Multilayer Materials for Electrostatic Switches
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Multilayer Materials For Electrostatic Switches Mark A Phillips, Brennan L Peterson, Christine M Esber, Jesse Hwang and Bruce M Clemens Department of Materials Science and Engineering, Stanford University, Stanford CA 94305 ABSTRACT Metal multilayers are a unique way to manufacture reliable conductive members for MEMS devices. These members are particularly suited for electrostatic switches. Unlike elemental Al, which experiences curvature problems during processing, the residual stress in these beams can be controlled and calibrated during stress measurements to have repeatable shapes and stresses upon release. In this study, curvature experiments were performed to determine the optimal deposition parameters (pressure, thickness) to produce metal multilayer beams with zero average stress and zero bending moment, which can be obtained by alternating tensile and compressive layers within. Resistivity measurements were made to determine the effect of interfaces on resistivity. INTRODUCTION MEMS (Micro-Electro-Mechanical Systems) combine electrical and mechanical functions at the micrometer scale. Due to their small size, they are incredibly sensitive and provide rapid response. However, the ability to control and characterize residual stresses is very important for the development of higher performance MEMS. Metal multilayers are a unique way of addressing the demands of reliable MEMS devices as they offer low resistivity, high strength, and control of residual stress. Initial experiments with elemental Al revealed significant downward bending. The deposition of Al (similar to that shown for W in Figure 5) produces a bilayer of tensile and compressive regions. When the film is released from the substrate the region in tension will contract and the region in compression will expand, resulting in downward curvature of the beam. Using information collected from in-situ stress measurement, it is possible to design a thin film in which multiple layers in tensile and compressive stress states balance each other, producing a system with minimal residual average stress and curvature. Here we present the initial results of free-standing cantilever beams produced by deposition of Mo/W multilayers with subsequent thin film processing. The aim was to produce micro-beams with minimal bending and low resistivity. Currently, many MEMS are manufactured from silicon, which has a high resistivity (~100000 μΩ cm). Doping can reduce silicon resistivity, but metals provide significantly lower resistivity, making them desirable materials for conducting members. Metal multilayers have higher resistivity than the average of the individual components, due to interface effects. The higher the number of layers, the more interfaces, and therefore higher resistivity. EXPERIMENTAL DETAILS In-situ Curvature Understanding development of stress during deposition allows prediction and control of residual stresses in MEMS devices. By alternating tensile and compressive layers of two or more EE5.1.1
different materials, residual stresses can be tuned
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