The failure mechanism of the TaCoN barrier for copper metallization was examined using films by direct current (dc) magnetron reactive sputtering at various nitrogen flow rates. The as-deposited TaCoN films had a glassy structure and were free from intermetallic compounds. Optimizing the nitrogen flow rate during sputtering maximized the thermal stability of the Si/Ta66.8Co11.4N21.8/Cu metallization system up to an annealing temperature of 750°C when the film was deposited using a nitrogen flow rate of 1 sccm, as revealed by using X-ray diffraction, a scanning electron microscope, a four-point probe and a transmission electron microscope. Structural analysis indicated that the failure mechanisms of the studied Si/TaCoN/Cu stacked films involved the initial dissociation of the barrier layer that was annealed at a specific temperature, and the subsequent formation of diffusion paths along which the copper penetrates through the TaCoN barrier layer to react with underlying Si. The high formation temperature of the Cu3Si phase demonstrated that the studied film was highly stable, indicating that the TaCoN thin film is highly promising for use as a diffusion barrier for Cu metallization.