Abstract:
A method of processing wafers in a rotating disc CVD reactor uses wafer carrier having a unitary plate defining wafer-holding features such as pockets on its upstream surface. The carrier connects to the spindle of the reactor during processing. After processing the carrier and wafers in the reactor, the wafers are removed from the carrier. The carrier is renewed by removing the hub from the plate, cleaning the plate and then reassembling the plate with the same or a different hub.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 12/001,761, filed on Dec. 12, 2007, the disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to chemical vapor deposition apparatus 
     Certain materials such as compound semiconductors are formed by exposing a surface of a workpiece, most commonly a disc-like wafer, to gases at elevated temperatures so that the gases react and deposit the desired material on the surface of the workpiece. For example, numerous layers of III-V semiconductors such as gallium nitride, indium nitride, gallium arsenide, indium phosphide and gallium antimonide and the like can be deposited onto a substrate to form electronic devices such as diodes and transistors and optoelectronic devices such as light-emitting diodes and semiconductor lasers. II-VI semiconductors can be deposited by similar processes. The properties of the finished device are profoundly influenced by minor variations in properties of the various layers deposited during the process. Therefore, considerable effort has been devoted in the art development of reactors and processing methods which can achieve uniform deposition over a large wafer surface or over the surfaces of numerous smaller wafers held in the reactor. 
     One form of reactor which has been widely used in the industry is the rotating disc reactor. Such a reactor typically includes a disc-like wafer carrier. The wafer carrier has pockets or other features arranged to hold one or more wafers to be treated. The carrier, with the wafers thereon, is placed into a reaction chamber and held with the wafer-bearing surface of the carrier facing in an upstream direction. The carrier is rotated, typically at rotational velocities of several hundred revolutions per minute, about an axis extending in the upstream to downstream direction. Reactive gases are directed in the downstream direction towards the wafers on the carrier from an injector head positioned at the upstream and the reactor. The wafer carrier is maintained at a desired elevated temperature, most commonly about 350° C. to about 1600° C. during this process. The rotation of the wafer carrier helps to assure that all areas of the exposed wafers are exposed to substantially uniform conditions and that helps to assure uniform deposition of the desired semiconductor material. After the wafers on a particular wafer carrier have been treated, the wafer carrier is removed from the reaction chamber and replaced by a new wafer carrier bearing new wafers and the process is repeated with the new wafer carrier. 
     Many rotating disc reactor designs incorporate a spindle with a disc-like metallic element, referred to as a “susceptor” permanently mounted on the spindle. The wafer carrier to be treated is disposed on the susceptor and held by the susceptor during the treatment process. Heating elements such as electrical resistance elements disposed downstream of the susceptor heat the susceptor and the wafer carrier during the process. More recently, as disclosed in U.S. Pat. No. 6,685,774, the disclosure of which is incorporated by reference herein, “susceptorless” reactors have been developed. In a susceptorless reactor, the wafer carrier is mounted directly onto the spindle of the reactor when the wafer carrier is placed into the reactor chamber for treatment. The surface of the wafer carrier facing downstream is directly exposed to the heating elements. The susceptorless reactor design provides significantly improved heat transfer from the heating elements of the reactor to the wafer carrier and significantly improved uniformity of heat transfer to all areas of the wafer carrier. 
     A wafer carrier for a susceptorless reactor must incorporate features which allow the wafer carrier to mechanically engage the spindle when the wafer carrier is placed into the reaction chamber. Such engagement must be provided without damaging the spindle. Moreover, the wafer carrier must be formed from materials which retain substantial strength and rigidity at the elevated temperatures employed and which do not react with the gases employed in the process. Although satisfactory wafer carriers for susceptorless reactors can be formed from materials such as silicon carbide-coated ceramic materials, still further improvement would be desirable. 
