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The contact resistance at metal-metal (W, Mo, Ru, Co, TiN) interfaces is determined using a new method based on blanket superlattice thin films where the resistivity ρ parallel to the interfaces is measured as a function of superlattice period Λ to quantify the electron interface scattering. Epitaxial W(001)/Mo(001) superlattices show a continuous resistivity increase from 7.10 to 8.62 µΩ-cm with decreasing Λ = 50-1.7 nm, indicating a contact resistance of 2.6×10-16 Ω-m2. Ru/Co multilayers show a much more pronounced increase from 15.0 to 47.5 µΩ-cm with Λ = 60-2 nm which is attributed to atomic intermixing leading to an interfacial Ru-Co alloy with a high measured ρ = 61 µΩ-cm and a Ru-Co contact resistance for interfaces deposited at 400 °C of 9.1 ×10-15 Ω-m2. Ru/TiN and Co/TiN interface resistances are dominated by the high ρ for TiN, and are therefore proportional to the TiN thickness. 

CuTi layers are co-sputter deposited on 20-nm-SiO2/Si(001) wafers at 350 ℃ to quantify their stability in direct contact with a dielectric and to explore the potential of CuTi as barrier- and liner-free interconnect metal. X-ray diffraction pole figures indicate a preferred 001 out-of-plane crystalline orientation and Rutherford backscattering confirms a stoichiometric composition. Vacuum annealing tests at 450 ℃ of CuTi layers indicate considerably higher thermal stability than for pure Cu layers, including negligible dewetting observed by scanning electron microscopy and negligible intermixing with the oxide substrate quantified by photoelectron spectroscopy. Four-point bend tests show a 25% higher interfacial toughness for CuTi/SiO2 than Cu/SiO2 interfaces. CuTi/SiO2 samples also exhibit a 300-times longer failure time than Cu/SiO2 during time-dependent dielectric breakdown tests using an externally applied 3 MV/cm electric field. The higher stability of CuTi in comparison to Cu is attributed to a higher cohesive energy in combination with an atomically thin self-limiting Ti oxide layer at the CuTi/SiO2 interface. 

The electron reflection probability r at symmetric twin boundaries Σ3, Σ5, Σ9, and Σ11 is predicted from first principles for the eight most conductive face-centered cubic (fcc) metals. r increases with decreasing interplanar distance of atomic planes parallel to the boundary. This provides the basis for an extrapolation scheme to estimate the reflection probability r r at random grain boundaries, which is relatively small, r r = 0.28–0.39, for Cu, Ag, and Au due to their nearly spherical Fermi surfaces, but approximately two times higher for Al, Ca, Ni, Rh, and Ir with a predicted r r = 0.61–0.72. The metal resistivity in the limit of small randomly oriented grains with fixed average size is expected to be proportional to the materials benchmark quantity ρ o λ × r r /(1 − r r ), where ρ o and λ are the bulk resistivity and bulk electron mean free path, respectively. Cu has the lowest value for this quantity, indicating that all other fcc metals have a higher resistivity in the limit of small randomly oriented grains. Thus, the conductivity benefit of replacement metals for narrow Cu interconnect lines can only be realized if the grains are larger than the linewidth or exhibit symmetric orientation relationships where r < r r .