Concentration and stress evolution during electromigration in passivated Al(0.25 at.% Cu) conductor lines

Abstract

We have used x-ray microbeam fluorescence and diffraction for in-situ measurements of electromigration-induced Cu diffusion and stress evolution in passivated, polycrystalline 10μm-wide, 200μm-long Al(0.25 at.% Cu) conductor lines. Cu migration is in the direction of the electron flow and is determined by the direction and magnitude of the current and by the temperature during electromigration. The effective charge and diffusivity of Cu in Al(Cu) have been obtained from analysis of the Cu concentration profiles. The evolution of electromigration-induced strains normal to the sample surface has been monitored by x-ray microbeam diffraction. A linear strain profile developed after about 9 hrs of electromigration with 1.5×105 A/cm2 at 300°C, corresponding to 3MPa/μm equi-biaxial stress. From the Cu profile measured at the same time, the critical Cu concentration for significantly slowing down Al grain boundary diffusion is estimated to be ∼0.15 at. %. These data also confirm that downstream Cu transport is accompanied by a counter flow of Al in the upstream direction.