Abstract:
A crystal pulling apparatus is disclosed which employs the Czochralski method. The crystal pulling apparatus is operated while a containing a crucible of molten material, while maintaining the growing chamber under a controlled pressure of less than atmospheric. In the event of a vacuum pump unexpectedly ceasing operation, power to the heater is terminated, thus allowing the molten material to solidify. In such an event, a second vacuum pump can readily be attached to the growing chamber thus restoring pressure control, and allowing power to the heater to be restored.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an apparatus and method for producing a single crystal by the Czochralski technique comprising an auxiliary vacuum port, and an auxiliary vacuum pump dedicated to the machine in the event of failure of the primary vacuum pump. 
     BACKGROUND OF THE INVENTION 
     In a conventional crystal growing apparatus employing the Czochralski (CZ) technique, charge material, such as silicon, gallium-arsenide, and the like, that is to be grown into a single crystal is loaded into a crucible. A circumferential heater surrounds the crucible, 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 one direction, and the crucible is rotated 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. 
     A main controller is connected to respective control circuits for drive mechanisms, limit switches, sensors, pressure control and the like, so as to completely control the crystal pulling apparatus. For safety reasons, the supply of power to the heater is interlocked with sensors to other key items such as the vacuum pump, inert purge gas, and a cooling water system. As such, if an anomaly occurs in the vacuum system, inert purge gas system, or the like, the power supplied to the heater is shut off for safety reasons. 
     During a main vacuum pump failure situation, in a relatively short time the molten charge material will begin to freeze into a solid form. Such solidification of the molten charge material can cause significant damage and potential danger. It is common for the charge material to be wasted, as well as the crucible and other parts supporting the crucible due to thermal expansion. The associated costs with a failure from inoperable machine time, lost charge material, broken or damaged crucible and related parts, and time needed to clean and repair the crystal growing apparatus are significant. Moreover, an abrupt solidification of a large amount of the charge material may cause a leak of the melt, which could in turn lead to grower damage, and potentially a steam explosion or other significant safety problem. 
     To maintain reduced pressure, a vacuum pump is run continuously during the crystal pulling process. This main vacuum pump is subjected to substantial quantities of silicon oxide dust, a byproduct of molten silicon. In the past, oil-sealed vacuum pumps were used. However, oil-sealed pumps require a substantial amount of power, and the oil is a contaminant to the vacuum chamber. 
     It is now common to use a dry vacuum pump as the main vacuum pump in a crystal growing apparatus. Dry vacuum pumps use less electrical power, which lowers the cost of ownership, and they do not have oil to contaminate the process chamber. In contrast to the oil seals used in an oil-sealed pump, a dry pump relies on extremely close tolerances between its rotors and stators to provide the necessary seals within the pump. However, the extremely small gaps between the rotors and stator of a dry vacuum pump can be filled by the silicon oxide dust, resulting in increased load on the pump motor. Left unchecked, this increased load could result in overload of the motor, tripping a breaker and causing a shutdown of the crystal growing process. Thus, there has been a demand for measures to secure greater safety, and to reduce the costs associated with such an incident. 
     SUMMARY OF THE INVENTION 
     The present invention has been accomplished in view of the above-mentioned problems, and it is an object of the present invention to provide an environment for maintaining a safe, stable process state within the crystal growing apparatus upon the loss of a main vacuum pump. 
     The present invention provides a method of connecting the crystal growing apparatus vacuum piping normally dedicated to the main vacuum pump to the auxiliary vacuum pump instead. After the auxiliary pump has been connected, the controller for the crystal pulling apparatus is able to re-establish gas flow, pressure control, and control of the heater. This prevents the freezing of the molten charge material, damage to the crucible or other equipment, and product loss. Even though the heating state is briefly interrupted during the switch over from the primary vacuum source to the auxiliary vacuum source, no problem will arise because the thermal capacity of the molten charge material is sufficiently large. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of a crystal pulling apparatus. 
     FIG. 2 is a schematic illustration of an embodiment of the present invention wherein the crystal pulling apparatus is operating under primary vacuum. 
