The effect of copper additions on electromigration in aluminum thin films

Copper additions reduce the rate of electromigration in aluminum thin film conductors subjected to dc currents of high density. The results of experiments conducted with the goal of understanding the mechanism of this effect are presented. The relationships between lifetimes and the mode of preparation, the composition, and the heat treatment of the samples are explored. The lifetimes of copper bearing aluminum conductors vary in the same way, with respect to temperature and current density, as conductors made of pure aluminum. Data obtained with an electron microprobe on samples which have failed electrically, after prolonged exposure to current, show the coincidence of crack formation and copper depletion in the surrounding area. Analysis along the length of current stressed samples, also with the microprobe, indicates that under the effect of current, copper atoms migrate at a considerably faster rate than aluminum atoms. Gold and silver additions in aluminum do not have the same effect as copper additions. It is believed that this difference is due to variations in adsorption of different atomic species on aluminum grain boundaries.