High reflectivity micromirrors fabricated by coating high aspect ratio Si sidewalls

Abstract

Micromachined Si vertical structures were sputter coated with Au onto sidewalls as reflective mirrors. These vertical micromirrors were integrated with comb drives as optical switching arrays. The micromirrors were first etched in a Cl2 plasma generated by an electron cyclotron resonance source, followed by a short B diffusion and a wet etch with ethylenediamine pyrocatechol to reduce sidewall roughness and to separate the mirrors from the substrate. The sputter-deposited Au films have nearly uniform thickness along the sidewalls of the micromirrors. There was only a 6% thickness variation from the top to the bottom of the mirror sidewalls. Reducing the sputtering pressure from 30 to 4 mTorr improved the coating uniformity for high aspect ratio, closely spaced mirrors. For 30-μm-tall micromirrors with 6 μm spacing, the Au thickness close to the bottom of the sidewalls decreased by 52% compared to the top of the sidewalls when sputtered at 4 mTorr. The roughness along the micromirror sidewalls was found to change from 7.7 to 16.1 nm as the pressure during sputtering was varied from 4 to 30 mTorr. The resultant reflectivity for these vertical micromirrors decreased from 85% to 70% at a wavelength of 1.55 μm. For high reflectivity micromirrors with smooth sidewalls and uniform thickness, sputter deposition of 400-nm-thick Au at 4 mTorr should be used.