Patent Publication Number: US-2009223447-A1

Title: Apparatus for producing silicon carbide single crystal

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is based on and claims priority to Japanese Patent Application No. 2008-55083 filed on Mar. 5, 2008, the contents of which are incorporated in their entirety herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus for producing a silicon carbide single crystal. 
     2. Description of the Related Art 
     A silicon carbide (SiC) single crystal has a high breakdown voltage and a high electron mobility. Thus, the SiC single crystal is expected to be useful for a semiconductor substrate of a power device. The SiC single crystal can be produced by a high temperature chemical vapor deposition method (high temperature CVD method) as described, for example, in U.S. Pat. No. 6,030,661 (corresponding to JP-A-11-508531). In the high temperature CVD method, the SiC single crystal is produced by an epitaxial growth of SiC at a high temperature. 
     In the method disclosed in U.S. Pat. No. 6,030,661, mixed gas supplied from a gas introducing pipe flows into a susceptor through a passage made of a heat insulating material. When the mixed gas flows from the heat-insulated passage into the susceptor, the mixed gas is rapidly heated. Thus, it is difficult to produce a high quality SiC single crystal. 
     For restricting the rapid temperature increase of the mixed gas, the mixed gas may be heated at the passage for increasing the temperature of the mixed gas that flows into the susceptor. However, when the temperature of the mixed gas is higher than about 500 degrees centigrade, silicon (Si) may deposit on a wall of the passage. In addition, when the temperature of the mixed gas reach a reaction temperature of Si and carbon (C), Si reacts with C and SiC may deposit on the wall of the passage. As a result, the passage may clog with the deposits. 
     In an apparatus for producing an SiC single crystal disclosed in US 2004/231583 A (corresponding to JP-A-2002-154899), a cooling mechanism is provided in the vicinity of a gas introducing pipe for introducing mixed gas into a crucible. The cooling mechanism provides a temperature gradient so that temperature increases toward the crucible. 
     When the cooler is provided, the gas introducing pipe can be cooled effectively. However, if the cooler is damaged and cooling water leaking from the cooler enters the crucible, a pressure in the apparatus rapidly increases due to evaporation of the cooling water, and thereby the apparatus may be damaged. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing problems, it is an object of the present invention to provide an apparatus for producing an SiC single crystal. 
     According to an aspect of the present invention, an apparatus for producing an SiC single crystal includes a vacuum chamber, a gas introducing pipe, a cooler, and a shielding part. The reaction container is disposed in the vacuum chamber and defines an internal space where an SiC single crystal substrate is disposed as a seed crystal. The reaction container has an opening portion through which the internal space of the reaction container communicates with a portion of an internal space of the vacuum chamber located below the reaction container. The gas introducing pipe connects an outside of the vacuum chamber and an inside of the vacuum chamber for supplying a mixed gas from the outside of the vacuum chamber to the SiC single crystal substrate through the opening portion of the reaction container. The mixed gas includes a silicon-including gas and a carbon-including gas. The cooler is disposed adjacent to the gas introducing pipe and is apart from the reaction container. The cooler is outside a supply passage of the mixed gas provided from the gas introducing pipe to the SiC single crystal substrate located above the gas introducing pipe. The cooler is configured to reduce a surrounding temperature by fluid flowing in the cooler. The shielding part is disposed between the cooler and the internal space of the reaction container so that the fluid leaking from the cooler is restricted from scattering in the internal space of the reaction container. The shielding part is outside the supply passage of the mixed gas. 
     In the present apparatus, even if the cooler is damaged, the fluid leaking from the cooler is blocked by the shielding part disposed between the cooler and the reaction container and is dropped to the portion of the internal space of the vacuum chamber located below the reaction container. Thus, a scattering of the fluid in the internal space of the reaction container can be restricted, and thereby a rapid pressure increase in the vacuum chamber can be restricted. 
     According to another aspect of the present invention, an apparatus for producing an SiC single crystal includes a vacuum chamber, a reaction container, a gas introducing pipe, and a cooler. The reaction container is disposed in the vacuum chamber and defines an internal space where an SiC single crystal substrate is disposed as a seed crystal. The reaction container has an opening portion through which the internal space of the reaction container communicates with a portion of an internal space of the vacuum chamber located below the reaction container. The gas introducing pipe connects an outside of the vacuum chamber and an inside of the vacuum chamber for supplying a mixed gas from the outside of the vacuum chamber to the SiC single crystal substrate through the opening portion of the reaction container. The mixed gas includes a silicon-including gas and a carbon-including gas. The cooler is disposed adjacent to the gas introducing pipe and is apart from the reaction container. The cooler is outside a supply passage of the mixed gas provided from the gas introducing pipe to the SiC single crystal substrate located above the gas introducing pipe. The cooler is configured to reduce a surrounding temperature by fluid flowing in the cooler. At least a portion of the cooler is located below the reaction container. The cooler includes a wall having a first section facing the internal space of the reaction container and a second section other than the first section. The first section has a first mechanical strength and the second section has a second mechanical strength. The first mechanical strength is greater than the second mechanical strength. 
