Effect of stress in passivation layer on electromigration lifetime for vias

The electromigration performance of the 1.2-μm-diameter via chain between two levels of an Al line was evaluated. It was found that the electromigration lifetime of the vias decreases owing to the increase in the total compressive stress in the passivation layer, even though the step coverage of Al film in the vias is improved by controlling the slope angle of the via hole to increase the electromigration lifetime. On the other hand, the lower limit of the step coverage of Al film in the vias is determined by the stress-induced failure rate. Therefore, in order to bring about high reliability of the vias, the total compressive stress in the passivation layer must be lowered and the step coverage of Al film in the vias must be improved