Abstract:
A susceptor for use in a Czochralski crystal growing apparatus is disclosed wherein erosion of the susceptor is minimized. The susceptor contains ventilation holes that allow process gases found between the susceptor and crucible to escape. The crucible may incorporate the use of a protective coating over part or all of the susceptor, such as a silicon carbide coating. The ventilation holes are placed at various heights along the susceptor wall to allow ventilation near the area of plastic deformation of the crucible.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a graphite susceptor used to form semiconductor ingots, and particularly to a susceptor that contains ventilation holes.  
         BACKGROUND OF THE INVENTION  
         [0002]    One common method for producing monocrystalline semiconductor ingots used to manufacture semiconductor wafers is the so-called Czochralksi (CZ) method. When growing an ingot using the Czochralski method, charge material such as silicon, gallium-arsenide, and the like, is loaded into a generally bowl-shaped crucible. The crucible is typically made out of quartz, and is supported and surrounded by a susceptor, typically made out of graphite or graphite composite. The susceptor typically is similarly shaped to the crucible, such that the general appearance is a bowl within a bowl. A circumferential heater surrounds the susceptor, and supplies heat to melt the charge material to a molten state. A seed crystal with the desired crystalline structure is then lowered into contact with the melt, and allowed to thermally stabilize. The seed is rotated in one direction, and the crucible is rotated in the opposite direction. The seed is then raised at a controlled rate, thus enabling growth of a crystal. Typically, crystal growth is accomplished at a pressure lower than atmospheric, with an inert purge gas supplied to flush the system of impurities.  
           [0003]    During silicon crystal growth, temperatures are elevated to approximately 1425° C. This is sufficiently high to allow the quartz crucible to experience plastic deformation, wherein it is still capable of containing the molten silicon, but the circumferential area around the base expands and stretches. Often the crucible will stretch enough to come in contact with, and be supported by the susceptor.  
           [0004]    The contact between the quartz crucible and the graphite susceptor at these elevated temperatures limits the useable lifetime of the susceptor. Silicon oxide (SiO) gas is emitted from the crucible during crystal growth, which causes a chemical reaction to occur with the graphite susceptor. Oxygen from the SiO gas erodes the graphite and forms carbon monoxide (CO) and carbon dioxide (CO 2 ). Silicon, on the other hand, combines with the graphite to form silicon carbide (SiC). Depending on the gas flow properties of the area, the gases can enhance erosion of the graphite susceptor, and shorten the useable lifetime even further.  
           [0005]    Attempts have been made to control erosion of graphite parts, including the susceptor, with limited success and with high costs. For example, in U.S. Pat. No. 5,476,679, Lewis et al. discloses a method for coating a susceptor with a glassy carbon coating. Although this method is effective in reducing metallic contamination being introduced to the crucible, it has not been fully effective in protecting erosion.  
           [0006]    Silicon Carbide coatings have also been employed in attempts to protect graphite parts and thereby increase useable lifetime. Unfortunately, the difference in thermal expansion coefficients of graphite and silicon carbide result in separation between the graphite and silicon carbide coating and cracking of the silicon carbide coating. Therefore, not only is coating with silicon carbide not fully functional, it is also quite expensive to apply, as it is typically applied by chemical vapor deposition techniques (CVD).  
           [0007]    In co-pending U.S. patent application Ser. No. 09/553,818 Kondo et al. at least partially obviate the limitations of silicon carbide coating by silicon carbide implantation, wherein silicon carbide is introduced into the graphite itself, and then gradually increase the percentage of silicon carbide and reduce the percentage of graphite approaching the surface, to the point where the surface is entirely silicon carbide. This technique increases the peel strength over a silicon carbide layer, and allows for a thinner layer to be produced, thereby reducing costs of implementation. There remains erosion, albeit at a slower rate.  
