A Monolithic High-Flow Knudsen Pump Using Vertical Al2O3 Channels in SOI

This paper describes the analysis, design, and microfabrication of a Knudsen pump for high flow generation. The Knudsen pump generates gas streams from the cold end to the hot end using free-molecular flow without moving parts. The designed Knudsen pump has densely arrayed vertically oriented 2-μm wide rectangular channels for providing high flow. The temperature gradient is provided between a thin-film metal heater and a gridded Si heat sink in the Knudsen pump structure. The Knudsen pump is fabricated on a single silicon-on-insulator wafer using a four-mask lithographic process. The sidewalls of the rectangular channels are constructed by atomic layer deposition of Al₂O₃ on sacrificial Si channels and subsequent Si etching. The fabricated Knudsen pumps have designed footprints ranging from 0.4-3.2 cm². At atmospheric pressure, the fabricated Knudsen pumps provide a maximum measured air flow rate of >200 sccm with response time of <;0.5 s. Experimental results have also shown that these Knudsen pumps are most suitable for pumping at moderate vacuum pressure of ≈200 Torr (27 kPa).