Abstract:
An improved wafer holder design is described which has manufacturing and performance advantages over present state-of-the-art holders used in various wafer processing applications. The new wafer holder design incorporates a series of short radial grooves. The grooves extend from the base of a circular channel, which runs along the outside diameter of the substrate wafer recess, to a fixed radial location which varies based on wafer size and thickness. The grooves provide a slight overlap with the wafer to facilitate the free exchange of gases beneath the wafer necessary for wafer loading and unloading operations. The short length of the radial grooves make the wafer holder easier to manufacture and offer more robust performance compared to the present state-of-the-art holders.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application is entitled to the benefit of, and claims priority to, provisional U.S. Patent Application Ser. No. 60/492,063 filed Aug. 1, 2003 and entitled “SUSCEPTOR FOR SUPPORTING WAFERS DURING SEMICONDUCTOR MANUFACTURE,” the entirety of which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE PRESENT INVENTION  
       [0002]     1. Field of the Present Invention  
         [0003]     The present invention relates generally to the manufacture of substrate wafers of the type used in producing semiconductor devices, and, in particular, to susceptors and other substrate wafer holders for use in a substrate supporting mechanism in a reaction chamber during semiconductor manufacturing processes.  
         [0004]     2. Background  
         [0005]     Typical wafer holders are described in U.S. Pat. Nos. 4,821,674 and 5,427,620. These wafer holders are typically used to support a single substrate wafer during various wafer processes during the manufacture of integrated circuits. Such process applications may include silicon application processes such as chemical vapor deposition (“CVD”) and physical vapor deposition (“PVD”), thermal process applications used for treatment of semiconductor wafer substrates such as rapid thermal processing (“RTP”) and high temperature etch processing, and the like.  
         [0006]     To save process time, substrate wafers must be loaded at an elevated temperature, and when the wafers are placed on a flat, smooth, unbroken surface, the heat-related convection currents affect the ability of the substrate wafer to settle uniformly. Thus, as shown in the aforementioned patents, a series of intersecting channels is typically machined or otherwise applied to the wafer contact surface of the holder, the intent of which is to provide a free flow of gases between the substrate and holder to avoid undesired movement of the substrate during loading and unloading operations. The presence of the intersecting channels alleviates the substrate settling issue by allowing the hot gases to escape from underneath the wafer. The intersecting channels also facilitate loading the wafers using the Bernoulli principle.  
         [0007]     The above cited patents depict a wafer holder with only a limited number of intersecting channels. However, in practice, the number of channels required is very large because of the need to maintain a uniform temperature profile over the surface of the substrate wafer which the holder supports. In addition, it is difficult to maintain the uniformity of the channeled surface, which is critical to avoiding issues such as image transfer to the process wafer. Both temperature uniformity and image transfer issues must be avoided to ensure proper electrical and physical properties of the process wafer along with any deposited coatings.  
         [0008]     U.S. Pat. No. 5,403,401 cites a number of manufacturing issues involving wafer holders made with a substrate contact face possessing a series of underlying, intersecting channels, a typical example of which is shown in  FIGS. 1A-1C . Specifically, wafer holders of this design are difficult to maintain flat during manufacture if the channels exist only on one face of the holder. After machining, these wafer holders, which are typically machined from graphite, receive a coating, such as silicon carbide (SiC), deposited by CVD at high temperatures. As the holder cools to room temperature, differential shrinkage between the holder substrate and the coating generally leads to a state of stress, wherein the coating is under compression and the substrate under tension (although this stress state may be reversed, depending upon the properties of the substrate and the coating). The amount of stress that develops is dependent in part upon the surface area. As a result, large surface area differences between the two faces of the same holder, such as those that may exist when only one face is machined, can lead to large differences in stress, which in turn cause the part to distort or warp.  
