Thermal cycling and stress relaxation response of Si-Al and Si-Al-SiO2 layered thin films

Abstract

The deformation of Si-Al and Si-Al-SiO₂ multi-layered thin films in response to controlled sequences of constant- and variable-amplitude thermal cycling and isothermal exposures has been studied experimentally by recourse toin situ measurements of curvature changes which made use of the laser scanning technique. In an attempt to systematically isolate salient mechanistic features, a select set of companion experiments have also been conducted on the Si-SiO₂ bi-layer system. In some cases, the layered solids have been subjected to as many as 14 thermal cycles between 20 and 450°C to examine the stability of thermally induced deformation. It is found that the variation of curvature with temperature reaches saturation after the first thermal cycle for the Si-Al bi-layer system. The presence of the SiO₂ passivation layer, however, drastically alters the plastic deformation characteristics of the Al layer with the result that: (i) sharp transitions arise in the variation of curvature with temperature during constant-amplitude thermal cycling; (ii) as many as 12 thermal cycles are needed to attain saturation in the curvature-temperature hysteresis loops; (iii) the extent of stress relaxation is significantly reduced during isothermal hold periods in the heating or the cooling phase of the thermal cycle; and (iv) the effects of certain types of variable-amplitude thermal cycling on elastoplastic deformation are essentially suppressed. An elastoplastic analysis, presented by Sureshet al. (J. Mech. Phys. Solids42, 979, 1994) for multi-layer systems, has been used to interpret some of the experimental results obtained in this paper. The predictions of this analysis for curvature changes during thermal cycling (without isothermal hold periods) are found to capture many trends experimentally observed in the Si-Al and Si-Al-SiO₂ layered systems. It is seen, however, that continuum analyses based upon assumptions of steady-state, power-law creep response for thethin Al film fail to capture the measured effects of the passivation layer on creep relaxation even at saturation.