     SUMMARY OF TEE INVENTION 
     One aspect of the invention provides a wafer carrier for a CVD reactor. The wafer carrier desirably includes a plate of a non-metallic refractory material, preferably a ceramic material such as silicon carbide. The plate has oppositely-facing upstream and downstream surfaces, and has a central region and a peripheral region. The plate has wafer-holding features adapted to hold a plurality of wafers exposed at the upstream surface of the plate in the peripheral region. The wafer carrier according to this aspect of the invention desirably also includes a hub attached to the plate in the central region, the hub having a spindle connection adapted to engage a spindle of a CVD reactor so as to mechanically connect the plate with the spindle. The hub may be formed at least in part from one or more materials other than the material of the plate. For example, the hub may include metallic elements and may also include an insert formed from a relatively soft material such as graphite defining the spindle connection. In operation, the hub mechanically connects the plate to the spindle without imposing potentially damaging concentrated loads on the plate. Desirably, the hub is removably attached to the plate. 
     A further aspect of the invention provides a chemical vapor deposition reactor incorporating a wafer carrier as discussed above, together with additional elements such as a reaction chamber, a spindle mounted within the reaction chamber for rotation about an axis extending generally in the upstream to downstream direction, an injector head for introducing one or more reaction gases into the reaction chamber, and one or more heating elements surrounding the spindle. The spindle connection of the wafer carrier is adapted to mount the wafer carrier on the spindle with the upstream surface of the plate facing toward the injector head and with the downstream surface of the plate facing toward the one or more heating elements. Preferably, when the wafer carrier is mounted on the spindle, the downstream surface of the plate in the peripheral region of the plate directly confronts the heating elements. Stated another way, the hub preferably does not extend between the peripheral region of the plate downstream surface and the heating elements. Thus, the hub does not interfere with radiant heat transfer between the heating elements and the plate. 
     Yet another aspect of the invention provides methods of treating wafers. A method according to this aspect of the invention desirably includes the steps of processing a plurality of wafer carriers, each including a hub and a plate removably attached to the hub, by engaging the hub of each wafer carrier with a spindle of a processing apparatus and rotating the spindle and wafer carrier while treating wafers carried on the plate, and removing wafers from each wafer carrier after that wafer carrier has been processed. The treatment preferably includes a chemical vapor deposition process. These steps desirably are repeated using new wafers. The method according to this aspect of the invention most desirably includes the further step of renewing each wafer carrier by removing the hub from the plate, then cleaning the plate, and then reassembling the plate with the same or a different hub. The step of cleaning the plate may include etching the plate. Because the hub is removed from the plate before cleaning, the steps used to clean the plate may include treatments which would attack the hub. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagrammatic view of a reactor and associated wafer carrier in accordance with one embodiment of the invention. 
         FIG. 2  is a view similar to  FIG. 1  depicting the system in a different operating state. 
         FIG. 3  is diagrammatic top plan view depicting the wafer carrier used in the system of  FIGS. 1 and 2 . 
         FIG. 4  is a fragmentary sectional view taken along line  4 - 4  in  FIG. 3 . 
         FIG. 5  is a fragmentary partially sectional view depicting portions of a wafer carrier in accordance with a further embodiment of the invention. 
         FIG. 6  is a view similar to  FIG. 4  but depicting portions of a wafer carrier in accordance with yet another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     A susceptorless reactor system according to one embodiment of the invention incorporates a reaction chamber  10 . Chamber  10  has a gas injector head  12  at its upstream end and an exhaust connection  14  open to the interior of the chamber adjacent its downstream end. Reaction chamber  10  is equipped with a spindle  16  having its axis  18  extending generally in the upstream to downstream direction of the chamber. Spindle  16  is connected to a motor drive  20  for rotating the spindle about axis  18 . The spindle is equipped with a suitable vacuum seal (not shown). A heating device  22  is mounted within chamber  10  in a fixed position so that the heating device surrounds spindle  16  adjacent its upstream end. By way of example, heating device  22  may include one or more electrical resistance heaters, one or more elements suitable for receiving RF energy and converting the same to heat or essentially any other device capable of evolving heat without contaminating the interior of chamber  10 . 
