Influences of specimen size and deformation mode on the strength of single-crystal silicon micro-beam structures

In this article, the influences of specimen size and deformation-mode on the fracture strength of single-crystal silicon (SCS) micro-beam structures are described. We have designed and developed a new materials test device that is able to apply pure torsion, torsion-bending-combined, and bending deformations to a micro-scale beam specimen. The differences in deformation-mode are determined by loading position and with or without plate support. The specimen made of SCS consists of a SCS plate supported by two SCS micro-beams, and a frame, and was fabricated by deep reactive ion etching (DRIE) with the same fabrication recipe. All the SCS specimens tested in those deformation-modes were fractured in a brittle manner at a laboratory temperature. The fractured strength was calculated assuming that all the specimens had an ideal flat surface, and the data were arranged using two-parameter Weibull distribution function. Both the specimen size and deformation-mode dependencies were apparently found. To estimate the “true” strength, finite element analyses (FEAs) using the models that included scalloping sidewalls were carried out. The revised strength data that were estimated by multiplying stress concentration factors into the original data suggested that no deformation-mode dependency on strength existed. Based on scanning electron microscopy (SEM) analyses, we have found that specimen size dependency was probably related to the number of nanoscale surface defects that were introduced during a DRIE process.