Abstract:
A semiconductor fabrication reactor according to the invention comprises a rotatable susceptor mounted to the top of a reactor chamber. One or more wafers are mounted to a surface of the susceptor and the rotation of the susceptor causes the wafers to rotate within the chamber. A heater heats the susceptor and a chamber gas inlet allows semiconductor growth gasses into the reactor chamber to deposit semiconductor material on said wafers. A chamber gas outlet is included to allow growth gasses to exit the chamber. In a preferred embodiment, the inlet is at or below the level of said wafers and the outlet is preferably at or above the level of the wafers. A semiconductor fabrication system according to the invention comprises a source of gasses for forming epitaxial layers on wafers and a source of gasses for dopants in said epitaxial layers. A gas line carries the dopant and epitaxial source gasses to a reactor for growing semiconductor devices on wafers, and the source gasses in the gas line are injected into the reactor chamber through a reactor inlet. The reactor comprises an inverted susceptor mounted in a reactor chamber that is capable of rotating. One or more wafers are mounted to a surface of the susceptor, the rotation of the susceptor causing the wafers to rotate within the chamber. A heater heats the susceptor and the source gasses deposit semiconductor material on the wafers.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to fabricating semiconductor devices and more particularly to an apparatus for fabricating semiconductor devices using metalorganic chemical vapor deposition (MOCVD).  
           [0003]    2. Description of the Related Art  
           [0004]    Numerous semiconductor devices can be fabricated in MOCVD systems using different material systems, with MOCVD systems more recently being used to fabricate Group III nitride based devices. Growth of Group III nitride based semiconductor devices in MOCVD systems is generally described in DenBaars and Keller,  Semiconductors and Semimetals , Vol. 50, Academic Press Inc., 1997, p. 11-35. One of the concerns in fabricating Group III nitride devices is the ability to produce uniform materials with minimal impurities in the device layers, while providing sharp interfaces between layers. Impurities and poor interfaces between layers can negatively impact device performance and can prevent consistent reproduction of semiconductor devices.  
           [0005]    Some conventional multi-wafer MOCVD reactors utilize a rotatable susceptor that is mounted at the bottom of the reactor chamber. [See Emcore Discover and Enterprise Series of the TurboDisc Tools, provided by Emcore Inc.]. Semiconductor wafers are held on the top surface of the susceptor and a heating element is arranged below the susceptor to heat the susceptor and the wafers. Reactant growth gasses enter the reactor to deposit the desired materials on the wafer with the susceptor rotating to provide a more uniform deposition of the materials on the wafer.  
           [0006]    One the disadvantages of these conventional MOCVD reactors is that a large and non-uniform boundary layer thickness of hot air can form over the wafers and susceptor as a result of the heating of the susceptor. During growth, heat from the susceptor causes gasses to rise and the boundary layers can extend to the top surface of the reactor chamber. Reactant growth gasses are injected into the reactor chamber, usually through a top inlet. When the lower temperature growth gasses encounter the hot gasses heat convention can occur, which causes turbulence within the reactor. This turbulence can result in non-uniform deposition of materials on the wafer. It is also difficult for the deposition gasses to diffuse through a larger boundary layer and as a result, much of the growth gasses do not deposit on the wafers. This increases the amount of growth gasses necessary to form the desired semiconductor device.  
           [0007]    A large boundary layer over a susceptor can also limit the susceptor&#39;s speed of rotation. As the rotation speed of a heated susceptor is increased, the boundary layer can cause turbulence that adds to the turbulence from the convection forces of the lower temperature growth gasses. This can lead to further non-uniformity in the device layers.  
           [0008]    Another disadvantage of conventional MOCVD reactors is that the growth gasses that do not deposit on the wafers (or susceptor) can deposit on the sidewalls or top surface of the reactor chamber above the susceptor. These deposits can adversely impact the reactor&#39;s ability to grow good quality layers. The deposits can react with gasses for subsequent layers and redeposit on the wafers during fabrication. The deposits can be introduced as impurities in the subsequent layers and the deposits can reduce the sharpness between layers. This can ultimately limit the reactor&#39;s ability to accurately reproduce the semiconductor devices.  
