5 nm ruthenium thin film as a directly plateable copper diffusion barrier

Abstract

Interfacial stability of electroplated copper on a $5\mathrm{nm}$ ruthenium film supported by silicon, $\mathrm{Cu}∕\left(5\mathrm{nm}\mathrm{Ru}\right)∕\mathrm{Si}$ , was investigated using Rutherford backscattering and high-resolution analytical electron microscopy. Transmission electron microscopy (TEM) imaging shows that a $5\mathrm{nm}$ Ru film is amorphous in contrast to the columnar microstructures of thicker films $\left(20\mathrm{nm}\right)$ . Direct Cu plating on a $5\mathrm{nm}$ Ru film yielded a homogeneous Cu film with over 90% plating efficiency. It is demonstrated that $5\mathrm{nm}$ Ru can function as a directly plateable Cu diffusion barrier up to at least $300°\mathrm{C}$ vacuum anneal. TEM reveals an interlayer between $\mathrm{Ru}∕\mathrm{Si}$ , which expands at the expense of Ru upon annealing. Electron energy loss spectroscopy analyses show no oxygen (O) across the $\mathrm{Cu}∕\left(5\mathrm{nm}\mathrm{Ru}\right)∕\mathrm{Si}$ interfaces, thereby indicating that the interlayer is ruthenium silicide $\left({\mathrm{Ru}}_x{\mathrm{Si}}_y\right)$ . This silicidation is mainly attributed to the failure of the ultrathin Ru barrier at the higher annealing temperature.