Abstract:
Wafer carrier arranged to hold a plurality wafers and to inject a fill gas into gaps between the wafers and the wafer carrier for enhanced heat transfer and to promote uniform temperature of the wafers. The apparatus is arranged to vary the composition, flow rate, or both of the fill gas so as to counteract undesired patterns of temperature non-uniformity of the wafers. In various embodiments, the wafer carrier utilizes at least one plenum structure contained within the wafer carrier to source a plurality of weep holes for passing a fill gas into the wafer retention pockets of the wafer carrier. The plenum(s) promote the uniformity of the flow, thus providing efficient heat transfer and enhanced uniformity of wafer temperatures.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates generally to semiconductor fabrication technology and, more particularly, to chemical vapor deposition (CVD) processing and associated apparatus for reducing temperature non-uniformities on semiconductor wafer surfaces. 
       BACKGROUND OF THE INVENTION 
       [0002]    Many semiconductor devices are formed by processes performed on a substrate. For example, in the fabrication of light-emitting diodes (LEDs) and other high-performance devices such as laser diodes, optical detectors, and field effect transistors, a chemical vapor deposition (CVD) process is typically used to grow a thin film stack structure using materials such as gallium nitride over a sapphire or silicon substrate. The substrate typically is slab of a crystalline material, commonly referred to as a “wafer,” typically in the form of a disc. 
         [0003]    One common process is epitaxial growth. For example, devices formed from compound semiconductors such as III-V semiconductors typically are formed by growing successive layers of the compound semiconductor using metal organic chemical vapor deposition or “MOCVD.” In this process, the wafers are exposed to a combination of gases, typically including a metal organic compound as a source of a group III metal, and also including a source of a group V element which flow over the surface of the wafer while the wafer is maintained at an elevated temperature. Typically, the metal organic compound and group V source are combined with a carrier gas which does not participate appreciably in the reaction as, for example, nitrogen. One example of a III-V semiconductor is gallium nitride, which can be formed by reaction of an organo gallium compound and ammonia on a substrate having a suitable crystal lattice spacing, as for example, a sapphire wafer. Typically, the wafer is maintained at a temperature on the order of 1000-1100° C. during deposition of gallium nitride and related compounds. 
         [0004]    Composite devices can be fabricated by depositing numerous layers in succession on the surface of the wafer under slightly different reaction conditions, as for example, additions of other group III or group V elements to vary the crystal structure and band gap of the semiconductor. For example, in a gallium nitride based semiconductor, indium, aluminum or both can be used in varying proportion to vary the band gap of the semiconductor. Also, p-type or n-type dopants can be added to control the conductivity of each layer. After all of the semiconductor layers have been formed and, typically, after appropriate electric contacts have been applied, the wafer is cut into individual devices. Devices such as light-emitting diodes (“LEDs”), lasers, and other optoelectronic devices can be fabricated in this way. 
         [0005]    The epitaxial growth process can be carried out in a CVD tool that includes a process or reaction chamber, which provides a sealed environment that allows infused gases to be deposited upon the substrate to grow the thin film layers. 
         [0006]    One type of CVD tool which has been widely accepted in the industry uses a device commonly referred to as a “wafer carrier.” Wafer carriers typically comprise a large disc with numerous wafer retaining regions or “pockets,” each pocket adapted to hold one wafer. The wafer retaining pockets are typically comprise a recess or cavity formed on the top surface dimensioned to receive a wafer, and are characterized as having an upwardly-facing floor and a radially inward-facing perimeter wall. 
         [0007]    Typically, the wafers are supported by protrusions that extend from the floor and the perimeter wall the wafer retaining pocket to suspend the wafer above the surface of the floor of the wafer retaining pocket. The wafer can bow downward (concave) in different process layers. Suspension above the pocket floor at a predetermined distance prevents wafer from unevenly sitting bottomed out in the pocket due to this shape, which can cause wafer toss and large temperature non-uniformities. Maintaining a certain distance between the wafer and the pocket floor also reduces the wafer temperature non-uniformity that results from an uneven wafer-pocket gap due to the bowing of the wafer. The suspension of the wafer also promotes uniformity of the temperature of the wafer by eliminating random contact points that would otherwise result between the wafer and the pocket floor. Moreover, certain wafer carriers implement pocket floors that are appropriately bowed or shaped to help with uniformity at certain process layers. If the wafer rests on the floor directly, layers that the floors were not designed for can incur large non-uniformities due to bottoming out. 
         [0008]    The wafer carrier is supported on a spindle within the reaction chamber so that the top surface of the wafer carrier with exposed surfaces of the wafers is oriented to face upwards, toward a gas distribution element of the CVD tool. While the spindle is rotated, the gas is directed downwardly onto the top surface of the wafer carrier and flows across the top surface toward the periphery of the wafer carrier. The wafer carrier is maintained at the desired elevated temperature by heating elements, typically electrical resistive heating elements disposed below the bottom surface of the wafer carrier. These heating elements are maintained at a temperature above the desired temperature of the wafer surfaces, whereas the gas distribution element typically is maintained at a temperature well below the desired reaction temperature so as to prevent premature reaction of the gases. Therefore, heat is transferred from the heating elements to the bottom surface of the wafer carrier and flows upwardly through the wafer carrier to the individual wafers. 
         [0009]    The importance of maintaining uniform conditions at all points on the top surfaces of the various wafers during the CVD process has long been recognized. Minor variations in composition of the reactive gases and in the temperature of the wafer surfaces cause undesired variations in the properties of the resulting semiconductor device. For example, if a gallium and indium nitride layer is deposited, variations in wafer surface temperature will cause variations in the composition and band gap of the deposited layer. Because indium has a relatively high vapor pressure, the deposited layer will have a lower proportion of indium and a greater band gap in those regions of the wafer where the surface temperature is higher. If the deposited layer is an active, light-emitting layer of an LED structure, the emission wavelength of the LEDs formed from the wafer will also vary. Thus, considerable effort has been devoted in the art heretofore towards maintaining uniform conditions. 
         [0010]    While considerable effort has been devoted to design an optimization of such systems, further improvement would be desirable. In particular, it is desirable to provide better uniformity of temperature across the surface of each wafer. 
