Monolithic SOI through-wafer Knudsen pumps with mechanically robust Si channels

Abstract

Knudsen pumps (KPs) generate gas flow using thermal transpiration in narrow channels with a longitudinal temperature gradient. With no moving parts, KPs offer long lifetime and are attractive for complex microfluidic systems. Although high performance was previously reported in microfabricated KPs that utilized suspended dielectric membranes to construct the pumping channels, this approach can limit the mechanical robustness and complicate the fabrication process. In this work, we present monolithically microfabricated KPs without using suspended membranes. These types of structures significantly improve the mechanical robustness, allow a wider process window, and simplify the microfabrication process. The pumping channels in this work are 14 µm in length, 1.2 × 200 µm2 in cross-section, vertically oriented, and with rigid silicon and dielectric sidewalls; the overall structures are easily manufacturable. This work primarily focuses on two core KP designs, which incorporate 4 and 6 parallel pumping channels in ≈ 11 and 17 mm2 footprints, respectively. The scaling capability is also investigated preliminarily in a design with 45 parallel pumping channels. The KPs were fabricated on a silicon-on-insulator wafer by a six-mask lithographic process with standard fabrication steps. This process eliminated the use of complicated trench refill, polish, and sacrificial dry etch steps that were required for the previously reported KPs with suspended membranes. At atmospheric pressure, the 4-channel KPs provided a blocking pressure of 620 Pa and maximum flow rate of 0.041 sccm through 0.001 mm2 channel area, using 2 W applied power. This maximum flow rate per unit channel area (sccm/mm2) is superior to that of the KPs with suspended membranes. For all the KP designs, the experimentally measured pumping performance well matched the modeling results, with <15 % discrepancy. The fabrication simplicity and reliability of the mechanically robust KPs can enable monolithic integration onto complex lab-on-a-chip systems.