Abstract:
A barrel susceptor for supporting semiconductor wafers in a heated chamber having an interior space. Each of the wafers has a front surface, a back surface and a circumferential side. The susceptor includes a body having a plurality of faces arranged around an imaginary central axis of the body. Each face has an outer surface and a recess extending laterally inward into the body from the outer surface. Each recess is surrounded by a rim defining the respective recess. The susceptor also includes a plurality of ledges extending outward from the body. Each of the ledges is positioned in one of the recesses and includes an upward facing support surface for supporting a semiconductor wafer received in the recess. Each of the support surfaces is separate from the outer surface of the respective face.

Description:
BACKGROUND 
       [0001]    The present invention generally relates to an epitaxial barrel susceptor and more particularly to an epitaxial barrel susceptor including modified pockets. 
         [0002]    Chemical vapor deposition is a process for growing a thin layer of material on a semiconductor wafer so lattice structures of the deposited material are identical to the wafer. Using this process, a layer having different conductivity may be applied to the semiconductor wafer to achieve the necessary electrical properties. Chemical vapor deposition is widely used in semiconductor wafer production to build up epitaxial layers so devices can be fabricated directly on the epitaxial layer. For example, a lightly doped epitaxial layer deposited over a heavily doped substrate permits a CMOS device to be optimized for latch up immunity as a result of a low electrical resistance of the substrate. Other advantages, such as precise control of a dopant concentration profile and freedom from oxygen are achieved. Conventional epitaxial deposition processes are disclosed in U.S. Pat. Nos. 5,904,769 and 5,769,942, which are incorporated by reference. 
         [0003]    Epitaxial deposition takes place in a reactor. One type of reactor is called a barrel reactor or a batch reactor. A barrel reactor has a reaction chamber that houses a susceptor for holding wafers during epitaxial deposition.  FIG. 1  illustrates a conventional barrel susceptor, which is designated in its entirety by the reference number  10 . The susceptor  10  has generally vertical but slightly sloping faces  12 . Each face  12  has two or more shallow recesses  14  aligned in a generally vertical column. Each recess  14  is generally circular and has a diameter suitable for receiving a single wafer (not shown). Each recess also has a planar surface  16  on which a back surface of the corresponding wafer rests. In most cases, the susceptor  10  is constructed of silicon-carbide-coated graphite to reduce contaminants such as iron from the graphite being released into the surrounding process gases. Conventional barrel susceptors are described in U.S. Pat. No. 6,129,048, which is incorporated by reference. 
         [0004]    Prior to epitaxial deposition, semiconductor wafers are loaded in the recesses of the susceptor and the susceptor is lowered into a deposition chamber. The epitaxial deposition process begins by introducing a cleaning gas, such as hydrogen or a hydrogen and hydrochloric acid mixture, to a front surface of the wafer (i.e., a surface facing away from the susceptor) to pre-heat and clean the front surface of the wafer. The cleaning gas removes native oxide from the front surface, permitting the epitaxial silicon layer to grow continuously and evenly on the surface during a subsequent step of the deposition process. The epitaxial deposition process continues by introducing a vaporous silicon source gas, such as silane or a chlorinated silane, to the front surface of the wafer to deposit and grow an epitaxial layer of silicon on the front surface. The recesses of the susceptor may include orifices for simultaneously injecting hydrogen gas over a back surface of the wafer. During both steps, the susceptor rotates in the reaction chamber. 
         [0005]    Barrel reactors are advantageous because they can simultaneously process a plurality of wafers and thus have a higher throughput, typically between six and fifteen wafers in a single run. However, one problem encountered using conventional barrel reactors is that frequently wafers processed do not meet current thickness uniformity requirements. Current thickness uniformity requirements mandate a thickness uniformity of less than about two percent and preferably, less than about one percent. Typical thickness uniformity of wafers processed in a barrel reactor using a conventional barrel susceptor is between about two percent and about five percent for a 200 millimeter (mm) diameter wafer having a ten millimeter edge exclusion. In recent years, most silicon wafers are processed in single wafer reactors to meet the stricter thickness uniformity requirements. The single wafer reactor has a much lower throughput than the barrel reactor because only one wafer is processed in a single run in the single wafer reactor. 