         [0009]     To alleviate this problem, the above-cited patent suggests that similar machining detail should be added to both faces of the wafer holder in order to avoid the differences in stress and thus ensure that the part remains flat after coating. Unfortunately, adding machining detail to both sides of the wafer holder can significantly increase its manufacturing cost. Another solution cited in the above patent is to tailor the thickness of the coating so that a controlled coating differential is maintained between the two opposite faces. In principle, this is an appropriate fix; however, in practice it can be difficult to maintain a consistent coating thickness differential between faces. There is also the problem that the amount of thickness differential required is a function of the differential in coefficient of thermal expansion (“CTE”) between the holder substrate and the surface coating. Unfortunately, certain holder substrate materials, such as graphite, have CTE&#39;s which span a range, which complicates this process. For example, the range for graphite is affected by the type of coke used in its manufacture, binder levels, particle sizing, and processing temperatures.  
         [0010]     There is also a need to minimize the total contact area between the wafer holder and substrate wafer in order to maintain a uniform temperature profile across the surface of the wafer as well as to minimize any markings to the backside of the substrate wafer. In order to minimize total contact area, the number of channels is intentionally high, which means that the grids formed at the channel intersections are kept small. It is the tops of these individual grid areas, formed by the channels, that provide support for the wafer substrate. The problem with small grids is that they are relatively weak areas of the holder surface, and thus are prone to damage. This in turn can affect the lifetime of the wafer holder if one or more grids become damaged.  
         [0011]     One additional drawback of wafer holders having a high number of intersecting channels on one or more faces is that such wafer holders are more prone to developing pinholes in the surface coating. This will also cause the wafer holder to be rejected, since once the coating is breached, the substrate beneath the coating is exposed to the process environment. The higher occurrence of pinholes through the surface coating on parts machined with a high number of channels is due to coating thickness variations along with cleaning issues, which are more problematic at the base of the machining detail. (Pinhole formation occurs over a period of time during use of the holder. This is generally a surface erosion problem, which can be affected by cleanliness.)  
         [0012]     One final drawback to wafer holders machined with a high number of intersecting channels is that it is often desired to machine a concave-shaped profile into the face of the holder that is in contact with the substrate wafer, particularly for large diameter wafers and/or lower temperature processes which require a higher level of temperature uniformity. The presence of a high amount of surface detail greatly increases the complexity of machining such a profile, which further adds to the cost of the part.  
       SUMMARY OF THE PRESENT INVENTION  
       [0013]     The wafer holder described in the present invention has a number of advantages over present state-of-the-art wafer holders. For example, it requires a smaller amount of machining and requires less complex machining, making it easier to incorporate concave surface profiles, especially for large substrate wafer diameters. This, in turn, provides improved part-to-part consistency. It is also more damage tolerant, making it easier to maintain dimensional control during manufacture, and provides improved wafer holder performance.  
         [0014]     Broadly defined, the present invention according to one aspect is a wafer holder for holding semiconductor substrate wafers in a chemical vapor deposition system, including: a holder body having a top surface; a circular wafer recess in the top surface of the holder body, the wafer recess having an outer perimeter and an interior area; and a plurality of slots arranged in the top surface of the holder body, each beginning adjacent the outer perimeter of the wafer recess and extending toward and terminating in the interior area of the wafer recess.  
         [0015]     In features of this aspect, the wafer holder further includes a circular groove extending around the outer perimeter of the wafer recess; substantially all of the slots extend radially from the circular groove toward the interior area of the wafer recess; the holder body is formed from at least one of the following: graphite, silicon, silicon nitride, silicon carbide, quartz or aluminum oxide; a surface coating may be applied to at least the top surface of the holder body; the surface coating is formed from at least one of the following: silicon carbide, silicon nitride, pyrolytic graphite, pyrolytic carbon, diamond, aluminum nitride, aluminum oxide, silicon dioxide or tantalum carbide; the wafer recess may encompass a concave surface; the concave surface of the wafer recess is adapted to aid in gas flow beneath a substrate wafer disposed in the wafer recess; the concave surface of the wafer recess is adapted to help maintain a uniform temperature profile across the surface of a substrate wafer disposed in the wafer recess; the number of slots is selected to minimize negative effects on the thermal profile of a substrate wafer disposed in the wafer recess; and the number of slots is selected to allow sufficient gas flow beneath the substrate wafer to aid in proper loading and unloading operations.  
