Impact on Electromigration Performance of Combining CuSiN and Ti-Barrier Metal in Cu Interconnects

CuSiN and Ti-barrier metal (BM) were used in combination to resolve the tradeoff of CuSiN between resistivity and electromigration (EM) performance. Two types of CuSiN were prepared for this experiment. The first type, called CuSiN(h), was a thick layer and improved EM lifetime, but excess Si appeared during the formation of CuSiN(h) and diffused into the Cu line. Consequently, the resistivity was high. The second type, called CuSiN(l), exhibited no Si diffusion in the Cu line, but there was no improvement in EM performance in the case of Ta-BM. Although using CuSiN(h) and Ti-BM in combination slightly mitigated the resistivity increase caused by CuSiN(h), the extent of mitigation was insufficient, and resistivity remained high. On the other hand, the combination of CuSiN(l) and Ti-BM provided the ideal distribution of Si and Ti for high EM performance: specifically, Si and Ti were uniformly distributed along grain boundaries on the Cu surface. As a result, the median time-to-failure due to EM was about two orders longer as compared with the Ta-BM sample without CuSiN, whereas resistivity remained low. In EM testing, Cu atom transport was suppressed throughout the grain boundaries on the Cu surface, and the activation energy was 1.45 eV.