Self-tensioning Support Post Design to Control Residual Stress in MEMS Fixed-Fixed Beams
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Self-tensioning Support Post Design to Control Residual Stress in MEMS Fixed-Fixed Beams Ryan M. Pocratsky1 and Maarten P. de Boer1 1 Carnegie Mellon University, Dept. of Mechanical Engineering, 5000 Forbes Avenue, Pittsburgh, PA 15235, U.S.A. ABSTRACT Fixed-fixed beams are ubiquitous MEMS structures that are integral components for sensors and actuation mechanisms. However, residual stress inherent in surface micromachining can affect the mechanical behavior of fixed-fixed structures, and even can cause buckling. A selftensioning support post design that utilizes the compressive residual stress of trapped sacrificial oxide to control the stress state passively and locally in a fixed-fixed beam is proposed and detailed. The thickness and length of the trapped oxide affects the amount of stress in the beam. With this design, compression can be reduced or even converted into tension. An analytical model and a 3D finite element model are presented. The analytical model shows relatively good agreement with a 3D finite element model, indicating that it can be used for design purposes. A series of fixed-fixed beams were fabricated to demonstrate that the tensioning support post causes a reduction in buckling amplitude, even pulling the beam into tension. Phase shifting interferometry deflection measurements were used to confirm the trends observed from the models. Controlling residual stress allows longer fixed-fixed beams to be fabricated without buckling, which can improve the performance range of sensors. This technique can also enable local stress control, which is important for sensors. INTRODUCTION Commercial devices and devices in development utilize fixed-fixed beams in a variety of applications. The performance of these devices depends on the stress state of the beam. The Polychromator, a chemical sensor developed by Hocker et. al. [1], uses a series of fixed-fixed beams to form a programmable diffraction grating. MEMS pressure switches and microvalves employ buckled membranes to control the on/off state of the device [2, 3]. RF capacitive switches use fixed-fixed beams to form the capacitor [4]. Fixed-fixed beams are also used in the actuation mechanisms of MEMS display elements [5]. The stiffness of a fixed-fixed beam depends strongly on residual stress of the structural layer. Sources of residual stress are extrinsic and intrinsic. An extrinsic source is the coefficient of thermal expansion (CTE) mismatch between film layers and the substrate. Processing steps such as deposition and annealing are performed at elevated temperatures while devices are typically operated near room temperature. Intrinsic sources result from non-equilibrium thin film growth processes including nucleation and grain growth. These sources combine to give the final residual stress state. Published methods to address residual stress issues in integrated circuit fabrication foundries include adjusting the fabrication process and adjusting the design to minimize buckling. Optimization of the deposition conditions, post-deposition annealing
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