Chemical and morphological study of the sensitisation, activation and Cu electroless plating of Al2O3 polycrystalline substrate

A study of the different stages of the electroless deposition of copper on polycrystalline alumina activated by a two-step method (sensitisation + activation) has been performed from both a chemical and a morphological point of view. The combination of XPS measurements, AES depth profiling and AFM imaging has allowed to detect the distribution of the formed compounds and to perform depth estimations and quantification of the surface topographic parameters. During sensitisation by immersion in SnCl2 acid solution, it has been found that oxidised species are firstly adsorbed onto the substrate. Other Sn2+ ions are then directly oxidised to form stannic compounds over this layer and, eventually, stannous compounds and Sn0 are also found on surface. After sensitisation by immersion in PdCl2 acid solution, palladium is found to be in oxidised forms (PdCl2 and PdO). Afterwards, Pd is reduced by the formaldehyde present in the plating solution, allowing catalysis for Cu deposition. Furthermore, marked topographical variations have been encountered throughout the process. The initial formation of a flat Sn-based surface during the sensitisation process is followed by an important redissolution of this layer during activation and the final formation of a thick copper film.