Controlling sidewall smoothness for micromachined Si mirrors and lenses

Micromachined vertical mirrors and lenses in Si were fabricated as micro-optical components. A Cl₂ plasma generated by an electron cyclotron resonance source was used to etch these mirrors and lenses in Si and the etched Si sidewalls were characterized by atomic force microscopy and scanning electron microscopy. A trilayer resist process has been developed to provide smooth Ni etch mask edges by postbaking the top imaging resist. The resultant Si etched sidewall showed a roughness of 18.69 nm, as compared to the sidewall roughness of 29.95 nm without postbaking. As the plating current density was varied from 5 to 400 mA/cm², the etched Si sidewall roughness increased from 24.31 to 43.69 nm. Optimized etch conditions were investigated for smooth Si sidewalls. Sidewall was much rougher with a roughness of 98.99 nm when 50 W rf power was applied for dry etching, whereas the roughness was only 29.95 nm when 100 W rf power was used. Additionally, thermal oxidation followed by oxide removal was able to reduce etched Si sidewall roughness. After oxide removal, the sidewall roughness decreased to 5.93 nm after 135 min wet oxidation at 1100 °C. However, oxide grown at 900 °C was less efficient for reducing the sidewall roughness as compared to 1100 °C. The deep etch-shallow diffusion process was applied to fabricate micromirrors and microlenses. It was found that B diffusion at 1.175 °C for 2 h decreased the Si sidewall roughness from 29.95 to 10.79 nm and selective wet etch lowered sidewall roughness further to 5.01 nm. Vertical micromirrors that were 40 μm tall and 2 μm wide were bonded onto the glass substrate and released. Microlenses that were 40 μm thick with radius of curvature of 50 μm have also been demonstrated. © 1996 American Vacuum Society.