Photoassisted electrochemical micromachining of silicon in HF electrolytes

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We have demonstrated the ability to fabricate stress-free micromechanical cantilever beams by selective etching of silicon p—n structures in HF solutions utilizing a photoassisted electrochemical process. A particular novelty of this technique is that n or p regions of a p—n structure may be selectively etched at controlled rates by appropriate choice of cell bias, p—n junction bias, and illumination intensity. p-Si is selectively etched by either one of two ways. Illumination of the p—n junction serves to bias the p-layer anodically relative to the n-substrate, resulting in etch rates of up to 0.6 μm/min. Alternatively, p-Si etch rates up to 10 μm/min are attained without illumination by short circuiting the p—n junction and anodically biasing the n-Si substrate. n-Si, on the other hand, is selectively etched at rates up to 10 μm/min by illuminating and reverse biasing the p—n junction, driving the p-layer cathodic. At etch rates below approximately 1 μm/min, porous silicon layers form, which can be subsequently removed with chemical etchants. These processes are characterized by high-resolution etch-stops with smooth surfaces, rendering them potentially attractive for micromachining purposes. 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.