High-Aspect-Ratio Thin-Film Stiffening Structures Made of Atomic-Layer-Deposited Alumina and Its Application to a Scanning Micromirror

This work presents the first high-aspect-ratio alumina stiffening structures, fabricated by combining atomic layer deposition (ALD) of alumina with silicon deep reactive ion etching (Si-DRIE), designed to improve MEMS micromirror devices. By selective etching of an alumina-coated silicon mold, we produced vertical alumina fins with a depth up to $500 \mu \text{m}$ and a width down to $0.13 \mu \text{m}$ , corresponding film-aspect-ratios up to 3800. We applied this stiffening structure to a lightweight MEMS micromirror with $215 \mu \text{m}$ thickness and 2 mm diameter. We characterized the optical flatness and dynamic deformation of this device, which are quantities of interest for compact image projection systems.