Effect of barrier thickness on electromigration reliability of Cu/porous low k interconnects

The effects of Ta barrier thickness on electromigration reliability of dual-damascene Cu/porous methylsilsesquioxane interconnects were investigated. With decreasing Ta barrier thicknesses, the threshold product of current density and line length ( j L ) c was found to be reduced due to less structural confinement from thinner barriers. The effect can be accounted for by the effective modulus of the structure except for the 75-Å Ta barrier where the ( j L ) c product is reduced more than expected, probably due to the presence of defects in the barrier. Results from the early failure test structures revealed a bimodal failure distribution for samples with the 75-Å and l00-Å barriers. Focused ion beam microprobe and transmission electron microscopy observations revealed that the weak-mode early failure was caused by Cu outdiffusion through structural defects in the thin Ta barrier.