Cryogenic etch process development for profile control of high aspect-ratio submicron silicon trenches

Abstract

A cryogenic etch process using low temperature (T⩽−100°C) and SF6 and O2 gases is presented for fabricating high aspect ratio silicon microstructures, including photonic devices and micro- and nanoelectromechanical systems. The process requires only a single electron beam resist mask and results in open area etch rates of 4μm∕min. Various etch process parameters, including O2 flow, rf forward power, substrate temperature, and chamber pressure were studied, and the resulting effect on the etch quality was evaluated in terms of sidewall verticality and surface roughness. The optimized process uses low temperature (T=−110°C) and low chamber pressure (P=7mTorr) and enables sidewall verticality greater than 89.5° with roughness of 1–10nm. A silicon etch selectivity of 26:1 was obtained for 380nm thick electron beam resist. Using the optimized process, a silicon-on-insulator Fabry-Pérot optical cavity with integrated rib waveguides and deeply etched silicon/air distributed Bragg reflector mirrors was fabricated and tested. The device exhibits sharp resonance peaks with full width at half maximum Δλ=0.45nm, free-spectral range of 26 nm, finesse F=58, and quality factor Q=3400 (at λ0=1531.6nm). The optical measurements and extracted mirror reflectance (R≈95%) confirm the high quality of our optimized etch process.