Strain relaxation mechanisms of lead and lead alloy thin films on silicon substrates

Abstract

Strain relaxation mechanisms were investigated using X-ray diffraction, transmission electron microscopy and scanning electron microscopy for lead and lead alloy films deposited onto oxidized silicon wafers at room temperature and then thermally cycled between room and liquid helium temperature. The strain relaxation mechanisms were found to be strongly dependent on temperature and on the stress or strain level applied to the films. Two dominant relaxation mechanisms were observed in the films: dislocation glide (at low temperature and high stress) and diffusion creep (at high temperature and low stress). The strain was observed to be elastically supported by the films at low temperature and low stress. In the dislocation glide region, most dislocations were observed to glide across the grains on {111} planes inclined to the film surface. All dislocation motions were confined in each grain by the surface oxide, the substrate and/or grain boundaries. Based on the observed dislocation behavior, a new thin film strength model was developed using a minimum energy criterion. From this model, it was concluded that the grain size, the film thickness and the shear moduli of the surface oxide and the substrate have strong effects on the thin film strength. In the diffusion creep region, two relaxation processes were observed. The primary (fast) relaxation process is believed to occur by grain boundary diffusion creep and results in the formation of hillocks when the film is under compressive stress. The secondary (slow) relaxation process was observed to be volume diffusion creep. In the elastic region, large elastic strain variations were observed along directions normal and parallel to the surface of the blanket films. Additional strain variation was observed at the edges of line-shaped films. In this paper the studies carried out in the dislocation glide region are reviewed. © 1982.