Reactively-sputtered TiN films were studied as a copper diffusion barrier in the Cu/TiN/Ti/Si and Cu/TiN/Ti/SiO2/Si multi-layer structures. From the viewpoint of the microstructure of TiN, the diffusion barrier property of TIN against copper improved when the grain boundary of TiN (as the diffusion path of copper) was extended and densified, which was confirmed by the increase of the breakdown temperature of the TiN diffusion barrier detected by various characterization methods. The 40-nm thick TiN with double deposition (extension of the grain boundary of TiN) and stuffing (the densification of the grain boundary of TiN) by RTP treatment (NH3, 600 degrees C, 1 min) was found to be stable up to 575 degrees C for 2 h by the C-V method.