Selection of solutes for improving electromigration resistance of metals: A new insight

Abstract

There has long been as much art as science for choosing solute elements to improve the resistance to electromigration of metals such as aluminum. Alloying additions have been selected with the intention of maximizing the degree of segregation to the grain boundaries, exploiting differences in atom size to limit the grain-boundary excess volume, and forming grain-boundary precipitates which potentially can act as reservoirs of solute or as internal diffusion barriers. We show, using published data concerning the performance of a number of Al alloys measured under similar conditions, that the above strategies do not provide a direct correlation with lifetime. A further necessary condition for an addition which forms a precipitate in a eutectic system is its ability to dissolve into the grain boundary to replenish material driven away by electromigration. In peritectic systems, significant supersaturation can exist in the grains, and the grain boundaries. Equilibration of such systems decreases their effectiveness. In such systems, grains rather than precipitates can act as reservoirs.