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Screw type vacuum pump with introduced inert gas
| Details |
Inventors: Naya, Koutarou; Shiinoki, Kazuaki; Hayakawa, Tadashi; Masujima, Kiyoshi; Uchida, Riichi; Matsubara, Katsumi;
Assignee: Hitachi, Ltd. (Tokyo, JP)
Primary Examiner: Vrablik; John J.
Assistant Examiner:
Attorney, Agent or Firm: Antonelli, Terry, Stout & Kraus
The screw type vacuum pump comprises a pump casing having a suction inlet and a discharge outlet, and a pair of rotors incorporated within the pump casing and meshing with each other to rotate in synchronized manner. The vacuum pump evacuates a reaction vessel, to which process gas is applied, to obtain a negative pressure condition of a predetermined level. Upon operation of the pump, inert gas is introduced into a compression working chamber directly or through a sealing portion disposed adjacent the discharge outlet. The introduced inert gas is adiabatically compressed therein to generate heat to heat the rotors and a casing wall. Further the inert gas effects a reduction of partial pressure of the process gas in the pump. Accordingly, a side reaction product is prevented from accumulating on the rotors and the casing wall. |
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DETAILED DESCRIPTION Accordingly an object of the present invention is to provide a screw type vacuum pump which can prevent such reaction products from accumulating on inner surfaces thereof so as to improve its reliability.
Another object of the present invention is provide a screw type vacuum pump which can prevent lubricating oil from entering into the working chambers thereof to obtain a clean vacuum.
Yet another object of the present invention is to provide a screw type vacuum pump in which lubricating oil separates from inert gas so as to improve the reliability of the lubrication.
To this end, according to the present invention, the pump is provided in a working chamber under the compression stroke with an inert gas introducing means through which inert gas in introduced thereinto.
A typical process for producing a silicon nitride film in a low-pressure CVD device may be expressed as follows: 3SiH.
sub.
2 Cl.
sub.
2 +10NH.
sub.
3 .
fwdarw.
SiN.
sub.
4 +6NH.
sub.
4 Cl+6H.
sub.
2 Ammonium chloride is generated as a side reaction product of this process.
The higher the pressure becomes, the higher the deposition of ammonium chloride due to the vapor pressure characteristics thereof.
As a result, in a screw type vacuum pump, ammonium chloride accumulates on the surfaces of the rotor portions and the casing portions which cooperate with each other to define working chambers contributing to the compression and the discharge strokes, respectively.
When an inert gas such as nitrogen gas is introduced into the working chambers, a partial pressure or a concentration of ammonium chloride in the mixture of such introduced inert gas and ammonium chloride is lowered, so that it becomes harder, for the ammonium chloride to deposit.
Further, the inert gas in the working chambers is adiabatically compressed by pumping operation to heat the rotors and the casing wall.
As a result, even though ammonium chloride is deposited as a side reaction product, it hardly adheres to or accumulates on the rotors and the casing wall
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