         [0006]    Accordingly, there is a need for a barrel reactor susceptor capable of producing wafers meeting current thickness uniformity requirements. 
       SUMMARY 
       [0007]    The present invention includes a barrel susceptor for supporting a plurality of semiconductor wafers in a heated chamber having an interior space. Each of the wafers has a front surface, a back surface opposite the front surface and a circumferential side extending around the front surface and the back surface. The susceptor is sized and shaped for receipt within the interior space of the chamber and for supporting the plurality of semiconductor wafers. The susceptor comprises a body having a plurality of faces arranged around an imaginary central axis of the body. Each face has an outer surface and a recess extending laterally inward into the body from the outer surface. Each recess is surrounded by a rim defining the respective recess. Further, the susceptor comprises a plurality of ledges extending outward from the body. Each of the ledges is positioned in one of the recesses and includes an upward facing support surface for supporting a semiconductor wafer received in the recess. Each of the support surfaces is separate from the outer surface of the respective face. 
         [0008]    In another aspect, the present invention includes a barrel susceptor for supporting a plurality of semiconductor wafers in a heated chamber having an interior space. Each of the wafers has a front surface, a back surface opposite the front surface and a circumferential side extending around the front surface and the back surface. The susceptor is sized and shaped for receipt within the interior space of the chamber and for supporting the plurality of semiconductor wafers. The susceptor comprises a body having a plurality of faces arranged around an imaginary central axis of the body. Each face has an outer surface and a recess extending laterally inward into the body from the outer surface. Each recess is surrounded by a rim defining the respective recess. Further, the susceptor comprises a plurality of ledges extending outward from the body in each of the recesses. Each of the ledges includes an upward facing support surface for supporting a semiconductor wafer received in the recess. 
         [0009]    The present invention also includes a barrel susceptor for supporting a plurality of semiconductor wafers in a heated chamber having an interior space. Each of the wafers has a front surface, a back surface opposite the front surface and a circumferential side extending around the front surface and the back surface. The susceptor is sized and shaped for receipt within the interior space of the chamber and for supporting the plurality of semiconductor wafers. The susceptor comprises a body having a plurality of faces arranged around an imaginary central axis of the body. Each face has an outer surface and a plurality of adjoining recesses extending laterally inward into the body from the outer surface. Each recess is at least partially surrounded by a rim defining the respective recess. In addition, the susceptor comprises a plurality of ledges extending outward from the body. Each of the ledges is defined by at least one of the rims defining the recesses and including an upward facing support surface for supporting a semiconductor wafer received in at least one of the recesses. 
         [0010]    In yet another aspect, the present invention includes a barrel susceptor for supporting a plurality of semiconductor wafers in a heated chamber having an interior space. Each of the wafers has a front surface, a back surface opposite the front surface and a circumferential side extending around the front surface and the back surface. The susceptor is sized and shaped for receipt within the interior space of the chamber and for supporting the semiconductor wafers. The susceptor comprises a body having a plurality of faces arranged around an imaginary central axis of the body. Each face has an outer surface and a recess extending laterally inward into the body from the outer surface. Each recess is no more than about 0.045 inch deep. The susceptor further comprises a plurality of ledges extending outward from the body. Each of the ledges includes an upward facing support surface for supporting a semiconductor wafer received in one of the recesses. 
         [0011]    In still another aspect, the present invention includes a barrel susceptor for supporting a plurality of semiconductor wafers in a heated chamber having an interior space. Each of the wafers has a front surface, a back surface opposite the front surface, a thickness measured between the front surface and the back surface, and a circumferential side extending around the front surface and the back surface. The susceptor is sized and shaped for receipt within the interior space of the chamber and for supporting the plurality of semiconductor wafers. The susceptor comprises a body having a plurality of faces arranged around an imaginary central axis of the body. Each face has an outer surface and a recess extending laterally inward into the body from the outer surface to a depth equal to the thickness of the respective wafer. The susceptor also includes a plurality of ledges extending outward from the body. Each of the ledges includes an upward facing support surface for supporting a semiconductor wafer received in one of the recesses. 
