Electrodeposition of Cu(111) Onto a Ru(0001) Seed Layer for Epitaxial Cu Interconnects

The electrodeposition of Cu onto epitaxial Ru(0001) seed layers was investigated from a sulfuric acid-based solution containing dilute copper(II) sulfate and chloride ions. Using galvanostatic deposition at −350 μA/cm2, Cu was deposited epitaxially onto a 30 nm-thick Ru(0001) seed layer, despite a compressive misfit strain between −6.9% and −8.3%, depending on the extent of strain relaxation of the Ru layer. However, rather than depositing as a single crystal, Cu grew as a bicrystal having a common out-of-plane orientation of Cu(111) and two equivalent in-plane orientations. The Cu grain size was large, on the order of micrometers, and the grain boundaries were identified as incoherent ∑3{211} twin boundaries. The Cu initially grew as isolated islands, coalescing into a contiguous film at thicknesses around 50 nm. The Cu film was rough, and thickness and coverage varied over the electrodeposited region. After the initial island growth, Cu void fraction and film roughness both decreased with thickness as the deposit transitioned into a planar film with nanometric islands growing on the film surface. However, at thicknesses exceeding 200 nm, anisotropic growth of large, faceted Cu islands on the planar Cu film again increased the surface roughness. The epitaxially deposited Cu bicrystal showed an improvement in resistivity when compared with polycrystalline Cu similarly electrodeposited onto a polycrystalline Ru seed.