FRACTURE INITIATION AT SHARP NOTCHES IN SINGLE-CRYSTAL SILICON

In the context of linear anisotropic elasticity, a universal singular stress field may exist in the region surrounding a sharp reentrant cor ner (notch). In general, both the radial and tangential variation of t he stress fields differ for mode I (symmetric) and made II (antisymmet ric) deformations, and for general anisotropy the mode I and II deform ations are coupled. A failure criterion based on critical values of th e stress intensities may be appropriate in situations where the region around the corner dominated by the singular fields is large compared to the size of intrinsic flaws and any inelastic zones. We determined the mode I stress fields and stress intensities for two sets of notche d silicon flexure specimens using a combination of an asymptotic analy sis using the Stroh formalism, dimensional considerations, and continu um finite element analysis. We carried out a companion experimental st udy to assess the suitability of a critical stress intensity failure c riterion. Specifically, assuming such a criterion is valid, we extract ed critical values of the mode I stress intensities for a series of no tched silicon flexure specimens with notch angles of 70.53 degrees and 125.26 degrees. The specimens were fabricated by anisotropic etching of silicon wafers in KOH, resulting in notch angles of 70.53 degrees b etween (111) and (<(11)over bar>1) planes and 125.26 degrees between ( 111) and (100) planes. The data show that good failure correlation is obtained through the use of a single parameter, the critical mode I st ress intensity. (C) 1998 American Institute of Physics. [S0021-8979(98 )08506-5].