Effect of electronegativity on electron surface scattering in thin metal layers

Abstract

In situ transport measurements on 10-nm-thick epitaxial Cu(001), Co(001), and Rh(001) layers exhibit a characteristic increase in the sheet resistance ΔRs/Ro = 43%, 10%, and 4% when adding 4.0, 13.0, and 13.0 monolayers of Ti, respectively. Similarly, exposing these layers to 0.6 Torr O2 results in a 26%, 22%, and <5% increase in Rs. This suggests that adatoms on Cu and Co surfaces considerably disturb the surface potential, leading to diffuse electron scattering and a resulting resistance increase while these effects are negligible for Rh. A similarly small resistivity increase Δρ/ρ < 7% is measured during air exposure of 10-nm-thick epitaxial layers of electronegative metals including Ru, Rh, Ir, W, and Mo, while Δρ/ρ increases to 11%–36% for more electropositive metals including Cu, Ag, Co, Ni, and Nb. The Δρ for Ni, Co, and Nb is larger than what is expected for a complete transition from specular to diffuse surface scattering, indicating a breakdown of the semiclassical Fuchs–Sondheimer model, which needs to be replaced by a two-dimensional conductor description. The measured inverse correlation between electronegativity and Δρ/ρ suggests that the magnitude of the surface potential perturbation is the primary parameter affecting electron surface scattering in thin metal layers. More specifically, the charge transfer from electropositive metal surfaces to adatoms perturbs the surface potential and causes electron surface scattering and a resistance increase. Conversely, electronegative metals facilitate smooth surface potentials with specular electron reflection and a minimized resistance increase. They are, therefore, promising as conductors for highly scaled interconnect lines.