       SUMMARY OF THE INVENTION 
       [0011]    Various embodiments of the invention provide a uniform distribution of inert, high conductivity gas, referred to herein as a “fill gas,” to a plurality of wafer pockets in a wafer carrier. The uniform distribution of fill gas to the pockets promotes the uniform diffusion of heat transferred between the pocket floors and the wafers to provide efficient heat transfer and enhanced uniformity of temperatures, both across the surface of a given wafer as well as between the plurality of wafers. The fill gas delivered in various embodiments of the invention can have a high conductivity relative to the process gases that would otherwise occupy the interstitial space between the pocket floor and the wafer. The higher conductivity fill gas enhances the heat transfer therebewteen and reduces sensitivity to variations in the thermal conductive coupling. 
         [0012]    United States Patent Application Publication No. 2011/0206843 to Gurary et al. (Gurary), assigned to the owner of the instant application, discloses a system that delivers fill gas to a plurality of wafer retaining pockets of a wafer carrier. Each wafer retaining pocket is in fluid communication with a respective conduit that is sourced through the spindle of the CVD tool, and terminates at the face of the pocket floor to define a single exit aperture, preferably at the center of the wafer retaining pocket. It has been found that such an arrangement can cause localized heating or cooling of the wafer at locations immediately above the exit aperture. Localized heating or cooling adversely affects the temperature profile of exposed face of the wafer, and can also cause the wafer to bow slightly due to the presence of thermal gradients. 
         [0013]    Various embodiments of the present invention mitigate the effect of localized heating or cooling by delivering the fill gas through a plurality of weep orifices that pass through the floor of the retainer pocket. The “weep orifices” are so named because the flow therethrough is slow enough so as to avoid or reduce convection heat transfer between the wafer and the fill gas. In one embodiment, the weep orifices are distributed in a uniform, matrixical arrangement to promote uniform distribution of the fill gas within the wafer retaining pocket. In order to source the plurality of weep orifices among the plurality of wafer retaining pockets, various plenum configurations and distribution manifold arrangements are implemented to prevent those weep orifices that are closer to the source of the fill gas from starving those weep orifices which are located further from the source of the fill gas. 
         [0014]    Structurally, various embodiments of the invention include a wafer carrier for use in a system for growing epitaxial layers on a plurality of wafers by chemical vapor deposition. The wafer carrier can include a body portion formed symmetrically about a central axis, the body portion including generally planar top and bottom surfaces that are substantially perpendicular to the central axis. A plurality of wafer retention pockets can be recessed relative to the top surface of the body portion, each of the wafer retention pockets including a floor portion having an upper surface generally parallel to the top surface of the body portion. A center receptacle can be recessed relative to the bottom surface of the body portion, the center receptacle being concentric with the central axis. In one embodiment, a plurality of flow passages extending radially outward from and are in fluid communication with the center receptacle. At least one plenum chamber can be defined within the body portion, the at least one plenum chamber being in fluid communication with at least one of the plurality of flow passages. In certain embodiments, a plurality of manifold passages are in fluid communication with the at least one plenum chamber, the manifold passages having an orientation that is substantially parallel with the top surface of the body portion of the wafer carrier and extending underneath the surface of the floor portion of one of the plurality of wafer retention pockets. A plurality of weep orifices are formed in the floor portion of the one of the plurality of wafer retention pockets, each of the weep orifices being in fluid communication with one of the plurality of manifold passages and passing through the upper surface of the floor portion of the wafer retention pocket. The weep orifices can be oriented substantially parallel with the central axis. The plurality of manifold passages can be parallel to each other. 
         [0015]    In one embodiment, the plenum chamber or chambers can be characterized as having a first hydraulic diameter and each of the manifold passages have a second hydraulic diameter, the first hydraulic diameter being about 2 to about 100 times the second hydraulic diameter. The manifold passages can have a second hydraulic diameter and each of the weep orifices have a third hydraulic diameter, the second hydraulic diameter being the same to about 50 times the third hydraulic diameter. Each of the wafer retention pockets can be at least partially surrounded by a thermal isolation slot, the thermal isolation slot being defined in and recessed from the bottom surface of the wafer carrier. 
         [0016]    The at least one plenum chamber can be a single plenum chamber is centrally located within the body portion and symmetric about the central axis, wherein the single plenum chamber is defined by a plenum cavity formed in a central region of the body portion and a closure portion disposed within the plenum cavity, the plenum cavity being in fluid communication with the plurality of manifold passages. In one embodiment, the cavity is recessed from the bottom surface of the body portion. In another embodiment, the single plenum chamber is defined by a two-piece plenum structure, the two-piece plenum structure being disposed within the body portion of the wafer carrier. 
         [0017]    Alternatively, the wafer carrier can include a plurality of distributed plenum chambers, each being associated with a respective one of the wafer carrier pockets. Each plenum chamber can surround the wafer pocket, the plenum chamber providing thermal isolation of the wafer pocket from a body portion of the wafer carrier. 