     In the present apparatus, the first section of the wall of the cooler facing the internal space of the reaction container has the mechanical strength greater than the mechanical strength of the second section other than the first section. Thus, even if the cooler is damaged, the fluid is restricted from scattering in the internal space of the reaction container, and thereby a rapid pressure increase in the vacuum chamber can be restricted. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Additional objects and advantages of the present invention will be more readily apparent from the following detailed description of preferred embodiments when taken together with the accompanying drawings. In the drawings: 
         FIG. 1  is a diagram illustrating a cross-sectional view of an apparatus for producing an SiC single crystal according to a first embodiment of the present invention; 
         FIG. 2  is a diagram illustrating an enlarged view of a part of the apparatus including a shielding plate and a cooler; 
         FIG. 3  is a diagram illustrating a cross-sectional view of the cooler taken along line III-III in  FIG. 2 ; 
         FIG. 4  is a flowchart illustrating an exemplary process for reducing a leakage of cooling water; 
         FIG. 5  is a diagram illustrating an enlarged view of a part of an apparatus for producing an SiC single crystal according to a modification of the first embodiment; 
         FIG. 6  is a diagram illustrating an enlarged view of a part of an apparatus for producing an SiC single crystal according to a second embodiment of the present invention; and 
         FIG. 7  is a diagram illustrating an enlarged view of a part of an apparatus for producing an SiC single crystal according to a third embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     An apparatus  1  for producing an SiC single crystal according to a first embodiment of the present invention will be described with reference to  FIG. 1  to  FIG. 3 . The apparatus  1  includes a vacuum chamber  10 , a crucible  30 , a gas introducing pipe  50 , a cooler  70 , and a shielding plate  90 . The crucible  30  is disposed in the vacuum chamber  10 . The crucible  30  can function as a reaction container. The crucible  30  has a through hole  30   a.  In the following description, an extending direction of the through hole  30   a  is defined as a vertical direction, and a direction perpendicular to the vertical direction is defined as a horizontal direction. The gas introducing pipe  50  introduces source gas into the crucible  30 . The cooler  70  cools the gas introducing pipe  50  for providing a temperature gradient. The shielding plate  90  blocks cooling water leaking from the cooler  70 . 
     The vacuum chamber  10  has an approximately cylindrical shape and defines an internal space. The vacuum chamber  10  has a lower chamber  11  and an upper chamber  12 . The lower chamber  11  is a portion to hold the crucible  30 . The upper chamber  12  is a portion to take out a completed SiC. 
     The upper chamber  12  is made of SUS (stainless steel), for example. The upper chamber  12  has a take-out port  12   a  on a sidewall thereof. The take-out port  12   a  is provided for taking out an SiC single crystal that grows in the vacuum chamber  10 . The upper chamber  12  has an upper opening portion. The upper opening portion is covered with a top flange  13 . The top flange  13  is made of SUS, for example. An exhaust pipe  14  is coupled with a portion of the upper chamber  12  other than the take-out port  12   a.  The exhaust pipe  14  connects an outside of the vacuum chamber  10  and an inside of the vacuum chamber  10 . The exhaust pipe  14  is coupled with a vacuum pump (not shown) through a pressure control valve  15  including an actuator. The pressure control valve  15  can function as a pressure control element. The vacuum chamber  10  is evacuated by the vacuum pump and the pressure control valve  15 . Thus, a pressure in the vacuum chamber  10  is controlled by the vacuum pump and the pressure control valve  15 . 
     The lower chamber  11  is made of quartz, for example. The lower chamber  11  has a lower opening portion. The lower opening portion is covered with a bottom flange  16 . The bottom flange  16  is made of SUS, for example. The crucible  30  is disposed in the lower chamber  11  and is surrounded by a first heat insulating member  17 . 
     The crucible  30  has an approximately cylindrical shape having a bottom. The crucible  30  opens at an upper portion thereof. The crucible  30  defines an internal space  31  surrounded by an upper end of the crucible  30 , a sidewall of the crucible  30 , and a bottom of the crucible  30  including the through hole  30   a.  Thus, the internal space  31  is also a portion of the internal space of the vacuum chamber  10 . A pedestal  32  is located in the internal space  31  of the crucible  30  so as to be apart from the sidewall of the crucible  30 . On a lower surface of the pedestal  32 , an SiC single crystal substrate  33  as a seed crystal is attached. The internal space  31  of the crucible  30  provides a reaction room where the SiC single crystal grows on the SiC single crystal substrate  33 . 
     The crucible  30  is made of, for example, a high purity graphite that can withstand temperature of about 2400 degree centigrade. When the crucible  30  is made of the high purity graphite, the amount of impurities generated from the heated crucible  30  can be restricted. Thus, the SiC single crystal that grows in the crucible  30  can be restricted from taking in the impurities. 
     The pedestal  32  is fixed at a lower end of a shaft  18  extending in the vertical direction. The shaft  18  is coupled with a rotating and vertically moving device (not shown). The rotating and vertical moving device can rotate and vertically move the shaft  18 . That is, the rotating and vertical moving device can move the shaft  18  upward from a position illustrated in  FIG. 1  so that the grown SiC moves to a take-out room defined by the upper chamber  12 . In addition, the rotating and vertically moving device can move the SiC single crystal substrate  33  from the take-out room to the internal space  31  of the crucible  30 . Furthermore, the rotating and vertically moving device can rotate the shaft  18  in a state where the SiC single crystal substrate  33  is located in the internal space  31  so that the SiC single crystal substrate  33  rotates and the SiC single crystal grows. The shaft  18  has a pipe shape, for example. The shaft  18  has a lower section made of quartz and an upper section made of SUS. 
