TiB2 films co-sputtered from boron and TiB2 targets were used as diffusion barriers between Cu thin films and Si substrates. Material characteristics of TiB2 films and metallurgical reactions of CuTiB2/[Si] system annealed in the temperature range 400-800 degrees C fur 30 min were investigated by glancing angle X-ray diffraction, Auger electron spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and sheet resistance measurements. The chemical composition and resistivity of the co-sputtered Ti-B films are sensitive to the bias applied on the substrate. A nanocrystalline TiB2 film with a resistivity of 300 mu Omega cm was obtained when a negative bias of 200 V was applied to the substrate during sputtering. After depositing a copper overlayer, we observed that the sheet resistance of the Cu(180 nm)/TiB2(60 nm)/[Si] system stayed at a constant value after annealing up to 600 degrees C for 30 min: however, the sheet resistance increased by almost five orders of magnitude after annealing at 700 and 800 degrees C. At that point, the surface morphology was seriously deteriorated and formation of Cu3Si was also observed. The co-sputtered TiB2 diffusion barrier accordingly breaks down after annealing at 700 degrees C for 30 min.