TaN x films sputtered from a TaN target were used as diffusion barriers between Cu thin films and Si substrates. Material characteristics of TaN x films and metallurgical reactions of Cu/TaN x /Si systems annealed in the temperature range 400–900°C for 60min were investigated by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, cross-sectional transmission electron microscopy, and sheet resistance measurements. We found that the deposition rate decreased with increasing bias. TaN, β-Ta, and Ta 2 N phases appeared and/or coexisted in the films at specific biases. A step change in N/Ta ratio was observed whenever a bias was applied to the substrate. After depositing a copper overlayer, we observed that the variation percentage of sheet resistance for Cu (70nm)/TaN x (25nm, x=0.37 and 0.81)/Si systems stayed at a constant value after annealing up to 700°C for 60min; however, the sheet resistance increased dramatically after annealing above 700 and 800°C for Cu/TaN 0.37 /Si and Cu/TaN 0.81 /Si systems, respectively. At that point, the interface was seriously deteriorated and formation of Cu 3 Si was also observed.