Attempts to improve the reliability of narrow passivated Al/Cu conductors as defmed by the performance in an accelerated electromigration test are reviewed. The results are explained in terms of the effect of thermally induced stress and stress voiding on electromigration lifetime. Other processing variations which produced small or inconsequential changes are also discussed. In addition, it was observed that electromigration perfonnance in samples susceptible to stress voiding was degraded as a function of storage time at room temperature.

Electromigration-induced failure was investigated in Al/Cu/Si conductor stripes deposited over trenches cut into an oxide layer and compared with those deposited onto flat areas. The results indicate a reduction in lifetime which is proportional to the size of the step to be covered and this reduction is in excess of what is expected from simple current density scaling. In addition, the electromigration performance of conductors deposited over topology did not correlate well with the performance on flat structures.

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.