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Proceedings Paper

Statistical characterization of fracture of brittle MEMS materials
Author(s): Peter T. Jones; George C. Johnson; Roger T. Howe
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Paper 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.

Paper Details

Date Published: 18 August 1999
PDF: 10 pages
Proc. SPIE 3880, MEMS Reliability for Critical and Space Applications, (18 August 1999); doi: 10.1117/12.359368
Show Author Affiliations
Peter T. Jones, Univ. of California/Berkeley (United States)
George C. Johnson, Univ. of California/Berkeley (United States)
Roger T. Howe, Univ. of California/Berkeley (United States)

Published in SPIE Proceedings Vol. 3880:
MEMS Reliability for Critical and Space Applications
Russell A. Lawton; William M. Miller; Gisela Lin; Rajeshuni Ramesham, Editor(s)

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