         [0016]     In other features of this aspect, the dimensions of the slots are selected to minimize negative effects on the thermal profile or backside markings of a substrate wafer disposed in the wafer recess; the dimensions of the slots are selected to provide effective gas flow beneath substrate wafers for the purpose of aiding in proper loading and unloading operations; each of the plurality of slots is between 0.030 in. and 1.000 in. in length, and preferably between 0.035 in. and 0.065 in. in length; each of the plurality of slots is between 0.010 in. and 0.030 in. in width, and preferably between 0.015 in. and 0.025 in. in width; and each of the plurality of slots is at least 0.001 in. deep, and preferably between 0.004 in. and 0.008 in. deep.  
         [0017]     In other features of this aspect, the wafer holder is a susceptor; the wafer holder further includes a circumferential ledge for supporting the edges of a wafer; the plurality of slots are disposed at least partly in the circumferential ledge; and the wafer recess. is a first wafer recess, the plurality of slots is a first plurality of slots, and the wafer holder further includes a second circular wafer recess in the top surface of the holder body adjacent the first circular wafer recess, the wafer recess having an outer perimeter and an interior area, and a second plurality of slots arranged in the top surface of the holder body, each beginning adjacent the outer perimeter of the second wafer recess and extending toward and terminating in the interior area of the second wafer recess.  
         [0018]     The present invention according to another aspect is a wafer holder for holding semiconductor substrate wafers in a chemical vapor deposition system, including: a holder body having a top surface; a circular wafer recess in the top surface of the holder body, the wafer recess having an outer perimeter and an interior area; and a plurality of non-interesting slots arranged in the top surface of the holder body, each beginning adjacent the outer perimeter of the wafer recess and extending toward and terminating in the interior area of the wafer recess.  
         [0019]     In features of this aspect, the wafer holder further includes a circular groove extending around the outer perimeter of the wafer recess; substantially all of the slots extend radially from the circular groove toward the interior area of the wafer recess; the holder body is formed from at least one of the following: silicon, silicon nitride, silicon carbide, quartz or aluminum oxide; a surface coating may be applied to at least the top surface of the holder body; the surface coating is formed from at least one of the following: silicon carbide, silicon nitride, pyrolytic graphite, pyrolytic carbon, diamond, aluminum nitride, aluminum oxide, silicon dioxide or tantalum carbide; the wafer recess may encompass a concave surface; the concave surface of the wafer recess is adapted to aid in gas flow beneath a substrate wafer disposed in the wafer recess; the concave surface of the wafer recess is adapted to help maintain a uniform temperature profile across the surface of a substrate wafer disposed in the wafer recess; the number of slots is selected to minimize negative effects on the thermal profile of a substrate wafer disposed in the wafer recess; and the number of slots is selected to allow sufficient gas flow beneath the substrate wafer to aid in proper loading and unloading operations.  
         [0020]     In other features of this aspect, the dimensions of the slots are selected to minimize negative effects on the thermal profile or backside markings of a substrate wafer disposed in the wafer recess; the dimensions of the slots are selected to provide effective gas flow beneath substrate wafers for the purpose of aiding in proper loading and unloading operations; each of the plurality of slots is between 0.030 in. and 1.000 in. in length, and preferably between 0.035 in. and 0.065 in. in length; and each of the plurality of slots is between 0.010 in. and 0.030 in. in width, and preferably between 0.015 in. and 0.025 in. in width; and each of the plurality of slots is at least 0.001 in. deep, and preferably between 0.004 in. and 0.008 in. deep.  
         [0021]     In other features of this aspect, the wafer holder is a susceptor; the wafer holder further includes a circumferential ledge for supporting the edges of a wafer; the plurality of slots are disposed at least partly in the circumferential ledge; and the wafer recess is a first wafer recess, the plurality of slots is a first plurality of slots, and the wafer holder further includes a second circular wafer recess in the top surface of the holder body adjacent the first circular wafer recess, the wafer recess having an outer perimeter and an interior area, and a second plurality of slots arranged in the top surface of the holder body, each beginning adjacent the outer perimeter of the second wafer recess and extending toward and terminating in the interior area of the second wafer recess.  
