Effects of Passivation Layer on Stress Relaxation and Mass Transport in Electroplated Cu Films

Recent studies have shown that the Cu/cap layer interface is the dominant diffusion path for electromigration (EM) in Cu interconnects, making it important to develop effective methods to evaluate the effect of passivation layer on interfacial mass transport and EM lifetime for Cu interconnects. This work shows that a set of isothermal stress relaxation tests together with appropriate modeling analysis can be used to evaluate the kinetics of interface and grain boundary diffusion and the effects of passivation layer. Thermal stresses in electroplated Cu films with and without passivation were measured using a bending beam technique, subjected to thermal cycling and isothermal annealing at selected conditions. Thermal cycling experiments provide information to select the initial stresses and temperatures for isothermal stress measurements. A kinetic model coupling grain boundary diffusion with surface and interface diffusion was developed and found to account well for the isothermal stress relaxation experiments within a certain range of temperatures and stresses. Based on the kinetic model, both the grain boundary diffusivity and the Cu/SiN interface diffusivity were deduced from isothermal stress relaxation measurements. While the deduced grain boundary diffusivity reasonably agrees with other studies, the diffusivity at the Cu/SiN interface was found to be considerably lower than the grain boundary diffusivity within the temperature range of the present study.