ON THE DISPERSION OF ELECTROMIGRATION FAILURE TIMES OF AL-ALLOY CONTACTS TO SILICON

Electromigration failure data for integrated circuit metallizations may only be collected in experimentally convenient intervals by the use of highly accelerated conditions of temperature and current density. Large extrapolations are required to apply these data to circuit operating conditions. A common assumption made with these extrapolations is that the dispersion of the log-normal failure distribution, sigma, does not vary with temperature. In this letter we investigate the temperature dependence of the dispersion of failure times due to electromigration of the Al allow layer in contacts to silicon. We show that the dispersion is temperature dependent for these structures and exhibits increasing values with decreasing temperature. The magnitude of the dispersion is consistent with that calculated on the basis of a dominant contribution from fluctuations in the temperature between test structures. It is implied that the intrinsic, temperature-independent contribution to the dispersion is small for contacts to silicon and for contacts between metal levels (vias).