Enhancing Adhesion of Electroless Copper Film on Smooth Polyimide Surfaces by Photocatalytic Oxidation

The application of advanced high-frequency (5 G) requires a low-profile Cu foil to reduce the high-speed signal loss due to skin effect. However, the adhesion of electroless copper film on the smooth surface of polyimides (PI) remains a challenge. Here, a two-step composite modification method, consisting of TiO2 photocatalysis and Na2S2O8 photooxidation treatment, is successfully applied to induce a high adhesion strength of electroless copper film on the smooth PI surface. To meet the requirements of the flexible electronics, the average surface roughness of the PI film is increased from 4.2 to 11.5 nm through optimizing the modification. Whereas the surface contact angle decreases from the original 76.9 degrees to 18.7 degrees and the adhesion strength between the electroless copper film and PI film is increased from 0 to 0.90 KN m(-1). Spectroscopic investigations reveal that the amide bonds on the surface PI get oxidized by radicals, while an abundant -COOH and -NH2 groups form. The high oxidation potential, strong stability of the sulfate anion radicals, as along with their increased concentration in the treatment solution are contributed to the extraordinary modification performance. Therefore, the TiO2 photocatalysis and Na2S2O8 photooxidation composite treatment is an effective and environmentally friendly method for surface modification of PI films.