A cryogenic etch process using low temperature (T <= 100 degrees C) and SF6 and O-2 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 4 mu m/min. Various etch process parameters, including O-2 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 degrees C) and low chamber pressure (P=7 mTorr) and enables sidewall verticality greater than 89.5 degrees with roughness of 1 - 10 nm. A silicon etch selectivity of 26:1 was obtained for 380 nm thick electron beam resist. Using the optimized process, a silicon- on- in sul ator Fabry-Perot 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 Delta lambda=0.45 nm, free-spectral range of 26 nm, finesse F=58, and quality factor Q=3400 (at lambda(0)= 1531.6 nm). The optical measurements and extracted mirror reflectance (R approximate to 95%) confirm the high quality of our optimized etch process. (c) 2007 American Vacuum Society.