Via resistance and reliability trends in copper interconnects with ultra-scaled barrier layers

We present a combined experimental and theoretical study of via resistance modulation in Cu interconnects with ultra-scaled diffusion barriers and wetting layers. In particular, we demonstrate that reducing the thickness of the TaN-based diffusion barrier below 1 nm results in a decrease in the measured via resistance, while reducing the thickness of the Co wetting layer below 1 nm has virtually no impact on via resistance. These results are explained using first-principles transport calculations, which show that a 1 nm thick TaN layer is more effective in blocking electrons than a 1 nm thick Co layer. Measurements of time-dependent dielectric breakdown indicate that scaling either TaN or Co layers below 1 nm in thickness results in degraded reliability. These results suggest that there is minimal value in scaling the thickness of Co wetting layers below 1 nm, while scaling TaN diffusion barriers below 1 nm results in a trade-off between performance and reliability.