Abstract:
This invention provides a method and apparatus for substantially eliminating deposition on the edge of a wafer supported on a pedestal in a processing chamber. Process gas flow onto the wafer surface is inhibited from reaching the wafer edge and backside, by means of a shadow ring placed over the wafer without touching it. Deposition on the edge and backside of the wafer are therefore substantially eliminated. The shadow ring defines a cavity which circumscribes the wafer edge, into which purge gas is flowed. This purge gas flows out from the cavity through the gap between the shadow ring and the upper surface of the wafer. Alignment pins are placed on the wafer supporting surface of the pedestal. These pins have sloping surfaces and are arranged to guide the wafer to a centered position on the pedestal when the wafer is placed on the pedestal. These pins also serve to align the shadow ring to the pedestal and thence to the wafer. The shadow ring has a plurality of keyed formations which mate to the pins, and as the shadow ring and pedestal are brought together, the pins serve to align the shadow ring. This precise rotational alignment and centering of the shadow ring results in substantial elimination of edge deposition. The keyed formations have elliptical cross-sections to provide for radial movement of the pins with respect to the keyed formations due to thermal expansion.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a division of U.S. patent application Ser. No. 08/729,210 filed Oct. 15, 1996, now U.S. Pat. No. 6,033,480, and a continuation in part of U.S. patent application Ser. No. 08/626,789 filed on Apr. 2, 1996, now U.S. Pat. No. 5,888,304, and titled “Heater with Shadow Ring and Purge Above Wafer Surface,” and a c-i-p of U.S. patent application Ser. No. 08/200,079 filed on Feb. 23, 1994, now U.S. Pat. No. 5,800,686, and titled “Chemical Vapor Deposition Chamber with Substrate Edge Protection,” the disclosures of which are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a method and apparatus for depositing useful layers of materials on substrates, for example, semiconductor wafers, used in the manufacture of semiconductor dies. More particularly, this invention relates to a method and apparatus for substantially eliminating unwanted deposition on the edge of the wafer. 
     2. Background 
     Overview of Chemical Vapor Deposition 
     Chemical vapor deposition, commonly referred to as “CVD,” is one of a number of processes used to deposit thin layers of material on a semiconductor wafer, and may be based on thermal, plasma, or optically assisted decomposition, or reaction of chemicals. To process wafers in, for example, a thermal CVD process, a chamber is provided with a susceptor configured to receive a wafer. The wafer is typically placed onto and removed from the susceptor by a robot blade and supported by the susceptor during processing. In these typical systems, the susceptor and the wafer are heated to a temperature of between 200-650° C. prior to processing. Once the wafer is heated to an appropriate temperature, a processing fluid, typically a gas, is introduced into the chamber through a gas manifold often situated above the wafer. The processing gas thermally decomposes upon contact with the heated wafer surface to deposit a thin material layer thereon. 
     A primary goal of wafer processing is to obtain as many useful die as possible from each wafer. Many factors affect the ultimate yield of die from each wafer processed. These factors include processing variables, which affect the uniformity and thickness of the material layer deposited on the wafer, and particulate contaminants that can attach to a wafer and contaminate one or more die. Both of these factors must be controlled in CVD and other processes to maximize the die yield from each wafer. 
     Particulate Continuation due to Unwanted Deposition Near the Wafer Edge 
     One of the causes of particulate contaminants in the chamber is improper deposition at the edge of a wafer. Because edge deposition conditions are difficult to control, due in part to the fact that wafer edges are typically chamfered, incomplete or improper deposition can occur around a wafer&#39;s edge. This may lead to deposited layers not adhering properly to each other and/or not adhering properly to the wafer. 
     This problem is illustrated in FIG.  1 ( a ), which is a schematic partial cross-section of a typical semiconductor wafer. For this example, the wafer  1  is shown with three consecutive deposited layers  2 ,  3 , and  4  in which the upper layer  4  is a tungsten layer formed after depositing intermediate layers  2  and  3  (or more layers, if necessary) on the wafer. Such a three-layer process for the deposition of tungsten is common as tungsten does not readily adhere to the silicon (or oxidized silicon) surface of the wafer. Accordingly, the very thin “primer” layer  2  is deposited, followed by the second layer  3  of a material to which tungsten layer  4  readily adheres. As can be seen from FIG.  1 ( a ), however, the tungsten layer  4  has “wrapped” around onto the beveled outer edge  5  of the wafer to come into direct contact with the silicon wafer. 
