I. Introduction
THE RECENT focus of the MEMS world on optical applications of micromachined devices has pushed the field out of surface micromachining technology [1]–[3]. This is mainly due to the need for very flat mirrors and the desire for the large actuation forces available using high aspect-ratio micromachining. Conversion of lateral actuation to rotation by applying actuation at points displaced from torsional suspensions' shear centers. Torsional suspensions are etched to Upper-beam level, and actuation arms are etched to Lower-beam level to apply push–pull force transferred from adjacent in-plane actuators (not shown in the figure). Fabrication schematic for three-level beam structures. Four masks, two on front- and two on back-side are prepared first, followed by back- and front-side DRIE high aspect ratio silicon etches. By moving to silicon-on-insulator (SOI) technology, the flatness issue is mostly ameliorated [4]. The biggest remaining obstacle in SOI MEMS is the demonstration of out-of-plane motion.