Analysis of the adhesion structure between the cycloolefin polymer and the copper seed layer formed using medium-vacuum sputtering

We have developed a novel approach for the fabrication of 6G communication antenna substrates by directly sputtering a copper seed layer onto a resin with low dielectric properties and low adhesiveness, without relying on a metal adhesion layer such as titanium. This study presents a detailed analysis of the effects of two surface modification methods—vacuum ultraviolet (VUV) irradiation and oxygen plasma treatment—performed before sputtering the copper seed layer on the interfacial structure between a cycloolefin polymer (COP) film and copper plating layer. Using scanning transmission electron microscopy, energy-dispersive x-ray spectroscopy, and selected area electron diffraction, we confirmed that both surface modification methods promote the formation of polycrystalline Cu2O at the interface between the COP film and the copper plating layer. VUV irradiation enhances the smoothness of the COP surface, resulting in the uniform distribution of hydrophilic functional groups. These functional groups stably interact with the sputtered copper, leading to the oxidation of copper during annealing and the formation of a consistent Cu2O layer. In contrast, oxygen plasma treatment selectively etches regions of the COP surface with high oxygen affinity, creating fine structures and concentrating the hydrophilic functional groups in the recessed areas. After sputtering, copper interacts with these localized functional groups, and during annealing, limited diffusion causes the copper to oxidize and aggregates into a Cu2O particle structure. Thus, this study demonstrates that the structures of polycrystalline Cu2O formed at the interface vary depending on the surface modification methods and elucidates the mechanisms behind these differences through analysis of surface morphology and the distribution of functional groups induced by each method.