Plasma Etching Techniques to Form High-Aspect-Ratio MEMS Structures

Abstract

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 (MERLE). 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/O₂ in MERLE, 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/O₂ 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 MERLE for high aspect-ratio MEMS structures. The etching rates of Si substrate in MERLE 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 ICE etcher.