Evacuation apparatus

An evacuation apparatus for evacuating a process gas in a vacuum chamber of a substrate processing apparatus for fabrication of semiconductors or liquid crystal panels to produce a vacuum in said vacuum chamber, said evacuation apparatus comprising:

• a two-stage Roots vacuum pump as a booster pump for evacuation of said vacuum chamber, said booster pump having an inlet port connected to said vacuum chamber via a pipe, an outlet port, and a pair of two-stage Roots-type pump rotors driven by a first motor and supported by bearings, the Roots-type pump rotors comprising an inlet-side Roots rotor as a first stage and an outlet-side Roots rotor as a second stage, and an axial width of said inlet-side Roots rotor being larger than an axial width of said outlet-side Roots rotor, said two-stage Roots vacuum pump is able to be started by said first motor only when a pressure of said outlet port of said two-stage Roots vacuum pump is lowered to a predetermined pressure,
• a screw vacuum pump as a main pump for evacuation of said outlet port of said two-stage Roots vacuum pump, said screw vacuum pump having an inlet port connected to said outlet port of said two-stage Roots pump via a pipe, an outlet port, and a pair of screw pump rotors driven by a second motor and supported by bearings, said screw vacuum pump is able to be started by said second motor at an atmospheric pressure in said inlet port of said screw vacuum pump,
• wherein said two-stage Roots vacuum pump is configured to operate at a pumping speed larger than a pumping speed of said screw vacuum pump,
• and an inert gas is supplied to said two-stage Roots vacuum pump and said screw vacuum pump so that said bearings of said two-stage Roots vacuum pump and said screw vacuum pump are prevented from being exposed to said process gas, and a controller for controlling each rotational speed of said first and second motors so that said screw vacuum pump starts first then said two-stage Roots vacuum pump starts after a time period when said pressure of said outlet port of said two-stage Roots vacuum pump is lowered to said predetermined pressure,
• wherein a pumping speed of said inlet-side rotor of said booster pump is in a range of 20,000 Liters/minute to 100,000 Liters/minute, and
• a pumping-speed ratio of said inlet-side rotor to said outlet-side rotor of said booster pump is in a range of 2:1 to 10:1.