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Fatigue Processes in Silicon MEMS Devices Emily D. Renuart1, Alissa M. Fitzgerald2, Thomas W. Kenny3, Reinhold H. Dauskardt1 1 Department of Materials Science and Engineering, Stanford University, CA 94305-2205. 2 Sensant Corporation, 650 Saratoga Ave., San Jose, CA 95129. 3 Department of Mechanical Engineering, Stanford University, CA 94305. ABSTRACT MEMS devices may experience significant alternating loads during service, associated with both applied and vibrational loading. Long-term reliability and lifetime predictions require understanding of possible fatigue mechanisms in these structures. Although silicon is not generally considered susceptible to fatigue crack growth, recent studies suggest that there may be fatigue processes in silicon MEMS structures. The phenomenon, however, has still not been extensively studied. In this work, we used a compressive double cantilever beam geometry to examine stable crack growth. Crack length and loads were carefully monitored throughout the test in order to distinguish between the apparent role of environmentally assisted crack growth (stress corrosion) and mechanically induced fatigue. Results revealed similar step-like crack extension versus time for the cyclic and monotonic tests. The fatigue crack-growth curve extracted from the crack extension data exhibited a nearly vertical slope with no evidence of fatigue crack-growth. Fracture surfaces for the monotonic and cyclic tests were similar, further suggesting that a true mechanical fatigue crack-growth mechanism did not occur. INTRODUCTION Microelectromechanical systems (MEMS) form a growing range of devices, including micromirrors for digital displays, airbag accelerometers, and miniature blood pressure monitors inserted into the blood stream. Silicon is commonly used as the principal material for MEMS devices because it is easy to manufacture using well-established microelectronic processing techniques. In addition, silicon has very good mechanical properties similar to those of high strength steel, including a strength of 1-2 GPa and an elastic modulus of 170 GPa [1]. Finally, of importance to some MEMS devices that experience significant cyclic mechanical or vibrational loading during service, silicon is not generally considered susceptible to fatigue damage. Silicon demonstrates nearly perfectly brittle fracture, showing very little crack tip plasticity at room temperature due to low dislocation mobility and density. Therefore, it is generally believed that single crystal silicon will not fatigue by typical metal-like fatigue mechanisms. It is possible, however, that other mechanisms could lead to fatigue damage in silicon. For example, it may occur by cyclic degradation of bridging ligaments remaining in the crack wake or environmental effects due to oxide damage during cyclic loading, similar to processes reported in brittle ceramics [2,3].

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Table I. Previous Research on Single Crystal Silicon Research Fatigue effect observed? Number of cycles Mutoh et al (1990) [4] No 107 cycles Brown et al (1993) [5]