Diffusion barrier properties of nanostructured amorphous Ta-Si-N thin films with different N concentrations have been investigated in metallurgical aspects of Cu metallization. When the N content exceeds 40 at. &percent;, the Ta-Si-N film remains in the nanostructure phase even after annealing at 1100 °C for 1 h and effectively prevents Cu diffusion after annealing at 900 °C for 30 min. The reason for the excellent thermal stability is that excess N atoms disturb the grain growth ofTaSi2phase and keep the Ta-Si-N film in the nanostructure phase during the high temperature annealing. The Ta-Si-N film with N content less than 40 at. &percent; fails to prevent the Cu diffusion after annealing at 700 °C for 30 min. ©1997 American Institute of Physics.