<p>Direct Cu bonding stands as a pivotal technology for enabling high-density vertical interconnects in advanced 3D heterogeneous integration. However, achieving reliable low-temperature Cu bonding requires a careful balance between suppressing surface oxidation and allowing sufficient atomic diffusion for bonding. The surface must be effectively protected against oxidation while still permitting atomic diffusion across the interface. In this study, we demonstrate that the thickness of the hydrogenated amorphous carbon (a-C:H) passivation layer is the key factor governing both oxidation suppression and atomic diffusion behavior. By systematically tuning the Ar/CH₄ plasma treatment time, we identify a decisive transition in film properties from a thin, diffusion-permeable layer to a thick, diffusion-barrier layer. This transition reveals that the thin a-C:H layer possesses a defect-rich structure characterized by sp² clusters, whereas increased thickness induces a transition to a rigid, sp²-dominant network. This thickness-dependent structural evolution dictates the dominant bonding mechanism. In the case of a thin a-C:H layer, a synergistic triple-diffusion process may be enabled, involving Cu transport through defects associated with sp²-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.</p>