         [0018]    In another embodiment of the invention, a method for controlling the temperature distribution in a wafer carrier includes providing a wafer carrier that defines a central axis and includes a wafer pocket, the wafer pocket being in fluid communication with a plurality of weep orifices, each of the plurality of weep orifices being in fluid communication with a plenum chamber, the plenum chamber being contained within the wafer carrier and in fluid communication with a center receptacle, the center receptacle being concentric about the central axis; and charging the plenum chamber with a fill gas via the center receptacle, thereby causing the fill gas to enter the wafer pocket via the plurality of weep orifices. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  is a schematic of a CVD apparatus in an embodiment of the invention; 
           [0020]      FIG. 2  is a partial sectional view of a spindle fitted to a wafer carrier in an embodiment of the invention; 
           [0021]      FIG. 3  is an exploded isometric view of the top of a two-piece wafer carrier in an embodiment of the invention; 
           [0022]      FIG. 4  is an isometric view of a closure portion of the two-piece wafer carrier of  FIG. 3  in isolation; 
           [0023]      FIG. 5  is an exploded isometric view of the bottom of the two-piece wafer carrier of  FIG. 3 ; 
           [0024]      FIG. 6  is an isometric sectional view of the two-piece wafer carrier of  FIG. 3 ; 
           [0025]      FIG. 7  is an enlarged partial sectional view of the two-piece wafer carrier of  FIG. 7 ; 
           [0026]      FIG. 8  is a plan sectional view of the wafer carrier of  FIG. 3 , the cross-section being parallel to the upper surface of the body and at the axial location of the manifold passages; 
           [0027]      FIG. 9  is an enlarged, partial view of the plan sectional view of  FIG. 8 ; 
           [0028]      FIG. 10  is a plan sectional view of the wafer carrier of  FIG. 3 , the cross-section being parallel to the upper surface of the body and at an axial location between the manifold passages and the upper surface of the floor portions; 
           [0029]      FIG. 11  is an isometric exploded view of a two-piece wafer carrier having interior and exterior pockets in an embodiment of the invention; 
           [0030]      FIG. 12  is a plan sectional view of the wafer carrier of  FIG. 11  at the axial location of the manifold passages; 
           [0031]      FIG. 13  is an exploded isometric view from an upper perspective of a three-piece wafer carrier in an embodiment of the invention; 
           [0032]      FIG. 14  is an exploded isometric view from a bottom perspective of the three-piece wafer carrier of  FIG. 13 ; 
           [0033]      FIG. 15  is a partially exploded view from an upper perspective of a multiple plenum wafer carrier in an embodiment of the invention; 
           [0034]      FIG. 16  is an isometric view of a wafer retention pocket of the multiple plenum wafer carrier of  FIG. 15 ; 
           [0035]      FIGS. 16A through 16C  are isometric views of alternative wafer retention pockets in embodiments of the invention; 
           [0036]      FIG. 17  is an isometric sectional view of the multiple plenum wafer carrier of  FIG. 15 ; and 
           [0037]      FIG. 18  is an enlarged partial sectional view of the multiple plenum wafer carrier of  FIG. 15 . 
       
    
    
     DETAILED DESCRIPTION 
       [0038]    Referring to  FIGS. 1 and 2 , a schematic of a chemical vapor deposition (CVD) apparatus  11  is depicted an embodiment of the invention. The CVD apparatus  11  includes a reaction chamber  10  having a gas distribution element  12  arranged at a top end  13  of the reaction chamber  10 . In certain embodiments, the gas distribution element  12  is connected to sources  14   a ,  14   b ,  14   c  for supplying process gases to be used in the wafer treatment process, such as a carrier gas and reactant gases such as a metalorganic compound and a source of a group V metal. The gas distribution element  12  is arranged to receive the various gases and direct a flow of process gasses generally in the downward direction. The gas distribution element  12  can also be connected to a coolant system  16  arranged to circulate a liquid through the gas distribution element  12  so as to maintain the temperature of the element at a desired temperature during operation. A similar coolant arrangement (not depicted) can be provided for cooling walls  17  of reaction chamber  10 . The reaction chamber  10  can also equipped with an exhaust system  18  arranged to remove spent gases from the interior of the chamber through ports (not depicted) at or near the bottom of the chamber so as to permit continuous flow of gas in the downward direction from the gas distribution element  12 . 
         [0039]    In the depicted embodiment, a spindle  20  is arranged within the chamber so that a rotational axis  22  of the spindle  20  extends in the upward and downward directions. The spindle  20  can be mounted to the reaction chamber  10  by a conventional rotary pass-through device  25  that incorporates bearings and seals (not depicted) for rotation of the spindle  20  about the rotational axis  22 , while maintaining a seal between the spindle  20  and the reaction chamber  10 . The spindle  20  has a fitting portion  24  at a top end  23 , the fitting portion  24  being adapted to releasably engage a wafer carrier. In the depicted embodiment, the fitting portion  24  is a generally frustoconical, tapering toward the top end  23  of the spindle  20 . The spindle  20  can be connected to a rotary drive mechanism  26  such as an electric motor drive, which is arranged to rotate the spindle about axis  22 . In some embodiments, the spindle  20  has an internal gas passageway  28  terminating at an opening  30  at the top end  23  of the spindle  20  and within fitting portion  24  ( FIG. 2 ). 
         [0040]    In one embodiment, the spindle  20  has a gas entry port  38  in fluid communication with the gas passageway  28 , the gas entry port  38  being remote from the upper end of the spindle and being below the chamber  10  and the pass-through  25 . A rotary connection assembly  40  can define an annular space  42  surrounding the gas entry port  38 , so that the port  38  remains in communication with annular space  42  during rotation of the spindle  20 . The rotary connection assembly  40  can include an inlet  44  communicating with the annular space  42  and hence with the gas entry port  38  and gas passageway  28  of the spindle. The rotary connection assembly  40  can also include conventional seals  46 . Although the seals  46  are diagrammatically depicted as O-rings, the seals may be of any type available to the artisan, such as a lip seal or packing gland. 
         [0041]    In various embodiments, the inlet  44  of the rotary connection assembly  40  is connected to fill gas supply sources  48   a  and  48   b  ( FIG. 1 ), arranged to supply different gas components having different thermal conductivities. Flow control elements  50   a  and  50   b  are provided in the depicted embodiment to regulate the flow of gas from each of sources  48   a  and  48   b  independently. The flow control elements  50   a ,  50   b  can be connected to signal outputs of a control system  52 . In one embodiment, the flow control elements  50   a  and  50   b  are arranged for adjustment during operation in response to control signals applied by the control system  52 . For example, flow control elements  50   a  and  50   b  can be conventional electrically-controllable valves or mass flow controllers provided in the connections between the rotary connection assembly  40  and the sources  48   a  and  48   b . However, any other arrangement for regulation of flow from a gas source in response to a control signal can be utilized. 
         [0042]    The spindle  20  can also be provided with internal coolant passages  54  and  56  ( FIG. 2 ) extending generally in the axial direction of the spindle  20  within gas passageway  28 . The internal coolant passages  54  and  56  can be connected to a coolant source  60  via a second rotary connection assembly  58  ( FIG. 1 ). A fluid coolant can be circulated in a loop through the coolant passages  54  and  56 , originating and returning to the coolant source  60 . The coolant source  60  can include conventional devices for regulating the flow and maintaining the temperature of the circulating coolant. 
