Statistical characterization of fracture of brittle MEMS materials

Peter T. Jones; George C. Johnson; Roger T. Howe

doi:10.1117/12.359368

18 August 1999 Statistical characterization of fracture of brittle MEMS materials

Peter T. Jones, George C. Johnson, Roger T. Howe

Proceedings Volume 3880, MEMS Reliability for Critical and Space Applications; (1999) https://doi.org/10.1117/12.359368
Event: Symposium on Micromachining and Microfabrication, 1999, Santa Clara, CA, United States

Abstract

The fracture of brittle MEMS materials is often characterized by ultimate strength measures such as the maximum stress or strain in an element at failure. It has been known for many decades that a better way to characterize the strength of a brittle material on the macro-scale is to make use of statistical measures. This is due to the nature of brittle materials in which failure occurs when a critically sized flaw exists in the region that is under tensile stress. The distribution of flaws is often random, so the strength of a brittle material can only be properly characterized by statistical measures. Working with MEMS devices, where the site scale is small, it becomes even more important to use a statistical approach. Doing so can explain two observed effects. First, there is an apparent size effect on the strength of the material. The larger the structure that is under a given stress, the larger the region where a critically sized flaw may exist, resulting a higher probability of failure. Second, two identical beams with different stress states, loaded to the same maximum stress can have dramatically different average strengths. In this paper, Weibull statistics are used to characterize the strength of one MEMS material-- polycrystalline silicon. The relevant statistical measures are obtained from the fracture of a large number of cantilever beams. It is shown that, for this material, the average failure strength of a beam loaded in uniaxial tension should be on the order of 40% lower than the average strength of identical beams loaded in cantilever bending.

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Peter T. Jones, George C. Johnson, and Roger T. Howe "Statistical characterization of fracture of brittle MEMS materials", Proc. SPIE 3880, MEMS Reliability for Critical and Space Applications, (18 August 1999); https://doi.org/10.1117/12.359368

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