Mechanically probing time-dependent mechanics in metallic MEMS J.P.M. Hoefnagels1*, L.I.J.C. Bergers1,2, N.K.R. Delhey1, M.G.D. Geers1 1 Eindhoven University of Technology, Department of Mechanical Engineering, P.O.Box 513, 5600MB, Eindhoven, The Netherlands 2 Foundation for Fundamental Research on Matter (FOM), P.O.Box 3021, 3502 GA, Utrecht, The Netherlands * E-mail: j.p.m.hoefnagels@tue.nl, tel:+31-40-2475894, Fax:+31-40-2447355 ABSTRACT The reliability of metallic micro-electromechanical systems (MEMS) depends on time-dependent deformation such as creep. To this end, a purely mechanical experimental methodology for studying the time-dependent deformation of free-standing microbeams has been developed. It is found most suitable for the investigation of creep due to the simplicity of sample handling and preparation and setup design, whilst maximizing long term stability and displacement resolution. The methodology entails the application of a constant deflection to a μm-sized free-standing aluminum cantilever beam for a prolonged period of time. After this load is removed, the deformation evolution is immediately recorded by acquiring surface height profiles through confocal optical profilometry. Image correlation and an algorithm based on elastic beam theory are applied to the full-field beam profiles to yield the tip deflection as a function of time. The methodology yields the tip deflection as function of time with ~3 nm precision. 1. RELIABILITY AND TIME-DEPENDENT MECHANICS The application of metals as structural components in MEMS is common for ‘radio-frequency MEMS’ (RF-MEMS). Figure 1 shows an example. The reliability of these devices has been shown to critically depend on timedependent mechanics, such as fatigue and creep [1]. Fatigue affects the device life time through its limitation on the number of device operation cycles, e.g. the number of open/closed cycles of an RF-MEMS switch. Creep can directly affect the operational characteristic, e.g. through a shift in pull-in voltage of an RF-MEMS switch which results in a reduced power handling [2]. Whereas fatigue effects may pose less of a problem than expected at small geometrical length scales [3], the detrimental influence of creep seems to increase upon miniaturization [3]. Figure 1: Scanning electron micrograph of an RF-MEMS switch (courtesy of EPCOS Netherlands B.V.). The difference between micro- and macroscale creep is generally attributed to the size-effect: the interaction between microstructural length scales and dimensional length scales [4,5]. The physical micro-mechanisms of creep in these free-standing microbeams are, however, poorly understood, let alone implemented in models. In the literature some reports can be found discussing creep and relaxation effects in thin aluminum films [6-11]. Proceedings of the SEM Annual Conference June 7-10, 2010 Indianapolis, Indiana USA ©2010 Society for Experimental Mechanics Inc. 43 T. Proulx (ed.), MEMS and Nanotechnology, Volume 2, Conference Proceedings of the Society for Experimental Mechanics Series 2, DOI 10.1007/978-1-4419-8825-6_7, © The Society for Experimental Mechanics, Inc. 2011
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