Electromigration properties of multigrain aluminum thin film conductors as influenced by grain boundary structure

Electromigration rates in polycrystalline interconnect lines are controlled by grain-boundary diffusion. As such, reliability of such interconnects is a direct function of the grain-boundary character distribution in the lines. In the present work, drift velocity experiments were performed on multicrystalline lines of pure Al to determine the electromigration activation energy of the lines. Lines cut from films processed by partially ionized beam deposition techniques were analyzed. One set of lines was analyzed in the as-deposited condition while the other film was annealed before testing. The measured drift velocities varied dramatically between these two types of films, as did the grain-boundary character distributions measured by orientation imaging. The data were analyzed based on Borisov's equation to obtain mean grain boundary energies. Grain-boundary energy of the film with poor electromigration performance was calculated to be that reported for random boundaries, while that for the more reliable film was calculated to be that reported for twin boundaries in Al. Percolation theory was used to aid explanation of the results based upon the fraction and connectedness of special boundaries in the films.