Study of electromigration-induced resistance and resistance decay in Al thin-film conductors

Using an ac Bridge technique, the resistance of thin-film (500 nm thick) Al and AlCu(5%) conductor stripes was monitored during high direct current density (1×106 A cm-2) stressing. The resistance was found to increase approximately linearly with time during stressing with a rate that was thermally activated. Surprisingly the activation energy varied considerably between nominally identical samples. After 3 h of stressing, the current was turned off and an exponential decay in the resistance was observed with a time constant of several hours. The decay was also thermally activated with an energy that varied from film to film. Both results suggest that the activation energy for resistance change during electrical stress can be significantly different in nominally identical films.