     The through hole  30   a  is provided at an approximately center portion of the bottom of the crucible  30  and is surrounded by a peripheral portion of the bottom. A portion of the internal space of vacuum chamber  10  located below the crucible  30  is a low-temperature area. The internal space  31  of the crucible  30  communicates with the low-temperature area through the through hole  30   a.  The gas introducing pipe  50  connects the outside of the vacuum chamber  10  and the inside of the vacuum chamber  10 . The gas introducing pipe  50  has an upper end portion  51  located below the crucible  30 . The gas introducing pipe  50  supplies the source gas from the outside of the vacuum chamber to the SiC single crystal substrate  33  in the internal space  31  of the crucible  30 . The source gas is a mixed gas including a silicon-including gas, a carbon-including gas, and at least one of hydrogen, helium, and argon. The silicon-including gas is monosilane gas, for example. The carbon-including gas is propane gas, for example. 
     The gas introducing pipe  50  is apart from the crucible  30  and the cooler  70  is disposed adjacent to the gas introducing pipe  50 . The crucible  30  is arranged so that the source gas can be supplied from the gas introducing pipe  50  to the SiC single crystal substrate  33  in the internal space  31  and cooling water leaking from the cooler  70  can drop below the internal space  31 . In the present embodiment, the gas introducing pipe  50  is arranged in such a manner that the upper end portion  51  is located below the bottom of the crucible  30 , as illustrated in  FIG. 1  and  FIG. 2 . Thus, the gas introducing pipe  50  is located outside the internal space  31  of the crucible  30 . The gas introducing pipe  50  is made of SUS, for example. 
     In the vacuum chamber  10 , the cooler  70  is disposed adjacent to the gas introducing pipe  50 . The cooler  70  reduces a surrounding temperature by circulating cooling water therein, and thereby the cooler  70  provides a predetermined temperature gradient to the gas introducing pipe  50 . As a result, a different in temperature of the source gas between the gas introducing pipe  50  and the crucible  30  can be reduced and a clog of the gas introducing pipe  50  due to deposition of Si and SiC can be restricted. The temperature of gas introducing pipe  50  increases toward the crucible  30 . The cooler  70  is apart from the crucible  30  so that a high temperature of the internal space  31  is maintained and a damage of the crucible  30  due to the difference in temperature is restricted. As illustrated in  FIG. 2 , the cooler  70  includes a body  71  having an approximately U-shape in cross section. The body  71  is made of SUS, for example. An upper end of the body  71  is joined at an upper end of a sidewall of the gas introducing pipe  50 . A lower end of the body  71  is joined at a portion below the joint portion of the upper end of the body  71 . The body  71  and the sidewall of the gas introducing pipe  50  define a storing space  72  therebetween. The cooling water circulates through the storing space  72 . The body  71  of the cooler  70  and the gas introducing pipe  50  can function as a wall of the cooler  70 . As illustrated in  FIG. 3 , the cooler  70  has an approximately ring shape surrounding the gas introducing pipe  50 . The body  71  has an upper section  71   a  facing the crucible  30 , a lower section  71   b  facing the bottom flange  16 , and a side section  71   c  connecting the upper section  71   a  and the lower section  71   b.  In an example illustrated in  FIG. 3 , the body  71  has a substantially uniform thickness at the upper section  71   a,  the lower section  71   b,  and the side section  71   c.  A thickness of the gas introducing pipe  50  in the horizontal direction is greater than the thickness of the body  71 . 
     The cooler  70  is coupled with an inlet pipe  73  and an outlet pipe  74 . Each of the inlet pipe  73  and the outlet pipe  74  penetrate through the lower section  71   b  of the body  71 . The inlet pipe  73  connecting the vacuum chamber  10  and the inside of the vacuum chamber  10 . The inlet pipe  73  supplies the cooling water from an outside of the vacuum chamber  10  to the storing space  72  of the cooler  70 . The outlet pipe  74  connecting the outside of the vacuum chamber  10  and the inside of the vacuum chamber  10 . The outlet pipe  74  drains the cooling water from the storing space  72  of the cooler  70  to the outside of the vacuum chamber  10 . As illustrated in  FIG. 3 , the inlet pipe  73  and the outlet pipe  74  are arranged on opposite sides of the gas introducing pipe  50 . As illustrated in  FIG. 1 , the inlet pipe  73  is provided with a supply control valve  75  at the outside of the vacuum chamber  10 . The supply control value  75  can function as a supply control element. The supply control valve  75  controls the amount of cooling water to be supplied to the storing space  72  of the cooler  70 . The outlet pipe  74  is provided with a check valve  76  at the outside of the vacuum chamber  10 . The check valve  76  prevents the cooling water from being drawn in an opposite direction of a drain direction. 
     A predetermined section of the gas introducing pipe  50  from the upper end portion  51  to a portion located below a lower end of the cooler  70  is surrounded by a second heat insulating member  35 . On an inner surface of the second heat insulating member  35 , a graphite layer  34  is disposed. Thus, at the predetermined section, the first heat insulating member  17  and the second heat insulating member  35  having the graphite layer  34  are located between the lower chamber  11  and the gas introducing pipe  50  integrated with the cooler  70 . An upper end of the second heat insulating member  35  including the graphite layer  34  is coupled with the peripheral portion of the bottom of the crucible  30 . Thus, a clearance is provided between the internal space  31  of the crucible  30  and a portion of internal space of the vacuum chamber  10  located below the second heat insulating member  35 , that is, the low-temperature area. Thereby, heat at the crucible  30  is restricted from escaping below the second heat insulating member  35 . If the cooler  70  is damaged and a leakage of the cooling water occurs, the cooling water drops to the portion of the internal space of the vacuum chamber  10  located below the second heat insulating member  35 . 
