The effects of the stress dependence of atomic diffusivity on stress evolution due to electromigration

Abstract

The evolution of mechanical stress resulting from electromigration in a polycrystalline thin film Al interconnect is simulated as a function of current density assuming zero stress boundary conditions. A polygranular cluster region is introduced into lines with otherwise bamboo microstructures, and results in a pair of atomic flux divergence sites and a tensile-compressive stress dipole. In the tensile stress region, the atomic diffusivity is higher than that in the compressive stress region, leading to more rapid electromigration, so that the tensile stress is initially slightly larger than the compressive stress in the dipole. Eventually though, the tensile stress drops to a small value while the compressive peak in the cluster increases above its initial values due to a stress buildup at the end of the cluster. For a given line, the maximum compressive stress resulting from electromigration is larger than the maximum tensile stress. Increasing the current density leads to an increasing difference in the maximum compressive and tensile stresses.