         [0043]    Various embodiments of the invention include heating elements  70  mounted within the reaction chamber  10  which surrounds the spindle  20  below the fitting portion  24 . The reaction chamber typically includes an entry opening  72  leading to an antechamber  76 , and a door  74  for closing and opening the entry opening. The door  74  is depicted schematically in  FIG. 1  as movable between the closed position (solid lines) and an open position  74 ′ (broken lines). In the closed position, the door  74  isolates the interior of the reaction chamber  10  from antechamber  76 . The door  74  can equipped with control and actuation devices for movement between the open position and closed positions. In practice, the door  74  can comprise a shutter movable in the upward and downward directions as disclosed, for example, in U.S. Pat. No. 7,276,124. The CVD apparatus  11  can further include a loading mechanism (not depicted) for moving a wafer carrier from the antechamber  76  into the reaction chamber  10 , engaging the wafer carrier with the spindle  20  in the operative condition, and for moving a wafer carrier off of the spindle  20  and into the antechamber. 
         [0044]    A wafer carrier  80  is mounted to the spindle  20  in an operative position. The wafer carrier  80  includes a body  82  which can be substantially circular about a central axis  84 . The body  82  is characterized as having a generally planar top surface  88 , a generally planar bottom surface  90 , and an outer peripheral edge  91 . The top and bottom surfaces  88  and  90  can extend generally parallel to each other and can be generally perpendicular to the central axis  84  of the body  82 . The body  82  also has a plurality of wafer retention pockets  92 , each adapted to hold a corresponding one of a plurality of wafers  94 . 
         [0045]    Various configurations of the wafer carriers  80  of the present invention are described below. 
         [0046]    Referring to  FIGS. 3-10 , a two-piece wafer carrier  80   a  comprising a body portion  82   a  and a closure portion  98  is depicted in an embodiment of the invention. The body portion  82   a  defines a plurality of wafer retention pockets  92   a  recessed from a top surface  88   a . The body portion  82   a  further comprises a plenum cavity  96  recessed from a bottom surface  90   a . The closure portion  98  is disposed within the plenum cavity  96  to define a plenum chamber  100  within the two-piece wafer carrier  80   a.    
         [0047]    Each wafer retention pocket  92   a  includes a floor portion  102  and a peripheral wall  104 . The floor portion  102  is characterized as having a thickness  106  and an upper surface  108  facing in the upward direction. A plurality of protrusions  112  extend in the upward direction from the upper surface  108  and radially inward from the peripheral wall  104 . The protrusions  112  can be configured and distributed about the periphery of the wafer retention pocket  92   a  as described in U.S. patent application Ser. No. 13/450,062, filed on Apr. 18, 2012 and owned by the owner of the present application. 
         [0048]    The floor portion  102  of a given wafer retention pocket  92   a  includes a plurality of manifold passages  114  that extend into the thickness  106  of the floor portion  102  substantially parallel to the upper surface  108 . A plurality of weep orifices  116  extend in the upward direction from each of the manifold passages  114 , passing through the upper surface  108  of the floor portion  102 , thereby establishing fluid communication between the wafer retention pocket  92   a  and the respective manifold passage  114 . 
         [0049]    Flow passageways, such as the weep orifices  116 , manifold passages  114  and plenum chambers (discussed below) can be characterized as having hydraulic diameters D H , defined as 
         [0000]        D   H =4 ·A/P   Eq. (1)
 
         [0000]    where A is the cross-sectional area of the flow passageway and P is the wetted perimeter of the flow passageway. The hydraulic diameters of the weep orifices  116  and manifold passages  114  are denoted by numerical references  118  and  120 , respectively ( FIG. 7 ). The hydraulic diameters  118  of the weep orifices  116  can be of substantially smaller dimension than the hydraulic diameters  120  of the manifold passages  114 . In a non-limiting example, the weep orifices  116  have a diameter on the order of 0.6 mm (0.025 inches), whereas the manifold passages  114  have a diameter on the order of 1.5 mm (0.06 inches). In various embodiments, the ratio of the hydraulic diameter  120  of the manifold passage  114  to the hydraulic diameter  118  of the weep orifices  116  is in the range of about 1 to about 50. 
         [0050]    The manifold passages  114  can be substantially parallel with respect to each other and spaced apart at uniform intervals. Likewise, the weep orifices  116  can be uniformly spaced along the respective manifold passage  114 . In such an arrangement, the weep orifices  116  define a matrixical arrangement on the upper surface  108  of the floor portion  102 . In the depicted embodiment, the manifold passages  114  and the weep orifices  116  of each of the manifold passages  114  are spaced at a substantially equal dimension, thereby defining a square matrixical arrangement. 
         [0051]    Alternatively, the weep orifices  116  can be in other than a matrixical arrangement or otherwise non-uniformly spaced ( FIGS. 16A through 16C ). For example, the weep orifices  116  can be arranged in a series of concentric circular patterns in between certain limiting radii on the pocket. Alternatively, they can be of different densities in different sections of the pocket. 
         [0052]    The plenum cavity  96  of the two-piece wafer carrier  80   a  is recessed into the body portion  82   a  from a bottom surface  90   a . The plenum cavity  96  is characterized as having a perimeter wall  124  and a ceiling portion  126 , the ceiling portion  126  having a lower surface  128 . The body portion  82   a  can further define a center receptacle  130  concentric with the central axis  84  and extending upward from the lower surface  128  of the ceiling portion  126 . In the depicted embodiment, the center receptacle is blind (i.e., does not pass through the top surface  88   a  of the body portion  82   a ). A plurality of threaded holes  132  ( FIG. 5 ) can be formed on the ceiling portion  126 , the threaded holes  132  being configured to receive fasteners  134  ( FIG. 3 ). In one embodiment, the threaded holes  132  are blind so as not to disturb the upper surface  88   a  of the body portion  82   a . The perimeter wall  124  defines exposed edges  136  of the floor portions  102  of the wafer retention pockets  92   a . In one embodiment, each of the manifold passages  114  of the various wafer retention pockets  92   a  are oriented to pass through the exposed edges  136  of the respective floor portion  102 , thereby establishing fluid communication between the plenum cavity  96  and the wafer retention pockets  92   a  via the manifold passages  114  and weep orifices  116 . 
