On the Unification of Material Strength Testing for MEMS Applications
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On the Unification of Material Strength Testing for MEMS Applications Kuo-Shen Chen Department of Mechanical Engineering, National Cheng-Kung University Tainan, Taiwan, 70101, ROC ABSTRACT In MEMS, the geometry and applied loads between strength testing coupon and structures for design are usually different and the strength data cannot be directly applied without modification for designing brittle MEMS structures. In this paper, a method based on equal failure probability is proposed to find the equivalent strength for design by scaling the test strength with a parameter called stress ratio. By the same approach, it is also possible to correlate MEMS material strength data obtained by different testing schemes. Keyword: Fracture strength, Failure probability, MEMS, Weibull statistics INTRODUCTION Material strength data is critical for structural design. Due to significant difference in length scale and fabrication processes, mechanical properties of materials for microelectromechanical system (MEMS) are usually different from their bulk states and the data obtained from macroscale testing cannot be directly applied for MEMS structural design. As a result, significant effort has been conducted in developing strength characterization schemes for MEMS materials [1,2]. These schemes are different in both specimen dimension and loading type. In addition, for the same material with same fabrication process, different test methods yielded different results [2]. This brings difficulty in MEMS structural design. Due to constraints in specimen fabrication and the available loading and instrumentation, MEMS material testing methods are usually different from the convenient testing schemes. Although testing structures and loading methods are different, their basic methodologies are the same. As shown in Figure 1, the entire characterization process includes two major efforts: failure load measurement and load to failure strength conversion. Both experimental investigation and mechanical modeling are critical for a successful strength characterization. Most MEMS materials are brittle. Their strength is controlled by the local behavior of dominated flaws, which depend on structural size, fabrication processes, and loading conditions. Strength of these MEMS materials is, therefore, a stochastic variable. This implies that the test strength cannot be directly applied unless the testing coupon and the desired structure is similar. If these material strength data cannot be used universally, individual material strength testing must be performed prior to any brittle MEMS structural designs. This imposes severe constraints for the development of MEMS. In addition, a well-established material strength database is also critical. Although forming standard test procedures maybe the best solution to solve this problem, it is difficult due to lack of similarity in scale, geometry, and loading between most MEMS devices. However, at this moment, it is possible to develop a method to correlate strength data from different testing procedures and
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