     The problem with this wrap around, as described above, is that tungsten does not adhere to the silicon wafer surface and could readily chip and flake during handling of the wafer, resulting in particulate contaminants. 
     In contrast, FIG.  1 ( b ) illustrates an idealized edge cross-section, in which all three layers terminate at or close to the same point with respect to the wafer&#39;s edge, preferably above the chamfer, with the metal layer  4  being the furthest back from the edge of the wafer. 
     Unwanted Deposition Near the Wafer Edge due to Heating 
     A factor which affects the uniformity of the deposition material layer is the method used for heating the wafer. One method involves heating the wafer with infared lamps through quartz walls of a reaction chamber. If the heat distribution is non-uniform, the wafer may warp, leading to non-uniform deposition on the top of the wafer as well as unwanted deposition from process gas seeping under upraised portions of the edge of the wafer. 
     Another method for heating the wafer is to use a heater pedestal for both supporting and heating the wafer. An example of this arrangement is described in the co-pending U.S. patent application Ser. No. 08/200,862 filed on Feb. 23, 1994 and titled “Improved Chemical Vapor Deposition Chamber,” the disclosure of which is hereby incorporated by reference. In this arrangement, the wafer is supported on a flat supporting surface of a heater pedestal mounted on a vertical stalk within the chamber. The pedestal is heated from within by means of an electrical heating coil, and the wafer, in turn, is heated by the hot supporting pedestal. In order to provide improved uniformity of heating of the wafer, the pedestal embodies of a vacuum chuck in which a “vacuum” is drawn at the interface between the underside of the wafer and the flat supporting surface of the pedestal. The resulting pressure differential across the wafer draws the wafer onto the pedestal resulting in improved uniformity of heating of the wafer. But as a result of this vacuum drawn at the backside of the wafer, processing gas can be drawn around the edge of the wafer and into the interface between the wafer and the pedestal. This can result in unwanted edge and backside deposition, leading to the generation of particle contaminants as described earlier. Accordingly, the improved uniformity of heating is accompanied by a possible increase in unwanted edge and backside deposition. 
     3. Prior Art 
     A number of prior art solutions to this problem have been proposed: 
     1. Purge Gas Ring 
     One solution to the problem of unwanted edge deposition is to use a purge gas ring which defines a manifold between itself and the heater body. During processing, a purge fluid, such as a gas, is injected through the heater body into the manifold and is projected toward the edge of the wafer at the outlet of the manifold. Consequently, processing gas is inhibited from contacting the extreme edge portion of the wafer. This method theoretically reduces unwanted deposition on the peripheral edge and lower surface of the wafer to produce the deposit pattern shown in FIG.  1 ( b ). In practice, however, the purge gas ring is not as successful as might be desired. 
     2. Shadow Ring 
     Another solution to this problem is to provide a shadow ring which is located over and in contact with a narrow peripheral area of the wafer to prevent deposition thereon. As shown in FIG.  1 ( c ), use of the shadow ring provides an undeposited annular strip around the wafer edge, which is sometimes desirable from a wafer handling perspective. However, the shadow ring is also not as successful as might be desired because of wafer warpage and the fact that the volatile deposition gas still tends to migrate under the lip of the shadow ring and deposit unwanted material on the wafer edge and backside. Furthermore, any misalignment of the shadow ring with respect to the wafer may result in incomplete shadowing of the wafer edge. 
     The need therefore exists for a method and apparatus for controlling the deposition of materials at or around the edge of a semiconductor wafer during CVD and/or other wafer processing operations. 
     SUMMARY OF THE INVENTION 
     This invention provides a method and apparatus for masking the edge of a wafer supported on a substrate support, such as a heater pedestal, in a processing chamber. Process fluid, such as a gas, is flowed onto the wafer surface, and is inhibited from depositing near the wafer edge by a shadow ring placed over but not in contact with the wafer. The shadow ring defines a wafer edge circumscribing cavity into which purge gas is flowed. This purge gas flows out from the cavity through the gap between the shadow ring and the upper surface of the wafer. 
     Thus, the shadow ring inhibits deposition of process gases on the wafer in two distinct ways. First, the shadow ring physically obstructs process gas, flowing down from above the wafer, from depositing on the masked portion of the wafer. Second, the shadow ring is used to direct purge gas flow to inhibit process gas from seeping under the shadow ring and depositing near the wafer edge. 