         [0022]     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]     Further features, embodiments, and advantages of the present invention will become apparent from the following detailed description with reference to the drawings, wherein:  
         [0024]      FIG. 1A  is a top view of a typical prior art wafer holder;  
         [0025]      FIG. 1B  is a side cross-sectional view of the prior art wafer holder of  FIG. 1A , taken along line  1 B- 1 B;  
         [0026]      FIG. 1C  is a side cross-sectional view of the prior art wafer holder of  FIG. 1B , shown with a wafer positioned thereon;  
         [0027]      FIG. 2  is a block diagram of a conventional chemical vapor deposition system;  
         [0028]      FIG. 3  is a perspective view of the wafer holder of  FIG. 2 , shown in isolation, in accordance with the preferred embodiments of the present invention;  
         [0029]      FIG. 4A  is a top view of the wafer holder of  FIG. 3 ;  
         [0030]      FIG. 4B  is a partial side cross-sectional view of the wafer holder of  FIG. 4A , taken along line  4 B- 4 B;  
         [0031]      FIG. 4C  is a partial side cross-sectional view of the wafer holder of  FIG. 4B , shown with a wafer positioned thereon;  
         [0032]      FIG. 5  is a top view of a multiplexed wafer holder, where two wafer recesses are arranged side by side in a single holder body, in accordance with an alternative embodiment of the present invention;  
         [0033]      FIG. 6A  is a perspective view of a prior art stepped-type wafer holder; and  
         [0034]      FIG. 6B  is a partial side cross-sectional view of the wafer holder of  FIG. 6A , taken along line  6 B- 6 B.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]     Referring now to the drawings, in which like numerals represent like components throughout the several views, the preferred embodiments of the present invention are next described. The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.  
         [0036]     The preferred embodiments of the present invention will be described with reference to an otherwise-conventional chemical vapor deposition (“CVD”) system  10 , because CVD is an example of a typical process for which the wafer holder of the present invention may find application. However, it should be understood that the wafer holder of the present invention may be used in a wide variety of wafer processes, including physical vapor deposition (“PVD”), rapid thermal processing (“RTP”), high temperature etch processing, and other thermal processing, and generally including any other process in which a semiconductor wafer must be lifted from a generally flat surface.  
         [0037]      FIG. 2  is a block diagram of a conventional chemical vapor deposition (“CVD”) system  10 . The CVD system  10  includes a reaction chamber  12  and a support mechanism  14 . The reaction chamber  12  may be of any conventional design, including the type sometimes referred to as a horizontal flow reaction chamber such as that disclosed in U.S. Pat. No. 5,427,620, the entirety of which is incorporated herein by reference.  
         [0038]     The support mechanism  14  is preferably rotatable and may be likewise generally similar to that disclosed in the &#39;620 patent. Of particular relevance, the support mechanism  14  includes a wafer holder  16  disposed within the reaction chamber  12 . The wafer holder  16  may be of a type often referred to as a susceptor, which typically is heated inductively, or of other types known to those skilled in the art.  
         [0039]      FIG. 3  is a perspective view of the wafer holder  16  of  FIG. 2 , shown in isolation, in accordance with the preferred embodiments of the present invention. As illustrated therein, the wafer holder  16  primarily includes a solid body  18  in the general shape of a flattened cylinder. The body  18  may be formed from any conventional wafer holder body material, most commonly including pure graphite but alternatively composed of silicon, silicon nitride, silicon carbide, quartz, aluminum oxide or another ceramic material or metal or metal alloy. The selection of material conventionally depends on the end-user&#39;s process requirements.  