     FIG. 3 is a schematic illustration of an embodiment of the present invention wherein the crystal pulling apparatus is operating under auxiliary vacuum. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An embodiment of the present invention will now be described with reference to the drawings. 
     As shown in FIG. 1., a crystal pulling apparatus  10  includes a bottom chamber  12 . The bottom chamber  12  houses a quartz crucible  14 , which is supported by a vertically moveable and rotatable susceptor and shaft assembly  16 . A cylindrical heater  18  made of, for example, graphite is disposed around the susceptor  16 , and is in turn surrounded by an 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 the main vacuum pump (not shown). 
     A top chamber  24  is disposed above the bottom chamber  12  while an isolation valve  22  is disposed therebetween. The top chamber  24  provides a space for accommodating a pulled 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 pulled 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 the auxiliary vacuum pump (not shown) that allows the top chamber to be evacuated of air and purge gases, so it may be rejoined with bottom chamber  12 . 
     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 . 
     When a single crystal is to be grown in the crystal pulling 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 with an inert gas. Subsequently, a charge material, such as silicon, is placed in the crucible  14 , and heated by the heater  18 , thereby making a material melt  36 . 
     The seed crystal  34  is lowered by the winding drum  28  until the end of the seed crystal  34  is lowered into the melt  36 . After allowing the seed crystal  34  to reach temperature equilibrium with the melt  36 , the winding drum  28  slowly begins to wind up the wire  30 , thus enabling a crystal  38  to be pulled. During the pulling operation, the winding mechanism  26  and thus the seed are rotating in the opposite direction of the susceptor assembly  16 . 
     A main controller (not shown) controls and monitors, among other things, the vacuum in the bottom chamber  12 . When vacuum failure occurs in the bottom chamber  12 , the power to the heater  18  is shut off. 
     Now turning to FIG. 2, exhaust gases flow from the bottom chamber through conduit  40  through a valve  42  and into the main vacuum pump  48 . In a preferred embodiment of the present invention, in the event of vacuum failure, power is shut off to the heater, and valve  42  closes to prevent backflow of exhaust gas back into the bottom chamber. In such a failure, a conduit  58  containing a very low cracking pressure check valve  54 , can be readily attached to conduit  40  through flange  52 . The opposite end of conduit  58  is then attached to conduit  70  through flange  60 , after making sure vacuum pump  68  is off, and opening cap  62 . When the conduit  58  is attached to both conduit  40  and conduit  70 , the operator can then open valve, thereby allowing the bottom chamber  12  to be evacuated by the auxiliary vacuum pump  68 , as illustrated in FIG.  3 . Purge gas flow can be re-initiated and the power to heater  18  can now be turned on, thus allowing the heater to maintain the melt  36  in a molten condition, thereby preventing freezing of the melt and damage to, for example, the crucible  14  and susceptor assembly  16 , through thermal expansion of the melt  36 . 
     After pressure control has been regained in the bottom chamber  12 , the failed main vacuum pump  48  can be disconnected from conduit  40  through flange  44 , and from the exhaust system (not shown) through flange  46 . The main vacuum pump  48  can now be replaced or fixed, and reinstalled. After main vacuum pump  48  has been reinstalled, power to the heater  18  is again stopped, valve  50  is closed and valve  42  is reopened. The power to the heater  18  is again supplied, and the main vacuum pump  48  now provides pressure control for the bottom chamber  12 . Vacuum pump  68  can now be shut off, and conduit  58  can now be removed, thus returning the crystal pulling apparatus to normal operating conditions. After allowing growing conditions to stabilize, crystal pulling may resume. 
     An alternate form of the present invention would provide permanent fixed conduit and valves to auxiliary vacuum pump  68 , with the main controller programmed such that in detection of a failed main vacuum pump  48 , all requisite valves arc actuated as described above in the manual method automatically, with a warning alarm activated to inform the operator. 
     Yet another alternative form of the present invention would allow for the  58  to be attached to flange  44  after the removal of primary vacuum pump  48  on one end, and attached to flange  60  of the auxiliary pump  68 , thus eliminating the need for valve  50 , flange  52 , and valve  54 . 
     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 specification 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 which come within the meaning and range of the equivalence of the claims are to be embraced within their scope.