Isotropic etching of silicon in fluorine gas for MEMS micromachining

Etching of silicon with molecular fluorine for the micromachining of micro electro mechanical systems (MEMS) has been evaluated. The etching process is carried out in a continuous flow etching system that uses a 25 vol% mixture of F2 in N2 and operates at room temperature and atmospheric pressure. Fluorine etches silicon isotropically at a rate of 0.2 µm min−1 and is a viable etchant for bulk silicon micromachining. The F2 etch results in the formation of pits about 10–50 µm in size within the silicon features and roughness in the micrometer and sub-micrometer length scales. SiO2, Pt, Ni, Al and Ta do not etch or roughen after several hours of F2 exposure. In addition, F2 does not etch low-stress silicon nitride; a solid layer forms on the silicon nitride surface upon exposure to F2 that protects the underlying silicon nitride from further attack. In contrast, a commercial XeF2 etching system etches low-stress silicon nitride with a silicon nitride:silicon selectivity of approximately 1:200. The F2 etching system is used to release tubes of a MEMS fuel processor that have 2 µm thick low-stress silicon nitride walls.