Blocking Pressure Enhancement in SOI Through-Wafer Monolithic Knudsen PUMPs

This paper reports the enhancement of the blocking pressure of Knudsen pumps (KPs) fabricated as through-wafer structures in silicon-on-insulator (SOI) wafers. Such pumps use thermal transpiration in narrow pumping channels with a longitudinal temperature gradient. For the pumps to efficiently provide high blocking pressure, the pumping channels must be both narrow and long, and must have thermally insulating sidewalls. The KPs described here incorporate dense arrays of 25 µm long pumping channels fabricated in the device layer of the SOI wafer. The channels have hydraulic diameters ≈1.2 µm, and 10 nm thick dielectric sidewalls that are highly insulating. At 0.5 W applied power, a 5-stage KP reported in this work provides a maximum flow rate of 0.36 sccm and blocking pressure of 2 kPa, which is suitable for certain microscale gas chromatographs. This performance, i.e. 4 kPa/W, is 45% higher than prior record of comparable KPs.