A Si-micromachined 48-stage Knudsen pump for on-chip vacuum

This paper describes a thermal transpiration-driven multistage Knudsen pump for vacuum pumping applications. This type of pump relies upon the motion of gas molecules from the cold end to the hot end of a channel in which the flow is restricted to the free molecular or transitional regimes. To achieve a high compression ratio, 48 stages are cascaded in series in a single chip. A five-mask, single silicon wafer process is used for monolithic integration of the designed Knudsen pump. The pump has several monolithically integrated Pirani gauges to experimentally measure the vacuum pumping characteristics of the pump. It has a footprint of 10.35 × 11.45 mm2. For an input power of 1350 mW, the fabricated pump self-evacuates the encapsulated cavities from 760 to ≈50 Torr, resulting in a compression ratio of 15. It also pumps down from 250 to ≈5 Torr, resulting in a compression ratio of 50. Each integrated Pirani gauge requires ≈3.9 mW of power consumption, and its response is sufficiently sensitive in the operating pressure range of 760–1 Torr. (Some figures may appear in colour only in the online journal)