           [0009]    A metal organic vapor phase epitaxy (MOVPE) system for the growth of Group III-V compound semiconductor materials is described in Aria et al.,  Highly Uniform Growth on a Low - Pressure MOPVE Multiple Wafer System,  Journal of Crystal Growth 170, Pgs. 88-91 (1997). The wafers are held in a susceptor and placed facedown (inverted) in the growth chamber, with the flow gasses flowing under the growth surfaces. The susceptor rotates, thereby rotating the wafers to attain a more uniform growth. Gasses are injected into the chamber from one of the sidewalls of the chamber, through a triple flow channel, and the gas exhaust in on the opposite sidewall. Group V species (hydride gasses) and H 2  carrier gas, Group III (organometals) and H 2  carrier gas, and purging gas flow into the reactor through the triple flow channel&#39;s upper, middle and lower channels, respectively.  
           [0010]    One disadvantage this of system is that because the inlet flow channels are on one chamber side wall and the outlet is on the opposite side wall at about the same height, gas flow is created across the chamber between inlet and outlet. Some of the gasses tend to flow through the chamber without having the opportunity to deposit reactants on the wafers. Also, the leading edges of the wafers experience gasses with the highest concentration of reactants, which results in non-uniform deposition across the wafers.  
           [0011]    The fluid flow and mass transport for “chimney” chemical vapor deposition (CVD) reactors is discussed in Holstein,  Modeling of Chimney CVD Reactors,  Journal of Crystal Growth 125, Pgs. 311-319 (1992). A chimney reactor has wafers held on heated susceptors (usually two) that are vertically mounted on the interior side walls of the reactor. The intent of the chimney reactor design is to create upward convective gas flow near the susceptor to help promote rapid gas switching for growth of abrupt heterojunctions. A cold carrier gas containing reactants enters at the base of the reactor and flows upward into the heated region.  
           [0012]    One of the disadvantages of this design is that asymmetric flow conditions result in the primary gas flow being located near one side of the reactor and reverse flow near the opposite side. This results in different deposition rates at the two susceptors. Also, with upward gas flow, the growth rate uniformity at the leading edge of the susceptor is much greater than at its trailing edge due to depletion of the reactants.  
           [0013]    Growth of GaAs based semiconductor devices in an MOCVD reactor is also discussed in Lee et al. MOCVD in Inverted Stagnation Point Flow, Journal of Crystal Growth, Pgs 120-127 (1886). The reactor is based on inverted stagnation point flow geometry where the reactants flow up towards wafers clamped to an inverted heated susceptor. However, this reactor is stagnation flow, where the susceptor does not rotate, which can reduce the uniformity of the device layers.  
         SUMMARY OF THE INVENTION  
         [0014]    The present invention seeks to provide an improved method and apparatus for the fabrication of semiconductor devices, and in particular the fabrication of semiconductor devices in MOCVD reactors. One embodiment of a semiconductor fabrication reactor according to the present invention comprises a rotatable susceptor mounted to the top of a reactor chamber. One or more wafers are mounted to a surface of the susceptor and the rotation of the susceptor causes the wafers to rotate within the chamber. A heater heats the susceptor and a chamber gas inlet allows semiconductor growth gasses into the reactor chamber to deposit semiconductor material on said wafers. The inlet is preferably at or below the level of said wafers. A chamber gas outlet is included to allow growth gasses to exit the chamber. The outlet is preferably at or above the level of the wafers.  
           [0015]    Another embodiment according to the invention comprises a semiconductor fabrication system that includes a source of gasses for forming epitaxial layers on wafers and a source of gasses for dopants in said epitaxial layers. A gas line carries the dopant and epitaxial source gasses to a reactor for growing semiconductor devices on wafers, and the source gasses in the gas line are injected into the reactor through a reactor inlet. The reactor comprises an inverted susceptor mounted in a reactor chamber that is capable of rotating. One or more wafers are mounted to a surface of the susceptor, the rotation of the susceptor causing the wafers to rotate within the chamber. A heater heats the susceptor and the source gasses deposit semiconductor material on the wafers. A chamber outlet allows the growth gasses to exit the chamber. In a preferred embodiment, the inlet at or below the level of said wafers and the outlet is above the level of said wafers.  
