Effect of liner thickness on electromigration lifetime

Electromigration lifetime was measured as a function of liner thickness for $\mathrm{Cu}/{\mathrm{SiO}}_2$ interconnect structures. A significant increase in mean lifetime was observed for structures in which the liner thickness at the base of the test via was less than approximately 6 nm, with a current density $\text{<5\hspace{0.05em}}\mathrm{mA}\mathrm{/\mu }{\mathrm{m}}^2$ in the power line connected to the test via. This is attributed to the continuous flow of Cu across the thin and possibly discontinuous liner at the base of the via. For extremely thin liner coverage, $\text{<1.4\hspace{0.05em}}\mathrm{nm}$ at the base of the via and 2.5 at the bottom of the test line, the mean lifetime was observed to decrease as a rapid diffusion path was created which partially offset the beneficial effects of continuous flow. Failure distributions appeared to be trimodal and this was confirmed through failure analysis. In the case of thin liner coverage $\text{(<6\hspace{0.05em}}\mathrm{nm}\mathrm{),}$ early fails, which are typically characterized by slitlike voids at the via/line interface, were not observed.