     The shielding plate  90  is disposed between the cooler  70  and the internal space  31  of the crucible  30 . The shielding plate  90  is outside a supply passage of the source gas provided from the gas introducing pipe  50  to the SiC single crystal substrate  33 . The shielding plate  90  restricts the cooling water from scattering in the internal space  31  of the crucible  30  when the body  71  of the cooler  70  is damaged and a leakage of the cooling water occurs. The shielding plate  90  can function as a shielding part. The shielding plate  90  is made of a material that has a high thermal conductivity and a high thermal resistance. For example, the shielding plate  90  may be made of a carbon material, SiC, or molybdenum. The shielding plate  90  may be configured so that the cooling water being in contact with the shielding plate  90  is difficult to evaporate compared with a case where the cooling water is in contact with the crucible  30 . For example, the shielding plate  90  may be made of a material having a specific heat lower than the crucible  30 . The shielding plate  90  may also be made of a carbon porous material having a bulk density of about 1.0 g/cm 3 . As described above, the cooler  70  and the gas introducing pipe  50  are located below the crucible  30 . The shielding plate  90  has a ring shape and is fixed to a wall of the through hole  30   a  provided at the bottom of the crucible  30 . A thickness of the shielding plate  90  in the vertical direction is less than a thickness of the bottom of the crucible  30 . The shielding plate  90  expands in the horizontal direction and is fixed to the wall of the through hole  30   a,  for example, by screwing. In the horizontal direction, a center of the shielding plate  90  having the ring shape is approximately corresponding to a center of the gas introducing pipe  50  and a center of the SiC single crystal substrate  33 . As illustrated in  FIG. 3 , an internal circumference of the ring shape of the shielding plate  90  is located outside the gas introducing pipe  50  in the horizontal direction. The supply passage of the source gas is defined as an area connecting an opening provided at the upper end portion  51  of the gas introducing pipe  50  and horizontal ends of the SiC single crystal substrate  33 , that is, an area between the dashed lines in  FIG. 2 . 
     As illustrated in  FIG. 1 , a temperature sensor  19  is disposed in the portion of the internal space of the vacuum chamber  10  located below the crucible  30 . With a controller  26 , the temperature sensor  19  can function as a leakage detecting element for detecting a leakage of the cooling water if the body  71  of the cooler  70  is damaged and the cooling water leaks. The temperature sensor  19  includes a thermocouple. On the bottom flange  16 , a ring-shaped wall  20  is disposed. The ring-shaped wall  20  is made of, for example, SUS. The ring-shaped wall  20  and the bottom flange  16  provide a lower storing portion  21 . The temperature sensor  19  is disposed in the lower storing portion  21  so as to be located just under the clearance between the cooler  70  and the second heat insulating member  35 . 
     In a portion of the internal space of the vacuum chamber  10  located above the crucible  30 , a cold trap  22  and an upper storing portion  23  are provided. If the body  71  of the cooler  70  is damaged and the leaking cooling water evaporates into vapor, the cold trap  22  traps and cools the evaporated water. The upper storing portion  23  stores water trapped by the cold trap  22 . In the present embodiment, the upper storing portion  23  is disposed at a lower end portion of the upper chamber  12 . The upper storing portion  23  has a predetermined depth. The cold trap  22  is disposed just over the upper storing portion  23 . 
     An upper RF coil  24  is put around an outer peripheral portion of the vacuum chamber  10  at a height similar to the SiC single crystal substrate  33 . When electricity is supplied to the upper RF coil  24 , the upper RF coil  24  can heat the SiC single crystal substrate  33  during the growth of the SiC single crystal. Below the upper RF coil  24 , a lower RF coil  25  is put around the outer peripheral portion of the vacuum chamber  10 . When electricity is supplied to the lower RF coil  25 , the lower RF coil  25  can heat a lower section of the crucible  30  and an upper section of the gas introducing pipe  50 . Each of the upper RF coil  24  and the lower RF coil  25  is a high-frequency dielectric heating coil and can function as a heating element. 
     The controller  26  has a determining section and a control section. The determining section determines whether the cooling water leaks from the cooler  70  based on a detecting signal from the temperature sensor  19 . The control section controls the actuators based on the determining result of the determining section. That is, the temperature sensor  19  and the determining section of the controller  26  can function as the leakage detecting element. The controller  26  is electrically coupled with the actuator for controlling an opening degree of the pressure control valve  15 , an actuator for controlling outputs of the upper RF coil  24  and the lower RF coil  25 , and the actuator for controlling an opening degree of the supply control valve  75 . The control section of the controller  26  controls the actuators based on the determining result of the determining section. That is, the control section of the controller  26  include a first control part for controlling the supply amount of the cooling water, a second control part for controlling the outputs of the upper RF coil  24  and the lower RF coil  25 , and a third control part for controlling the opening degree of the pressure control valve  15 . The apparatus  1  further includes a warning device  27 . The warning device  27  warns that a leakage of the cooling water occurs based on a leakage detecting signal from the controller  26 . 
