Thermal stress and stress relaxation in copper metallization for ULSI interconnects
Thermal stress and mass transport are key issues for Cu metallization yield and reliability. In this study, thermal stresses in Cu films, and line structures with three types of inter-level dielectric (ILD), SiOF, CDO (carbon doped oxide) and SiLK, and the linewidths of 0.2µm and 0.4µm, were investigated using a bending beam technique, Xray diffraction (XRD) and finite element analysis (FEA). During thermal cycling, plastic yield was found to play an important role for the plastic deformation of Cu films. The deformation was strongly affected by the presence of impurities in the films. The stress in Cu lines was found dependent on annealing and the properties of the ILD, but not sensitive to the change of linewidth in submicron range. FEA results indicated that the stresses in the ILD’s as a function of Cu linewidth were quite different in the interconnects. Stress-induced void formation was studied in passivated Cu films during thermal cycling and isothermal annealing. The void density was strongly affected by the ramp rate, film stress and thermal history during thermal cycling. A kinetic model was developed for the void growth, and an activation energy of 0.75eV was deduced. The local stress gradients due to the mechanical anisotropy, and in the void vicinity were evaluated by FEA models. In order to characterize the Cu/passivation interface diffusivity, isothermal stress relaxations of Cu films and line structures were studied. The isothermal stress relaxation behaviors were shown significantly different for the films with different passivation layers, SiNx, SiC, modified SiC and a metal cap layer. Diffusional kinetic models were developed for the stress relaxation in thin films and then combined with experimental results to deduce the interfacial diffusivities, which were found to be much smaller than the grain boundary diffusivity. In the line structures with different ILD’s, the effects of the passivation layers were found to be consistent with the results of the Cu films. The mechanical confinement of the passivation layers on the stress in the Cu lines was evaluated using FEA models. Results from this study demonstrate that stress relaxation measurement is an effective method to evaluate electromigration (EM) performance in Cu metallization.