Plasma etching techniques to form high-aspect-ratio MEMS structures

This chapter describes an in-situ trench etching and release technique to fabricate high aspect-ratio beams for high performance MEMS accelerometers using the SiO2 etching mask from Magnetically Enhanced Reactive Ion Etcher (MERIE). Work has started from comparison of the profiles of trenches masked by SiO2 to those masked by Si3N4 in forming MEMS structures. In our experimental conditions using HBr/SiF4/O-2 in MERIE, the etching process with the SiO2 mask was proven to be able to form deeper anisotropic trenches than that with the the Si3N4 mask. Amount of oxygen available from the etching gas and/or the mask appeared to be a major factor controlling the floor etching of trenches as well as the sidewall passivation of beams. Excessive oxygen generated from the SiO2 mask led to sidewall encroachment of the beams. On the other hand, lack of oxygen in the Si3N4 mask resulted in etch-stopping and micro-grassing, before forming the high-aspect-ratio MEMS structures. In the conventional process, lateral encroachment due to the release etching was severe underneath the SiO2 mask. In the in-situ process developed in this research, the sidewall of the beams was passivated by inhibiting layers formed during the HBr/SiF4/O-2 trench etching, and the beams were not attacked by the subsequent SF6 release etching. Major constituents of the passivating layers that were produced by the in-situ process were Si and O. The in-situ process dispensed with both sidewall deposition and floor etching steps of the conventional process. Dependency of etching rates on open-ratio and pattern-size was studied in MERIE for high aspect-ratio MEMS structures. The etching rates of Si substrate in MERIE remained unchanged despite a change in the overall open-ratio on the wafer in the range of 10% to 50%, and this was different from the results of an ICP etcher.