     An exemplary method of producing an SiC single crystal using the apparatus  1  will now be described. The SiC single crystal substrate  33  used as a seed crystal is attached to the lower surface of the pedestal  32 . Then, the SiC single crystal substrate  33  is arranged at a predetermined portion in the internal space  31  of the crucible  30  by controlling the shaft  18 . The vacuum chamber  10  is evacuated through the exhaust pipe  14 . The upper RF coil  24  and the lower RF coil  25  are supplied with electricity so as to heat the SiC single crystal substrate  33 , the crucible  30 , and the upper section of the gas introducing pipe  50 . The temperature of the crucible  30  is maintained at a predetermined temperature higher than 1420 degree centigrade that is the melting temperature of Si. For example, the temperature of the crucible  30  is maintained at a temperature about 2400 degree centigrade at which SiC is capable of sublimating. In addition, the pressure in the vacuum chamber  10  is set to be a predetermined pressure, for example, by introducing argon gas. Then, the source gas is introduced from the gas introducing pipe  50  to the internal space  31  of the crucible  30 . Thereby, the SiC single crystal grows on the SiC single crystal substrate  33 . 
     As an example of a control by the controller  26 , an exemplary method of controlling the supply control valve  75  will be described with reference to  FIG. 4 . 
     When a production of the SiC single crystal starts, that is, when the source gas is introduced from the gas introducing pipe  50  to the internal space  31  of the crucible  30 , the temperature sensor  19  starts to detect temperature and outputs the detecting signal to the controller  26  at S 10 . 
     At S 20 , the determining section of the controller  26  determines whether the cooling water leaks from the cooler  70  based on the detecting signal from the temperature sensor  19 , for example, by comparing the detecting signal with a threshold value. If the determining section of the controller  26  determines that the cooling water does not leak from the cooler  70 , the process at S 10  and S 20  is repeated and the production of the SiC single crystal continues. 
     If the determining section of the controller  26  determines that the cooling water leaks from the cooler  70 , the control section of the controller  26  outputs a signal to the actuator for controlling the opening degree of the supply control valve  75  provided at the inlet pipe  73  of the cooling water. Thereby, the supply control valve  75  is closed, and the supply of the cooling water to the storing space  72  of the cooler  70  is stopped at S 30 . 
     At S 40 , the controller  26  outputs the signal to the warning device  27  so as to inform that a leakage of the cooling water occurs. Then, the warning device  27  warns that a leakage of the cooling water occurs, for example, by using a buzzer or a warning light. Furthermore, the production of the SiC single crystal is stopped. 
     In  FIG. 4 , the exemplary method of controlling the opening degree of the supply control valve  75  based on the determination of whether a leakage of the cooling water occurs is illustrated as an example. The opening degree of the pressure control valve  15 , the outputs of the upper RF coil  24  and the lower RF coil  25  can be controlled in a manner similar to the above-described method. When the determining section of the controller  26  determines that a leakage of the cooling water occurs at S 20 , the control section of the controller  26  outputs a signal to the actuator of the pressure control valve  15  provided at the exhaust pipe  14  so that the opening degree of the pressure control valve  15  becomes a fully open state and gas in the vacuum chamber  10  is exhausted to the outside of the vacuum chamber  10 . In addition, the control section of the controller  26  outputs a signal to the actuator of the upper RF coil  24  and the lower RF coil  25  so that the outputs of the upper RF coil  24  and the lower RF coil  25  are reduced or stopped. 
     In the apparatus  1  according to the present embodiment, the water-cooling type cooler  70  is arranged adjacent to the upper end portion of the gas introducing pipe located below the crucible  30 . The shielding plate  90  extending to the horizontal direction is fixed to the wall of the through hole  30   a  provided at the bottom of the crucible  30  so that the shielding plate  90  does not disturb the supply of the source gas to the SiC single crystal substrate  33 . Thus, even if the cooler  70  for cooling the gas introducing pipe  50  is damaged and leaking cooling water scatters above the cooler  70 , the scattering cooling water is reflected by the shielding plate  90  as illustrated by the dashed arrow in  FIG. 2 . The temperature of the shielding plate  90  is lower than the temperature of the crucible  30 . The cooling water drops to the lower storing portion  21  through the clearance between the cooler  70  and the second heat insulating member  35  having the graphite layer  34 . The lower storing portion  21  is provided at the low-temperature area in the vacuum chamber  10  located below the crucible  30 . In this way, the apparatus  1  can restrict the cooling water from scattering into the internal space  31  of the crucible  30  located in the high-temperature area above the cooler  70 . Thus, the cooling water leaking from the cooler  70  does not scatter into the internal space  31  of the crucible  30 , and a rapid pressure increase in the vacuum chamber  10  due to evaporation of the cooling water can be reduced. 
     The shielding plate  90  is located above the cooler  70 . However, the shielding plate  90  has the ring shape and the inner circumference is larger than the upper end portion  51  of the gas introducing pipe  50  so that the shielding plate  90  does not disturb the supply of the source gas. Thus, the source gas can be supplied from the gas introducing pipe  50  to the SiC single crystal substrate  33  located in the internal space  31  of the crucible  30 . 
     The gas introducing pipe  50  and the cooler  70  is located below the crucible  30 . Thereby, a facing area of the cooler  70  and the internal space  31  of the crucible  30  except for the supply passage of the source gas is reduced. Thus, the scattering of the cooling water into the internal space  31  of the crucible  30  can be restricted by providing the shielding plate  90  only above the cooler  70 . Therefore, an area of the shielding plate  90  can be reduced and a configuration of the shielding plate  90  can be simplified. 
