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Modeling of Fluid Damping in Resonant Micromirrors with Out-of-Plane Comb-Drive Actuation
* 1 , 1 , 2
1  Politecnico di Milano, Dipartimento di Ingegneria Civile e Ambientale, Piazza Leonardo da Vinci 32, 20133 Milano, Italy
2  STMicroelectronics, Advanced Systems Technology, Via C Olivetti 2, 20041 Agrate Brianza, Italy

Abstract: Comb-drive micromirrors are becoming of interest for a broad range of light manipulation applications. For some of these applications a vacuum packaging, which is able to guarantee higher quality factors, turns out to be not favorable for technical and functional reasons. Furthermore, micromirrors for picoprojectors application are required to function at high frequencies in order to achieve high resolution images. Accordingly, a study of the energy dissipated due to the interaction between the moving parts of the micromirror and the surrounding air, leading to fluid damping, is an important issue. Even if air damping has been thoroughly studied, an extension to large air domain distortion linked to large tilting angles of torsional micromirrors, is still partially missing. In such situations, the flow formation turns out to be far more complex than that assumed in analytical models. This task is here pursued by adopting three-dimensional computational fluid dynamics models; specifically, two models, holding at different length scales, are adopted to attack the problem through an automated dynamic remeshing method. The time evolution of the torque required to compensate for the fluid damping term is computed for a specific micromirror geometry. The relative contributions due to drag, shear and squeeze film terms are calculated as well. These results are shown to compare well with the available experimental data in terms of the overall sensor quality factor.
Keywords: resonant micromirror, combdrive, quality factor, CFD