Evaluating the Mechanical Properties of MEMS by Combining the Resonance Frequency and Microtensile Methods
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Evaluating the Mechanical Properties of MEMS by Combining the Resonance Frequency and Microtensile Methods Dongil Son, Jong-jin Kim, Dong Won Kim, Tae Won Lim, Dongil Kwon School of Materials Science and Engineering, Seoul National University, Seoul 151-742, Korea ABSTRACT Tensile, fracture and fatigue properties of single- and polycrystalline silicon and LIGA-Ni were evaluated by the resonance frequency and microtensile methods. A new method for evaluating the fracture toughness that combines these two methods was proposed. A pre-crack was generated in an electrostatically driven test specimen and a load was applied by piezoelectrically driven microtensile equipment. Before the microtensile test, a new surface micromachining technique including a two-step sacrificial layer removal was used. The precracked specimen was attached to microtensile equipment by a UV-adhesive glass grip. The fatigue pre-crack was successfully introduced and the fracture toughness could be derived on the basis of fracture mechanics. The fracture toughness of the pre-cracked specimen was relatively low compared with that of the notched specimen, so that we were able to determine the effect of the notch tip radius. The fatigue properties of LIGA-Ni film were also evaluated. A tensiletensile fatigue load was applied by a piezoelectric actuator, and real-time load-displacement curves were displayed via computer. The dependence of S-N curves, crack propagation rates and fatigue-notch factor on the applied load for 10 µm Ni film was analyzed. INTRODUCTION Information on mechanical properties of thin films has become indispensable in the design of microelectromechanical systems (MEMS), where thin films are structural as well as electrical materials. To achieve long-term reliability in various MEMS devices, it is necessary to understand the fracture, fatigue, elastic, and plastic properties of thin films. New test methods suitable for micrometer-scale specimens, including testing techniques, specimen design and equipment, have been required. For example, cantilever beam tests are used for elastic modulus and yield strength, nanoindentation tests for elastic modulus and hardness, microtensile tests for stress-strain curves, electrostatically actuated comb-driven structures for elastic and fracture properties, etc. [1-8]. In this study, we fabricated two types of specimens. One for microtensile testing only and the other for a microtensile-compatible resonating device driven by alternating electrostatic force. With the latter device, we obtain the change in resonant frequency as a function of cycles to detect the fatigue pre-crack. Once the fatigue pre-crack is introduced in the specimen, it is etched one more time to achieve the load-displacement curve with microtensile equipment. Although a fatigue pre-crack is necessary to measure the fracture toughness on the basis of fracture mechanics, difficulties in specimen fabrication and handling have limited the test procedures. Furthermore, equipment for measuring the very small load and displa
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