Lower limits of line resistance in nanocrystalline Back End of Line Cu interconnects

The strong non-linear increase in Cu interconnect line resistance with a decrease in linewidth presents a significant obstacle to their continued downscaling. In this letter we use first principles density functional theory based electronic structure of Cu interconnects to find the lower limits of their line resistance for metal linewidths corresponding to future technology nodes. We find that even in the absence of scattering due to grain boundaries, edge roughness or interfaces, quantum confinement causes a severe reduction in current carrying capacity of Cu. We discuss the causes of transport orientation dependent anisotropy of quantum confinement in Cu. We also find that when the simplest scattering mechanism in the grain boundary scattering dominated limit is added to otherwise coherent electronic transmission in monocrystalline nanowires, the lower limits of line resistance are significantly higher than projected roadmap requirements in the International Technology Roadmap for Semiconductors.