Electromigration Induced Failure as a Function of Via Interface

Abstract

Failure of test structures composed of pairs of W vias connected by lengths of Al/0.5%Cu conductor were compared as a function of the position of the via with respect to the conductor link. In one configuration, the current was passed from first level to a conductor link on second level and in the other the current was passed from second level to one on the first. In each case the links were identical in cross section, length and composition. There was a significant difference in the performance with respect to electromigration lifetime between these configurations as well as a difference in the failure mode. In the first level links, where the current passed first through a a 350A thick TiN ARC (Anti-Reflective Coating) layer and then a 150A Ti layer, the lifetime was significantly shorter than that of the second level samples where the current was passed across an interface of Al3Ti intermetallic compound. In addition, the failure mode was different, the first metal links failing directly under the via whereas in second metal link, the failures were away from the via in the metal link itself.

Comment on “Effect of current crowding on vacancy diffusion and void formation in electromigration” [Appl. Phys. Lett. 76, 988 (2000)]

“…If the via is poorly made, voids will form exactly at the flux divergence. 15,16 Therefore, we can see that the generation of voids at locations where no current flows, but relatively near to a flux divergence is completely within the bounds of a ''standard'' model of electromigration induced stress and that current crowding plays a small if not insignificant role in electromigration failure.…”