     The interior of chamber  10  is connected to the interior of a preload chamber  24  by a loading lock  26 . Lock  26  is equipped with a gas-tight shutter which can be selectively opened to permit communication between chambers  10  and  24  and closed to block such communication. The preload chamber  24  is provided with an appropriate loading door (not shown) so that wafer carriers can be placed into the preload chamber and removed therefrom. Also, the preload chamber  24  is connected to an atmospheric control system (not shown) so that an atmosphere corresponding to the atmosphere within chamber  10  can be provided within chamber  24 . Chambers  10  and  24  are provided with an appropriate robotic handling device (not shown) for moving wafer carriers between the chambers and for placing wafer carriers onto spindle  16  and removing the wafer carriers from the spindle. 
     The system further includes one or more wafer carriers  30 . As discussed in greater detail below, each wafer carrier includes unitary plate or body  32  defining an upstream surface  34  and an oppositely facing downstream surface  36 . The upstream surface  34  is provided with features such as pockets  38  arranged to hold wafers so that the surfaces of the wafers face generally upstream. Each wafer carrier also includes a hub  40  exposed adjacent the center of body  32 , the hub  40  being adapted to mate with the upstream end of spindle  16 . In the loading position depicted in  FIG. 1 , a wafer carrier  34  with wafers in pockets  38  is disposed within chamber  24 . In the operative, deposition position depicted in  FIG. 2 , the same wafer carrier  30  is disposed within reaction chamber  10  and is engaged on spindle  16 . While the wafer carrier is in the active or deposition position depicted in  FIG. 2 , the body  32  of the wafer carrier overlies heating elements  22 . In this condition, the heating elements are operated to heat the wafer carrier to the desired elevated temperature. Spindle  16  is rotated so as to thereby rotate the wafer carrier and the wafers thereon about axis  18 . Reactive gasses pass downstream from injector head  12  and pass over the upstream facing surface of the wafer carrier and over the surfaces of the wafers disposed in the pockets of the wafer carrier. The gasses react at the surfaces of the wafers, thereby forming the desired material on the surfaces of the wafers. Merely by way of example, in a deposition process for forming a III-V semiconductor, the reactive gasses may include first and second gasses. The first gas may include one or more organometallic compounds, most typically metal alkyls selected from the group consisting of gallium, indium and aluminum alkyls, in admixture with a carrier gas such as nitrogen or hydrogen. The second gas may include one or more hydrides of a group V element, such as ammonia or arsine, and may also include one or more carrier gasses. Following deposition, the wafer carrier with the finished wafers is returned to preload chamber  24  and a different wafer carrier with new wafers is placed onto the spindle  16 . The features of the deposition apparatus apart from the wafer carrier and the related mating features of the spindle may be generally similar to those disclosed in the aforementioned U.S. Pat. No. 6,685,774, the disclosure of which is hereby incorporated by reference herein. 
     As best seen in  FIGS. 3 and 4 , wafer carrier  30  has a central axis  42  which is coincident with the axis  18  of the spindle when the wafer carrier is mounted on the spindle. Plate  32  is a plate of one or more refractory materials, preferably one or more non-metallic refractory materials. As used in this disclosure, the term “non-metallic” material includes compounds of metals with non-metals, such as oxides, nitrides and carbides of metals, and also includes carbon and other non-metallic elements and compounds thereof. Also, as used in this disclosure, a plate “of” one or more materials should be understood as referring to a plate in which the one or more materials constitute at least the majority of the thickness of the plate over at least the majority of the area of the plate, and in which the one or more materials contribute at least a substantial portion of structural strength of the plate. Thus, unless otherwise specified, a plate of one or more non-metallic materials may include minor layers or other minor features formed from other materials. The material of the plate desirably is resistant to the temperatures and chemical environment encountered in the wafer processing operation and in operations used to clean the wafer carrier. Although the material of the plate should have substantial structural strength, it may be a brittle material with high sensitivity to localized stresses. As explained below, the structure of the wafer carrier desirably protects the plate from high localized stresses imposed by the spindle in use. Non-metallic refractory materials selected from the group consisting of silicon carbide, boron nitride, boron carbide, aluminum nitride, alumina, sapphire, quartz, graphite, and combinations thereof are preferred. Most desirably, the plate is a unitary slab of a single non-metallic refractory material. Unitary plates formed from silicon carbide are particularly preferred. In some cases, the plate may include a coating. The coating material desirably is resistant to the temperatures and chemicals encountered in use and cleaning of the wafer carrier as, for example, a coating of a metal carbide, oxide or nitride such as titanium carbide or tantalum carbide. Such a coating is particularly desirable where plate is formed from graphite. 
