Stress relaxation during isothermal annealing in electroplated Cu films

The deformation mechanisms of electroplated Cu thin films on TaN/SiO2/Si were investigated by performing isothermal annealing between 180 °C and 400 °C. The stress relaxation behavior during isothermal annealing was analyzed in terms of the grain growth, power-law creep and diffusional creep. The approximated relaxed stress caused by grain growth was too small to consider as the main deformation mechanism in the Cu films. In the case of the diffusional model, the relaxation curve should be able to be fit by two exponent curves only under compressive stress. Furthermore, an extremely low activation energy was obtained. The stress relaxation curves obtained by power-law creep were the best fit. The stress exponent, n, for power law creep, was around 6 during heating and this suggested that the dislocation core diffusion was activated for the power-law creep. On the other hand, the large n value of over 20 during cooling was attributed to the breakdown of the power-law due to the high tensile stress in the cooling stage. Therefore, among the possible mechanisms for stress relaxation in electroplated Cu films, climb-controlled dislocation creep was found to be the most plausible one.