The Cu metallization in future VLSI technology requires a liner or diffusion barrier. We have studied TaN thin Films of thickness from 5 nm to 3 nm as the diffusion barrier. We deposited sandwiched TaN/Cu/TaN thin films on oxidized Si wafers, annealed them in air with ramping temperature, and measured in-situ resistance changes. Scanning and transmission electron microscopes were used to characterize the surface and microstructure of the thin films and Rutherford backscattering spectrometer (RBS) was used to profile the elemental composition and after oxidation. The oxidation of the Cu sandwiched between TaN films of 5, 10, 20, and 30 nm occurs at 300, 400, 450 and 475°C, respectively. The activation energy of oxidation of Cu with the TaN barrier of 10, 20, and 30 nm are 1.73, 2.13, and 2.26 eV, respectively, but the 5 nm TaN was not calculated due to large data scattering. For comparison, the oxidation temperature and activation energy for bare Cu films in air are 250°C and 0.74 eV, respectively. It suggests that 20 nm of TaN is quite effective as a barrier layer. However, the reaction between Cu and the underlying SiO2/Si layers was also detected in RBS; this may arise as another serious reliability criterion to the system.