Abstract: A composite dry vacuum pump of this invention has closed chambers accommodating a pair of screw rotors and a pair of roots rotors respectively mounted coaxially to the front ends of the screw rotors. The roots rotors and the screw rotors are rotated at the same speed in opposite directions by a motor to aspirate fluids from a suction port on one end wall and discharge the fluid through a discharge port on the outer surface of the other end wall. The characteristics of power requirement of the portion of the roots rotors and that of the portion of the screw rotors changes inversely to the suction port pressure variation, therefore the characteristic of the total power requirement of the pump of this invention is almost flat over the all pressure range and the motor having smaller output power can operate the pump according to the present invention.

Abstract: A gas exhaust system for use in a semiconductor manufacturing process wherein mechanical or chemical processing is applied on silicon wafers or the like arranged in a air-tight process chamber thereof, comprises a vacuum chamber having a gas suction port formed on an upper side thereof and a gas exhaust port formed on a lower side thereof. In the vacuum chamber, the first and second rotors are arranged vertically and supported rotatably at their lower end port ions in a cantilevered condition. The first and second rotors are driven to rotate in the opposite directions to thereby form a certain degree of vacuum condition in the vacuum chamber, whereby the residual gasses in the process chamber are discharged therefrom through the gas exhaust system.