     Six alignment pins are pressed into and protrude from the wafer supporting surface of the heater pedestal. These pins have sloping surfaces and are arranged so that they guide the wafer to a centered position on the heater pedestal when the wafer is placed on the pedestal. These pins are preferably made from a material which minimizes friction between the pins and the wafer, so that the wafer slides easily into position. 
     In a preferred embodiment, the pins are made from stainless steel, while the pedestal ais made from aluminum. 
     These pins also serve to align the shadow ring to the heater pedestal and thence to the wafer. The shadow ring has formed in its underside a plurality of concave features which mate to the pins. As the shadow ring and heater pedestal are brought together, the pins serve to precisely align the shadow ring. The shadow ring is thereby centered and rotationally aligned with respect to the wafer, so that it exactly covers the desired portion of the wafer. At process temperature, the shadow ring rests on the support ring and there is a small gap between the tops of the pins and the shadow ring. The pins have elliptical cross-sections which are radially aligned with the pedestal to accommodate relative movement between the pins and the concavities due to thermal expansion. 
     The shadow ring is vertically supported from below (and thus prevented from touching the wafer) by a support ring which circumscribes the heater pedestal. The support ring may be made from aluminum or aluminum alloy, and is welded to the pedestal after aligning it with the pedestal by means of a pin pressed into the pedestal. The interface between the support ring and the pedestal seals the bottom edge of the purge gas manifold defined between the pedestal and the support ring. 
     The shadow ring typically also has lower thermal conductivity than the heater pedestal, which reduces heat transfer from the heater pedestal to the shadow ring. As the rate of deposition is temperature-dependent, reducing the temperature of the shadow ring reduces deposition thereon. In a preferred embodiment, the heater pedestal is made of aluminum, the support ring is made of 6061-T6 aluminum alloy, and the shadow ring is made of 314 stainless steel or a ceramic. 
     The heat transfer is further inhibited by minimizing the contact area between the support ring and the shadow ring, which may be accomplished by designing the two rings to have minimal overlap. 
     This invention finds particular application in, but is not limited to, semiconductor wafer processing chambers which process such wafers using CVD processes. 
     One advantage of this invention is that it substantially eliminates the occurrence of deposition near the edge of the wafer. This in turn eliminates particle contaminants originating from unwanted near-edge deposition that inadequately adheres to the surface. 
     Another advantage of this invention is that the aligning pins serve to align the wafer with the pedestal, the shadow ring with the pedestal, and thus, the shadow ring with the wafer. This prevents “off centered” shadowing of the edge portion of the wafer, as well as incomplete shadowing of a non-circular wafer by a non-circular shadow ring that is rotationally misaligned with respect to the wafer. 
     These advantages and further details of the present invention will become apparent to one skilled in the art from the following detailed description and accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
     FIG.  1 ( a ) is a partial cross-section of a semiconductor wafer illustrating non-ideal deposition of material layers at the edge of the wafer; 
     FIGS.  1 ( b ) and  1 ( c ) are partial cross-sections of a semiconductor wafer illustrating improved deposition of material layers at the edge of the wafer; 
     FIG. 2 is an partial sectional view of a heater pedestal in a semiconductor chamber, in accordance with the invention; 
     FIGS.  3 ( a ) and  3 ( b ) are detailed views of the upper portions of the heater pedestal shown as regions A and B, respectively, in FIG. 2, illustrating in particular, the configuration of the shadow ring; and 
     FIG. 4 is a bottom view of the shadow ring of FIGS.  3 ( a ) and  3 ( b ). 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Introduction 
     A heater pedestal  16  in accordance with the invention is shown in FIG.  2 . As will be apparent from the description below, the pedestal  16  can be used to reduce the deposition of material on a semiconductor wafer&#39;s underside and edge. The pedestal  16  is shown to include edge protection in the form of a circumferential shadow ring  24  positioned adjacent to the periphery of a wafer  20  received on the supporting surface  22  of the heater pedestal  16 . 
     In operation, a wafer  20  is positioned on the heater pedestal  16  and the shadow ring  24  is positioned to mask an outer peripheral edge of the wafer  20 . During processing, a continuous flow of purge gas about the peripheral edge of the wafer is provided through a gap  80  defined between shadow ring  24  and the surface of the masked portion of the wafer  20 . This purge flow inhibits deposition on the edge of the wafer  20 , and on the underside of the wafer  20  immediately adjacent to its edge. 
