Grain boundary diffusion and electromigration in Cu-Sn alloy thin films and their VLSI interconnects

Abstract

Grain boundary diffusion of 67Cu and 113Sn radioactive tracers has been measured in Cu-2wt%Sn alloy thin films. Addition of Sn resulted in reduction of grain boundary diffusion of both species and increase in the activation energies by about 0.4eV compared to that for grain boundary self diffusion in Cu at 0.95eV. Grain boundary diffusion of Sn was substantially faster compared to Cu. Electromigration studies have also been conducted in Cu-Sn alloys conducting stripes of 5μm width, 30nm in thickness and lenghts varying from 3-300μm with current densities of 5×10 5 - 2.1×106 A/cm2 and at temperatures ranging from 250-450°C. Upon addition of Sn, the activation energies for electromigration also increased by about 0.5eV although their absolute values were smaller than those measured in the radioactive tracer diffusion experiments. Furthermore, Sn enhanced the electromigration life times very significantly. From these comparative studies, close similarities between the grain boundary diffusion and the electromigration in thin films at low temperatures and the accompanying grain boundary solute segregation in both cases, are unequivocally established.