The design of a piezoelectrically actuated scanning micro-mirror

Abstract

Micro Electro Mechanical Systems (MEMS) are becoming the standard in today's devices. This is in part due to their low power consumption but more importantly their small size allows them to take on physical properties that can't be achieved by any other means. One application of MEMS is scanning mirrors, that are found in micro and Pico projectors and are significantly bringing down the cost of LIDaR technology, the main sensing technology in self driving cars. Scanning Micromirrors employ one of various actuation techniques in order to achieve resonance. An emerging technology that is becoming increasingly popular in this application is actuation by piezoelectric materials. This is due to a number of factors, mainly due to the low driving voltage required, the linearity of the effect and its nature to be a reciprocal transducer, allowing to measure the performance of the mechanism and allowing for calibration. This project is centred around the research, design, and testing that goes into the design of a micromirror that is piezoelectrically actuated, that aims for a relatively high resonant frequency of around 230 kHz, with a mirror of small dimensions, a diameter of around 250 µm, that can be used for beam steering in close quarter applications or imaging in small form factor cameras such as those used in keyhole surgery. The micromirror was designed using CoventorWare, a simulation software that can be used to build and carry out various simulations on MEMS devices. The PIEZOMUMPs manufacturing process was used to design and build the mirror. The mirror is made up of a silicon base, that will house the mirror and the actuator wings, that are excited electrically with opposing charges at the resonant frequency of the mirror to induce motion in one dimension. A layer of piezoelectric material is then deposited on top. An oxide layer fills in the spaces that the piezoelectric layer does not fill so as not to short the electrodes with the conductive silicon base. Electrodes are then placed atop the structure to induce a charge in the piezoelectric material.