Conventional turbomolecular vacuum pumps include a housing having an inlet port, an interior chamber containing a plurality of axial pumping stages and an exhaust port. The exhaust port is typically attached to a roughing vacuum pump. Each axial pumping stage includes a stator having inclined blades and a rotor having inclined blades. The rotor and stator blades are inclined in opposite directions. The rotor blades are rotated at high speed by a motor to pump gas between the inlet port and the exhaust port. A typical turbomolecular vacuum pump may include nine to twelve axial pumping stages.
Variations of the conventional turbomolecular vacuum pump are known in the art. In one prior art configuration, one or more of the axial pumping stages are replaced with disks which rotate at high speed and function as molecular drag stages. This configuration is disclosed in U.S. Pat. No. 5,238,362 issued Aug. 24, 1993 to Casaro et al. A turbomolecular vacuum pump including an axial turbomolecular compressor and a molecular drag compressor in a common housing is sold by Varian, Inc. under Model No. 969-9007. Turbomolecular vacuum pumps utilizing molecular drag disks and regenerative impellers are disclosed in German Patent No. 3,919,529 published Jan. 18, 1990.
Molecular drag compressors include a rotating disk and a stator. The stator defines a tangential flow channel and an inlet and an outlet for the tangential flow channel. A stationary baffle, often called a stripper, disposed in the tangential flow channel separates the inlet and the outlet. As is known in the art, the momentum of the rotating disk is transferred to gas molecules within the tangential flow channel thereby directing the molecules toward the outlet
Another type of molecular drag compressor includes a cylindrical drum that rotates within a housing having a cylindrical interior wall in close proximity to the rotating drum. The outer surface of the cylindrical drum is provided with a helical groove. As the drum rotates, gas is pumped through the groove by molecular drag.
A prior art high vacuum pump is shown in FIG. 4. A housing 10 defines an interior chamber 12 having an inlet port 14 and an exhaust port 16. The housing 10 includes a vacuum flange 18 for sealing the inlet port to a vacuum chamber (not shown) to be evacuated. The exhaust port 16 is typically connected to a roughing vacuum pump (not shown). In cases where the vacuum pump is capable of exhausting to atmospheric pressure, the roughing pump is not required. Located within housing 10 is an axial turbomolecular compressor 20, which typically includes several axial turbomolecular stages, and a molecular drag compressor 22, which typically includes several molecular drag stages. Each stage of the axial turbomolecular compressor 20 includes a rotor 24 and a stator 26. Each rotor and stator has inclined blades as is known in the art. Each stage of the molecular drag compressor 22 includes a rotor disk 30 and a stator 32. The rotor 24 of each turbomolecular stage and the rotor 30 of each molecular drag stage are attached to a drive shaft 34. The drive shaft 34 is rotated at high speed by a motor located in a motor housing 38.
Turbomolecular vacuum pumps and related types of vacuum pumps are used in a wide variety of applications. In many applications, the physical size of the vacuum pump is an important system design consideration. For example, vacuum pumps are frequently used in semiconductor processing equipment that is located in or adjacent to clean room facilities, and strict limitations are placed on the size of the equipment. Another application requiring small size is portable instruments, such as miniature mass spectrometers. In such applications, the electric motor adds significantly to the size, weight and cost of the vacuum pump.
A prior art large capacity turbomolecular pump has been driven by a gas turbine, which in turn was driven by an air compressor. Because of the need for an air compressor, the prior art pump was expensive and required a tight rotary seal between the pump and turbine sections.
Accordingly, there is a need for vacuum pumps which are compact, which are low in cost and which are simple to manufacture.