In situ study of stress relaxation mechanisms of pure Al thin films during isothermal annealing

It is known that the compressive stress relaxation of Al thin films occurs by various deformation mechanisms, such as hillock formation, microstructural changes, and creep. In order to clarify the characteristics and quantify the contributions of each deformation mechanism, the stress relaxations of Al thin film during isothermal annealing at 194 °C were investigated by wafer curvature measurement and in situ hillock observation. For the stress relaxation measured by the wafer curvature method, fast relaxation and subsequent slow relaxation processes were observed. In contrast, for the stress relaxation measured by hillock formation, only one relaxation process was observed, which had a similar time constant to that of the slow relaxation process observed in the wafer curvature measurement. These results indicate that hillock formation is the most plausible mechanism for slow stress relaxation.