Electromigration is a complex process consisting of the nucleation, growth, and movement of voids. While interconnect microstructure plays a major role in determining void nucleation time and location of void nucleation, recent studies have shown that this is not the only contributing factor. Thermodynamics and kinetics-based calculations by Flinn and Gleixner et al. have shown that electromigration void nucleation by vacancy condensation at both homogeneous and heterogeneous sites will not occur at a reasonable rate without a mechanism for reducing or altogether eliminating the energy barrier for nucleation. By implanting argon ions into the aluminum interconnect lines, we introduce an initial defect population (argon bubbles) of controlled size and location.
Tests were performed in a high-voltage SEM (120 keV), which enables in-situ observation of the voiding process through the passivation layer. Images taken throughout the in-situ tests were analyzed to determine void nucleation times and locations. In the argon-implanted interconnects, ten of the 15 voids that nucleated were within implanted regions. Voids nucleated in the interior of the line within the implanted regions, as well as at the passivation/sidewall interface where voids are typically seen in conventional electromigration tests. In addition, voids in implanted regions nucleated much more quickly than those in unimplanted regions. These observations support the idea of argon bubbles reducing the nucleation barrier. TEM was used to analyze the microstructure of both control and implanted interconnect lines.