Photo-assisted electrochemical machining of micromechanical structures

Stress-free micromechanical cantilever beams have been fabricated by selective etching of silicon p-n structures in HF solutions utilizing a photo-assisted electrochemical process. With this technique, n or p regions can be selectively etched at controlled rates by appropriate choice of cell bias, p-n junction bias, and illumination intensity. p-Si can be selectively etched by utilizing illumination of the p-n junction to anodically bias the p-layer relative to the n-substrate. Etch rates of up to 5 μm/min resulted in the formation of porous layers readily removed with chemical Si etching solutions. n-Si can be selectively etched by illuminating and applying a reverse bias across the p-n junction, driving the p-layer cathodic. Etch rates up to 10 μm/min and high resolution etch stops with smooth surfaces were obtained. The effects of key variables, including doping type, cell bias, p-n junction bias, and illumination intensity, on etch rate, selectivity, and surface finish are discussed