High-aspect ratio vertical comb-drive actuator with small self-aligned finger gaps

A vertical comb-drive actuator with thin, high-aspect ratio comb fingers and small self-aligned gaps is presented. Key to the actuator design is the self-aligned, offset comb-drive fingers, which are fabricated with small gaps (≤ 2 μm) using a single lithography step. The offset comb fingers are fabricated using two thick conducting layers separated by a thin dielectric layer. The lower fingers are formed from the device layer (20 μm) of an silicon-on-insulator (SOI) substrate, while an in situ-doped polysilicon layer (20 μm), deposited in an epitaxial reactor (Epipoly), is used for the upper comb fingers. The Epipoly films have been optimized and characterized for application as structural and electrical components. The offset comb fingers are formed using a combination of deep-reactive ion etching (DRIE), thin oxide barrier layer growth, and an isotropic dry silicon etch (XeF₂) of selected areas of the Epipoly layer. The actuator has been implemented in a high fill-factor (>90%) micromirror array for optical telecommunications applications. Large continuous scan angles (±10°) with actuation voltages <60 V have been measured with no pull-in phenomena observed.