Modelling the upper yield point and the brittle–ductile transition of silicon wafers in three-point bend tests

Abstract

Three-point bend tests carried out by Folk [R.H. Folk, The brittle to ductile transition in silicon: evidence of a criticaly yield event. PhD thesis, University of Pennsylvania (2000).] on initially dislocation-free specimens of semiconductor-grade single crystals of Si showed that, for a given strain rate, above a critical temperature, T _c, and at a certain stress, a large load drop occurs upon yielding and the sample deforms plastically. Below T _c specimens fail by brittle fracture after a small load drop, or at even lower temperatures without apparent prior plastic flow. This paper describes a simple dislocation model and computer simulation to explain the occurrence of a yield drop, the strain-rate dependence of T _c and Folk's etch pit observations of the plastic zone. Good agreement is obtained between the temperature dependence of the upper yield stress and the strain-rate dependence of T _c with the predictions of the model. These parameters are controlled by dislocation velocity.

Acknowledgements

We are grateful to Professor David Pope for sending us a copy of Folk's thesis, and to Professor Vaçek Vitek for emailing copies of the micrographs. We would like to pay tribute to Dr. Folk for his excellent experimental study. Our thanks are also due to Professor G. D. W. Smith for providing laboratory facilities.