         [0053]    The closure portion  98  of the two-piece wafer carrier  80   a  includes an upper face  142 , a lower face  144  and a periphery  146 . The periphery  146  is shaped to the contour of the perimeter wall  124  of the plenum cavity  96 , such that the closure portion  98 , when disposed in the plenum cavity  96 , provides a close tolerance fit with the perimeter wall  124  of the plenum cavity  96 . In one embodiment, the closure portion  98  can include a thin lip running along the periphery  146  that mates with recessed shoulders formed on the exposed edges  136  of the floor portions  102  of the wafer retention pockets  92   a . The lip and the shoulders can be dimensioned to overlap when fitted together, providing a mating fit. 
         [0054]    In the depicted embodiment, the closure portion  98  includes a hub portion  152  and a plurality of spacer portions  154 , all extending upward from the upper face  142  of the closure portion  98 . The hub portion  152  includes an exterior surface  156  and an interior surface  158  defining an aperture  162  that passes through a top face  164  of the exterior surface  156 . The exterior surface  156  can be dimensioned for a close tolerance fit with the center receptacle  130  of the body portion  82   a . In one embodiment, an outer radial face  163  of the exterior surface  156  of the hub portion  152 , when fully engaged with the fitting portion  24 , extends only partway into the length of the center receptacle  130 , thereby defining a gap  165  between the top face  164  of the hub portion  152  and the center receptacle  130 . The interior surface  158  of the hub portion  52  can be configured to mate with the fitting portion  24  of the spindle  20  (e.g., complementary to the frustoconical or other shape of the fitting portion  24 ). 
         [0055]    The spacer portions  154  are characterized as having substantially equal thicknesses  166  ( FIG. 4 ) and each defining a raised face  168 , the raised faces  168  lying substantially on a common plane. In one embodiment, the spacer portions  154  are of equal shape, and extend radially outward from and are uniformly distributed about the hub portion  152 . The distribution of the spacer portions  154  define plurality of channels  172  between the spacer portions  154 , the channels  172  extending radially outward from the hub portion  152  to a peripheral portion  175  of the closure portion  98  that extends radially beyond the spacer portions  154 . 
         [0056]    A plurality of slots  174  can be formed on the outer radial face  163  of the exterior surface  156  of the hub portion  152 , the slots  174  extending axially (i.e., parallel to the central axis  84 ) and each being adjacent a radially proximal end of a respective one of the channels  172 . The closure portion  98  can further include a plurality of through holes  178  arranged to align with the threaded holes  132  of the body portion for insertion of the fasteners  134  therethrough. 
         [0057]    In one embodiment, the floor portions  102  are partially surrounded by thermal isolation slots  182  and  184  that extend in the upward direction from the bottom surface  90   a  and the ceiling portion  126 , respectively. The thermal isolation slots  182 ,  184  extend into the body portion  82   a  but need not pass through the top surface  88   a  of the body portion, thereby leaving the top surface  88   a  structurally undisturbed. 
         [0058]    In fabrication, the body portion  82   a  of the two-piece wafer carrier  80   a  can be initially formed as a blank circular disk having the parallel top and bottom surfaces  88   a  and  90   a  and an outer peripheral edge  91   a . A plurality of reference axes  186  are designated that extend through and perpendicular to the central axis  84 , and which can be uniformly distributed about the central axis  84 . The axial location of the reference axes  186  (i.e., the location along the central axis  84  where the reference axes  186  intersect) corresponds to the axial location where the manifold passages  114  are to pass through the floor portions  102  of the wafer retention pockets  92   a . Each of the wafer retention pockets  92   a  is designated to be laterally centered about a corresponding one of the reference axes  186 . In the depicted embodiment, for each wafer retention pocket  92   a , the manifold passages  114  passing therethrough includes a designated central manifold passage  114 ′ ( FIG. 9 ) that is formed along the respective reference axis  186  with other manifold passages  114  for the corresponding wafer retention pocket  92   a  being formed parallel to and at the same axial location as the respective reference axis  186 . 
         [0059]    The manifold passages  114  can be bored from the outer peripheral edge  91   a , extending toward the center of the body portion  82   a . The length any one of the respective manifold passages  114  is typically long enough to pass through the location of where the perimeter wall  124  of the plenum cavity  96  is or will be formed. In one embodiment, each of the manifold passages  114  include an enlarged diameter portion  188  proximate the outer peripheral edge  91   a.    
         [0060]    For those manifold passages  114  that lie along an axis that passes through a thermal isolation slot  182 , the enlarged diameter portion  188  of a given manifold passage  114  extends far enough into the body portion  82   a  to entirely pass through the location where the thermal isolation slots  182  are or will be formed. Accordingly, the depth (distance into the body from the outer peripheral edge  91   a ) of the enlarged diameter portions  188  varies with the location of the respective manifold passage  114  with respect to the closest reference axis  186 . After forming the enlarged diameter portions  188 , plugs  192  can be inserted into the enlarged diameter portions  188  that extend substantially the entire length of the respective enlarged diameter portion  188 . The plugs prevent fill gas from exiting the ends of the manifold passages  114  in operation. 
         [0061]    After the manifold passages  114  have been formed and the plugs  192  inserted, the weep orifices  116  and the wafer retention pockets  92   a  are formed from the top surface  88   a  of the body portion  82   a , the weep orifices  116  extending into manifold passages  114 . The plenum cavity  96 , center receptacle  130 , threaded holes  132  and thermal isolation slots  182  and  184  are formed from the bottom surface  90   a.    
         [0062]    Other than forming the various aspects of the body portion  82   a  after formation and plugging of the manifold passages  114 , there is no particular sequence to their formation. Likewise, the features and aspects of the closure portion  98  can be formed using standard machine practices, again without particular sequence. 