     Even if the cooler  70  is damaged, a leakage of the cooling water can be detected by the temperature sensor  19  and the controller  26 . When the controller  26  detects a leakage of the cooling water, the controller  26  outputs the signal to the actuator of the supply control valve  75  provided at the inlet pipe  73  so that the supply of the cooling water to the cooler  70  is stopped. Thereby, the amount of the cooling water leaking from the cooler  70 , that is, the amount of cooling water to be evaporated can be reduced. As a result, a rapid pressure increase in the vacuum chamber  10  can be restricted. 
     In addition, when the controller  26  detects a leakage of the cooling water, the controller  26  controls the actuator for controlling the outputs of the upper RF coil  24  and the lower RF coil  25  so that the outputs of the upper RF coil  24  and the lower RF coil  25  are reduced or stopped. Thereby, the evaporation of the leaking cooling water can be restricted and a rapid pressure increase in the vacuum chamber  10  can be restricted. 
     Furthermore, when the controller  26  detects a leakage of the cooling water, the controller  26  controls the actuator of the pressure control valve  15  provided at the exhaust pipe  14  so that gas in the vacuum chamber  10  is exhausted to the outside of the vacuum chamber  10 . Thereby, the pressure in the vacuum chamber  10  can be reduced and a rapid pressure increase in the vacuum chamber  10  can be restricted. 
     The outlet pipe  74  coupled with the cooler  70  is provided with the check valve  76  at a portion located outside the vacuum chamber  10 . Thus, the outlet pipe  74  drains the cooling water from the storing space  72  of the cooler  70  to the outside of the vacuum chamber  10  and a suction of fluid from the outlet pipe  74  can be restricted. The supply of the cooling water to the cooler  70  is stopped by fully closing the supply control valve  75 . Even if the storing space  72  of the cooler  70  becomes empty, fluid, for example, air is not suctioned from the outlet pipe  74  to the storing space  72  of the cooler  70 . Thereby, the apparatus  1  can prevent the possibility that the suctioned air leaks from the cooler  70 , the leaking air reacts with the source gas in the internal space  31  of the crucible  30 , and the apparatus  1  is damaged due to a heat generation and or a volume expansively. 
     The cold trap  22  and the upper storing portion  23  are provided between the crucible  30  and the exhaust pipe  14 . Even if the cooler  70  is damaged, the cooling water evaporated in the internal space  31  of the crucible  30  is changed into water at the cold trap  22  and is stored at the upper storing portion  23 . Thus, the rapid pressure increase due to the evaporation of the cooling water can be restricted. 
     In the apparatus  1  illustrated in  FIG. 1 , the gas introducing pipe  50  and the cooler  70  are located below the crucible  30 , and the shielding plate  90  is fixed at the bottom of the crucible  30 , as an example. The location of the shielding plate  90  is not limited to the above-described example. For example, the shielding plate  90  may be fixed to the gas introducing pipe  50  or the cooler  70 . In an apparatus according to a modification of the first embodiment, the shielding plate  90  is made of a porous material such as graphite and has an inverted L shape in cross section, as illustrated in  FIG. 5 . The shielding plate  90  has a short section  90   a  and a long section  90   b.  The shielding plate  90  is fixed, for example, by screwing, in such a manner that the short section  90   a  facing the upper section  71   a  of the body  71  of the cooler  70  is in contact with the upper end portion  51  of the gas introducing pipe  50  and a part of the upper section  71   a  of the body  71 , and the long section  90   b  is in contact with the side section  71   c  of the body  71 . An opening portion of the short section  90   a  corresponds to the opening portion of the gas introducing pipe  50 . A lower end of the long section  90   b  is located below the lower section  71   b  of the body  71 . In the present case, the cooling water leaking from the cooler  70  sinks in the shielding plate  90 , and then the cooling water drops downward. Thus, the scattering of the cooling water reflected by the shielding plate  90  can be restricted. Because the cooling water sinks in the shielding plate  90 , the cooling water can escape downward even through a clearance is not provided between the cooler  70  and the shielding plate  90 . In addition, because the leaking cooling water can escape to only lower side of the cooler  70 , the scattering of the cooling water into the internal space  31  of the crucible  30  can be effectively restricted. Furthermore, the shielding plate  90  made of the porous material has a high heat insulation. Thus, a heat damage of the body  71  of the cooler  70  can be reduced. 
     In the apparatus illustrated in  FIG. 5 , the shielding plate  90  made of the porous material is fixed so as to be in contact with the cooler  70 . When the shielding plate  90  is made of a material in which the cooling water does not sink, that is, a material reflecting the cooling water, the long section  90   b  is separated from the side section  71   c  of the body  71  so as to provide a clearance therebetween. In such a case, the cooling water leaking from the upper section  71   a  of the body  71  can escape downward trough the clearance. 
     In the apparatus illustrated in  FIG. 5 , the shielding plate  90  made of the porous material has the inverted L shape in cross section. The shielding plate  90  made of the porous material may have other shape. For example, the shielding plate  90  having a flat plate shape as illustrated in  FIG. 2  may be made of a porous material. 
     In the apparatus  1  according to the present embodiment, a leakage of the cooling water from the cooler  70  is detected using the temperature sensor  19  and the controller  26 . If a leakage of the cooling water is detected, in accordance with the signals from the controller  26 , the supply of the cooling water to the cooler  70  is stopped, the gas in the vacuum chamber  10  is exhausted, and the outputs of the upper RF coil  24  and the lower RF coil  25  are reduced or stopped. However, the apparatus  1  is required to include at least the shielding part, for example, configured by the shielding plate  90 . The apparatus  1  may include at least one of the above-described three control parts. 
