The first "dry pumps" based on the Roots/claw principle were launched in 1984 and have made a strong contribution to a revolution in two areas of positive-displacement pumping in the semiconductor industry. These are (a) the clean pumping of systems and load locks and (b) the continuous pumping of processes which generate copious quantities of particulate, condensate or corrosive material as reaction by-products. These pumps are now moving into their third generation. In this paper, the current state-of-the-art is reviewed, with particular reference to methods of achieving ultraclean pumping and to solutions to applications where ~ 100 g/day of particulate (such as aluminium chloride or silica) must be transmitted through the pump with very high reliability. Issues arising from exhaust management of large arrays of pumps, such as those encountered in modern semiconductor fabrication facilities, are also discussed.

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The ability of ‘‘dry’’ primary vacuum pumps of the roots/claw configuration to handle semiconductor processes which previously could be pumped only with very frequent maintenance and great expense, has represented a quantum advance in pumping technology. A detailed knowledge of the mechanism by which dusty by-products are generated and transported through the machine, together with the characteristics of particulate deposition, is required when such pumps are serving production wafer-processing equipment. Experiments are described which detail the performance of a pump under conditions of high rates of dust loading together with techniques, by which reliability can be enhanced.

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An absolute vacuum gauge exploiting molecular drag between coaxial discs, a configuration suggested by Langmuir, is being further investigated. The device operates in a range similar to that of the spinning rotor gauge. It consists of a silicon disc 92 mm in diameter suspended on a calibrated torsion fiber a few mm above a metal disc of the same size which rotates at speeds up to 50 000 rpm. Molecules leaving this rotor communicate a torque to the suspended disc by molecular drag. The angular deflection thus produced is sensed optically and is about 2° at 10−6 mbar. The critical factors relevant to absolute measurement, such as tangential momentum accommodation, geometry, and edge effects have been investigated and experimental comparisons with a spinning rotor gauge demonstrate that absolute pressure measurements with uncertainty better than ±5% are possible at pressures ∼ 10−6 mbar. Measurements of similar uncertainty will be possible at 10−7 mbar.