Dependence of Time-Dependent Dielectric Breakdown Lifetime on the Structure in Cu Metallization

Abstract

Time-dependent dielectric breakdown (TDDB) in Cu metallization and its dependence on the presence of a barrier metal, the kind of barrier metal, barrier metal thickness, the low-k dielectric and the low-k barrier dielectric, is investigated. There is a distinct difference between TDDB degradation mechanisms with and without barrier metals. The degradation mechanism is drift through the bulk without the barrier, but drift along the CMP surface with the barrier. The barrier property against TDDB is better for Ta and TaN than that for TiN. The TDDB characteristic is independent of the barrier metal thickness, but depends strongly on the electric field strength in the structure. The degradations, related to Cu ion diffusion, are mainly caused by electrical stress, not thermal stress. In a low-k structure, the leakage current path is along the CMP surface due to the electric field concentration. The TDDB lifetime with a barrier metal depends on the breakdown voltage of the low-k material. The TDDB lifetime decreases as the k-value becomes lower. Although the use of low-k barrier dielectrics decreases the TDDB lifetime, a satisfactory TDDB lifetime is achievable with a low-k barrier dielectric as thin as 25 nm. All low-k structures in our study satisfy the 10-year projected TDDB reliability. However, the TDDB lifetime for technology generations beyond the 65 nm-node CMOS may have inadequate 10-year reliability.