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Post-release deformation and curvature correction of an electrothermally actuated MEMS bilayer platform
A. Kumar, Ashudeep, D. Bansal, P. Kumar, Anuroop, Khushbu,
Published in Elsevier B.V.
Volume: 221
Surface micromachined devices are known to have residual stress-induced deformation. This paper presents the effects of residual stress on the flatness of an electrothermally actuated large aperture MEMS bilayer platform. The platform consists of a SiO2–Al composite plate of area 500 × 500 μm2 suspended over a cavity through bimorph actuators. The bilayer platform consists of 0.95 μm thick aluminum film on a thermally grown 0.75 μm thick silicon dioxide on a silicon substrate. Bimorph actuators also consist of laminated layers of silicon dioxide and aluminum of thicknesses the same as on the platform. The ensuing compressive stress in silicon dioxide (240 MPa) & tensile stress in aluminum (35 MPa) manifests itself in significant post-release curling of the bilayer platform. Finite Element Simulation is done in Coventorware® to analyze the room temperature post-release deformation behavior of the platform. In order to correct the platform curvature, two methods of stress counterbalancing i.e. (i) metal reinforcement framing, and (ii) deposition of a stress compensation layer are proposed and their effectiveness is investigated using FEM simulations. The simulation results show that a 1 μm thick gold reinforcement frame results in a 13 μm peak-to-valley height difference between center and corner of the platform, which improves to 6 μm for a gold reinforcement thickness of 3 μm. The maximum height difference reduces to 1 μm for silicon dioxide of thickness 0.75 μm. According to FEM results, the presence of a stress compensation layer at the top is more effective in curvature correction compared to metal reinforcement framing; however, the post-release elevation of the stress-compensated platform is 5 μm below the post-release elevation of the reinforced platform and 25 μm below the zero reference plane. To verify the simulation results, the platform is fabricated with a deposition of 1 μm thick silicon dioxide layer at the top. The fabricated platform exhibit significant improvement in post-release deformation with a stress compensation layer compared to an unbalanced platform. © 2019 Elsevier B.V.
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JournalData powered by TypesetMicroelectronic Engineering
PublisherData powered by TypesetElsevier B.V.