Effect of Nonlinear Structural Stiffness on the Response of Capacitive MEMS Devices
M. Shavezipur, P. Nieva, A. Khajepour, S.M. Hashemi
University of Waterloo, CA
Keywords: tunable capacitor, nonlinear stiffness, residual stress, linear response
Abstract:In this paper, a design technique is presented that can linearize the response of MEMS capacitive elements such as tunable capacitors, capacitive sensors and out-of-plane electrostatic actuators. The method combinations the electrode flexibility and nonlinear stiffness created by a set of mechanical stoppers to reduce the nonlinearity of the characteristic response. however, due to the simple fabrication and post-processing (i.e. release and packaging) only parallel-plate tunable capacitors are studied here. The initial curvature of the moving plate resulting from its inherent residual stress and the position of the stoppers are tailored and optimized, using ANSYS FEM simulations, to provide a smooth C-V curve with high linearity and fairly constant sensitivity throughout the working range. Moreover, because the overall structural stiffness of the device is increased, higher tunability is also achieved. The experimental results of capacitors fabricated with PolyMUMPs display a good agreement with FEM simulations and also verify the applicability of the proposed design technique, where a notable improvement in the linearity of the C-V response and higher tunabilities are observed. The design technique proposed in this paper is easy to implement, does not require a complicated fabrication process and can be applied to different capacitive MEMS devices.