           [0016]    In a preferred embodiment according to the invention, the susceptor has a face down surface facing the bottom of said chamber, and the wafers are mounted to the face down surface. As fully described below, by inverting the susceptor the depth of the boundary layer is reduced, which reduces the turbulence generated when lower temperature growth gasses encounter the boundary layer. The growth gasses can also more easily penetrate the boundary layer and the susceptor can be rotated at a higher rotation rate. This arrangement also helps reduce the level of impurities in the semiconductor material that are introduced from deposits within the reactor chamber.  
           [0017]    These and other further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings, in which: 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIG. 1 is a simplified schematic of an embodiment of an MOCVD semiconductor fabrication system according to the present invention;  
         [0019]    [0019]FIG. 2 is a sectional view of one embodiment of a reactor according to the present invention;  
         [0020]    [0020]FIG. 3 is a sectional view of another embodiment of a reactor according to the present invention having a central rotation rod gas inlet;  
         [0021]    [0021]FIG. 4 is a below perspective view of an embodiment of a susceptor according to the present invention that can be used in the reactor in FIG. 3;  
         [0022]    [0022]FIG. 5 is a sectional view of another embodiment of a reactor according to the present invention having a central bottom gas inlet;  
         [0023]    [0023]FIG. 6 is a sectional view of another embodiment of a reactor according to the present invention having bottom showerhead gas inlet;  
         [0024]    [0024]FIG. 7 is a sectional view of another embodiment of a reactor according to the present invention having sidewall gas inlet; and  
         [0025]    [0025]FIG. 8 is a sectional view of another embodiment of a reactor according to the present invention having a height adjustable susceptor.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]    MOCVD reactors with inverted susceptors according to the present invention can be used in many different semiconductor fabrication systems, but are particularly adapted for use in MOCVD fabrication systems of the type shown in FIG. 1. MOCVD is a nonequilibrium growth technique that relies on vapor transport of precursers and subsequent reactions of Group III alkyls and Group V hydrides in a heated zone. Composition and growth rate are controlled by controlling mass flow rate and dilution of various components of the gas stream to the MOCVD reactor.  
         [0027]    Organometallic Group III growth gas sources are either liquids such as trimethylgallium (TMGa) and trimethylaluminum (TMAl), or solids such as trimethylindium (TMIn). The organometallic sources are stored in bubblers through which a carrier gas (typically hydrogen) flows. The bubbler temperature controls the vapor pressure over source material. Carrier gas will saturate with vapor from the organometallic source and transport vapor to the heated substrate.  
         [0028]    Group V growth gas sources are most commonly gaseous hydrides, for example NH 3  for nitride growth. Dopant materials can be metal organic precursers [diethylzine (DEZn), cyclopenin dienyl magnesium (Cp 2 Mg)j or hydrides (silane or disilane). Growth gasses and dopants are supplied to the reactor and are deposited as epitaxial layers on a substrate or wafer. One or more wafers are held on a structure of graphite called a susceptor that can be heated by a radio frequency (RF) coil, resistance heated, or radiantly heated by a strip heater, which in turn heats the wafers.  
         [0029]    The MOCVD semiconductor fabrication system  10  comprises a reactor chamber  12  having a susceptor  14  that is mounted to the top of the chamber  12  and is inverted. The susceptor  14  can hold a plurality of wafers  16  that can be made of many different materials such as sapphire, silicon (Si), silicon carbide (SiC), aluminum gallium nitride (AlGaN), gallium arsenide (GaAs). For Group III nitride based semiconductor devices a preferred wafer is made of SiC because it has a much closer crystal lattice match to Group III nitrides compared to other materials, which results in Group III nitride films of higher quality. SiC also has a very high thermal conductivity so that the total output power of Group III nitride devices on SiC is not limited by the thermal dissipation of the wafer. The availability of semi insulating SiC wafers also provides the capacity for device isolation and reduced parasitic capacitance that make commercial devices possible. SiC substrates are available from Cree, Inc., of Durham, N.C. and methods for producing them are set forth in the scientific literature as well as in a U.S. Pat. Nos. Re. 34,861; 4,946,547; and 5,200,022.  
