We report the effects of Al doping on electromigration in damascene Cu interconnects. Al doping was performed by thermal diffusion from a CuAl seed layer positioned underneath the Cu interconnects. To investigate the dependencies of the Al concentration, the seed CuAl layer thickness was increased from 40 to 90 nm on a planar surface. The effects of Al doping on the incubation time, drift velocity, and critical product of electromigration were investigated. The drift velocity of Cu mass transport in CuAl alloys decreases with an increase in the concentration of Al atoms. The observed critical product of electromigration is 1500 A/cm, and it is independent of the Al concentration. The measured activation energies of the normalized drift velocities for CuAl seed layer thicknesses of 40, 60, and 90 nm are 1.2, 1.4, and 1.5 eV, respectively. The Al concentrations at Cu/SiCN interface, grain boundary, Ta/Cu interface, and bulk were investigated along the length of a line by the electron microprobe technique. The time dependent Al concentration at the Cu/SiCN interface near the cathode end of the line is observed. These characteristics indicate that the doped Al affects the electromigration-induced Cu diffusion and does not affect the driving force.
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