     In the apparatus  1  according to the present embodiment, the cooler  70  and the gas introducing pipe  50  are integrated. In the present case, the cooler  70  can effectively cool the gas introducing pipe  50  and can provide the predetermined temperature gradient. Alternatively, the cooler  70  may be separated from the gas introducing pipe  50 . Also in such a case, the shielding part, for example, configured by the shielding plate  90  is disposed in such a manner that the shielding part is outside the supply passage of the source gas, and the shielding part restricts the cooling water from scattering into the internal space  31  of the crucible  30  and drops the cooling water below the internal space  31 . 
     Second Embodiment 
     An apparatus  1  for producing an SiC single crystal according to a second embodiment of the present invention will be described with reference to  FIG. 6 . Because the apparatus  1  according to the present embodiment has many portions in common with the apparatus  1  according to the first embodiment, a description of the common portions will be omitted and different portions will be mainly described. 
     In the apparatus  1  according to the present embodiment, a portion of the cooler  70  is located in the internal space  31  of the crucible  30 , that is, above an outer surface of the bottom of the crucible  30 . 
     In an example illustrated in  FIG. 6 , a portion of the gas introducing pipe  50  from the upper end portion  51  to a predetermined position is located in the internal space  31  of the crucible  30  through the through hole  30   a.  The cooler  70  is integrated with the gas introducing pipe  50  in a manner similar to the first embodiment. Thus, a portion of the cooler  70  from the upper section  71   a  to a predetermined position of the side section  71   c  is also located in the internal space  31  of the crucible  30 . Between the crucible  30  and the cooler  70  integrated with the gas introducing pipe  50 , a clearance is provided so that the internal space  31  communicates with the low-temperature area of the vacuum chamber  10  located below the crucible  30 . 
     The shielding plate  90  has the inverted L shape in cross section in a manner similar to the example illustrated in  FIG. 5 . The short section  90   a  extends in the horizontal direction from the outer end portion  51  of the gas introducing pipe  50  to an outside of the cooler  70  so as to face the whole area of the upper section  71   a  of the body  71 . The long section  90   b  extends in the vertical direction from a position upper than the cooler  70  to a position lower than the cooler  70  so as to face the whole area of the side section  71   c  of the body  71 . Thus, the whole are of the short section  90   a  and a portion of the long section  90   b  are located in the internal space  31  of the crucible  30 . The shielding plate  90  is fixed, for example, by screwing in a state where the short section  90   a  is in contact with the upper end portion  51  of the gas introducing pipe  50  and a portion of the upper section  71   a.  The shielding plate  90  is made of a material that can reflect the cooling water. A clearance is provided between the short section  90   a  and the other portion of the upper section  71   a  and between the long section  90   b  and the side section  71   c.  The cooling water leaking from the cooler  70  can escape downward through the clearance. 
     In the apparatus  1  according to the present embodiment, a portion of the cooler  70  is located in the internal space  31  of the crucible  30 . However, the scattering of the cooling water in the internal space  31  of the crucible  30  can be restricted by the shielding plate  90 , and the cooling water can be dropped to the low-temperature area of the vacuum chamber  10  located below the crucible  30 . Thus, a rapid pressure increase due to evaporation of the cooling water can be restricted. 
     The shielding plate  90  is provided to at least the portion of the cooler  70  facing the internal space  31  of the crucible  30 . Thus, the lower end of the long section  90   b  is located at a height corresponding to the outer surface of the bottom of the crucible  30  or lower than the outer surface of the bottom. A configuration of the present apparatus  1  except for the shape of the shielding plate  90 , the locations of the gas introducing pipe  50 , the cooler  70 , and the shielding plate  90  may be similar to the above-described configuration of the apparatus  1  according to the first embodiment. 
     Third Embodiment 
     An apparatus for producing an SiC single crystal according to a third embodiment of the present invention will be described with reference to  FIG. 7 . Because the apparatus  1  according to the present embodiment has many portions in common with the apparatus  1  according to the first embodiment, a description of the common portions will be omitted and different portions will be mainly described. 
     In the apparatuses  1  according to the first embodiment and the second embodiment, even if the cooling water leaks from the cooler  70 , the cooling water is blocked by the shielding plate  90  so that the cooling water does not scatter in the internal space  31  of the crucible  30 . In the apparatus  1  according to the present embodiment, the thickness of the body  71  of the cooler  70  is different by location so that even if the cooler  70  is damaged, the cooling water leaks from an opposite side of the internal space  31  of the crucible  30 . In an example illustrated in  FIG. 7 , the shielding plate  90  is not provided. 
     In the example illustrated in  FIG. 7 , the gas introducing pipe  50  and the cooler  70  are located below the crucible  30 . The body  71  of the cooler  70  has an approximately U-shape in cross section. The body  71  is made of SUS, for example. The upper end of the body  71  is joined at the upper end of the sidewall of the gas introducing pipe  50 . The lower end of the body  71  is joined at a portion lower than the joint portion of the upper end of the body  71 . The body  71  and the sidewall of the gas introducing pipe  50  define the storing space  72  through which the cooling water circulates. The cooler  70  has a ring shape surrounding the gas introducing pipe  50 . The cooler  70  is coupled with the inlet pipe  73  and the outlet pipe  74 . Each of the inlet pipe  73  and the outlet pipe  74  penetrate through the lower section  71   b  of the body  71 . The inlet pipe  73  supplies the cooling water from the outside of the vacuum chamber  10  to the storing space  72  of the cooler  70 . The outlet pipe  74  drains the cooling water from the storing space  72  of the cooler  70  to the outside of the vacuum chamber  10 . 
