Plasticity Length Scale in LIGA Nickel MEMS Structures
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Plasticity Length Scale in LIGA Nickel MEMS Structures J. Lou*, P. Shrotriya*, S. Allameh*, N. Yao*, T. Buchheit+ and W.O. Soboyejo* * The Princeton Materials Institute and The Department of Mechanical and Aerospace Engineering, Princeton University, Olden Street, Princeton, NJ 08544 + Mechanical Reliability and Modeling Dept., Sandia National Laboratories, Albuquerque, NM 87185 ABSTRACT The plasticity length scale parameters associated with rotational and stretch gradients in LIGA nickel MEMS structures plated from sulfamate bath are presented in this paper. These are obtained from micro-bend and micro-tensile experiments on LIGA nickel films with different thicknesses. The composite length-scale parameter, lc is measured to be ~ 4.8 µm for as-plated films (25, 50, 100 and 175 µm thick). The implications of the results are then discussed for the modeling of plasticity in LIGA nickel structures. INTRODUCTION In the last two decades, LIGA (an acronym of the German words “Lithogrphic, Galvanoformung, Abformung”) nickel micro-electro-mechanical systems structures have been developed for applications in larger and thicker devices [1-4]. These include applications in micro-switches [1-4] and accelerometers that are used for the deployment of air bags [1]. In many of these applications, a basic understanding of the deformation mechanisms at the micron scale is critical to the long-term application of LIGA Ni MEMS structures. Since the potential range of thicknesses used in actual Ni MEMS structures is between ~10 µm to ~700 µm, it is essential to understand how the material will behave in this range. Clearly, the traditional continuum mechanics generally developed for larger scale material will no longer apply at the micron scale. This has stimulated recent work on plasticity at the micron scale [5-10]. In addition to statistically stored dislocations, the contributions of the geometrically necessary dislocations associated with strain gradients must be included to understand plasticity at the micron scale. Recent efforts to explain the plasticity length scale effects have introduced two length-scale parameters: one associated with the rotational gradients, lR, and the other associated with stretch gradients, ls. This effect has been explored by indentation experiments [68], which are associated with the stretch gradients and micro-bend experiments [9] which introduced a composite plasticity length scale parameter. This paper presents a recent study on the extraction of the composite length scale parameter for the LIGA Ni MEMS films with thickness ranging from 25~175 µm. A composite length scale parameter is extracted using a micro-bend technique developed by Stölken and Evans [9]. The implications of the results are then discussed for the modeling of plasticity in LIGA nickel structures. MATERIALS The LIGA Ni specimens used in this study were electrodeposited using sulfamate bath chemistry at Sandia National Laboratories, Livermore, CA. The details of the fabrication process and chemistry composition are outlined by Chri
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