     Although the upstream and downstream surfaces  34  and  36  are depicted as completely planar surfaces apart from the pockets  38  in upstream surface  34 , this is not essential. The thickness of plate  32  can vary over a wide range. However, in one example, plate  32  has an outside diameter of about 300 mm and is about 8 mm thick. 
     Plate  32  has a central region  44  encompassing central axis  42  and a peripheral region surrounding the central region  44 . Although the border of central region  44  is depicted in broken lines in  FIG. 3  for illustrative purposes, there may not be a visible boundary between the central region and the peripheral region. The wafer engaging features or pockets  38  are disposed in the peripheral region of the plate  32 . Plate  32  has a central bore  46  extending through the plate from upstream surface  34  to downstream surface  36  in the central region so that the central bore encompasses the axis  42 . 
     Hub  40  most preferably is removably attached to the central region of plate  32 . Hub  40  includes an upstream hub element  48  having a generally cylindrical portion received in central bore  46  of plate  32  and also having a flange  50  overlying a portion of the upstream surface  34  of the plate immediately surrounding the central bore. Hub  40  further includes a downstream hub element  52  having a generally cylindrical portion extending into central bore  46  and having a flange  54  which overlies a portion of the downstream surface  36  of plate  32  within the central region of the plate. Hub elements  48  and  52  have a slight clearance fit within central bore  46 . For example, the outside diameters of the hub elements (apart from the flanges) may be about 25 microns (0.001 inches) or so smaller than the inside diameter of central bore  46 . Hub elements  48  and  52  are held together and urged toward one another by fasteners such as screws  56 , of which only one is visible in  FIG. 4 , spaced around central axis  42 . Thus, flanges  50  and  54  are forcibly engaged with the upstream and downstream surfaces  34  and  36  of plate  32 . The hub elements may be formed from materials other than the materials of the plate. Hub elements  50  and  52  desirably are formed from metals which can survive the temperatures to be encountered in service and which will not corrode or contaminate the interior of the reaction chamber during use. For example, the hub elements may be formed from metals selected from the group consisting of molybdenum, tungsten, and rhenium, combinations of these metals and alloys of these metals. In other embodiments, the hub elements may be formed the same materials as the plate. 
     Hub  40  further includes an insert  58  defining a tapered hole with an open end facing in the downstream direction (toward the bottom of the drawing in  FIG. 4 ), the hole having an interior diameter which decreases progressively in the upstream direction. Insert  58  desirably is formed from a material which can withstand the temperatures attained during service, but which is somewhat softer than the materials used to form the hub elements  48  and  52 . For example, insert  58  may be formed from graphite. Insert  58  is retained within hub elements  48  and  52  by an insert retainer plate  62  which in turn is fastened to the downstream hub element  52  by one or more screws. 
     In the operative, deposition position depicted in  FIGS. 2 and 4 , the wafer carrier  30  is mounted on spindle  16 . Spindle  16  has a tapered end  66 , and this tapered end is received within the tapered hole  60  of the insert. In the particular embodiment illustrated, the included angle of tapered end  66  is slightly less than the included angle of tapered hole  60  in the insert, so that the spindle engages insert  58  only at the extreme upstream end of the spindle and there is a slight clearance fit around tapered end  66  near the downstream or opened end of hole  60 . In the operative position, the downstream surface  36  of plate  32  confronts the heating elements  22  of the reaction chamber. Because the hub  40 , and particularly the downstream hub element  54  is disposed only within the central region of the plate  32 , the downstream surface  36  of plate  32  within the peripheral region is not covered by the hub. Thus, as seen in  FIG. 4 , the downstream surface  36  plate in the peripheral region directly confronts the heating elements  22 , with no solid structures intervening between the downstream surface  36  of the plate peripheral region and the heating elements  22 . Thus, there is a direct path for radiant heat transfer from the heating elements to the peripheral region of the plate. This promotes efficient heat transfer between heating elements  22  and plate  32 . Stated another way, the hub  40  does not extend between the heating elements and the downstream surface of the plate in the peripheral regions and does not interfere with heat transfer from the heating elements to the plate. Use of a hub tends to retard heat transfer from the plate to spindle  16 . Thus, as best seen in  FIG. 4 , there are physical interfaces between the plate  32  and the hub elements  48  and  52 , an additional interface between the hub elements and insert  58 , and yet a further interface between the insert  58  and spindle  16 . All of these interfaces have the desirable effect of reducing heat transfer from the plate to the spindle. 