         [0012]    Other objects and features will be in part apparent and in part pointed out hereinafter. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0013]      FIG. 1  is a perspective of a conventional barrel reactor susceptor; 
           [0014]      FIG. 2  is a perspective of a susceptor of a first embodiment of the present invention; 
           [0015]      FIG. 3  is a perspective of a susceptor of a second embodiment; and 
           [0016]      FIG. 4  is a perspective of a susceptor of a third embodiment. 
       
    
    
       [0017]    Corresponding reference characters indicate corresponding parts throughout the drawings. 
       DETAILED DESCRIPTION 
       [0018]    Referring now to the drawings, and more particularly to  FIG. 2 , a barrel susceptor for supporting a plurality of wafers in a barrel reactor is generally indicated by the reference number  20 . The susceptor  20  includes a polyhedral body  22  having a plurality (e.g., three) trapezoidal faces, generally designated by  24 , positioned symmetrically about an imaginary central axis  26  of the body. Triangular corner pieces  28  may separate the faces  24  of the susceptor  20 . It should be understood that the body  22  may have other shapes without departing from the scope of the present invention. The body  22  has a hollow interior (not shown) that is in fluid communication with a port  30  through a top  32  of the body. 
         [0019]    Referring to  FIG. 2 , each face  24  of the susceptor  20  includes a planar outer surface  40  having an imaginary longitudinal axis  42  extending from a bottom  44  of the face to a top  46  of the face opposite the bottom. Each face  24  slopes inward toward the central axis  26  of the body  22  at the top  46 . Each face  24  also has a column of shallow recesses  50 ,  52 ,  54  centered on the corresponding longitudinal axis  42  and two evaluation piece recesses  56  adjacent the bottom  44  of the face. Although the evaluation piece recesses  56  may have other dimensions without departing from the scope of the present invention, in one embodiment each evaluation recess has a depth of about 0.050 inch and a diameter of about 1.3 inch. The evaluation recesses are used for holding smaller evaluation wafers, which can be destructively tested to evaluate the epitaxial process without sacrificing a production wafer. 
         [0020]    Although the bottom recess  50  may have other depths without departing from the scope of the present invention, in one embodiment the bottom recess has a depth of about 0.045 inch. The bottom recess  50  is bordered by a rim, generally designated by  60 , having a narrow rounded lower portion  62  adapted for holding a semiconductor wafer (not shown) during epitaxial processing. The rim  60  of the bottom recess  50  has narrow straight side portions  64  and a rounded upper portion  66 , all of which are spaced from the wafer during epitaxial processing to allow gas circulation at an upper edge as well as at leading and trailing edges of the wafer in the bottom recess  50 . 
         [0021]    The middle recess  52  is positioned above the bottom recess  50 . Although the middle recess  52  may have other depths without departing from the scope of the present invention, in one embodiment the middle recess has a depth of about 0.045 inch. The middle recess  52  has a generally circular rim  70  adapted to hold and surround a semiconductor wafer (not shown) during epitaxial processing. The bottom and middle recesses  50 ,  52 , respectively, are separated by a curved ledge or protrusion  72  formed between the circular rim  70  of the middle recess and the rounded upper portion  66  of the rim  60  of the bottom recess. Although the ledge  72  may have other dimensions without departing from the scope of the present invention, in one embodiment the ledge has a width of about 0.138 inch and is flush with the face  24  of the susceptor  20 . Accordingly, the ledge  72  has an upward facing support surface  74  having a width of about 0.045 inch for supporting a circumferential side of a semiconductor wafer during epitaxial processing. 
