Thermal cycling fatigue and deformation mechanism in aluminum alloy thin films on silicon

Thermal cycling was performed between room temperature and 723 K in a sputter deposited thin film of Al-l mol % Si alloy on a silicon substrate. After given numbers of cycling, residual stress was determined at room temperature by measuring the film curvature using a laser deflection apparatus. Residual stress was found to increase with increasing the cycle number up to the 4th cycle, followed by a continuous decrease by further cycling. Based on the microstructure observation, the initial increase of residual stress was caused by the increase of lattice dislocations and their tangling. The following decrease of residual stress was caused by crack formation and delamination. Stress relaxation experiments were also performed during isothermal annealing at various temperatures. Analysis of the relaxation curves indicates three temperature regions representing different deformation mechanisms. The boundaries between the neighboring regions were found to agree with the boundaries in a deformation mechanism map calculated for an Al thin film. Based on the obtained knowledge of the deformation mechanisms, the origin of the microstructure changes and the structural failure by thermal cycling are discussed.