     The thickness of the upper section  71   a  of the body  71  facing the internal space  31  of the crucible  30  is defined as T 1 . The thickness of the lower section  71   b  of the body  71  facing the bottom flange  16  is defined as T 2 . The thickness of the side section  71   c  connecting the upper section  71   a  and the lower section  71   b  and facing the second heat insulating member  35  is defined as T 3 . In the example illustrated in  FIG. 7 , the thicknesses T 1 , T 2 , and T 3  satisfy a relationship of T 1 &gt;T 3 &gt;T 2 . That is, the thickness T 1  of the upper section  71   a  is the thickest and the thickness T 2  of the lower section  71   b  is the thinnest. Therefore, the upper section  71   a  has the highest mechanical strength and the lower section  71   b  has the lowest mechanical strength. The thickness of the gas introducing pipe  50  in the horizontal direction is greater than the thickness T 1  of the upper section  71   a.    
     In the body  71  of the cooler  70 , the lower section  71   b  having the lowest mechanical strength is more likely to be damaged, and the upper section  71   a  having the highest mechanical strength is less likely to be damaged. Thus, even if the cooler  70  is damaged, the cooling water is less likely to scatter toward the internal space  31  of the crucible  30 . Therefore, the scattering of the cooling water in the internal space  31  can be restricted and a rapid pressure increase in the vacuum chamber  10  due to evaporation of the cooling water can be restricted. 
     The thickness of the lower section  71   b  is thinner than the thickness of the other section of the body  71 . Even if the lower section  71   b  is damaged, the cooling water does not scatter upward. Thus, a rapid pressure increase in the vacuum chamber  10  due to evaporation of the cooling water can be effectively restricted. However, the relationship among the thicknesses T 1 , T 2 , and T 3  is not limited to T 1 &gt;T 3 &gt;T 2 . For example, the thicknesses T 1 , T 2 , and T 3  may satisfy a relationship of T 1 ≧T 2 &gt;T 2  or a relationship of T 1 &gt;T 3 ≧T 2 . At least the thickness of the lower section  71   b  is thinnest in the body  71 . 
     In the example illustrated in  FIG. 7 , the cooler  70  is located below the crucible  30 . Alternatively, a portion of the cooler  70  may be located in the internal space  31  of the crucible  30 . In such a case, a mechanical strength of a section of the body  71  facing the internal space  31  of the crucible  30 , that is, a section of the body  71  above the outer surface of the bottom of the crucible  30  is set to be greater then a mechanical strength of the other section of the body  71 . For example, in a case where the upper section  71   a  and a portion of the side section  71   c  are located in the internal space  31  of the crucible  30 , a thickness of a section of the body  71  located below the outer surface of the bottom of the crucible  30 , at least, a thickness of the lower section  71   b  is set to be thinner than the thickness of the section of the body  71  facing the internal space  31 . 
     In the example illustrated in  FIG. 7 , the mechanical strength of the body  71  is varied by location by controlling the thickness. The difference in the mechanical strength may be provided by other way. For example, in a case where the body  71  is more likely to be damaged at a welded portion, the welded portion may be provided at a section of the body  71  other than the section facing the internal space  31  of the crucible  30 . 
     A configuration of the present embodiment may be combined with the configuration of the first embodiment and/or the configuration of the second embodiment. 
     Other Embodiments 
     Although the present invention has been fully described in connection with the preferred embodiments thereof with reference to the accompanying drawings, it is to be noted that various changes and modifications will become apparent to those skilled in the art. 
     In the above-described embodiments, cooling water is used as an example of the fluid circulating in the storing space  72 . However, the fluid is not limited to cooling water. 
     In the above-described embodiments, the cooler  70  is disposed adjacent to the upper end portion  51  of the gas introducing pipe  50 , as an example. However, the location of the cooler  70  with respect to the gas introducing pipe  50  is not limited to the above-described examples. At least the cooler  70  is required to be disposed adjacent to the gas introducing pipe  50  so that the cooler  70  is outside the supply passage of the source gas provided from the gas introducing pipe  50  to the SiC single crystal substrate  33  located above the gas introducing pipe  50 . 
     A sensor for providing the leakage detecting element includes, but not limited to the temperature sensor  19 . For example, a fluid level sensor for detecting a level of cooling water may be stored in the lower storing portion  21 . 
     In the above-described embodiments, the leakage detecting element is provided by the temperature sensor  19  and the determining section of the controller  26 . That is, the determining section is included in the controller  26 . Alternatively, the temperature sensor  19  may include the determining section and the controller  26  may include only the control section. 
     In the above-described embodiments, the controller  26  has a control section that can function as the first control part for controlling the actuator of the supply control valve  75  provided at the inlet pipe  73 , the second control part for controlling the actuator for controlling the outputs of the upper RF coil  24  and the lower RF coil  25 , and the third control part for controlling the actuator for controlling the actuator of the pressure control valve  15  provided at the exhaust pipe  14 . Alternatively, the first control part, the second control part, and the third control part may be provided separately.