On the basis of atomistic calculations, an explanation was suggested for why Cu prolonged the electromigration lifetime of Al-Cu interconnects, as compared with Al. It was noted that Cu preferentially segregated to grain-boundary interstitial sites. The overall grain boundary diffusivity decreased with increasing Cu segregation at grain boundary sites. Calculations predicted that, in Al-Cu lines, the Cu would diffuse first; with Al diffusion being essentially suppressed because of a higher diffusion activation energy. The activation energy for Cu incubation diffusion was calculated to be 0.95eV. The predictions were in excellent agreement with experiment.

A New Investigation of Copper’s Role in Enhancing Al-Cu Interconnect Electromigration Resistance from an Atomistic View. X.Y.Liu, C.L.Liu, L.J.Borucki: Acta Materialia, 1999, 47[11], 3227-31