Co-W Barrier Layers for Metallization of Copper Interconnects: Thermal Performance Analysis

The back-end-of-line (BEOL) copper interconnect structure has been subjected to downscaling for the last two decades, while the materials used for conforming and assuring its physical integrity during processing have faced significant obstacles as the single-digit nanometer process node is implemented. In particular, the diffusion barrier layer system comprised of Ta/TaN has faced major constraints when it comes to the electrical performance of the smaller Cu lines, and thus alternative formulations have been investigated in recent years, such as Ru-Ta or Co-W alloys. In this work, we assess how PVD (physical vapor deposition) deposited equimolar Co-W films perform when exposed to different vacuum annealing temperatures and how these films compare with the Ta adhesion layer used for Cu seeding in terms of dewetting resistance. The stacks were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) coupled with energy dispersive X-ray spectroscopy (EDX) mapping. The Cu film at the surface of the Cu/Co-W system exhibited grain growth starting at 300 °C, with the formation of abnormally large Cu grains starting at 450 °C. Sheet resistance reached a minimum value of 7.07 × 10^-6 Ω/sq for the Cu/Co-W stack and 6.03 × 10^-6 Ω/sq for the Cu/Ta stack, both for the samples annealed at 450 °C.