     Detailed Description 
     1. Pedestal and Shadow Ring Architecture 
     In FIG. 2, a heater pedestal  16  is shown to comprise a main body  61  supported by a vertical stalk  18 . Within the stalk  18  are a purge gas conduit  51 , a housing containing a thermocouple (not shown) for measuring the temperature of the pedestal, and a conduit  56  for supplying power to a heating element  54  located within the main body  61  of the pedestal  16 . The main body  61 , which is typically made from aluminum, has a supporting surface  22  on which a semiconductor wafer can be supported. The surface  22  of the pedestal  16  may have formed therein a plurality of concentric vacuum grooves (not shown) for applying a vacuum to draw the wafer  20  onto the supporting surface  22  for improved uniformity of heating during operation. These vacuum grooves may take the form of concentric grooves and be supplied by a vacuum conduit (not shown) as described in pending U.S. patent application Ser. No. 08/626,789. The surface  22  may be textured to reduce the area of contact between the wafer  20  and the surface  22  to reduce sticking. 
     Wafer transfers are achieved by means of a conventional processing apparatus robot arm (not shown) which enters the chamber through a slit valve opening (not shown). This robot arm is used to place wafers  20  in position for processing. 
     A removable shadow ring  24  circumscribes the main body  61  and rests on supporting ring  28 , which in turn is electron beam welded to the main body  61  after aligning it by means of a pin  60  embedded in the main body  61 . Alternatively, the pin  60  may be embedded in an aluminum ring (not shown) which is welded to main body  61 . Supporting ring  28  and main body  61  define a purge gas manifold  26 , which would be difficult to machine from a single piece of material. The purge gas manifold  26  is fed by eight purge gas conduits  86  formed in main body  61 , and serves to distribute and equalize the pressure/flow of purge gas received from purge gas conduits  86 . These purge gas conduits  86  are in communication with a purge gas supply (not shown) via purge gas conduit  51 . 
     As shown in FIGS.  3 ( a ) and  3 ( b ), purge gas flows through passage  84  and feeds a purge gap  80  defined between the shadow ring  24  and the upper surface of the masked portion of the wafer  20 . The purge gap  80  is dimensioned to allow sufficient flow of purge gas to prevent deposition on the edge and underside of the wafer. In a preferred embodiment of the invention, the purge gap  80  is 0.010 to 0.030 inches (0.25 to 0.76 mm) when the apparatus is at operating temperature (typically 265° C.). 
     Although the figures illustrate a preferred embodiment suitable for use with a circular wafer, it is noted that the purge gap  80  follows the perimeter of the wafer. Accordingly, if a non-circular wafer (such as a wafer with a “wafer flat” formed in it) is being processed, the purge gap  80  will be formed to follow the irregular perimeter. Furthermore, though the preferred embodiment comprises a supporting surface  22  having a flat surface for supporting the wafer, the invention may be used with supporting surfaces having different configurations, such as a wafer supporting surface having a pocket for the wafer. 
     2. Wafer and Shadow Ring Alignment 
     The main body  61  of the pedestal  16  has six alignment pins  32 , each of which has sloping sides  33 . Each pin  32  is pressed into a hole  38  drilled in main body  61  and protrudes from the surface  22  of the pedestal  16  as indicated in FIG.  3 ( a ). The sloping sides  33  of each pin  32  serve to guide the wafer  20  and shadow ring  24  into position when engaged by the pin  32 . Each pin  32  has an axial passage  34  which permits residual gas to escape from the region under the pin  32  in the hole  38 , so that excess pressure does not build up under the pin  32  and force it out when the chamber is heated to operating temperature. This axial passage  34  can also eliminate “virtual leak,” when trapped non-process gases may leak into the process chamber from under the pin  32 . 
     In the preferred embodiment, the pin  32  is 0.187″ (4.75 mm) and has a circular cross-section. However, pin  32  may have an elliptical or other non-circular cross-section to, for example, increase the area of contact with the edge of the wafer  20 . Further, the pin  32  may be domed or given a sharply angled shape. In the illustrated embodiment, the sloped sides  33  of pin  32  have an angle of approximately 20 degrees from the major axis of the pin  32 . In this embodiment, the pin  32  is made from a material, such as stainless steel, which minimizes friction with respect to wafer  20  and shadow ring  24 . 