           [0008]    U.S. Pat. No. 6,221,478 (Kammeyer) discloses a method of putting a silicon carbide layer on the surface by a technique of applying a silicon-containing paste to a selected surface area of a graphite part. The silicon containing paste contains silicon containing powder and carbon containing liquid. The paste is applied to the desired areas, and is then heated so that the silicon in the paste melts and reacts with carbon on the surface of the graphite component, converting the graphite to form a protective silicon carbide coating. This method also allows coating of desired areas only. Once again, however, there remains erosion.  
           [0009]    Accordingly, there is a need for reduced erosion of graphite parts in general and susceptors in specific, including the need for enhanced gas flow properties.  
         SUMMARY OF THE INVENTION  
         [0010]    The present invention provides a susceptor that includes a plurality of vent holes cut into the side walls of the susceptor to facilitate improved gas flow characteristics. The vent holes are cut into the susceptor are placed such that they will not be obstructed by the plastic deformation of the crucible, such that if the crucible expands and comes in contact with the susceptor the vent holes will remain functional.  
           [0011]    The apparatus of the present invention may be employed alone, or in conjunction with coating of silicon carbide or through chemical vapor implantation of silicon carbide. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is a partial sectional view of a Czochralski crystal growing apparatus.  
         [0013]    [0013]FIG. 2 is a partial sectional view of a crucible housed within a suspector before heating.  
         [0014]    [0014]FIG. 3 is a partial sectional view of a crucible housed within a susceptor at elevated temperatures.  
         [0015]    [0015]FIG. 4 is a sectional view of a susceptor containing vent holes. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    Turning now to FIG. 1, a crystal growing apparatus  10  includes a bottom chamber  12 . The bottom chamber  12  houses a quartz crucible  110 , which is supported by a susceptor  100 . The susceptor  100  is in turn supported by a vertically moveable and rotatable shaft assembly  16 . A cylindrical heater  18  made of, for example, graphite is disposed around the susceptor  100 , which in turn is surrounded by and insulating cylinder  20 . The bottom chamber  12  also has a conduit  40  for evacuating air during start up, and process gas during crystal pulling operations utilizing a vacuum pump (not shown).  
         [0017]    A top chamber  24  is disposed above the bottom chamber  12  while an isolation valve  22  is disposed there between. The top chamber  24  provides a space for accommodating a grown crystal. The isolation valve  22  functions to allow a vacuum tight separation between the top chamber  24  and the bottom chamber  12  thus enabling a grown crystal to be removed from the top chamber  24  without losing vacuum or temperature in the bottom chamber  12 . The top chamber  24  has a conduit  70  that goes to a vacuum pump (not shown) that allows the top chamber to be evacuated of air and purge gases, so it may be rejoined with the bottom chamber  12 . When the isolation valve  22  is opened, a purge gas such as argon is introduced through conduit  70 , flowed through the entire growing apparatus  10 , and exited through conduit  40 .  
         [0018]    A winding mechanism  26  is disposed above the top chamber  24 , and includes a winding drum  28  within the winding mechanism  26 . The winding mechanism  26  is rotatable around a vertical axis with respect to the top chamber  24 . A wire  30  is wound onto the winding drum  28 , and extends downward. A seed chuck  32  for holding a crystal seed  34  is attached to the lower end of the wire  30 .  
         [0019]    When a single crystal is to be grown in the crystal growing apparatus  10 , the isolation valve  22  is in an open position so as to allow the seed  34  to be lowered into the bottom chamber  12 . Both the bottom chamber  12  and the top chamber  24  are evacuated and purged of air, and an inert gas is then flowed through the apparatus for the remainder of the growing process. A charge material, such as silicon, is placed in the crucible  110 , and heated by the heater  18 , thereby making a molten material  36 .  
         [0020]    The seed crystal  34  is lowered by winding drum  28  until the end of the seed crystal  34  is lowered into the molten material  36 . After allowing the seed crystal  34  to reach temperature equilibrium with the molten material  36 , the winding drum  28  slowly begins to wind up the wire  30 , thus enabling a crystal  38  to be pulled or grown. During the growing operation, the winding mechanism  26  and thus the seed are rotating in the opposite direction of the shaft assembly  16 .  