         [0022]    The top recess  54  surrounds an upper portion of the middle recess  52 . Although the top recess  54  may have other depths without departing from the scope of the present invention, in one embodiment the top recess has a depth of about 0.0165 inch. The top recess  54  is bordered by a rim  80  having narrow straight side portions  82  and a narrow straight upper portion  84  spaced from the wafer during epitaxial processing to allow gas circulation at an upper edge as well as at leading and trailing edges of the wafer. Although the ledge  72  of the middle recess  52  and the rounded lower portion  62  of the rim  60  of the bottom recess  50  may have other dimensions without departing from the scope of the present invention, in one embodiment each of theses features has an arc length of about 1200 and a radius approximately equal to that of the wafers intended to be processed using the susceptor  20  (e.g., about 3.97 inches). In other embodiments, the dimensions of the different recesses may differ from each other. As the use and manufacture of the susceptor  20  are conventional and well known in the art, they will not be described in further detail. 
         [0023]    In one particular embodiment, a difference between the depth of the middle recess  52  and the depth of the top recess  54  generally equals a thickness of the wafer so the outer surface of the wafer is substantially flush with the surface of the top recess. As a result of the wafer surface being flush with the top recess surface, gas flows smoothly across the wafer and is not disrupted at the leading edge of the wafer. Because the gas flows smoothly, the boundary layer remains thinner than it would if the flow were disrupted. Reactant gases must diffuse in through the boundary layer and reactant byproducts must diffuse out through the boundary layer. The thinner boundary layer improves diffusion, resulting in a thicker boundary layer. 
         [0024]    Tests using the susceptor  20  described above yielded epitaxial thickness profiles that varied by less than about one micrometer over a 200 millimeter (mm) diameter wafer having a three millimeter edge exclusion. Moreover, the actual epitaxial thickness increased near the leading and trailing side edges of the wafer. This compares favorably with tests performed on the conventional susceptor  10  described in the background. The conventional susceptor yielded epitaxial thickness profiles that varied by more than about 1.2 micrometers (μm) over a 200 millimeter diameter wafer having a three millimeter edge exclusion. Further, the epitaxial thickness decreased by nearly one micrometer near the edges of the wafer compared to the thickness locations spaced from the edge. 
         [0025]      FIG. 3  illustrates a second embodiment of the present invention, in which a barrel susceptor for supporting a plurality of wafers in a barrel reactor is generally indicated by the reference number  120 . Because the susceptor  120  is similar to the susceptor  20  of the previous embodiment, like components will be indicated by corresponding reference numerals incremented by 100. The susceptor  120  includes a polyhedral body  122  having a plurality trapezoidal faces, generally designated by  124 , positioned symmetrically about an imaginary central axis  126  of the body. Triangular corner pieces  128  may separate the faces  124  of the susceptor  120 . The body  122  may have other shapes without departing from the scope of the present invention. 