     The use of a solid plate such as a solid plate of a non-metallic refractory material such as silicon carbide or other materials having high thermal conductivity provides significant advantages. The solid plate tends to promote temperature uniformity. A solid silicon carbide plate can be fabricated with a well-controlled surface morphology. Also, a solid silicon carbide plate is durable and can be subjected to cleaning processes such as wet etching to remove materials deposited on the plate during wafer processing. The hub may be detached from the plate prior to any such cleaning processes. Typically, the apparatus includes numerous wafer carriers, so that some wafer carriers are available for treating wafers while others are being cleaned. Depending on process conditions, the cleaning process can be performed after each use of the wafer carrier to treat a batch of wafers, or can be performed less frequently. Also, after cleaning, the plate may be reassembled with the same hub or with another similar hub to provide a renewed wafer carrier. 
     The hub provides a secure mounting for the plate on the spindle of the reaction chamber. Because the spindle does not directly engage the plate, the spindle does not tend to crack the plate during use. The relatively soft material of insert  58  assures that the spindle of the reaction chamber will not be damaged when the wafer carrier is engaged with the spindle. Although insert  58  may become worn with repeated use of the wafer carrier, the insert  58  can be readily removed and replaced. 
     Numerous variations and combinations of the features discussed above may be employed. For example, as seen in  FIG. 5 , a hub element  152  which extends within the central bore  146  of the plate may be provided with a polygonal exterior surface  153  so as to provide relatively large clearances  155  between the hub element and the surface of central bore  146  except at the corners of the polygonal element. This arrangement further reduces conductive heat transfer from plate  132  to the hub element  152 . Other shapes such as fluted or splined shapes may be used to provide a similar reduction in conductive heat transfer. Likewise, the surfaces of flanges  50  and  54  ( FIG. 4 ) which are in contact with the surfaces of the plate may be ridged or fluted so as to reduce conductive heat transfer between the plate and the hub and thus reduce conductive heat transfer to the spindle.&#39; 
     It is not essential to provide a central bore in the plate. Thus, as shown in  FIG. 6 , a plate  232  is provided with a set of small bores  233  extending between its upstream and downstream surfaces in the central region. An upstream hub element  248  and downstream hub element  252  are provided on the upstream and downstream of plate  232  and connected to one another by bolts  256  extending through holes  233 . In this arrangement as well, the hub is removably attached to the plate. As used in this disclosure with reference to a plate and hub, the term “removably attached” means that the hub can be removed from the plate without damaging the plate and without damaging the major structural elements of the hub. Removable attachments other than bolted attachments can be used. For example, the removable attachment may include pins, wedges, clips or other mechanical fastening arrangements. Also, the connection between the hub and the spindle may not incorporate a tapered fitting as discussed above with reference to  FIG. 4 . Thus, in the embodiment of  FIG. 6 , the hub has an insert  258  with a set of recesses that engage mating pins  266  on the end of the spindle  106 . Any other type of mechanical connection between the hub and the spindle can be employed. 
     In the embodiment discussed above with reference to  FIGS. 1-4 , the upstream hub element has a low, flat profile. However, as seen in  FIG. 6 , the upstream hub element  248  may have a domed shape so as to facilitate gas flow in the vicinity of the central axis  242  In yet another embodiment, one or both of the hub elements may directly engage the spindle without an intervening insert. As these and other variations and combinations of the features discussed above can be utilized without departing from the present invention, the foregoing description of the preferred embodiment should be taken by way of illustration rather than by way of limitation of the invention as defined by the claims. 
     Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.