective shims  218 . Through-holes  226  in formed through the support plates  215 ,  217  receive lift pins  150 .  
      When the substrate support  212  is fabricated from a material that is of insufficient strength to rigidly support itself under operating conditions, the substrate support  212  is subject to deformation at points that are not rigidly supported by the base structure  214 . In addition, the support plates  215 ,  217  may experience some slight deformation at operating temperature and pressure over time. When the substrate support  212  endures this deformation, the substrate  140  may deform to comply with the shape of the substrate support  212 . The thickness of the base structure  214  and/or the thickness of portions of the substrate support may be varied to overcome this circumstance.  
       FIG. 7  shows a substrate support  712  which, in one embodiment functions as a lower electrode in the chamber  100 . The substrate support  712  is pre-shaped with an upper surface  711  that has a varied thickness. More specifically, the nonplanar substrate support  712  has an increased thickness portion  713  at its edges to form a concave shape. The substrate support  712  is fabricated from a material of insufficient rigidity to support itself at operating conditions which may cause the substrate support  712  to deform and bow over the edges of the relatively rigid base structure  814 . Thus, the substrate support  712  is fabricated in a concave shape to anticipate possible thermal and pressure induced deformation of the substrate support  712  and the base structure  814  during processing. It is contemplated that this pre-shaping will translate a desired planar orientation to a substrate during processing as the heated substrate will conform to the planar orientation of the substrate support  712  during processing. It is understood that increased thickness portions  713  may be provided at other selected locations along the substrate support  712 .  
       FIG. 8  shows a base structure  814  having a base support plate  815  and lateral support plates  817  that form a nonplanar integral upper surface. The outer edges of the lateral support members  817  have increased thickness portions  813 . The thickness of the increased thickness portions  813  of the plates  817  may be appropriately varied and could be disposed at other locations along the support plates  817 .  
      In another aspect of the present invention, the chamber  100  may have a gas distribution plate  118  that is manufactured in a nonplanar shape. For example, the gas distribution plate  118  may be slightly concave. In this instance, a corresponding convex shape can be given to the substrate support  212 . To accomplish this, the base structure  214  may be configured to have limited dimensions such that edges of the substrate support  212  are not fully supported. Thus, some slight bowing of the substrate support  212  into a convex shape may occur at operating conditions. Alternatively, the base structure  214  may be fabricated from a material that likewise allows some slight fatiguing of the plates  217 , thereby further permitting bowing of the supported substrate support  212 . In either instance, the substrate support  212  is pre-shaped to provide a more parallel orientation between the substrate  140  and the gas distribution plate  118 . The extent of bowing can thus be controlled by the profile and material of the base structure  214 .  
       FIG. 9  shows a side, cross-sectional view of a substrate processing chamber  900  in an alternate embodiment. The chamber  900  is again configured to receive a large-area substrate, and to provide a plasma-enhanced chemical vapor deposition (PECVD) on a substrate  140 . The chamber  900  includes a side wall  930 , a lid  934 , a power supply  977 , a gas inlet  942 , a gas diffusion plate  944  having a plurality of nozzles  946 , an exhaust  980 , a processing region  925 , lift pins  922 , an upper electrode  972 , a lower electrode  912 , and a substrate support shaft  902 . When the lower electrode  912  is held at a potential different from that of the upper electrode  972 , a plasma can be between the upper electrode  972  and the lower electrode  912 .  
      The gas diffusion plate  944  is shown as nonplanar and in this example is convex; however, the gas diffusion plate  944  may alternately be concave. The gas diffusion plate  944  may be fabricated from 6061 aluminum alloy or other corrosion-resistant material. The lower electrode  912  also functions as a substrate support and has a nonplanar upper surface as well. The nonplanar upper surface may be due to variable thickness in the lower electrode  912 , or due to variable configurations of a lattice-type base structure  214 . The nonplanar surface of the lower electrode  912  produces a nonplanar profile in the substrate  140 . The nonplanar profile of the substrate  140 , in turn, generally matches the nonplanar profile of the gas diffusion plate  944 . The extent of bowing can again be controlled by the profile and material of the base structure  214 . Thus, the substrate  140  and the gas diffusion plate  944  are substantially parallel.  
      A method is also provided for shaping an electrode in a plasma-enhanced chemical vapor deposition (PECVD) chamber. The method includes the step of providing an upper electrode in the chamber. The method also includes the step of providing a substrate support in the chamber, which may function as a lower electrode, to receive a large-area flat substrate. The substrate support is fabricated from a thermally conductive metal that is of insufficient strength to rigidly support itself under operating conditions. The method also includes the steps of providing a base structure for supporting the substrate support. Preferably, though not required, the base structure of the substrate support is of sufficient strength to rigidly support itself under operating temperature conditions. In one aspect, the substrate support and supported substrate are shaped by providing a nonplanar shape to the base structure. In another aspect, a nonplanar shape of the substrate support is created by providing shims on top of a lattice-type base structure.  
      In one aspect of the method, the substrate support bows into a substantially planar shape when supported by the base structure under operating conditions. In another aspect, the substrate support bows or is otherwise formed into either a convex or a concave shape when supported by the base structure under operating conditions. In either instance, the substrate support conforms to a shape that is substantially parallel to the upper electrode and/or an upper gas diffusion plate when supported by the base structure under operating conditions. Other selected shapes may include a saddle shape or a cup shape to anticipate deflection in the upper electrode under operating conditions.  
      In one aspect, the method further comprises the step of providing a nonplanar gas diffusion plate in the chamber above the lower electrode, the plate having a plurality of gas distribution nozzles; and injecting process gas through the nonplanar gas diffusion plate and into a processing region of the chamber. The substrate support conforms to a shape that is substantially parallel to the gas diffuser under operating conditions. In one embodiment, the gas diffuser is convex, and the substrate support bows into the base structure in a concave manner to support the substrate in an orientation that is substantially parallel to the convex gas diffuser.  
      As can be seen, by appropriately shaping the surface of the electrode upon which is placed the substrate and/or the surface of the opposing electrode, it is possible to produce adequately uniform, useful process results. It is incidentally noted that shaping of the electrode may be used to allow whatever gas present in the chamber that would otherwise be trapped underneath a large substrate in an unpredictable manner to be voided as the substrate is placed on the support electrode. Such haphazardly trapped gas pockets can adversely affect the uniformity of the plasma. Shaping of the electrode may also prevent a substrate that would tend to distort due to temperature non-uniformities from losing physical contact with the substrate support or prevent a substrate with an as-manufactured non-flat shape from never achieving good physical contact to the support electrode.  
      While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.