Fabrication and modeling of rib-stiffened thin films

Abstract

Structures created from thin films using surface micromachining techniques typically have low out-of-plane bending stiffness, making them unsuitable for applications such as high-speed laser scan mirrors. The bending stiffness of a low-pressure chemical vapor deposited (LPCVD) film is altered by adding stiffening ribs to the back of the film. Ribs are incorporated by etching trenches into ⟨1 0 0⟩ silicon and then depositing a conformal film over the etched silicon. The ribs are arranged into square and hexagonal periodic patterns. A simplified numerical model of the stiffened film is proposed. The model assumes that the film behaves as a composite material composed of two perfectly connected homogeneous layers. Homogenized material properties for square and hexagonal cells are incorporated into the model to simulate bending in the composite film. Experimental cantilever beams are used to validate bending in numerical models. Numerical results for the cantilevers under bending loads are found to be in good agreement with experimental data. The application of this technique to create rib-stiffened micromirrors is illustrated.