Optical Detection of the Electromechanical Response of MEMS Micromirrors Designed for Scanning Picoprojectors

Single-axis rotational micromirrors actuated by comb finger structures have been designed in view of their application in reflective scanning picoprojectors for laser beam displacement along two perpendicular directions to obtain a raster scan scheme. A resonant mirror operating at a frequency around 25 kHz, suitable for horizontal scans, as well as a linear mirror, suitable for vertical scan at the typical video refresh rate (60 Hz), have been fabricated by Silicon-on-Insulator technology and are illustrated in this paper. We have in particular exploited the potentialities of semiconductor laser self-mixing interferometry, a powerful technique for characterizing the dynamic response of MEMS, for detecting the electromechanical response of both kinds of micromirrors. We report the results of the spot optical measurements performed on resonant and linear mirrors aimed at detecting the frequency of the fundamental rotational mode as well as of the in-plane and out-of-plane modes, close in frequency to the fundamental mode. We have experimentally demonstrated that the fabricated devices are suitable for high-resolution miniaturized projectors, in terms of frequency response and scanning angle.