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Micromachined tunable optical microfilters design and experimental processing

In this paper we present the design and the experiments performed to obtain a micromechanical voltage tunable Fabry-Perot interferometer integrated with a p-n photodiode on a silicon substrate. It can be used as a voltage tunable filter for the input radiation or as a voltage controlled attenuator to regulate the light from a monochromatic source. Different solution have been analyzed and experimented. The top mirror of the Fabry-Perot cavity is a doped poly-Si or Au/SiO2 movable membrane, electrostatically actuated, obtained using Si micromachining. A complex design process was performed: optical, electomechanical and technological. All these phases were performed interactively. Different materials were considered in order to perform an optimum design. Experimental micromachined interferometers were obtained using two techniques: (1) surface micromachining, and (2) anisotropic etching of (111)-oriented Si wafers, combined with an isotropic pre-etching step. These processes were optimized and matched to the photodiode fabrication process. Monolithic integrated interferometers coupled to p-n photodiodes were obtained.