The properties of ultrathin ruthenium (∼5 nm)/TaN(∼5 nm) bilayer as the copper diffusion barrier are studied. Cu, Ru, and TaN thin films are deposited by using the ion beam sputtering technique. The experimental results show that the thermal stability of the Cu/Ru/TaN/Si structure is much more improved than that of the Cu/Ru/Si structure, which should be attributed to the insertion of the amorphous TaN interlayer. The microstructure evolution of the Cu/Ru/TaN/Si structure during annealing is also discussed. The results show that the Ru/TaN bilayer can be a very promising diffusion barrier in the future seedless Cu interconnect technology.