Epitaxial metals for interconnects beyond Cu

Experimentally measured resistivity of Co(0001) and Ru(0001) single crystal thin films, grown on c-plane sapphire substrates, as a function of thickness is modeled using the semiclassical model of Fuchs–Sondheimer. The model fits show that the resistivity of Ru would cross below that for Co at a thickness of approximately 20 nm. For Ru films with thicknesses above 20 nm, transmission electron microscopy evidences threading and misfit dislocations, stacking faults, and deformation twins. Exposure of Co films to ambient air and the deposition of oxide layers of SiO₂, MgO, Al₂O₃, and Cr₂O₃ on Ru degrade the surface specularity of the metallic layer. However, for the Ru films, annealing in a reducing ambient restores the surface specularity. Epitaxial electrochemical deposition of Co on epitaxially deposited Ru layers is used as an example to demonstrate the feasibility of generating epitaxial interconnects for back-end-of-line structures. An electron transport model based on a tight-binding approach is described, with Ru interconnects used as an example. The model allows conductivity to be computed for structures comprising large ensembles of atoms (10⁵–10⁶), scales linearly with system size, and can also incorporate defects.