The electromigration performance of copper damascene interconnects has been studied with respect to hillock formation and is compared to void formation failure mode. The metal lines consisted of Chemical Vapor Deposition (CVD) copper deposited on CVD TiN and capped with SiN. SiO2 was used for copper lines insulation and final passivation. Two line widths (0.5 and 3µm) have been characterized. It is shown that higher activation energy values are obtained for void formation failure mode, respectively Ea = 0.86 eV for wide lines (poly-grain microstructure) and Ea =1.04 eV for narrow lines (quasi-bamboo microstructure) than for extrusion failure mode. Failure analysis performed with a Scanning Electron Microscope (SEM) showed that grain boundaries are active diffusion paths in polycrystalline copper lines whereas interface diffusion is believed to be the main diffusion path in narrow lines. Extrusions are shown to occur at the upper interface of copper damascene lines and to extend laterally as a consequence of cracks in dielectric layers and are thus responsible for short circuit between adjacent lines. Implications on extrapolated lifetimes at operating conditions are discussed.