Ru-Ta and Ru thin films (similar to 15 nm in thickness) as diffusion barrier for Cu metallization on SiO2/Si and Si substrates are studied. Experimental results show that the Ru-Ta film exhibits amorphous structure until annealing at 700 degrees C, whereas the Ru film crystallizes in as-deposited and annealed states. Sheet resistances of Cu/Ru and Cu/Ru-Ta stacking layers increase after annealing at 500 and 700 degrees C, respectively, regardless of whether the substrate is SiO2/Si or Si. Increase in resistance is concurrent with the formation of Ru2Si3 and Cu3Si when Cu/barrier stacks are deposited on Si substrate. Increase in resistance for Cu/barrier stacks deposited on SiO2/Si substrate is related to the diffusion of Cu through the crystallized barrier to the underlayers. Furthermore, current-voltage measurement also reveals that the Cu/Ru-Ta metallization has a lower leakage current than the Cu/Ru system. The failure mechanism and the effectiveness of Ta addition on barrier performance are discussed. (C) 2008 The Electrochemical Society. [DOI: 10.1149/1.2999348] All rights reserved.