Resonating AlN-thin film MEMS mirror with digital control

Abstract

Piezoelectrically actuated resonant micromirrors were designed to meet the light detection and ranging (LiDAR) system requirements. Key features were a 3-mm mirror aperture, a 40-deg field of view, and a 50-Hz refresh rate. The presented micromirror provides biaxial symmetrical beam steering with AE12.7 deg mechanical tilt angle, resulting in a 50-deg field of view with an adjustable Lissajous XY-scanning pattern for a forward-looking LiDAR system. The mirrors were fabricated using silicon on insulator wafers, and actuation was based on piezoelectric aluminium nitride thin film. The mirrors were vacuum packaged for high-quality factor resonator operation. The device design contained eight separate piezoelectric aluminium nitride elements arranged as differential pairs for each axis, where each actuator was equipped with a sensing element providing a mechanically coupled electrical feedback signal. The piezoelectric elements connected as actuators required only minimal power and were directly compatible with CMOS low-voltage logic, which eases integration to driving digital systems. The sense elements are used to monitor phase, amplitude, and frequency. A digital control system connected to each of these elements provides accurate frequency and phase control of independent orthogonal resonators, permitting control of the X and Y amplitudes and the refresh rate of the Lissajous pattern.