         [0021]    As previously mentioned, at the elevated temperatures during crystal growth the crucible and susceptor interact to form CO and CO 2  gases and SiC. The purge gas, typically argon, is used to assist in cooling the crystal and to flush out the CO, CO 2 , and SiC from the growing chamber. Typical argon flow ranges from 120 to 190 cubic liters per minute during growth, and will be significantly higher during purging processes.  
         [0022]    As shown in FIG. 2, susceptor  100  contains two mirror-image portions  102  and  104 . An expansion gap  106  separates the two portion  102  and  104 , and allows the portions  102  and  104  to expand and contract during heating and cooling without significantly changing the dimensions of the susceptor  100 . The susceptor  100  is held together at the bottom by sitting it a cup located at the top of the rotatable shaft assembly  16 . The quartz crucible  110  is housed within the susceptor  100 , and is packed with charge material  120  that is to be melted and grown into a monocrystalline ingot. Before heat is supplied to melt the charge material  120 , there is a gap between the wall of the crucible  110  and the susceptor  100 . Purge gas  130  can flow downward between the walls of the crucible  110  and susceptor  100 . The only exit for the purge gas  130  to escape is through the expansion gap  106  such that the gas flow between the walls of the crucible  110  and the susceptor  100  becomes turbulent, and changes from a laminar downward directed flow to a turbulent flow swirling between the walls until it encounters and escapes through the expansion gap  106 .  
         [0023]    [0023]FIG. 3 demonstrates the interaction of the susceptor  100  and the crucible  110  after sufficient heat has been supplied to change the charge material into a molten material  36 . At an area located approximate to the surface of the molten material  36 , the crucible wall plastically deforms and expands at  112 . As a crystal is extracted from the molten material  36 , the volume of the molten material  36  will decrease and the surface of the molten material  36  will move downward relative to the crucible wall  110  and the area of plastic deformation  112 . After the crucible  110  plastically deforms at  112 , however, it does not move downward to follow the relative position of the surface of the molten material  36 , but rather remains at a constant position. Purge gas  130  flows downward between the walls of the crucible  110  and the susceptor  100  to the point of plastic deformation  112 , and is turbulently forced out at the expansion gap  106 . Due to the extreme heat and chemical reactions between the susceptor  100  and crucible  110  as previously explained, the area of plastic deformation  112  is subject to a much more aggressive erosion than other parts of the susceptor  100 .  
         [0024]    As shown in FIG. 4, to provide more escape passages for the purge gas  130 , vent holes  108  are cut through the sidewall of the susceptor portions  102  and  104 . Vent holes  108  can be cut at a downward angle from the inside out to help facilitate better gas flow properties and more laminar flow. The vent holes  108  can also be cut at numerous distances from the top of the susceptor to facilitate various melt surface levels such that at least some of the vent holes  108  will be near the area of plastic deformation  112 , but will not be occluded by the wall of the crucible  100 . In this situation, there may be some vent holes  108  that are occluded by the wall of the crucible  100 , and others that are above the area of plastic deformation  112 . The physical shape of the vent holes  108  should be manufactured to reasonably facilitate desired gas flow properties. The size and quantity of vent holes  108  should not be so large as to risk structural failure of the susceptor  110 .  
         [0025]    It should be noted that the present invention may be used on graphite susceptors alone, or in conjunction with other erosion inhibiting methods. For example, a susceptor containing vent holes made out of a carbon based material such as graphite may be completely covered with a silicon carbide coating. Another embodiment may utilize providing a protective coating such as silicon carbide in the area proximal to the vent holes and along the expansion gap. Yet another embodiment includes enhancing erosion resistance by chemical vapor implantation of a protective layer such as silicon carbide.  
         [0026]    Although the invention has been described with reference to specific embodiments, other embodiments of the present invention will be apparent to those skilled in the art from a consideration of this specification or practice of the invention disclosed herein. It is intended that the written description be considered in all aspects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes that come within the meaning and range of the equivalence of the claims are to be embraced within their scope.