Exploring the Limits of Cobalt Liner Thickness in Advanced Copper Interconnects

We investigate the performance and reliability characteristics of Cu interconnects with Ta-based barrier layers and Co wetting layers at 7nm node dimensions with a focus on the impacts of reducing the Co thickness from 30Å down to 10Å. We demonstrate that while reducing Co liner thickness significantly reduces RC delay, there is a significant reduction in electromigration reliability below a thickness of 20Å if used in conjunction with a PVD (physical vapor deposition) TaN barrier. However, if the PVD treatment is followed by deposition of a thin ALD (atomic layer deposition) TaN, the Co layer thickness can be scaled down to 10Å without any penalty in either electromigration or time dependent dielectric breakdown (TDDB.) The combined PVD/ALD process with 10Å Co enables a 14% reduction in RC delay relative to our control split.