Fabrication technique for microelectromechanical systems vertical comb-drive actuators on a monolithic silicon substrate

This article presents a technique to fabricate a microelectromechanical systems vertical comb-drive actuator on a monolithic silicon substrate. This technique employs only two photomasks. The first photomask defines all the critical patterns, including a set of movable upper hollow comb fingers, a set of fixed lower comb fingers, and a suspension spring to avoid the alignment problem and maintain a small finger gap. The second photomask selectively covers the upper fingers to obtain the released upper hollow fingers. The vertical comb-drive actuator is fabricated by deep reactive ion etching process on a monolithic silicon wafer using these uniquely designed photomasks to avoid the residual stress and stiction problems. Different lateral gaps between the adjacent lower solid and upper hollow fingers are obtained with various finger widths. The height of the comb fingers is 10.0 μm. The vertical offset between the two sets of comb fingers can be adjusted by controlling the process conditions. Both symmetric and asymmetric staggered comb-drives are achieved through process modifications. The mechanism of notching effect is discussed and addressed by multiple spacer oxide deposition. Silicon residue effect, which occurs during the lowering down etching process, is investigated from the fabrication aspect and resolved by combining isotropic and anisotropic etching processes. This kind of vertical comb-drive actuator can be widely applied in optical switches, scanning micromirrors, and sensors.