The effect of interfacial layer on mass transport in Cu damascene line structures was investigated by measuring stress relaxation using bending beam and x-ray diffraction methods. Interfacial layers investigated included SiC, SiNx and a novel etch stop (ES) layer in Cu line structures with a CDO (carbon doped oxide) low k interlevel dielectric (ILD). The observed stress relaxation rates were compared with no passivation in order to examine the bonding effect on interfacial mass transport. A significant effect due to different interfacial layers was observed which can be attributed to the interfacial chemistry and correlated to electromigration lifetime and interfacial adhesion. A kinetic model is being developed to deduce interfacial diffusivity from stress relaxation rate by considering the contribution to mass transport via various diffusion paths including the Cu/passivation interface.