Si-Micromachined Knudsen Pumps for High Compression Ratio and High Flow Rate.

Abstract

This dissertation focuses on Si-micromachined Knudsen pumps. Knudsen pumps exploit thermal transpiration that results from the free-molecular flow in non-isothermal channels. The absence of moving parts, without frictional loss and mechanical failure, provides significantly higher reliability. For a high compression ratio, 48 stages are cascaded in series in a single chip of 10.35 × 11.45 mm2 area. A five-mask, single-wafer process is used for monolithic integration of the designed Knudsen pump. The pressure levels of each stage are measured by integrated Pirani gauges. Using 1.35 W, the fabricated pumps evacuates the encapsulated cavities from 760 to 50 Torr and from 250 to 5 Torr. Multistage Knudsen pumps are further explored using a two-part architecture. To increase the compression ratio, 162 stages are serially cascaded. The two-part architecture uses 54 stages designed for the pressure range of 760-50 Torr, and 108 stages designed for lower pressures. This approach provides greater compression ratio and speed than using a uniform design for each stage in the 48-stage Knudsen pump. The design has a footprint of 12 × 15 mm2. Using 0.39 W, the evacuated chamber is reduced from 760 to 0.9 Torr, resulting in a compression ratio of 844. The vacuum levels were sustained beyond 37 days of continuous operation. The dynamic calibration of microfabricated Pirani gauges is explored for increasing pressure measurement accuracy in the 162-stage Knudsen pump. Test results demonstrated that dynamic calibration can be significantly more accurate than conventional static calibration when Pirani gauges are embedded deep within a microfluidic pathway. Si-micromachined single-stage Knudsen pumps are explored for generating high-flow rates. A high density of thermal transpiration flow channels is arrayed in parallel for combined pumping operation. A design with 0.4 × 106 parallel channels in a footprint of 16 × 20 mm2 generates a measured 211 sccm air flow at a pressure difference of 92 Pa, using 37.2 W. The low-temperature atomic layer deposition (ALD) of Al2O3 is investigated for vacuum seals in wafer-level vacuum packaging applications. The conformal coverage provided by ALD Al2O3 is shown to seal micromachined cavities. Lifetime tests extending out to 19 months are reported.