         [0063]    In assembly, the closure portion  98  is aligned with and disposed within the complementary-shaped plenum cavity  96  so that the raised faces  168  of the spacer portions  154  register against the ceiling portion  126  of the plenum cavity  96 , and secured in place with the fasteners  134 . The hub portion  152  is thereby disposed within the center receptacle  130  of the body portion  82   a  to define the gap  165 , the gap  165  being in fluid communication with the gas passageway  28  of the spindle  20 . The slots  174  formed on the outer radial face  163  of the hub portion  152  cooperate with the interior surface of the center receptacle  130  and the channels  172  and the peripheral portion  175  of the upper face  142  of the closure portion  98  cooperate with the ceiling portion  126  of the plenum cavity  96  to define a plurality of flow passages  194  that extend from the gap  165  to the plenum chamber  100 . 
         [0064]    Thus, the plenum chamber  100  of the two-piece wafer carrier  80   a  is bounded by the ceiling portion  126  and perimeter wall  124  of the plenum cavity  96  and peripheral portion  175  of the upper face  142  of the closure portion  98 , with the plurality of flow passages  194  leading thereto. The plenum chamber  100  is thus continuous in the tangential direction and has a height substantially the same as the thickness  166  of the spacer portions  154 . 
         [0065]    After the closure portion  98  is secured to the body portion  82   a , the exterior surfaces of the assembled two-piece wafer carrier  80   a  can be coated with a material such as silicon carbide using a CVD process to resist the chemicals of the intended environment of operation. 
         [0066]    The treated surface exterior surface can have a thermal expansion coefficient that is different from that of the base material (e.g., SiC has a different thermal expansion coefficient than graphite). The difference in thermal expansion can cause the components to bow at temperatures that depart from the treatment temperature, particularly the peripheral portion  175  of the closure portion  98  because of the thinness of the material. Accordingly, in one embodiment, all exposed surfaces of the closure portion  98  (the upper face  142 , spacer portions  154  and exterior surface  156  of the hub portion  152 ) can be coated prior to assembly. 
         [0067]    Functionally, with both sides coated evenly, the differential expansion and contraction between the sides reduces bending due to differences in the coefficient of thermal expansion at temperatures different from the treatment temperature. The exterior surface  144  can also be masked during coating of the rest of the two-piece assembly. In this manner, after cool down following the treatment and coating of the entire assembly, the relatively thin peripheral portion of  175  of the closure portion  98  experiences the same thermal contraction on both sides, largely cancelling the bowing effect that would otherwise result from having a treated surface on only one side. 
         [0068]    In operation, the wafers  94  are loaded into the two-piece wafer carrier  80   a , and the wafer carrier  80   a  coupled to the fitting portion  24  of the spindle  20 . A fill gas  196  from one or both of the gas sources  48   a  and  48   b  is caused to flow into the gas passageway  28  via the rotary connection assembly  40 , wherefrom the fill gas  196  passes through the gap  165  and flow passages  194  and into the plenum chamber  100 . From the plenum chamber  100 , the flow of fill gas  196  pressurizes the plurality of manifold passages  114  so that the fill gas  196  flows through the plurality of weep orifices  116  to fill the voids between the upper surfaces  108  of the floor portions  102  of the wafer retention pockets  92  and the plurality of wafers  94 . The fill gas  196  exits the two-piece wafer carrier  80   a  around the perimeters of the wafers  94 . The heater elements  70  are energized and transfer heat to the bottom surface  90   a  of the two-piece wafer carrier  80   a , primarily by radiative coupling. The heat transferred to the bottom surface  90   a  of the two-piece wafer carrier  80   a  flows upwardly through the body portion  82   a , primarily by conduction through the solid portions and gases, contact conductance across the contacting surfaces (e.g., between the spacer portions  154  and the ceiling portion  126 ) radiation across the internal spaces (e.g., the plenum chamber  100  and flow passages  194 ) and by convection, conduction and advection to the fill gas  196 . Heat is radiated from the top surface  88   a  of the two-piece wafer carrier  80   a  and from the top surfaces of the wafers  94  to the lower temperature components that surround the two-piece wafer carrier  80   a  (e.g., the walls of the reaction chamber  10 ), and by advection to the process gas that flows out of the gas inlet element  12  and over the top surface  88   a  of the two-piece wafer carrier  80   a  and the top surfaces of the wafers  94 . The temperature at the top surfaces of the wafers  94  represents a balance between the heat transfer to the wafers  94  (primarily between the floor portions  102  of the wafer retention pockets  92   a ) the heat transfer away from the top surface of the wafer  94 . 
         [0069]    Functionally, the relative cross-sectional flow areas of the plenum chamber  100 , manifold passages  114  and weep orifices  116  are such that the uniformity of the flow exiting the weep orifices  116  is enhanced. That is, the effective cross-sectional flow area of the plenum chamber  100  is substantially greater than the combined cross-sectional flow area of the manifold passages  114 . Accordingly, the uniformity of a pressure distribution throughout the plenum chamber  100  is promoted throughout for better sourcing of the manifold passages  114  with fill the gas  196 . Likewise, the cross-sectional flow area of a given weep orifice  116  is substantially less than the cross-sectional flow area of the manifold passage  114 , so that manifold passage  114  does not experience a substantial pressure difference along its length; thus, the weep orifices  116  proximate the inlet to the manifold passage  114  do not starve the weep orifices  116  distal to the inlet. 
         [0070]    Because of the convective and advective heat transfer between to the fill gas  196  and the floor portions  102  of the wafer retention pockets  92 , the operating temperature of the floor portions  102  can be substantially different than the temperatures of the portions of the two-piece wafer carrier  80   a  adjacent thereto. The thermal isolation slots  182 ,  184  act to inhibit heat transfer between the floor portions  102  and the adjacent areas, thus promoting the uniformity of the temperature of the wafer retention pockets  92 . The design and influence of the thermal isolation slots is further described at U.S. patent application Ser. No. 13/618,799, filed on Aug. 29, 2012 and owned by the assignee of the instant application. 
         [0071]    The flow rate of the fill gas  196  is preferably low enough so that the back pressuring does not cause the wafers  94  to lift off of the protrusions  112 . The low flow rate also mitigates convective coupling on the backside of the wafers  94 , so that the heat transfer between the floor portions  102  and the wafers  94  is dominated by conduction through the fill gas  196 . By way of non-limiting example, for a system processing wafers of 4-inch (100 mm) diameter, with pockets slightly larger than 100 mm in diameter, the flow rate of the fill gas  196  into each pocket is typically less than about 100 standard cubic centimeters per minute (cc/min). More generally, the flow rate into each pocket can range from about 20 cc/min to about 1000 cc/min. 
