Experiments and theory of thermally-induced stress relaxation in amorphous dielectric films for MEMS and IC applications

Plasma-enhanced chemical vapor deposited (PECVD) silane-based silicon oxide (SiOx) films were chosen as an example to study the thermally-induced stress relaxation phenomena of amorphous dielectric films. Wafer curvature was measured optically both during and after various thermal conditions, including temperature cycling, constant peak temperature annealing, and varying peak temperature annealing experiments. From these measurements, we are able to obtain a complete stress evolutional history in the thin film, and further derive a series of mechanical/material properties such as viscoelasticity, density and viscosity, coefficient of thermal expansion (CTE), etc. We found that the stress in the amorphous silicon oxide films is sensitive to both viscosity and density changes. Stress relaxation was then theoretically investigated by a viscoelastic model, which was associated with a defects-based microstructural causal mechanism. Our theoretical model elucidates defects movement and consequent microstructure rearrangement as a source of damping, accompanied by viscous flow. This theory was applied to explain a series of experimental results, including stress hysteresis generation and reduction, stress relaxation, and CTE changes, etc.