Formation of ultra-smooth 45° micromirror on (1 0 0) silicon with low concentration TMAH and surfactant: Techniques for enlarging the truly 45° portion

This paper describes the fabrication of ultra-smooth 45° micromirrors on (1 0 0) silicon using low concentration tetramethylammonium hydroxide (TMAH) water solution with a new NCW-1002 surfactant, the etching property of which as an effective additive in TMAH is discussed. The formation of the 45° micromirror is investigated by examining the cross-sectional view of the micromirror slope in detail. Results show that only the middle portion of the micromirror is truly at 45° with the (1 0 0) plane, whilst the top and bottom portions of the micromirror deviate from 45°. The non-45° portions of the micro-mirror slope are due to (1) nearly isotropic etching behavior for planes close to (1 0 0) plane for surfactant added low concentration TMAH; (2) diffusion dependent etch effects arising from the overhanging oxide. Two techniques have been developed to extend the truly 45° portion of the micromirror. One technique involves successive removal of the overhanging oxide to effectively extend the truly 45° portion of the slope towards the micromirror top. The other technique involves an additional masking step to recess the non-45° lower portion of the micromirror.