Analysis and design of mems scan mirrors using periodically stiffened silicon nitride

Abstract

This research presents a novel fabrication method combining surface and bulk micromachining techniques to deposit mechanically stiffened silicon nitride films for use in MEMS fabrication. The stiffened silicon nitride film consists of a thin (~1.5 um) top sheet with stiffening fins molded to the back of the film. In the final configuration, the fins extend between 15 um and 40 um vertically from the back of the film. The molded fins are arranged into periodic square and hexagonal cell configurations ranging in size from 10 um to 250 um. The periodic cells significantly increase the bending stiffness of LPCVD silicon nitride films resulting in a film with a high strength-to-weight ratio. Larger aperture, silicon nitride micro mirrors are fabricated with the mechanically stiffened silicon nitride film. The mirrors demonstrate that deformation due to postrelease thermal strain and inertia during dynamic actuation can be mitigated by employing the proposed stiffening technique. Furthermore, the mirrors are fabricated with a minimal amount of processing making the proposed microfabrication technique an attractive solution for various MEMS applications Finally, homogenized material properties are obtained for the periodically stiffened silicon nitride film. The homogenized material properties are then used to simplify finite element models of biaxial and single axis torsion micro mirrors fabricated from the proposed film. The resulting finite element models are shown to be in excellent agreement with the experimental models. The presented numerical analysis method significantly simplifies model complexity while simultaneously reducing the computational cost associated with simulating MEMS built from a periodically stiffened thin film.