     The pins  32  are arranged around the surface  22  so that they guide the wafer  20  and shadow ring  24  to a centered position on the heater pedestal  16  when wafer  20  and shadow ring  24  engage the pins  32 . As the wafer  20  is placed in position on the pedestal, any misalignment of the wafer  20  causes its edge to come into contact with at least one pin  32 , so that the sloped sides  33  cause the wafer to move into an aligned position. 
     These pins  32  are also used to align the shadow ring  24  to main body  61 , and thence to the wafer  20 . As indicated in FIG. 4, the shadow ring  24  has a plurality of indentations  36  which mate with the pins  32 . As the heater pedestal  16  and shadow ring  24  are brought together after positioning the wafer  20  on the pedestal  16 , pins  32  engage indentations  36 . The indentations  36  may be tapered to correspond to the pins  32 , and have circular cross-sections. In a preferred embodiment, the indentations  36  have elliptical cross-sections to provide for the differing coefficients of thermal expansion of the shadow ring  24  and heater pedestal  16 . Each elliptical cross-section has its major axis radially aligned with respect to the pedestal  16 . Thus, the elliptical shape of indentation  36  allows for radial movement of the pin  32  with respect to indentation  36  due to thermal expansion. 
     Any misalignment of the shadow ring  24  is corrected by the engagement of pins  32  with indentations  36 . The shadow ring  24  is thereby centered and rotationally aligned. The pins  32  also limit rotational movement of the shadow ring  24 . This improves alignment of shadow ring  24  with the wafer  20 , which is particularly advantageous when non-circular wafers are used (such as a circular wafer with a flat side) with matching shadow rings, as it reduces the possibility of a portion of the wafer  20  protruding from under the shadow ring  24  (and thus not being shadowed). 
     At process temperatures, typically 265° C. in the illustrated embodiment, with accompanying thermal expansion of the various components, the shadow ring  24  rests directly on support ring  28 , and there is a small gap of approximately 0.020″ between the pin  32  and indentation  36 . 
     3. Heat Transfer Considerations 
     Contact area minimization and material selection are used to reduce heat transfer from the main body  61  to support ring  28  and shadow ring  24 . Reducing the contact area at the interface between support ring  28  and shadow ring  24  reduces the heat transfer across this interface. This, in turn, reduces unwanted deposition on the ring by keeping it relatively cool. Thus, support ring  28  contacts shadow ring  24  over a relatively small peripheral area of the support ring  28 . The support ring  28  or shadow ring  24  may further have textured contacting surfaces to reduce the area of contact and thus, heat transfer between them. Similarly, in the area above the purge gas manifold  26 , the support ring  28  contacts main body  61  only along a small circumferential flange  82 . The support ring  28  is supported from below by main body  61 . 
     Material selection is used to further reduce the heat transfer. Thus, shadow ring  24  is made of a material that has lower coefficients of thermal conductivity and thermal expansivion than the material of the main body  61 . In a preferred embodiment, shadow ring  24  is made from a nickel-chromium-iron alloy such as 316L stainless steel. The ring&#39;s lower thermal conductivity reduces heat transfer from the main body  61  to the shadow ring  24 . Similarly, pins  32  are made from 314 stainless steel, which not only reduces heat transfer to shadow ring  24  but also permits wafer  20  and shadow ring  24  to slide easily along their surfaces. 
     It should be noted that although the disclosed support ring  28  is made from aluminum or aluminum alloy, the support ring  28  may itself be made from a less thermally conductive material such as stainless steel, and affixed to the main body  61 . 
     Alternatively, the support ring  28  may have a component which is made from a less thermally conductive material for contacting the shadow ring  24  and for exposure to process gases. 
     4. Wafer Processing 
     FIG. 2 depicts a wafer positioned on a movable heater pedestal during normal operation, i.e., when the heater pedestal is in its uppermost position, after wafer loading has been accomplished. Although FIG. 2 does not depict the wafer loading process, details of this process may be visualized with the aid of the figure. 
