Electromechanical properties of amorphous and microcrystalline silicon micromachined structures
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Electromechanical properties of amorphous and microcrystalline silicon micromachined structures J. Gaspar1,2, M. Boucinha1, V. Chu1, J.P. Conde2 1 INESC, Instituto de Engenharia de Sistemas e Computadores (INESC), Rua Alves Redol 9, 1000-029 Lisbon, Portugal 2 Department of Materials Engineering, Instituto Superior Técnico (IST), Av. Rovisco Pais, 1049001 Lisbon, Portugal ABSTRACT Electrostatic actuation of bridge and cantilever structures is studied. The structures are composed of bilayers of amorphous or microcrystalline silicon and aluminum fabricated using surface micromachining on glass substrates. The structures are actuated by applying the sum of a DC and a low-frequency AC voltage. The resulting AC deflection is detected optically. The dependence of this deflection upon the applied voltages and the bridge length is studied and compared with the predictions of an electromechanical model. The deflection amplitude can be electrostatically controlled and detected with a precision estimated to be better than a nanometer. INTRODUCTION Microelectromechanical systems (MEMS) use planar microelectronics fabrication techniques to produce three dimensional structures which have both electronic and mechanical functionality. Currently, most MEMS are made by bulk micromachining of crystalline silicon substrates or by surface micromachining of low-stress, high-temperature polysilicon on a silicon wafer substrate [1]. Recently, surface micromachining principles have been used together with thin-film materials expanding the applications of MEMS to large area substrates [2,3]. A powerful aspect of MEMS technology is the fact that these structures allow the possibility of combining mechanical actuation with sensing and electronic control, all integrated with a common technology. Actuation can be achieved in different ways, such as thermal, piezoelectric, magnetic and electrostatic, to name a few of the more important ones. In this paper, electrostatic actuation will be studied in amorphous (a-Si:H) and microcrystalline (µcSi:H) silicon thin film bridge and cantilever structures on glass substrates using optical detection of the movement of the structure. The measurements are analyzed using an electromechanical model to extract the absolute deflection. EXPERIMENTAL PROCEDURES The structures are fabricated on glass substrates using surface micromachining techniques described elsewhere [3]. The bridges and cantilevers studied in this paper are bilayers composed of 1000 c of aluminum and (i) a-Si:H or (ii) µc-Si. The film thickness and structure dimension are summarized in Table 1. The electrostatic actuator is shown in fig. 1. A gate electrode is placed underneath the bridge or cantilever to supply the electrostatic force. The structure itself is made up of an a-Si:H or µc-Si structural material and a metallic top layer which serves as a counter-electrode to the gate. The minimum and maximum lengths, L, were 4 µm and 60 µm, respectively, for the cantilevers and 8 µm and 120 µm, respectively, for the bridges. The width, A26.4.1
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