Abstract
The stiff compromise between reliability and conductivity of copper interconnects used in sub-nanometer nodes has brought into focus the choice of encapsulation material. While reliability was the primary driver so far, herein, we investigate how electronic conductivity of Cu(111) thin films is influenced by the encapsulation material using density functional theory and Boltzmann transport equation. Atomically thin 2D materials, namely conducting graphene and insulating graphane both retain the conductivity of Cu films whereas partially hydrogenated graphene (HGr) results in reduction of surface density of states and a reduction in Cu film conductivity. Among transition metal elements, we find that atoms in Co encapsulation layer, which essentially act as magnetic impurities, serve as electron scattering centres resulting in a decrease in conductivity by at least 15% for 11 nm thick Cu film. On the other hand, Mo, Ta, and Ru have more favorable effect on conductivity when compared to Co. The cause of decrease in conductivity for Co and HGr is discussed by investigating the electronic band structure and density of states. Our DFT calculations suggest that pristine graphene sheet is a good encapsulation material for advanced Cu interconnects both from chemical protection and conductivity point of view.
Highlights
The continued downscaling of integrated circuits to increase the density of transistors on a chip has necessitated reduction in lateral dimensions of copper (Cu) interconnects to values well below the mean free path for electrons (40 nm in Cu)
A more fundamental question to ask is what materials or material properties are desirable for encapsulation purposes purely from the point of view of Cu conductivity? How does graphene, a conducting 2D material compare with Co? In addition to Co and graphene there has been a lot of interest in other platinum-group metals
Within density functional theory (DFT) and semi-classical Boltzmann transport theory, the effect of graphene, partially hydrogenated graphene, graphane, Co, Mo, Ta, and Ru encapsulations on the electronic transport properties of Cu(111) thin films of thicknesses up to 11 nm
Summary
The continued downscaling of integrated circuits to increase the density of transistors on a chip has necessitated reduction in lateral dimensions of copper (Cu) interconnects to values well below the mean free path for electrons (40 nm in Cu). Given the limitations on how much current density nanoscale cross-sections can support, one can think of the encapsulation layer that positively affects the electronic structure of surface copper atoms and improves effective conductivity. For example, it was reported[9] that the conductance of Cu could be enhanced by graphene capping and it was argued that www.nature.com/scientificreports/. The effect of atomically thin layer of Ru encapsulation lies in between Co and Gr whereas an atomic layer of Mo or Ta shows favorable effects on conductivity compared to Co and Ru. The cause of modulation in conductivity for 2D materials and transition metal encapsulations is discussed by investigating the electronic band structure and DOS for a Cu film of thickness, t = 4 nm
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