A monolithic Knudsen pump WITH 20 sccm flow rate using through-wafer ONO channels

This paper describes a lithographically microfabricated Knudsen pump for high gas flow. Knudsen pumps operate by thermal transpiration and require no moving parts. To achieve high gas flow, high-density arrays of microchannels (with over 4000 channels/mm²) are used in parallel. These vertically oriented microchannels have 2×120 μm² openings surrounded by 0.1 μm-thick silicon oxide-nitride-oxide (ONO) sidewalls. The thin ONO sidewalls provide thermal isolation between a heat sink formed within the Si substrate, and a Cr/Pt heater that provides a temperature bias for thermal transpiration. The Knudsen pump is monolithically microfabricated on a single wafer using a four-mask process. It has a footprint of 8×10 mm². It produces a measured air flow of 20 sccm (i.e., 0.8 sccm/mm²), with typical response times of 0.1–0.4 sec.