Resistivity scaling in CuTi determined from transport measurements and first-principles simulations

Abstract

The resistivity size effect in the ordered intermetallic CuTi compound is quantified using in situ and ex situ thin film resistivity ρ measurements at 295 and 77 K, and density functional theory Fermi surface and electron-phonon scattering calculations. Epitaxial CuTi(001) layers with thickness d = 5.8-149 nm are deposited on MgO(001) at 350 °C and exhibit ρ vs d data that are well described by the classical Fuchs and Sondheimer model, indicating a room-temperature effective electron mean free path λ = 12.5 ± 0.6 nm, a bulk resistivity ρo = 19.5 ± 0.3 μω cm, and a temperature-independent product ρoλ = 24.7 × 10-16 ω m2. First-principles calculations indicate a strongly anisotropic Fermi surface with electron velocities ranging from 0.7 × 105 to 6.6 × 105 m/s, electron-phonon scattering lengths of 0.8-8.5 nm (with an average of 4.6 nm), and a resulting ρo = 20.6 ± 0.2 μω cm in the (001) plane, in excellent agreement (7% deviation) with the measurements. However, the measured ρoλ is almost 2.4 times larger than predicted, indicating a break-down of the classical transport models. Air exposure causes a 6%-30% resistivity increase, suggesting a transition from partially specular (p = 0.5) to completely diffuse surface scattering due to surface oxidation as detected by x-ray photoelectron spectroscopy. Polycrystalline CuTi layers deposited on SiO2/Si substrates exhibit a 001 texture, a grain width that increases with d, and a 74%-163% larger resistivity than the epitaxial layers due to electron scattering at grain boundaries. The overall results suggest that CuTi is a promising candidate for highly scaled interconnects in integrated circuits only if it facilitates liner-free metallization.