     At the start of processing, heater pedestal  16  is lowered until the shadow ring  24  is caught by pumping plate  30 . As the pedestal is further lowered, wafer lift fingers (not shown) in pedestal  16  hit a vertical stop and remain stationary while the rest of the heater pedestal continues to move downward. Thus, the wafer lift fingers become vertically extended clear of the supporting surface  22 . When the heater pedestal  16  reaches its lowest position, a robot blade (not shown) holding an appropriately dimensioned semiconductor wafer  20  enters the chamber through an opening  68  in the chamber wall. As the heater pedestal  16  is raised, the extended wafer lift fingers pick up the wafer  20  from the robot blade and the blade is retracted out of the chamber. As the heater pedestal  16  continues to move upward, past the wafer lift finger stops, the wafer lift fingers retract into the main body  61 , lowering the wafer  20  with respect to the surface  22 . As the wafer  20  moves toward the surface  22 , the alignment pins  32  guide the wafer  20  to a centered position on the pedestal  16 . The pedestal  16  continues to rise, and the shadow ring  24  is lifted off pumping plate  30  by the support ring  28 , and centered and rotationally aligned by the alignment pins  32 . Eventually, the heater pedestal reaches its operating position as shown in FIG.  2 . At this point, processing may begin. 
     A typical CVD metal deposition process has two discrete deposition steps. During the first step, lasting about thirty (30) seconds, the pressure in the chamber is reduced to about 0.6 kPa (4.5 Torr) and a metal deposition gas introduced into the chamber. This step, known as a nucleation step, results in a very thin layer of metal being deposited on the wafer. This thin layer serves as a base layer onto which further layers of metal are deposited. 
     During the second deposition step, lasting less than 60 seconds and during which most of the metal is deposited, a “vacuum” may be drawn at the interface between the wafer and the supporting surface  22  of the pedestal  16 , if the pedestal  16  is provided with vacuum grooves (not shown). 
     The pedestal  16  is resistively heated by means of the electrically powered heating coil  54  mounted within the body of the pedestal  16 . The coil  54  is supplied with electrical power along a rigid, stainless steel connecting stem  56  located within the body of the stalk  18 . Although only one coil is shown in this figure, more than one coil could be used or even multiple heating elements which are nested together but powered separately. Alternatively, an infrared heater may be used. 
     When the pedestal  16  is heated, main body  61  expands radially outward due to increased temperature. The support ring  28 , which is made of stainless steel having a lower coefficient of thermal expansion and is at a lower temperature, does not expand as much. The outer face of the main body  61  thus comes into horizontal contact with the protruding inner edge  82  of the support ring  28 , creating a mechanical seal at the interface between the support ring  28  and the main body  61 . This seal inhibits process gas from seeping between main body  61  and support ring  28 . The support ring  28 , as mentioned before, is welded to and seals against main body  61  in the area below the purge gas manifold  26 . 
     During the second deposition step, an inert purge gas such as argon is supplied at approximately 4.2 cubic feet per minute (about two liters per second) by a purge gas supply (not shown) through the purge gas manifold  26  and out through purge gap  80 , at a pressure and flow rate sufficient to substantially inhibit migration of process gas into purge gap  80 . Once the purge gas has flowed out of purge gap  80 , it enters the processing chamber to be exhausted through standard exhausts in the chamber (not shown). 
     The supply of this purge gas has a particularly advantageous effect in that it substantially reduces or eliminates deposition of metal on the edge and outer portion of the underside of the wafer. This is because the purge gas flows out continually and at high mass flow rates through the purge gap  80 , effectively diluting deposition gas and preventing its diffusion into the vicinity of the wafer edge. In addition, deposition around the outer portion of the top of the wafer is directly inhibited because the shadow ring physically obstructs the process gas from flowing vertically downward in a direction perpendicular to the masked portion of the wafer. Therefore, deposition in the entire near-edge region is reduced by the combination of the shadow ring and the purge gas. 
     Although the present invention has been described above in terms of a specific embodiment (that of an apparatus for use in a CVD process), it is anticipated that alterations and modifications to this invention will no doubt become apparent to those skilled in the art. For example, the wafer need not be heated by an electrical element within the heater pedestal, but could be heated by infrared lamps outside a quartz processing chamber. As another example, the shadow ring need not be supported mechanically by the support ring, but could be supported, during processing, by the pins  32 , a buoyant lift of the purge gas, or mechanically attached to an upper gas manifold. Additionally, although the invention has been described for use in a thermal CVD chamber, the components herein are equally suited for use in plasma deposition and other wafer processing operations. Also, for plasma deposition, it is preferable for both the shadow ring and support ring to be made from a ceramic. It is therefore intended that the following claims be interpreted as covering all such alterations and modifications as fall within the true spirit and scope of the invention.