Stress Development and Relaxation during Reaction of a Cobalt Film with a Silicon Substrate

Abstract

Thin metal films react with silicon substrates to form various metal silicides. The sequence and kinetics of phase formation are still an area of intense research. Comparatively much less work has been done on the issue of stress development caused by the appearance of these new phases. A detailed review of the subject has been done ten years ago. We present here recent results obtained on Pd-Si, Co-Si, Ni-Si and discuss them in the light of what is known today on the elastic and plastic properties of thin films. A simple model published by S. - L. Zhang and F. M. d’Heurle takes into account the simultaneous stress formation due to the reaction and the relaxation of these stresses. It provides a qualitatively satisfying picture of stress evolution at least for the first phase which forms. The model relies on two basic elements: 1) stress formation due to the formation of a new phase, and 2) the stress relaxation mechanism at work in the growing silicide film. The sign of the stress can be understood from the variation in volume that occurs at the growing interface(s). The stress relaxation mechanisms at work in a growing film are complex. They are highly dependent on the microstructure (as we have shown when comparing Pd/Si(001) and Pd/Si (111)) but should be also highly size dependent (e.g. dislocation glide is more difficult in small scale structures). Inhomogeneous plastic relaxation in polycrystalline silicide films may be an important issue.