Elastic moduli, strength, and fracture initiation at sharp notches in etched single crystal silicon microstructures

We designed and fabricated a series of micromechanical test structures for microtensile testing by anisotropically etching epitaxial silicon. Specimens were fabricated to study Young's moduli, the uniaxial tensile strength, and the strength of T-structures which are tensile bars with an abrupt reduced cross section that have a 90 degrees corner at the point of reduction. They are a generic mimic of actual transitions that occur in micromechanical structures due to anisotropic etching. The test structures were loaded in uniaxial tension in a piezoactuated microtensile test apparatus. The applied force and crosshead displacement were recorded and displacements in the specimen gage section were directly measured using a speckle interferometry technique. During tensile loading of the T-structures, fracture always initiates at the sharp 90 degrees corners. This results in an interesting apparent strength scaling where the nominal strength of the structures increases as their width decreases. In order to understand the fracture initiation from the sharp 90 degrees corners of the silicon T-structures, we carried out a complete analysis of the elastic fields at the 90 degrees corners by coupling an asymptotic analysis (to compute the asymptotic radial and angular dependence of the elastic fields up to an arbitrary constant for each loading mode, the stress intensity), and full-field finite element calculations (to determine the magnitude of the stress intensities for specific geometries and loadings). Excellent results are obtained by using a single parameter, the critical mode I stress intensity, to correlate fracture initiation from the sharp 90 degrees corners of the T-structures. (C) 1999 American Institute of Physics. [S0021-8979(99)00306-0].