Diffusion studies of copper on ruthenium thin film a plateable copper diffusion barrier

Diffusion studies were carried out on physical vapor deposited Cu/Ru(~20 nm)/Si samples using secondary ion mass spectroscopy (SIMS) and transmission electron microscopy (TEM). Back side SIMS depth profiling revealed well-defined interfaces and showed that Cu interdiffusion was impeded by Ru thin film up to 450 deg C vacuum annealing. TEM showed a 20-22 nm Ru barrier layer with a columnar microstructure oriented vertically with respect to Si substrate. TEM results corroborate with SIMS data to indicate stability of the Ru film barrier for annealing temperatures up to 450 deg C. Direct Cu electroplating on ultrathin Ru barrier layers ( < 20 nm) was investigated in sulfuric acid. The electroplated Cu film is shiny, smooth, and without agglomeration under scanning electron microscopy. Excellent adhesion between interfacial layers was confirmed by the scribe-peel test. The interfacial characterization results indicate that Ru thin film is a promising candidate as a directly plateable Cu diffusion barrier.