         [0040]      FIG. 4A  is a top view of the wafer holder  16  of  FIG. 3 , and  FIG. 4B  is a partial side cross-sectional view of the wafer holder  16  of  FIG. 4A , taken along line  4 B- 4 B. Of particular interest, and as perhaps best understood with reference to  FIGS. 4A and 4B , the holder body  18  includes a top surface  20  and a bottom surface  22 . Centered on the top surface  20  is a wafer recess  24  formed by a slight depression in the holder body  18 , explained below, and defined by an outer circumferential rim  26 . At the perimeter of the recess  24 , immediately inside the outer rim  26 , is arranged a continuous circular groove  28 . The wafer recess  24  is adapted to support a wafer  30  therein, and the size of the wafer recess  24 , and thus the location of the circular groove  28 , is selected in conjunction with the selection of the size of the wafer  30 .  FIG. 4C  is a partial side cross-sectional view of the wafer holder  16  of  FIG. 4B , shown with a wafer  30  positioned thereon. As shown, the wafer  30  fits comfortably within the wafer recess  24 , with the outer edge of the wafer  30  preferably positioned directly above the circular groove  28 .  
         [0041]     Though not strictly necessary, the circular groove  28  helps ensure uniform gas flow under the wafer  30  during processing. By increasing the size of the groove  28 , the gas flow may be more easily controlled; however, temperature uniformity issues dictate against enlarging the groove too much. The size of the groove  28  is thus generally controlled by conventional principles, and the selection of an appropriately-sized groove may be dependent on the manufacturing process in which the wafer holder  16  is to be used. However, it is known that grooves  28  having a semicircular profile that is 0.035 in. wide and 0.035 in. deep have been used successfully as described herein below.  
         [0042]     The top surface  20  further includes a ring of narrow, non-intersecting slots  32  which extend inward from the groove  28  a fixed distance toward the center of the wafer recess  24 . The slots  32  are preferably radial in orientation, such that each slot  32  extends directly toward the geometric center of the wafer recess  24 , but it will be apparent that the orientation or angle of the slots  32  may vary considerably while still providing a path between the circular groove  28  and the interior area of the wafer recess  24 , and that such arrangements are within the scope of the present invention. The depth of each slot is preferably at least 0.001 in., and preferably in the range of 0.004-0.008 in., though other depths may be feasible. The fixed length of the slots  32 , along with their width and number, are dependent on the diameter and thickness of the wafer  30  in order to effectively lift the wafer  30  during loading and unloading utilizing the Bernoulli principle.  
         [0043]     It is important that the distance that the radial slots  32  extend underneath the wafer  30  (referred to herein as the amount of “slot overlap” with the substrate wafer  30 ) encompass both CTE-related growth and the possible off-center loading of the substrate wafer  30  in the substrate wafer recess  24 . The amount of slot overlap must also be controlled so as not to be so great that it affects temperature uniformity of the substrate wafer  30  or contributes to backside markings on the substrate wafer  30 . It is known that extending the slots  32  all the way to the actual center of the interior area of the wafer recess  24  is problematic. However, it is further believed that successful results may be achieved with slots  32  of an inch in length, at least with wafer holders  16  designed to accommodate 6-in. diameter wafers  30 . On the other hand, the overlap must be of a minimum length so as to provide sufficient gas flow to facilitate substrate wafer loading and unloading operations using Bernoulli principle. Although neither the minimum or maximum lengths are known, it is known that slots  32  of 0.040-0.060 in. have been used successfully as described herein below.  
         [0044]     It is also important to control the number of slots  32  in that, based on a given slot area, a minimum number of slots  32  is required in order to properly engage a substrate wafer  30  during loading and unloading operations. The minimum number is primarily dependent on the surface area of the slots  32  relative to the surface area of the wafer recess  24 , since this results in a corresponding lift force. Thus, the number of slots  32  may depend on the width and length of the slots  32  used, as well as the size of the wafer recess  24 . Wide slots  32 , however, are to be avoided as they will lead to localized thermal effects, and the length of the slots  32  should be controlled as described previously. Exact limits on the number of slots  32  and their width are unknown, but it is known that 96 slots  32 , each 0.019 in. wide, have been used successfully in wafer holders  16  designed to accommodate 6-in. diameter wafers  30  as described herein below.  
