Suspended Si Microstructures Over Controlled Depth Micromachined Cavities for MEMS Based Sensing Devices

Final shape of the silicon microstructures in wet anisotropic etching depends upon the relative etch rates (i.e., anisotropy) of different crystallographic planes. In order to fabricate the structures including curved shape (or sharp/rounded convex corners), dry anisotropic etching is used; because the wet etching exhibits severe undercutting at the sharp convex corners (or the edges not aligned along the 〈110〉 direction). The present research introduce the fabrication method of suspended silicon microstructures such as cantilever beams with sharp convex and concave corners, and with sharp convex and rounded concave corners of required thickness over controlled etch depth cavities. The fabrication process uses silicon direct wafer bonding and wet anisotropic etching in pure and surfactant added tetramethyl ammonium hydroxide (TMAH) water solutions. Surfactants in TMAH are used to reduce the etch rates of non-(100) planes, so that the undercutting at curved shape, edges aligned along non-〈110〉 directions and sharp convex corners can be minimized to a significant low levels. Various kinds of non-ionic surfactants NC-200, Triton-X-100 and polyethylene glycol (PEG) in a very small amount (0.1% by volume) were studied in 25 wt% TMAH solution.