Demonstration of NiCo as an Alternative Metal for Post-Cu Interconnects

Copper (Cu) is currently the dominant interconnect material in back-end-of-line processes due to its low bulk resistivity (ρ₀). Unfortunately, its resistivity increases significantly at small dimensions due to a long electron mean free path (EMFP, λ) of 39 nm, which leads to enhanced electron scattering at grain boundaries and surfaces, thereby limiting high-density integration. In this study, we demonstrate that single-phase hexagonal-close-packed (HCP) nickel-cobalt (NiCo) alloy thin films exhibit a significantly reduced resistivity size effect, outperforming Cu interconnect material for thicknesses below 8 nm (19.83 μΩ·cm at 4.9 nm). First-principles calculations predicted a lower ρ₀·λ of 5.68 × 10^-16 Ω m² for HCP NiCo (xx) than to Cu (6.73 × 10^-16 Ω m²), due to a significant contribution from the short EMFP of approximately 5 nm in the HCP NiCo alloy. To date, the growth of single-phase HCP NiCo film had not been achieved, as both HCP and face-centered-cubic (FCC) phases coexist at Co concentration of 50-80 atom %. However, single-phase HCP NiCo films were successfully grown by using an HCP Co seed layer, which reduced the lattice mismatch with the sapphire (0001) substrate. Moreover, NiCo thin films can be successfully dry etched, providing a significant advantage in device fabrication, since it does not require a damascene process unlike Cu. The NiCo thin films also exhibited high thermal stability above 500 °C. Therefore, HCP NiCo is considered a promising alternative to overcome the scaling limitations of Cu interconnects.