         [0045]     It is also believed that the slots  32  must be arranged in a generally uniform pattern around the perimeter of the wafer recess  24  in order to maintain temperature uniformity and the like. Thus, the spacing between slots  32  is preferably constant around the entire perimeter, and the length, width and depth of each slot  32  is preferably constant. However, it may be possible to use uniform patterns of slots  32  having a uniformly-varying pattern of lengths, widths, or spacings without departing from the scope of the present invention.  
         [0046]     The bottom surface  22  of the holder body  18  is of conventional design. A circular support groove  34  may be disposed in the bottom surface  22  such that the wafer holder  16  may be placed or mounted on an appropriate structure in the support mechanism  14 , such as the spider-type pedestal, typically composed of quartz, shown in U.S. Pat. No. 5,427,620.  
         [0047]     The holder body  18  may or may not be covered with a surface coating  36  composed of one of the following: silicon carbide, silicon nitride, pyrolytic graphite, pyrolytic carbon, diamond, aluminum nitride, aluminum oxide, silicon, silicon dioxide or tantalum carbide. Conventionally, the coating  36  is applied to the entire substrate, including but not limited to the top and bottom surfaces  20 ,  22 .  
         [0048]     The slight depression forming the wafer recess  24  may have a slightly concave profile, perhaps best seen in  FIG. 4C , that may be created by machining the top surface  20  of the holder body  18 . The depression further aids in releasing the substrate wafer  30  from the holder  16  by allowing gases to flow freely beneath the wafer  30  and preventing the wafer  30  from sticking to an otherwise flat surface. Concave surface profiles are desired for improved wafer temperature uniformity due to the tendency of wafers  30  to sag at high temperatures. The deformity caused by the sag will be a function of substrate wafer diameter as well as the end-user&#39;s process conditions. Moreover, because temperature uniformity may be optimized when the gap between the sagging wafer  30  and the top surface of the holder body  18  is relatively uniform, the profile of the wafer recess  24  preferably matches or approximates that of the sagging wafer  30 . Thus, because the profile of a wafer  30  when sagging is assumed to be roughly spherical, the concavity of the wafer recess  24  may be spherical as well. However, it will be apparent that closer analysis of the profile of a wafer  30  when sagging will likely reveal non-spherical characteristics, and thus the profile of the wafer recess  24  may be varied accordingly.  
         [0049]     In general, larger diameter wafers  30  and lower end-use process temperature conditions favor the use of profiles that have a larger dished shape. Reasons for this include the fact that larger wafers  30  are heavier so the need to minimize adhesion between the wafer  30  and the mating face of the substrate holder  16  is more critical. In addition, larger wafers  30  will sag more at temperature than smaller wafers  30 , so a larger dished profile may help achieve improved temperature uniformity. The concavity of conventional wafer recesses used in wafer processing may range from 0.002 in. to 0.010 in., with greater concavity required for particularly sensitive processing.  
         [0050]     Six susceptor-type wafer holders designed and manufactured according to the principles described herein were tested under standard process conditions. The wafer holder included a wafer recess sized to accommodate wafers having a diameter of 6 in. Each wafer holder included 96 radial slots, each 0.007 in. deep and 0.019 in. wide and 0.040 in. long. The wafer recess of each wafer holder was 0.017 in. deep and concave in profile by 0.002 in., and the continuous groove of each wafer holder was 0.035 in. wide and 0.035 in. deep. The six test parts worked successfully (successful release of a semiconductor wafer from the wafer holder using Bernoulli principle-based pickup) in up to 10,000 runs, which compares very favorably to the typical lifetime experienced in the industry, for conventional wafer holders such as the one illustrated in  FIGS. 1A-1C , of approximately 2,000 to 4,000 runs.  
