Control of Resistance by Oxide on the Surface of Cu Interconnects With CuSiN and Ti-based Barrier Metal

To achieve both low line resistance and high electromigration (EM) reliability, CuSiN was formed on Cu
x
O, which was purposely left on the Cu interconnect. Oxide on the Cu surface effectively suppressed Si diffusion into the Cu line: this diffusion had, in the absence of the oxide, appeared during CuSiN formation and increased line resistance. EM reliability, however, was degraded in the case of Ta barrier metal (BM). The degradation occurred because the number of Cu-Si bonds decreased, and the number of Si-O bonds increased. To counteract this, Ti-based BM was used in combination with CuSiN formed on Cu
x
O [CuSiN(Cu
x
O)], because Ti is uniformly distributed in Cu grain boundaries on the Cu surface when Ti is used together with CuSiN, and Ti oxide is preferentially formed to Si oxide. As a result, the combination of CuSiN(Cu
x
O) and Ti-based BM successfully retained the EM improvement brought about by CuSiN, whereas line resistance remained low. We have found a new solution that achieves compatibility between low line resistance and high EM reliability with ease by forming CuSiN on a Cu
x
O surface rather than by performing a sensitive adjustment of CuSiN formation.