The reactions in the Si/Ta/Cu metallization system produced by the sputtering process were investigated by means of sheet resistance measurements, X-ray diffraction (XRD), Rutherford backscattering spectroscopy (RBS), scanning electron microscopy (SEM) and optical microscopy. In particular, the reaction sequence was emphasized. The reaction mechanisms and their relation to the microstructure and defect density of the thin films are discussed on the basis of the experimental results and the assessed ternary Si–Ta–Cu phase diagram at 700°C. It was found out that the effectiveness of the Ta barrier is mainly governed by the defect density and their distribution in the Ta film. The failure was induced by the Cu diffusion through the Ta film and almost simultaneous formation of Cu3Si and TaSi2.