Plasma-Assisted Surface Functionalization for High-Strength, Thermally Stable Copper Plating on Cycloolefin Polymer Films

With the advancement of beyond 5G technologies, miniaturization of electronic devices, and proliferation of Internet of Things applications, transparent and flexible materials such as cycloolefin polymers (COPs) have garnered increasing attention as candidate antenna substrates. However, the inherent lack of chemically reactive functional groups on COP surfaces poses challenges for their use in antenna and circuit board fabrication. In this study, we propose a surface modification strategy that combines mild low-pressure plasma treatment with the subsequent introduction of functional molecular layers to enhance interfacial adhesion between copper (Cu) and COP substrates. Importantly, this approach maintains the inherent low surface roughness of COP while increasing chemical reactivity. We systematically investigated the influence of plasma treatment time (30 s to 10 min) on the COP surface properties, molecular layer bonding, and the adhesion performance of Cu/COP. A peel strength exceeding 1.2 kN/m was achieved when plasma treatment time was kept below 5 min, and adhesion values above 1.0 kN/m were retained even after thermal aging at 120 °C for 7 days. To clarify the mechanisms underlying adhesion performance, we thoroughly characterized the chemical structure at the Cu/COP interface and evaluated the effects of plasma treatment on both interfacial strength and thermal durability. Notably, atomic force microscopy-nanoscale infrared spectroscopy (AFM-nanoIR) was employed to directly probe chemical changes at the Cu/COP cross-section, providing submicron spatial resolution. This analysis revealed that oxidation at the interfaceparticularly intensified after thermal exposurewas a primary factor contributing to interfacial degradation and reduced adhesion performance. These findings highlight the critical role of plasma-induced surface activation in enhancing metal-polymer adhesion and offer insights into the limitations of current modification strategies. The results further suggest potential strategies for improving interfacial reliability in chemically inert polymer systems, paving the way for their broader use in diverse metal adhesion applications.