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

Micromachined Knudsen pumps are potentially useful for micro mass spectrometers and micro gas chromatographs. Knudsen pumps exploit the 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. In this talk, I will present my research toward the design, fabrication, and test of Si-micromachined Knudsen pumps. First, a 48-stage Knudsen pump and a two-part 168-stage Knudsen pump will be described. These pumps evacuate cavities from 760 to ‰^0.9 Torr, resulting in a compression ratio of ‰^844 the highest in micropumps. Second, dynamic calibration of embedded Pirani gauges will be explained. This method increases the accuracy of readings in complex fluidic networks. Third, low-temperature vacuum sealing using atomic layer deposition (ALD) of Al2O3 for wafer-level vacuum packaging will be presented. Finally, a single-stage Knudsen pump designed for high flow rates will be described. A high density of channels are integrated in a single chip and result in a measured 211 sccm air flow the highest in micropumps. I will discuss the results and conclude with future research directions.