         [0030]    During growth, the susceptor  14  is heated by heater  18  to maintain wafers  16  at a predetermined temperature. The temperature is typically between 400 and 1200 degrees centigrade (° C.), but can be higher or lower depending on the type of growth desired. The heater  18  can be any of the heating devices listed above, but is usually a radio frequency (RF) or resistance coil.  
         [0031]    A hydrogen or nitrogen carrier gas  20  is supplied to a gas line  22 . The carrier gas  20  is also supplied through mass flow controllers  24   a - c  to respective bubblers  26   a - c.  Bubbler  26   a  can have an organometallic Group III source as described above. Bubblers  26   b  and  26   c  may also contain a similar organometallic compound to be able to grow an alloy of a Group III compound. The bubblers  26   a - c  are typically maintained at a predetermined temperature by constant temperature baths  28   a - c  to ensure a constant vapor pressure of the organometallic compound before it is carried to the reactor chamber  12  by the carrier gas  20 .  
         [0032]    The carrier gas  20  which passes through bubblers  28   a - c  is mixed with the carrier gas  20  flowing within the gas line  22  by opening the desired combination of valves  30   a - c.  The mixed gas is then introduced into the reactor chamber  12  through a gas inlet port  32 , which can be located at different locations on the reactor, but in the system  10  is located at the bottom of the chamber  12 .  
         [0033]    A nitrogen containing gas  34  such as ammonia is supplied to the gas line  22  through a mass flow controller  36  and the flow of nitrogen containing gas is controlled by valve  38 . If the carrier gas  20  is mixed with the nitrogen containing gas  34 , and the organometallic vapor within the gas line  22  is introduced into the reactor chamber  12 , the elements are present to grow gallium nitride on the substrates  16  through thermal decomposition of the molecules in the organometallic and nitrogen containing gas.  
         [0034]    To dope alloys of gallium nitride on the wafers  16 , one of the bubblers  26   a - c  not being used for the organometallic compounds, can be used for a dopant material. Many different doping materials can be used such as beryllium, calcium, zinc, or carbon, with preferred materials being magnesium (Mg) or silicon (Si). Bubbler  26   b  or  26   c  can be used for an alloy material such as boron, aluminum, indium, phosphorous, arsenic or other materials. Once the dopant and/or alloy are selected and the appropriate valve  30   a - c  is opened to allow the dopant to flow into gas line  22  with the organometallic and nitrogen containing gas  34 , the growth of the doped layer of gallium nitride can take place on substrates  16 .  
         [0035]    The gas within the reactor chamber  12  can be purged through a gas purge line  40  connected to a pump  42  operable under hydraulic pressure. Further, a purge valve  44  allows gas pressure to build up or be bled off from the reactor chamber  12 .  
         [0036]    The growth process is typically stopped by shutting off the organometallic and dopant sources by closing valves  30   a - c,  and keeping the nitrogen containing gas  36  and the carrier gas  20  flowing. Alternatively, the reactor chamber  12  can be purged with a gas  46  that can be controlled through a mass flow controller  48  and valve  50 . The purge is aided by opening valve  44  to allow the pump  42  to evacuate the reaction chamber  12  of excess growth gasses. Typically, the purge gas  46  is hydrogen, but can be other gasses. Turning off power to the heater  18  then cools the substrates  16 .  
         [0037]    [0037]FIG. 2 shows one embodiment of a MOCVD reactor  60  in accordance with the present invention. The reactor  60  can be used to fabricate many different semiconductor devices from different material systems, but is particularly applicable to fabricating devices from the Group III nitride material system and its alloys, in an MOCVD fabrication system.  
