The effect of ash chemistries, N2/H2 and H2, on time-dependent dielectric breakdown (TDDB) lifetime has been investigated for Cu damascene structure with a carbon-doped CVD ultra low-k (ULK, k=2.5) intermetal dielectric. Two failure modes, interfacial Cu-ion-migration and Cu diffusion through the bulk intermetal ULK were attributed to the TDDB degradation for the H2 ash.The interfacial Cu-ion-migration was the only dominated failure mode for the N2/H2 ash. The nitrogen species in the N2/H2 plasma proved to be capable of forming a nitrided protection layer on the surface of the ULK. This nitrided layer suppressed further plasma damage during the ash process and thus lessened the TDDB degradation by preventing Cu diffusion through the bulk ULK.