We have characterized the electromigration performance of copper damascene interconnects using moderately and highly accelerated lifetime tests respectively at package and wafer level. Two metallizations have been studied: Chemical Vapor Deposition (CVD) copper deposited on CVD TiN (Process A) and electroplated (ECD) copper deposited on CVD TiN using 90 nm of CVD copper as a seed-layer (Process B). All metallizations were passivated with SiO2. Two line widths have been characterized: 0.6μm and 4μm.
For wide lines, we obtained similar activation energies (Ea) for both metallizations (0.63 for process A and 0.65 eV for process B). For narrow lines, the Ea value is 0.8eV for CVD copper whereas it is higher than 1eV for ECD copper. For wide lines of both metallizations, failure analysis performed with a Scanning Electron Microscope (SEM) gave clear evidences that microstructural gradients have a strong impact on voids and extrusions formation (i.e. that grain boundaries are an active diffusion path in spite of low Ea values). For narrow lines, diffusion at the upper interface is believed to be the main diffusion path.
From the reliability point of view, the extrapolated lifetimes of the metallization including ECD copper are much higher (1 to 2 orders of magnitude depending on the line width) than for CVD copper.