Impact of low-k dielectrics and barrier metals on TDDB lifetime of Cu interconnects

Time-dependent dielectric breakdown (TDDB) in Cu metallization and the dependence on the presence of barrier metal, barrier metal thickness, the kind of barrier metals and the low-k dielectrics, is investigated. There is a distinct difference in TDDB degradation mechanism with and without barrier metals. TDDB degradation of Cu interconnects without and with barrier metal is caused by bulk mode and CMP-surface mode, respectively. The TDDB characteristics with barrier metal are almost the same for different barrier metal thicknesses and depends much more strongly on the electric field strength than the MIS structure. Additionally, both degradations, related to Cu-ion diffusion, are mainly caused not by thermal stress but by electrical stress. The barrier properties of Ta and TaN are better than those of TiN against Cu-ion diffusion into dielectrics, for TDDB. In the case of a low-k structure, TDDB properties with barrier metal also depend on the CMP-surface. With low-k dielectrics, the electric field strength is concentrated near the CMP surface and the TDDB lifetime reduces as the k-value decreases. However, all low-k structures in this study are able to satisfy the 10-year TDDB reliability specifications for the capacitor.