SOI Through-Wafer Monolithic Knudsen Pumps for Microscale Gas Chromatography Applications

Abstract

Gas micropumps are essential components to generate gas flow and apply pressure in various microanalytical applications such as microscale gas chromatography (μGC). Knudsen pumps (KPs) are attractive for such applications because of the thermally driven, motionless operation that provides high reliability and long lifetime. In particular, silicon-on-insulator (SOI) through- wafer KP architectures are the most attractive type of KP for their high performance with the implementation of a number of well-controlled and lithographically defined pumping channels. This thesis explores two threads of SOI through-wafer KPs suitable for μGC applications. The first thread is KPs with suspended (S) membrane structures and the second thread is KPs with unsuspended (U) structure. The Gen S1 KP, the first generation KP of this thesis with suspended membrane structures, consists of four monolithically integrated stages that are fabricated by a five-mask lithographic process. Each pumping stage is fluidically connected to adjacent stages by lateral channels etched into glass dies that are bonded above and below the SOI die. The pumping channels are densely arrayed, vertically oriented, 1.2 μm wide rectangular channels with 10 nm thick Al2O3 sidewalls. A suspended membrane above the pumping channels houses the metal heater and provides thermal isolation to the substrate. While operating at ambient atmospheric pressure, the Gen S1 4-stage KP provides a blocking pressure of ≈3.3 kPa and maximum air flow rate of ≈0.75 sccm with 1.2 W input power. Such performance is suitable for providing separation gas flow in μGCs. The Gen S2 KP, the second generation KP of this thesis with suspended membrane structures, provides a major enhancement of blocking pressure performance by implementing the longest (25 xiii μm) vertical pumping channels in through-wafer KPs to date. The pumping channel length provides extremely high thermal isolation whereas the 1.2 μm width provides flow rates suitable for μGCs. At 0.5 W applied power, the Gen S2 5-stage KP provides a maximum flow rate of 0.36 sccm and blocking pressure of 2 kPa. This performance, i.e., 4 kPa/W, is 45% higher than prior record of comparable KPs. The Gen U1 is the mechanically robust KP with an unsuspended structure. With no suspended membrane, this structure significantly improves the mechanical robustness and simplifies the microfabrication process compared to the previously reported KPs that rely on suspended membranes. The unsuspended KP fabrication process eliminates the complicated trench refill, polish, and sacrificial etch steps that were required for micromachined KPs with suspended membranes. The reported KPs incorporate multiple parallel pumping channels to scale the gas flow. At atmospheric pressure, the 4-channel KP provides a blocking pressure of 620 Pa and maximum flow rate of 0.041 sccm through 0.001 mm2 channel area with 2 W applied power. Such a maximum flow rate per channel area is superior to that of the KPs with suspended membranes and complicated fabrication processes. The fabrication simplicity and reliability of this mechanically robust KP can enable monolithic integration onto lab-on-a-chip systems. Owing to their suspended membrane structures that provide high thermal isolation and a large number of parallel pumping channels, the Gen S1 and S2 KPs achieve high flow rate and power efficiency. In contrast, the Gen U1 KPs provide superior mechanical robustness and simple fabrication process. Both KP architectures provide similar level of blocking pressure, achieved by the similar temperature gradient along the pumping channels.