1. Field of the Invention
The present invention relates to a vacuum pump with a dust collecting function for use when a vessel for a process, in which various processes of productions by reaction or melting and crystallization processes are carried out under a reduced pressure atmosphere evacuated by a vacuum pump, is used. The process may be a process of epitaxial growth for producing monocrystalline film of silicon, in which an amount of dust is produced when the reaction process or a melting and crystallization process take place and the produced dust flows into the vacuum pump together with the existing gas.
2. Description of the Related Arts In general, the process of productions by reaction and the melting and crystallization processes in a vessel for processing under a reduced pressure are carried out in vacuum. Therefore, the specific gravity of the gas which flows into a vacuum pump is very small. When the gas, together with the dust, flows into the vacuum pump, the gas flows appropriately but has less ability to convey the dust, and therefore a greater portion of the dust is accumulated in the vacuum pump. In prior arts, the increased amount of the accumulated dust prevents the satisfactory running of the vacuum pump to cause difficulty in continuing the running of the vacuum pump so that frequent operations to remove the dust in the vacuum pump are needed.
Also, there is a problem that, if the sizes of grains of the dust which flows together with the gas into the vacuum pump are large, the internal structures of the vacuum pump, such as rotors, collect the grains of the dust to which can lead to a failure or a stoppage of the vacuum pump.
To prevent dust from flowing into the vacuum pump attempts have been made to separate the dust by providing filters or the like between the vacuum pump and the dust producing device. There is a problem, however, in that the dust causes blocking of the through-paths in the filter which are then greatly reduced. The effective evacuation performance of the vacuum pump for the process of production by reaction and the melting and crystallization in the vessel for processing prevent the reaction process and the melting and crystallization in the vessel for processing from continuing.
It is possible to provide a dust separator for separating dust utilizing the flow of gas, such as a cyclone type separator, between the vacuum pump and the vessel for processing. However, in this case, to reduce the loss of the pressure by the cyclone type separator, if the cross-sectional area of the gas flow in the separator is increased, no sufficient gas flow velocity is obtained so that the satisfactory separation of the dust cannot be realized in the cyclone type separator. Since the process is carried out under high degree of vacuum in the vessel for processing, the amount of the flow of the gas entering into the vessel, coming out from the vessel and being sent to the vacuum pump is relatively small. Therefore, the ability of the vacuum pump to transfer the dust to discharge the dust is low, and accordingly the dust tends to be accumulated in the vacuum pump to lead to a stoppage of the vacuum pump.
Since the dust is discharged together with the gas from the vacuum pump, a large amount of dust flows into the exhaust gas processing system. Therefore, there is a problem that the exhaust gas processing system is quickly contaminated and this prevents the functioning of the system.