         [0038]    The reactor  60  comprises a reactor chamber  62 , with a susceptor  64  that is, inverted and mounted from the reactor&#39;s top surface  66 . The susceptor  64  can be made of many heat conductive materials, with a suitable material being graphite. Semiconductor wafers  68  are mounted on the susceptor&#39;s face down surface  70  that faces the chamber&#39;s bottom surface  72 , with typical susceptors capable of holding approximately six three inch wafers and up to eighteen two inch wafers. The wafers can be held to the susceptor surface  70  by many different mechanisms including, but not limited to, mounting faceplates, clamps, clips, adhesives, tape, etc.  
         [0039]    The susceptor  64  is held within the reactor chamber  60  by a rotation rod  74  that can be rotated so that the susceptor  64  is also rotated. The susceptor is heated by a heating element  80  that is arranged between the susceptor  64  and the chamber&#39;s top surface. The heater  80  can be any of the heating devices listed above, but is usually a radio frequency (RF) or resistance coil. When the heater  80  heats the susceptor  64 , a hot gas boundary layer  82  forms over the susceptor surface  70  and the wafers  68 . During growth of semiconductor material on the wafers  68 , the growth gasses can enter the chamber  62  in many different ways and through different walls of the chamber  62 .  
         [0040]    By inverting the susceptor, the depth of the boundary layer  82  is reduced compared to conventional reactor chambers that have a susceptor at the bottom. As the susceptor  64  is heated and generates hot gas, the heated gas rises. Accordingly, the boundary layer  82  is compressed against the susceptor  64  and wafers  68  by the rising of the hot gas. The reduced boundary layer height reduces the turbulence generated when lower temperature growth gasses encounter the boundary layer  82 , which allows for more uniform deposition of materials on the wafers  68 . The growth gasses can also more easily penetrate the boundary layer  82  and as a result, more of the growth gasses deposit on the wafers  68 . This decreases the amount of deposition gasses necessary to form the desired semiconductor device.  
         [0041]    The reduced boundary layer also reduces gas convection that can occur when the susceptor  64  rotates. As a result, the susceptor  64  can be rotated much faster than conventionally arranged susceptors. In the reactor  10 , the susceptor can be rotated above 100 revolutions per minute (rpm) and up to several thousand rpm.  
         [0042]    The reduced boundary layer  82  also allows the deposition gasses to deposit on the wafers  68  under increased reactor chamber pressure to further facilitate efficient fabrication. Depending on the device being fabricated, the pressure can be below ⅛ of an atmosphere to more that 10 atmospheres.  
         [0043]    Another advantage of the inverted susceptor arrangement is that most of the growth gasses that do not deposit on the wafers rise past the susceptor  64  toward the top of the chamber  62 . These gasses can form deposits  84  on the side walls and top surface of the chamber  62  behind the susceptor. These deposits are less likely to interact with subsequent growth gasses to introduce impurities into the material deposited on the wafers  68  because the growth gasses will not encounter these deposits until they are past the wafers. That is, the gasses encounter these impurities when they are past the point when they are depositing reactants on the wafers. Gasses that do not deposit on the wafers or reactor walls can exit the chamber through a top gas outlet, although the outlet could at different locations on the chamber.  
         [0044]    [0044]FIG. 3 shows an embodiment of an MOCVD reactor  90  in accordance with the present invention that is similar to the reactor  60  in FIG. 2. The reactor has a rotation rod  92  that is hollow so that deposition gasses can enter the reactor chamber  94  through the rotation rod  92 .  
         [0045]    [0045]FIG. 4 shows a susceptor  96  that can be used in reactor  90 , which includes a central gas inlet  98  that allows gas from the rotation rod  92  to enter the reactor chamber  94  through the susceptor  96 . As the susceptor  96  rotates, the gasses from the inlet are drawn to the susceptor&#39;s perimeter and along the way, some of the growth gasses deposit on the wafers  100 . Gasses that do not deposit on the wafers, pass off the edge of the susceptor  96  and are drawn toward the chamber&#39;s top surface  102 . Like above, these gasses can form deposits  106  on the inside of the chamber&#39;s sidewalls  108   a,    108   b  and inside of the chamber&#39;s top surface  102 , that are downstream and behind the susceptor  96 . These deposits are less likely to adversely effect the fabrication of subsequent layers as described above. Gasses can exit the reactor chamber  94  through a gas outlet  110  that is preferably at the top of the reactor chamber, which promotes flow of the gasses past the wafers and then to the top of the chamber.  