         [0072]    Referring to  FIGS. 11 and 12 , a two-piece wafer carrier  80   b  having interior pockets  202  and exterior pockets  204 , the bottom views of which are is depicted in an embodiment of the invention. The two-piece wafer carrier  80   b  includes a body portion  82   b  having a bottom surface  90   b  and defining a plenum cavity  206 . A closure portion  208  is disposed over and within the plenum cavity  206 . The plenum cavity  206  is formed to define the radially inward-facing perimeters of the floor portions of the exterior pockets  204 , and to define the perimeters floor portions of the interior pockets  202 . The closure portion  208  includes many of the same aspects as the closure portion  98  of the two-piece wafer carrier  80   a  of  FIGS. 3-10 , including a hub portion  212 , spacer portions  214  that define channels  216  and a contoured periphery  218 . The closure portion  208  can also include peripheral spacer portions  222  that help maintain chamber spacing near the contoured periphery  218 . The closure portion  208  further includes openings  220  to accommodate the interior pockets  202 . 
         [0073]    The fabrication process for forming manifold passages  224  is similar to that described for the two-piece wafer carrier  80   a . The manifold passages  224  of the interior pockets  202  are formed extending the passages formed in an adjacent exterior pocket  204 , as depicted in  FIG. 12 . Note that designated central manifold passages  224 ′ (depicted with bold lines in  FIG. 12 ) that pass through both an interior and an exterior pocket  202  and  204  are not aligned to pass through the central axis of the two-piece wafer carrier  80   b , but instead is aligned to pass through the full diameter of both the respective interior and exterior pockets  202  and  204 . 
         [0074]    Referring to  FIGS. 13 and 14 , a three-piece wafer carrier  80   c  is depicted in an embodiment of the invention. The three-piece wafer carrier  80   c  includes an outer ring or body portion  82   c  with a bottom surface  90   c  and having wafer retention pockets  92   c  and a two-piece plenum structure  242 . The outer ring portion  82   c  includes many features that are similar to the two-piece wafer carrier  80   a  and which are identified with like-numbered numerical references. Instead of a plenum cavity, the outer ring portion  82   c  defines an interior opening  244  bounded by a radially inward-facing perimeter  246 . The manifold passages  114  pass through the radially inward-facing perimeter  246  to establish fluid communication with the interior opening  244 . 
         [0075]    The two-piece plenum structure  242  includes a top portion  252  and a bottom portion  254  that cooperate to define a plenum chamber therebetween. The top portion  252  includes a plurality of interior wafer retention pockets  256  that have the same features as the wafer retention pockets  92   c . The top portion  252  further includes a contoured periphery  258  that substantially conforms to radially inward-facing peripheries  262  of the wafer retention pockets  92   c . The contoured periphery  258  extends radially outward from the inward-facing perimeter  246  of the outer ring portion  82   c  to define a flange  264  on the top portion  252 . The outer ring portion  82   c  includes a contoured recess  266  from the top surface  88   a  and proximate the inward-facing perimeter  246 , the contoured recess  266  conforming to the shape and thickness of the flange  264 . 
         [0076]    The bottom portion  254  of the two-piece plenum structure  242  has generally the same features as the closure portion  208  of the two-piece wafer carrier  80   b , the likenesses of which are similarly labeled in  FIG. 13 . One difference is that the outer periphery  268  of bottom portion  254  is not contoured to the shape of the floor portions  102  of the wafer retention pockets  92   c , but rather for a close tolerance fit with the inward-facing perimeter  246 . 
         [0077]    In fabrication, both the top portion  252  and the bottom portion  254  of the two-piece plenum structure  242  can be treated on the interior surfaces in the same manner as the exterior surfaces to prevent bowing due the thermal expansion differences, as discussed above in relation to the closure portion  98  of the two-piece wafer carrier  80   a . The manifold passages  114  for both the outer ring  82   c  and the top portion  252  of the two-piece plenum structure  242  can be formed using methods previously described. 
         [0078]    In assembly, the top portion  252  of the two-piece plenum structure  242  is aligned with and disposed within the contoured recess  266  of the outer ring portion  82   c . The top portion  252  can be affixed to the outer ring portion  82   c  using fasteners (not depicted) that pass through the flange  264 . The bottom portion  254  is affixed to the top portion  252  to define a plenum chamber (not depicted) therebetween, the plenum chamber also being bounded by the inward-facing perimeter  246  of the outer ring portion  82   c . It is noted that, for the embodiment of the three-piece wafer carrier  80   c  as depicted, there is no fixed order for the assembly. That is, the top and bottom portions  252  and  254  of the two-piece plenum structure  242  can be affixed to each other before or after the top portion  252  is joined to the outer ring  82   c  portion. 
         [0079]    In operation, the three-piece wafer carrier  80   c  operates by the same principles as the two-piece wafer carriers  80   a  and  80   b . The fill gas  196  flows over the hub portion  212  and into the channels  216  to flood the plenum chamber. The manifold passages  114  of the wafer retention pockets  92   c  and  256  are thereby sourced with the fill gas  196 , which in turn sources the weep orifices  116 . 
         [0080]    It is noted that the three-piece wafer carrier  80   c  is depicted without thermal isolation slots (e.g., numerical references  182  and  184  of the two-piece wafer carriers  80   a  and  80   b ). It is recognized that thermal isolation slots can also be implemented into the three-piece wafer carrier  80   c.    
         [0081]    Referring to  FIGS. 15 through 18 , a multiple plenum wafer carrier  80   d  is depicted in an embodiment of the invention. The multiple plenum wafer carrier  80   d  includes a body portion  82   d  that defines a plurality of wafer retention pockets  282 , one each disposed within a respective through aperture  284  formed on the body portion  82   d . Each through aperture  284  is characterized as having an inner radial perimeter  285 . The multiple plenum wafer carrier  80   d  includes many of the same aspects as the wafer carriers  80   a ,  80   b  and  80   c  which are identified with like-numbered numerical references. 