         [0026]    Each face  124  of the susceptor  120  has a planar outer surface  140  having an imaginary longitudinal axis  142  extending from a bottom  144  of the face to a top  146  of the face opposite the bottom. Each face  124  also has a shallow recess  150  centered on the corresponding longitudinal axis  142 . Each face  124  slopes inward toward the central axis  126  of the body  122  at the top  146 . In addition, each face  124  has two evaluation piece recesses  156  adjacent the bottom  144  of the face. Although the recess  150  may have other depths without departing from the scope of the present invention, in one embodiment the recess has a depth of about 0.045 inch. The recess  150  is bordered by a rim, generally designated by  160 , having a narrow rounded lower portion  162  adapted for holding a semiconductor wafer (not shown) during epitaxial processing. The rim  160  of the recess  150  has narrow straight side portions  164  and a straight upper portion  166 . A curved ledge or protrusion  172  is provided in the recess  150  for holding a second semiconductor wafer (not shown) during epitaxial processing. Although the ledge  172  may have other dimensions without departing from the scope of the present invention, in one embodiment the ledge has a width of about 0.138 inch and is flush with the face  124  of the susceptor  120 . Accordingly, the ledge  172  has an upward facing support surface  174  having a width of about 0.045 inch for supporting a circumferential side of a semiconductor wafer during epitaxial processing. The side portions  164  and upper portion  166  of the rim  160  of the recess  150  are spaced from the wafers during epitaxial processing to allow gas circulation at an upper edge as well as at leading and trailing edges of the wafers in the recess. Although the ledge  172  and the rounded lower portion  162  of the rim  160  of the recess  150  may have other dimensions without departing from the scope of the present invention, in one embodiment each of these features has an arc length of about 1200 and a radius of about 3.97 inches. In other embodiments, the dimensions of the different recesses may differ from each other. A series of holes  180  arranged in a concentric circular pattern is positioned in the recess  150  above the ledge  172  and the rounded lower portion  162  of the rim  160 . The holes  190  are positioned behind the wafers when positioned in the recess  150  to reduce autodoping. As the holes  190  and other features of the susceptor  120  are conventional, they will not be described further. Tests on a susceptor  120  having the configuration described above yielded epitaxial thickness profiles that varied by less than about one micrometer over a 200 millimeter (mm) diameter wafer having a three millimeter edge exclusion. As with the susceptor  20  of the first embodiment, the actual epitaxial thickness increased near the leading and trailing side edges of the wafer. 
         [0027]      FIG. 4  illustrates a third embodiment of the present invention, in which a barrel susceptor for supporting a plurality of wafers in a barrel reactor is generally indicated by the reference number  220 . Because the susceptor  220  is similar to the susceptor  120  of the previous embodiment, like components will be indicated by corresponding reference numerals incremented by 100. The susceptor  220  includes a polyhedral body  222  having trapezoidal faces  224  positioned symmetrically about an imaginary central axis  226  of the body. Triangular corner pieces  228  separate the faces  224  of the susceptor  220 . The body  222  may have other shapes without departing from the scope of the present invention. 
         [0028]    Each face  224  of the susceptor  220  has a planar outer surface  240  having an imaginary longitudinal axis  242  extending from a bottom  244  of the face to a top  246  of the face opposite the bottom. Each face  224  also has a shallow rectangular recess  250  centered on the corresponding longitudinal axis  242  and bordered by a rim, generally designated by  260 . Each face  224  slopes inward toward the central axis  226  of the body  222  at the top  246 . Although the recess  250  may have other depths without departing from the scope of the present invention, in one embodiment the recess has a depth of about 0.045 inch. A plurality of ledges or protrusions  272  (e.g., four) is provided in the recess  250  for holding semiconductor wafers (not shown) during epitaxial processing. Although the ledges  272  may have other dimensions without departing from the scope of the present invention, in one embodiment each ledge has a width of about 0.125 inch, a length of about 0.250 inch and is flush with the face  224  of the susceptor  220 . Accordingly, each ledge  272  has an upward facing support surface  274  having a width of about 0.045 inch for supporting a circumferential side of a semiconductor wafer during epitaxial processing. Further, in one embodiment the ledges  272  are positioned at about 500 from the longitudinal axis  242  of the face  224  and at a radius of about 3.94 inches. The rim  260  of the recess  250  is spaced from the wafers during epitaxial processing to allow gas circulation completely around the wafers in the recess. A series of holes  290  is positioned in the recess  250  to reduce autodoping. Tests on a susceptor  220  having the configuration described above yielded epitaxial thickness profiles that varied by less than about one micrometer over a 200 millimeter (mm) diameter wafer having a three millimeter edge exclusion. As with the susceptor  20  of the first embodiment, the actual epitaxial thickness increased near the leading and trailing side edges of the wafer. 
         [0029]    When introducing elements of the present invention or the preferred embodiments(s) thereof, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of the elements. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. 
         [0030]    In view of the above, it will be seen that several advantages are achieved by the present invention. 
         [0031]    As various changes could be made in the above constructions, products, and methods without departing from the scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.