Release Optimization of Suspended Membranes in MEMS

Abstract

Releasing part(s) of micro-fabricated devices using etching techniques is one of the fundamental post-processing steps in micro-machining and it is important to have a comprehensive concept on how it can be done, since the final result will significantly influence the electrical and mechanical performance of devices. As micro--electromechanical systems (MEMS) are three dimensional structures, they are obtained by eliminating materials commonly used in this technology, such as crystalline silicon, polycrystalline silicon, silicon dioxide and silicon nitride. Micro-machining can be undertaken by etching the bulk of the substrate or sacrificial layers deposited at the surface of the wafer. Bulk etching removes great quantities of material and is usually applied to obtain thin membranes for pressure sensors, etching from the back surface of the substrate. On the other hand, surface micro-machining is based in the elimination of sacrificial layers deposited at the surface of a substrate underneath the layers that should be active. The present study will deal only with bulk etching. Therefore, as several materials are used to obtain finally the typical sensors and actuators in MEMS devices, etching must be selective depending on the material that should be removed. There are available methods for material etching based either on gaseous or liquid etchants. The former employs complex and expensive equipment, but with good yield, and the latter is a low cost process, but care should be taken since in some cases it uses toxic or even corrosive solutions and facilities are needed to exhaust or protect from the vapours produced during etching. However, both kinds of etching processes are widely used in MEMS and CMOS technology and this fact can be conveniently used to match both technologies, so complete MEMS devices can be integrated in the same substrate, reducing fabrication costs. In particular, wet etching can be either isotropic if material is removed uniformly in all crystallographic directions or anisotropic if etching is selective to a given crystallographic plane. With regard to anisotropic etching, two solutions are commonly used like potassium hydroxide (KOH) and trimethyl ammonium hydroxide (TMAH). Both solutions are regularly used in MEMS technology to obtain structures, such as thin membranes and