         [0051]     In view of such success, a susceptor-type wafer holder designed and manufactured according to the principles described herein was used in a silicon epitaxy deposition process. The wafer holder included a wafer recess sized to accommodate wafers having a diameter of 6 in. The wafer holder included 96 radial slots, each 0.060 in. long, 0.005 in. deep and 0.019 in. wide. The wafer recess was 0.017 in. deep and concave in profile by 0.0025 in. The continuous groove was 0.035 in. wide and 0.035 in. deep. The wafer holder was used successfully in 178 runs before use was discontinued for an unrelated issue. General performance was equal to that of conventional wafer holders such as the wafer holder illustrated in  FIGS. 1A-1C , but significantly, all wafer coatings carried out using the test wafer holder were of high quality with no indication of the temperature uniformity problems or image transfer issues described previously.  
         [0052]     Alternative configurations of the wafer holder of the present invention will be apparent to those of ordinary skill in the art. For example,  FIG. 5  is a top view of a multiplexed wafer holder  66 , where two wafer recesses  74  are arranged side by side in a single holder body  68 , in accordance with an alternative embodiment of the present invention. Each wafer recess  74  includes a circular groove  78  and a plurality of slots  82 , similar to the groove  28  and slots  32  provided in the wafer holder  24  previously described. Other than the inclusion of a second wafer recess  74  and the second plurality of slots  82 , the wafer holder  66  of  FIG. 5  is generally similar to the first wafer holder  16 .  
         [0053]     Such a configuration may be useful in maximizing the number of wafers  30  produced in a particular reaction chamber  12 . For example, a particular reaction chamber  12  may be designed to accommodate the manufacture of 8-in. wafers  30 , but a manufacturer sometimes wishes to use the reaction chamber  12  in the manufacture of wafers  30  that are only 4 in. in diameter. Manufacture of either wafer size may be accomplished through the use of an appropriately-sized wafer holder  16  of the type described herein. However, the manufacture of 4-in. wafers  30  in the 8-in. reaction chamber  12  may be optimized by using a wafer holder  66  such as the one shown schematically in  FIG. 5 . This approach may further be utilized to design wafer holders (not shown) having other numbers, sizes and arrangements of wafer recesses as will be readily apparent to those of ordinary skill in the art.  
         [0054]     The teachings of the present invention may be applied to other types of wafer holders as well. For example,  FIG. 6A  is a perspective view of a prior art stepped-type wafer holder  116 , and  FIG. 6B  is a partial side cross-sectional view of the wafer holder  116  of  FIG. 6A , taken along line  6 B- 6 B. The stepped-type wafer holder  116  includes a circumferential ledge  119  around the perimeter of a wafer recess  124  for supporting the outer edges of a wafer  30 . During processing, the wafer  30  conventionally sags into the wafer recess  124  in similar manner to that of the wafer holder  16  of the present invention. Typically, stepped-type wafer holders  116  use lift pins, disposed underneath the wafer  30  and arranged to project through openings  125  in the wafer recess  124 , to raise the wafer  30  off the top of the holder  116  from underneath, rather than using a Bernoulli-type pickup to raise the wafer  30  from above. Although not specifically illustrated, the principles of the present invention may likewise be applied to stepped-type wafer holders  116  by applying slots (not shown) to the ledge  119 . This would permit a stepped-type wafer holder  116  to use a conventional Bernoulli-type pickup during unloading operations.  
         [0055]     Based on the foregoing information, it is readily understood by those persons skilled in the art that the present invention is susceptible of broad utility and application. Many embodiments and adaptations of the present invention other than those specifically described herein, as well as many variations, modifications, and equivalent arrangements, will be apparent from or reasonably suggested by the present invention and the foregoing descriptions thereof, without departing from the substance or scope of the present invention. Accordingly, while the present invention has been described herein in detail in relation to its preferred embodiment, it is to be understood that this disclosure is only illustrative and exemplary of the present invention and is made merely for the purpose of providing a full and enabling disclosure of the invention. The foregoing disclosure is not intended to be construed to limit the present invention or otherwise exclude any such other embodiments, adaptations, variations, modifications or equivalent arrangements; the present invention being limited only by the claims appended hereto and the equivalents thereof. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for the purpose of limitation.