Electromigration Failure Under Pulse Test Conditions

Unpassivated aluminum thin-film conductors were subjected to pulsed dc currents at a frequency of 250 kHz, with duty factors ranging from 10% to 100%. Current densities of 4×106 A/cm2 and 1 × 107 A/cm2 were employed. Some tests were also performed at frequencies of 20 kHz and 100 kHz. The conductor lifetime was found to depend strongly on the pulse duty factor, with the mean time to failure, measured in on-time hours, showing a monotonic improvement as the duty factor of testing was decreased. At a pulse current density of 1 × 107 A/cm2, the results demonstrated the occurrence of electromigration in the direction of the "electron wind". A proportion of the improvement in the conductor lifetime is attributable to a decrease in the amount of conductor self-heating, particularly at a current density of 1 × 107 A/cm2; however, self-heating considerations cannot account for the entire improvement unless unusually large activation energies for grain boundary diffusion are assumed. The cause of the remainder of the improvement (not accounted for by self-heating) has not been determined; however, excess vacancy concentration, and its dependence on duty factor, may contribute to the lifetime improvement.