An Experimental Study of Fracture of LIGA Ni Micro-Electro-Mechanical Systems Thin Films
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INTRODUCTION
IN recent years, significant efforts have been made to develop LIGA (Lithographic, Galvanoformung Abformung) Ni micro-electro-mechanical systems (MEMS) as robust alternatives to silicon MEMS.[1,2] Note that LIGA is the German acronym for electrodeposition into x-ray etched molds. Potential applications include accelerometers for the deployment of air bags, microswitches for commercial and defense applications, microgears, and microsprings.[1] In many of these potential applications, material reliability is of primary importance. For example, in the case of a defense microswitch, the device must operate reliably under single loading scenarios. Under such conditions, there is a need to develop fracture mechanics approaches for the prediction of crack growth in thin films under monotonic loading. One of the ways of exploring crack-growth resistance involves the use of fracture mechanics approaches such as the J-Da curve.[3,4] The applicability of the J-integral[5] for characterizing the near-tip stress and strain fields of a mode I crack in a thin sheet has been demonstrated via three-dimensional finite-element analyses by a number of studies.[6,7,8] In a recent article, Li and Siegmund[9] summarized the range of applicability and limitations of the use of the stress intensity factor K, the J-integral, or its two-parameter (J-Q) extension for predicting the onset of crack growth in thin-sheet structures. These authors also demonstrated that quasi-static crack growth in thin-sheet metallic strucY. YANG, Graduate Student, and W.O. SOBOYEJO, Professor, are with the Department of Mechanical and Aerospace Engineering, Princeton University, and Princeton Institute for the Science and Technologies of Materials, Princeton, NJ 08544, USA. Contact e-mail: [email protected] S. ALLAMEH, Assistant Professor, is with the Department of Physics and Geology, Northern Kentucky University, Highland Heights, KY 41099, USA. B. BOYCE, Staff Scientist, is with the Materials and Process Sciences Center, Sandia National Laboratories, Albuquerque, NM 87185 USA. K.S. CHAN, Institute Scientist, is with the Southwest Research Institute, San Antonio, TX 78238, USA. Manuscript submitted February 28, 2006. Article published online June 13, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS A
tures can be analyzed via a cohesive zone crack model, providing that the material parameters relating the fracture response in the crack-tip cohesive zone can be deduced through an experimental/numerical modeling scheme. While there has been some work done on thin-sheet structures whose wall thicknesses are on the order of millimeter or centimeter scales, there have been no investigations on the fracture resistance of stand-alone metallic thin films. In particular, the authors are unaware of prior reports of J-Da curves for LIGA Ni MEMS thin films. Furthermore, it is unclear whether plasticity length scale effects are important in the cracktip deformation processes that occur in LIGA Ni MEMS thin films. There is, therefore, a need to develop fracture mech
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