Fabrication of single crystalline MEMS DM using anodic wafer bonding

Abstract

We report on the development of a new class of electrostatic MEMS deformable mirror (DM) fabricated through a combination of bulk micromachining, wafer bonding, and surface micromachining. The combination of these fabrication technologies introduces four major improvements over previous MEMS DMs, which are fabricated using surface micromachining alone. First, the MEMS DM structural components (mirror surface and actuator array) are made entirely of single crystalline silicon by use of the device layer of a whole 4-inch silicon-on-insulator (SOI) wafer bonded together via anodic bonding. Unlike current MEMS DMs fabricated entirely using surface micromachining, bulk micromachining steps in this fabrication process require no etch access holes, print through is inexistent, and no polishing steps are required. This leads to reduced diffraction of light from the mirror surface, improved mirror surface optical quality, and elimination of manufacturing processing steps. Second, through-wafer interconnects are used to connect the densely-packed electrostatic actuator array to driver electronics. This eliminates the need for wirebonding at the periphery of the DM, increasing the surface area available for actuators and removes the need for bulky wire bundles to connect the device to its driver. Third, by using the full area of a silicon wafer for each mirror, these MEMS DMs offer a larger optical aperture than any previously-reported MEMS DM. The larger aperture will achieve higher angular resolution, providing larger wavefront correction. Finally, the mirror and actuator thicknesses are not limited to several micrometers, unlike in surface micromachining. The thickness limits using this fabrication process is prescribed by the device layer thickness in SOI wafers, which vary between several micrometers to several hundred of microns.