Thickness-dependent structural evolution of a-C:H layers formed by one-step Ar/CH4 plasma reduction for direct Cu bonding

-rich clusters, carbon diffusion along Cu grain boundaries, and carbon diffusion into Cu lattice. This carbon-related lattice distortion is consistent with a characteristic local lattice expansion of ~ 6.8% observed by FFT analysis at the interface. Conversely, a thick a-C:H layer appears to act as a robust diffusion barrier, likely inhibiting the atomic intermixing required for effective bonding. This study demonstrates that optimization of the a-C:H thickness is critical for enabling sufficient atomic diffusion and achieving high-yield Cu direct bonding.