Design, Simulation, Fabrication, and Characterization of an Electrothermal Tip-Tilt-Piston Large Angle Micromirror for High Fill Factor Segmented Optical Arrays.

David Torres,Juan Pastrana,Sarah Dooley,Harris Hall,Lavern Starman

doi:10.3390/mi12040419

Abstract

Micro-electromechanical system (MEMS) micromirrors have been in development for many years, but the ability to steer beams to angles larger than 20° remains a challenging endeavor. This paper details a MEMS micromirror device capable of achieving large motion for both tip/tilt angles and piston motion. The device consists of an electrothermal actuation assembly fabricated from a carefully patterned multilayer thin-film stack (SiO2/Al/SiO2) that is epoxy bonded to a 1 mm2 Au coated micromirror fabricated from an SOI wafer. The actuation assembly consists of four identical actuators, each comprised of a series of beams that use the inherent residual stresses and coefficient of thermal expansion (CTE) mismatches of the selected thin films to enable the large, upward, out-of-plane deflections necessary for large-angle beamsteering. Finite element simulations were performed (COMSOL v5.5) to capture initial elevations and tip/tilt motion displacements and achieved <10% variance in comparison to the experiment. The measured performance metrics of the micromirror include tip/tilt angles of ±23°, piston motion of 127 µm at sub-resonance, and dynamics characterization with observed resonant frequencies at ~145 Hz and ~226 Hz, for tip/tilt and piston motion, respectively. This unique single element design can readily be scaled into a full segmented micromirror array exhibiting an optical fill-factor >85%, making it suitable for optical phased array beam control applications.

Highlights

MEMS micromirrors have been widely developed for several decades in both academia and industry for a wide range of applications
Finite element method (FEM) models for the undercarriage and the mirror-bonded devices were constructed in COMSOL 5.5 (COMSOL, Inc., Burlington, MA, USA)
The material parameters for the model were taken from the COMSOL library and the thickness for each layer was adjusted to the measured values

Summary

Introduction

MEMS micromirrors have been widely developed for several decades in both academia and industry for a wide range of applications During this significant period of time, many groups focused on single element designs while other researchers focused their efforts on closely packed arrays of micromirrors. In the past 10 years, several optical applications have arisen in which large-angle beamsteering can be both beneficial and required to meet new operational applications in areas such as medical instrumentation [4,5], LIDAR [6,7], projection display [8], smart windows [9], and free-space optical communications [10,11] These devices can be classified into two groups by their actuation degrees of freedom: Tip (1D) or tip and tilt (2D), where most of the 2D devices are capable of piston displacement. In a two-state digital manner, 2D micromirrors (tip/tilt) are capable of scanning an area with a single element [12,13,14], typically in a continuous (or highly discretized near continuous)

Methods

Results

Conclusion