Impregnation of Ni–P metal into polymer substrate via catalyzation in Sc-CO2 and electroless plating in Sc-CO2 emulsion

In this study we investigated the impregnation of Ni–P metal into polymer substrate via catalyzation in supercritical carbon dioxide (Sc-CO2) and electroless plating in Sc-CO2 emulsion, in comparison with calculated CO2 diffusion in polymer on the basis of Fick's law. Sc-CO2 has two major effects, high diffusivity and good chemical affinity with polymer, on the impregnation of Ni–P metal into the polyimide substrate. Our group fabricated an Ni–P thin film in Sc-CO2 with an electroless plating emulsion after catalyzing the Sc-CO2 with Pd complex. According to an EDX analysis, an increase in the Sc-CO2 catalyzation time led to increases in both the intensity and depth of the Ni-ion penetration into the polyimide. We found that the impregnation reaction of Ni by our novel method is CO2 diffusion controlling reaction. To clarify which of the two Sc-CO2 properties, diffusivity or chemical affinity, played a dominant role, we conducted an electroless plating reaction with emulsion using hexane as the catalyzation solvent whose diffusivity is lower than that of Sc-CO2. The good affinity with polyimide in the hexane catalyzation led to a film fabrication of Ni–P thin films, but numerous cracks were visible on the films under microscopic observation. According to an EDX analysis of the Ni–P catalyzed in hexane, the intensity of the penetrated Ni-ions into the polyimide was very low. The high diffusivity of the Sc-CO2 promoted the penetration of the Pd catalyst and Ni–P electroless plating solution into the polyimide substrate.