         [0082]    Each wafer retention pocket  282  includes an outer radial perimeter  286  upon which a tangential channel  288  is formed. Each tangential channel  288  is in fluid communication with the manifold passages  114  of the respective wafer retention pocket  282 . Each wafer retention pocket  282  can further include a flange or enlarged radius portion  292  formed on the outer radial perimeter  286  that mates within a recessed shoulder  294  formed on the respective through aperture  284 . 
         [0083]    Each through aperture  284  can be placed in fluid communication with the gap  165  of the center receptacle  130  via a respective flow passage  296 . In one embodiment, the flow passage  296  comprises a bored passageway  297 . In one embodiment, each flow passage  296  includes an axially extending portion  296   a  that extends substantially parallel to the central axis  84  and a radially extending portion  296   b  that extends substantially normal to the central axis  84 . During fabrication, the axially extending portions  296   a  can be formed as blind holes that extend from the top surface  88   d  ( FIG. 18 ). A cap  298  can be placed over the axially extending portions  296   a  to define an upper boundary of the gap  165  and placing the gap  165  in fluid communication with the flow passages  296 . The cap  298  can also enable a smooth upper surface  88   d  after any coating process is implemented on the multiple plenum wafer carrier  80   d.    
         [0084]    In reference to  FIGS. 16A through 16C , alternative configurations of wafer retention pockets  282   a  through  282   c , respectively, are depicted in embodiments of the invention. The wafer retention pockets  282   a  through  282   c  present weep orifices  16  that establish different patterns. Wafer retention pocket  282   a  ( FIG. 16A ) depicts the weep holes  116  as falling generally between a first radius R1 and a second radius R2, thus providing entry of the fill gas over a substantially annular region of the upper surface  108 . Wafer retention pocket  282   b  ( FIG. 16B ) depicts the weep holes  116  as falling generally outside a first radius R1, thus providing entry of the fill gas over a substantially outer annular region of the upper surface  108 . Wafer retention pocket  282   c  ( FIG. 16C ) depicts the weep holes  116  as falling generally within a rectangular pattern that is substantially centered on the upper surface  108 . 
         [0085]    While the various arrangements of the weep holes  116  are depicted and described in the context of the multiple plenum wafer carrier  80   d , it is understood that differing arrangements of the weep hole pattern can be applied to any of the wafer carriers  80  described and depicted herein. 
         [0086]    In assembly, each wafer retention pocket  282  is disposed in a respective one of the through apertures  284  so that the flange portion  292  rests on the recessed shoulder  294  of the respective through aperture. The tangential channel  288  cooperates with the inner radial perimeter  285  of the respective through aperture  284  to define a local plenum chamber  299  that surrounds the respective wafer retention pocket  282 . In one embodiment, the manifold passages  114  are oriented substantially perpendicular to the radially extending portions  296   b . The cap  298  is placed over the axially extending portions  296   a . The operational concept of the multiple plenum wafer carrier  80   d  is to source each wafer retention pocket  282  with the separate, local plenum chamber  299 , best seen in  FIG. 18 . The fill gas  196  enters the gap  165 , courses through the flow passage  296  and enters the local plenum chamber  299 . The local plenum chamber  299  has a much larger hydraulic diameter than the manifold passages  114 , and therefore substantially less resistance to flow than the manifold passages  114 . The substantially lower resistance to flow enables the plenum chamber  299  to charge to a substantially uniform pressure, thus sourcing each of the manifold passages  114  in a substantially uniform manner. The fill gas  196  enters the manifold passages  114  to source the weep orifices  116 , as in the other embodiments. 
         [0087]    Orienting the manifold passages  114  perpendicular to the radially extending portions  296  of the respective flow passage  296  prevents impinging any of the manifold passages  114  directly, thus aiding in spreading the flow of the fill gas  196  as it enters the plenum chamber  299  and enhancing the equal pressure sourcing of the manifold passages  114 . Also in the depicted embodiment of  FIGS. 15-18 , the manifold passages  114  pass entirely through the floor portions  103  of the wafer retention pockets  282 , and are sourced from both ends, further reducing pressure gradients along the length of the manifold passages  114 . 
         [0088]    The local plenum chamber  299  surrounds the respective wafer retention pocket  282  with fill gas  196  that flows tangentially around the wafer retention pocket  282 . The tangentially flowing fill gas  196  can act to thermally isolate the wafer retention pocket  282  from the body portion  82   d , thus enhancing the temperature uniformity of the resident wafer. Accordingly, while thermal isolation slots  182 ,  184  of the two-piece wafer carriers  80   a  and  80   b  can be implemented into the multiple plenum wafer carrier  80   d , the local plenum chamber  299  can provide the desired thermal isolation without need for thermal isolation slots. The bodies  82  of the various wafer carriers  80  are preferably formed from materials that do not contaminate the reaction chamber  10  and can withstand the temperatures of operation. Typical materials include graphite, silicon carbide, or other refractory materials. 
         [0089]    The following references, referred to above, are hereby incorporated by reference herein in their entirety except for the claims and express definitions included therein: U.S. Patent Application Publication No. 2011/0206843; U.S. Pat. No. 7,276,124; U.S. patent application Ser. No. 13/450,062; U.S. patent application Ser. No. 13/618,799. 
         [0090]    References to “top,” “bottom,” “upper” and “lower” are used for ease of description, and do not necessarily refer to gravitational frame of reference. Rather, these terms refer to directions relative to and between the gas distribution element  12  and the wafer carrier  80 . That is, the top end  13  of the reaction chamber  10  is typically, but not necessarily, disposed at the “top” of the chamber in the normal gravitational frame of reference. The “downward direction” as used herein refers to the direction away from the gas distribution element  12  toward the wafer carrier  80 , with the “upward direction” being the direction opposite the downward direction, regardless of whether these directions are aligned with the gravitational vector. Similarly, “top” or “upper” surfaces describe surfaces that face in the upward direction, and “bottom” or “lower” surfaces describe surfaces that face in the downward direction as defined. 
         [0091]    All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, can be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. 
         [0092]    Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) can be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. 
         [0093]    For purposes of interpreting the claims for the present invention, it is expressly intended that the provisions of Section 112, sixth paragraph of 35 U.S.C. are not to be invoked unless the specific terms “means for” or “step for” are recited in the subject claim.