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.
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