Thermal and electromigration induced strain and microstructure evolution in metal conductor lines

This thesis describes studies of electromigration (EM) in three important interconnect materials, Al, Cu and Pb free solder. In these studies, it was found that EM-induced strains saturate, forming a gradient along the upstream part of Al lines, and that no strain gradient formed in the downstream parts of the lines. An analytical Eshelby-type model and numerical FEM calculations showed that the EM-induced atomic diffusion occurs mainly along the top and bottom interfaces of the Al conductor lines. From the strain gradient formed during EM, the effective valence Z* for Al is determined to be 1.8+/-0.4, in agreement with several previous determinations using other methods.;Thermal strains in Al and Cu conductor lines were found to show similar behavior. Rather than forming biaxial strains after heating, compressive strains formed along the line, and tensile strains formed in the other two directions, which was explained by an anisotropic partial relaxation model.;Partial or full recovery of EM damage at 300C and 350C was observed in Cu lines, characterized by sudden increases and decreases of resistances during EM. Microstructure analysis and thermal calculations showed that the recovery of EM damage, and resistance decreases, were associated with local melting.;No strain gradient was formed during EM in Cu lines. It was found that EM current tends to cause the average residual thermal strain and RMS deviation of strain in Cu lines decrease. The current-induced strain decreases (athermal relaxation) were attributed to stronger interaction between electrons and Cu atoms located in a higher strain zones in the Cu line resulting in increased EM flux in these zones.