         [0046]    [0046]FIG. 5 shows another embodiment of an MOCVD reactor  120  in accordance with the invention, where the growth gasses enter the chamber  122  through a central bottom inlet  124  that is directed toward the wafers  126  on the rotating susceptor  128 . The growth gasses rise toward the susceptor  128  where gasses are deposited on the wafers  126 . Like the embodiment in FIG. 3, any gasses that do not deposit on the wafers  126  are drawn past the susceptor  128  where they can form deposits  130  on the inside of the chamber&#39;s sidewalls  132   a,    132   b  and inside of the chamber&#39;s top surface  134 . The reactor also has a top gas outlet  136 .  
         [0047]    [0047]FIG. 6, shows another embodiment of an MOCVD reactor  140  in accordance with the present invention, where the growth gasses enter the reactor chamber  142  through a bottom “showerhead” inlet  144 . The inlet  144  has multiple boreholes  145  for the growth gasses to pass into the chamber where they rise toward the wafers  146  on the rotating susceptor  148 . The bore-holes  145  in the inlet  144  provide for a more uniform application of the growth gasses across the susceptor  148 , which provides for a more uniform deposition of materials on the wafers  146 . Like above, the gasses that do not deposit on the wafers are drawn downstream and if they do not deposit on the walls of the reactor chamber  142 , they can exit the chamber through the top outlet  149 .  
         [0048]    [0048]FIG. 7 shows another embodiment of an MOCVD reactor  150  in accordance with the present invention, where the deposition gasses enter the reactor chamber  152  through a sidewall inlet  154 . Like above, the gasses that do not deposit on the wafers  156  on the rotating susceptor  158  are drawn downstream where they can form deposits  159  on the inside of the reactor&#39;s walls. The reactor can also have a top gas outlet  160 , which is arranged so that the gasses pass from the inlet  154  toward the top of the chamber  152 . The growth gasses rise toward the susceptor  158  where semiconductor material can be deposited on the wafers  156 .  
         [0049]    [0049]FIG. 8 shows still another embodiment of an MOCVD reactor  170  in accordance with the present invention, that includes a reactor chamber  171 , rotating susceptor  172 , wafers  174  on the susceptor, and a showerhead gas inlet  175 , all of which are similar those in reactor  140  of FIG. 6. In most respects, the reactor  170  operates in the same way as the reactor  140  in FIG. 6. However, in reactor  170  the susceptor  172  is mounted to the reactor&#39;s top surface  176  by a rod  178  that is movable in directions shown be arrows  177   a,    177   b,  to adjust the distance between the showerhead inlet  175  and the susceptor  172 . This adjustment can vary the concentration of reactants in the growth gasses that react with the wafers  174 , to vary the semiconductor growth conditions and rate.  
         [0050]    As further shown in FIG. 8 the susceptor  172  can be further adjusted in the direction of arrows  178   a,    178   b  to vary the angle between the susceptor  172  and the gas inlet  175 . Similarly, the angle of the gas inlet  175  can be adjusted in the direction of arrows  179   a,    179   b  to also adjust the angle between the susceptor  172  and the inlet  175 . These adjustments can also vary the semiconductor grown conditions and rate on the wafers  174 . The movable susceptor arrangement and angle adjustable susceptor and inlet arrangement can also be used in reactors  60 ,  120 ,  150 , above that have gas inlets through the susceptor, a bottom inlet and a side inlet, respectively. The reactors can include only one or all of these adjustment options.  
         [0051]    Although the present invention has been described in considerable detail with reference to certain preferred configurations thereof, other versions are possible. Many different gas inlets, gas outlets and susceptors can be used. The gas inlets and outlets can be arranged in many different locations. The reactor according to the invention can be used to grow many different semiconductor devices from many different material systems, in many different semiconductor fabrication systems. Therefore, the spirit and scope of the invention should not be limited to the preferred versions in the specification above or in the claims below.