Development of a focusing micromirror device with an in-plane stress relief structure in SOI technology

摘要

A new design concept for a dynamically focusing micromachined silicon membrane mirror with 6 mm diameter and electrostatic actuation was realized. To eliminate the influence of residual stress a special stress relief design of the membrane's suspension was developed in order to achieve a distortion-free optical mirror (distortion ＜ λ/10 (λ = 1064 nm). Even silicon membranes fabricated in SOI technology mostly suffer from buckling by residual compressive stress caused by mismatch in the coefficients of thermal expansion between silicon and the buried silicon oxide layer [1, 2].This often leads to severe distortion of stress sensitive devices such as membrane based micro mirror devices [3]. Even though a tensile pre-stress might improve the distortion in case of a non-deformed membrane, a tensile stress in the membrane increases the stiffness and thus reduces the sensitivity e.g. for capacitive sensors or for actuating devices. Different methods are reviewed for stress compensation or stress relief in membranes. We developed and fabricated a new stress relief structure which reduces the stress induced deformation of membranes and leads to substantially flat micromirrors of high optical quality. This is achieved by a special tangential beam suspension which allows an in-plane expansion or contraction of the membrane proportional to its inherent compressive or tensile stress. Optimized beam structures and the voltage dependence of the mirror's deflection were determined by 3D FEM simulations. For membranes with a compressive pre-stress of -20MPa simulations show a decrease in bow to values ＜ 18 nm in comparison with 700 nm for a conventional rigidly clamped membrane. A deflection of 16 μm within an aperture of 5 mm diameter is theoretically achieved by a voltage U_0 = 200 V resulting in a minimal focal length of 97 mm. The fabricated devices have been characterized by the means of interferometric optical measurement. The measurement results are in good agreement with the theoretical prediction of FEM simulations.