Electron scattering at Ru(0001) surfaces: Effect of Ti caps and oxygen exposure

In situ transport measurements on 7 nm thick epitaxial Ru(0001)/Al2O3(0001) films with and without Ti cap layers during low-pressure 0.05 mTorr O2 exposure are employed to examine the impact of surface chemistry on electron scattering at Ru(0001) surfaces. The sheet resistance Rs of Ru(0001) films increases by 1.0% during oxygen exposure and by 1.0%–2.6% during deposition of 0.1–0.5 nm thick Ti cap layers. These increases are attributed to a perturbation of the surface potential which causes diffuse electron surface scattering corresponding to a 0.08–0.20 decrease in the scattering specularity. In contrast, Ti oxidation reduces the resistance due to a decreasing density of localized surface states which facilitates specular scattering at Ru–TiOx interfaces. These results show that the interface scattering specularity and the overall Ru conductivity are maximized if the Ru conductor remains metallic but the Ti cap layer oxidizes. They demonstrate the potential of surface-state engineering to increase the conductivity of narrow interconnects. Overall, the electron scattering at Ru(0001) surfaces is only weakly affected by the chemistry of add-layers, with a measured decrease in scattering specularity of only −0.20 and −0.08 when coated with a Ti cap and exposed to oxygen, respectively. These values are 1.5–6 and 6–10 times smaller than those observed for competing metals including Cu, Co, and Rh. This superior stability and reduced sensitivity to surface state-induced scattering is a promising feature for future Ru metallization.