A static technique for the electro-mechanical characterization of MEMS devices for RF and microwave applications
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A static technique for the electro-mechanical characterization of MEMS devices for RF and microwave applications Anupama B. Kaul, Tomasz Klosowiak, and Joshua Liu Advance Technology Center, Motorola Labs, Schaumburg, IL 60196, USA ABSTRACT An approach for measuring force-dependent properties of microscopic structures commonly found in MEMS has been developed. The system has the capability of measuring forces and deflections of the order of micro-newtons and micro-meters, respectively. By implementing a visual inspection system, force is applied to localized areas on a beam, and the resulting force-deflection characteristic can be obtained. From this beam stiffness and effective elastic modulus can be calculated. These results were compared to simulation, which was performed using ANSYS FEM code. In addition, by applying a known mechanical force, direct correlation to voltage and thus electrostatic force can be obtained, which also elucidates the magnitude of the electrostatic feedback effect. Characterization of other force-dependent parameters such as DC contact resistance and isolation/insertion loss at RF and microwave frequencies was obtained experimentally, from which parameters such as lumped capacitance can be extracted. INTRODUCTION Microelectromechanical (MEMS) devices are revolutionizing every aspect of technology as we begin to realize the benefits of integrating micromechanical structures with electronics. A widely studied micromechanical structure is the cantilever beam, which was first demonstrated in 1979 to switch low-frequency electrical signals using electrostatic actuation [1]. The electrical performance of the cantilever beam switch is closely tied to the beam properties; for example actuation voltages or contact forces are directly dependent on beam stiffness and surface conditioning, and the latter parameters can be easily influenced by processing. The MEMS designer would thus benefit from an empirical quantification of such properties in order to ensure device reliability and optimize performance. In addition to existing techniques for quantifying such properties of microscopic structures, for example nanoindenters [2] and AFMs, we describe an approach that accurately quantifies force-dependent electrical and mechanical properties of the cantilever beam switch, and could be used to analyze other structures. Although the measurements described here are based on devices fabricated on printed wiring board substrates, the technique can be used to characterize MEMS devices formed using silicon or other material technologies. EXPERIMENTAL SET-UP The schematic of the set-up is shown in Fig. 1a. A standard laboratory micro-balance registers the force, and a precision dial-indicator mounted on a height gauge was used to monitor the deflection, each with a resolution of 1 µN and 0.6 µm, respectively. The force is delivered via a modified pogopin tip, which is on a fixture that is placed directly on the microbalance. By placing the height gauge on an x-y table, fine alignment of the force to a specific a
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