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To optimize copper diffusion barriers, TaNx thin films for diffusion barriers were prepared by using radio frequency sputtering with various flow ratios of N2/Ar as the reactive gas. The component transformed from Ta2N to TaN as observed from deposition rates, and N/Ta ratios as N2/Ar flow ratios from 0.075 to 0.3. Furthermore, the structural transformations from body-centered cubic through face-centered cubic to nanocrystalline for TaNx thin film demonstrated under increase of N2/Ar flow rate, induced three-step stages for formation of TaNx films in the sputtering process. The thermal stabilities of the TaNx thin film in Cu/TaNx/n+np+ diodes suggest that increasing flow ratio of N2/Ar from 0.075 to 0.3 enhanced the thermal stabilities from 450 to 550°C under leakage current conditions below 3 µA. The observation deduces that N2/Ar flow ratios dominated predominantly properties of TaNx films through crystal structure for barrier. [ABSTRACT FROM AUTHOR]