Silicon Strength Testing for Mesoscale Structural Applications

Abstract

AbstractThe development of power MEMS, such as the Microengine being developed at MIT, requires highly stressed structures to achieve high power densities. Material strength is, therefore, a critical issue for the design of such devices. Due to the stochastic nature of the strength of brittle materials, the length scales of the test specimens should be close to those of the structure in order to avoid excessive extrapolation of the test data. In this paper, strength characterization and supporting analysis of mesoscale biaxial flexure and radiused hub flexure single crystal silicon specimens are presented. The Weibull reference strength' of planar biaxial flexure specimens was found to lie in the range 1.2 to 4.6 GPa, depending on the surface quality. The local strength at stress concentrations was obtained by testing radiused hub flexure specimens. For the case of deep reactive ion etched (DRIE) specimens, the strength at fillet radii was found to be significantly lower than that measured from planar biaxial flexure specimens due to the inferior surface quality in such regions. It was found that strength could be significantly increased by the introduction of an additional isotropic etch after the DRIE step. The test results reported herein have important implications for the development of highly stressed microfabricated structures. The sensitivity of the mechanical strength to surface processing and etching techniques must be accounted for in the design cycle, particular with regard to the selection of the appropriate fabrication route. Furthermore, in the design of highly stressed MEMS devices, it is important to account for the stochastic nature of the material strength.