Adhesion studies of magnetron-sputtered copper films on Inconel substrates: Effects of substrate surface pretreatments

Abstract

The adhesion strength of sputtered copper films to Inconel substrates has been studied using the scratch adhesion test. The effects of substrate treatments before deposition such as chemical or ion bombardment etchings were investigated by means of a mean critical load derived from a Weibull-like statistical analysis. It was found that the mean critical load was very weak unless the amorphous layer produced by mechanical polishing on the substrate surface was eliminated. Chemical etching in a nitric-hydrochloric acid bath was shown to have practically no effect on the enhancement of the adhesion. In contrast, the addition in this bath of nickel and cooper sulphates allowed removal of the amorphous layer and an increase in the values of the mean critical load. However, it was observed that excessive chemical etching could cancel out the mean critical load enhancement. The results obtained in the case of ion bombardment etching pretreatments could be far higher than those obtained with chemical etching. Moreover, for a sufficiently long period of ion bombardment etching, the adhesion strength was so high that it was impossible to observe evidence of an adhesion failure. The roughness of the etched surface was measured with a profilometer and observations were made by scanning electron microscopy. In addition, the interfacial chemistry was studied by Auger electron spectroscopy. In the case of chemical etching, it was observed that, as a general rule, the mean critical load increases with increasing roughness of the surface. Similar findings had been obtained from earlier studies carried out on aluminium and nickel substrates. The very high values of the mean critical load obtained in the case of ion bombardment etching cannot be accounted for only by a roughness effect. The analyses performed by Auger electron spectroscopy suggest that, in that case, the elimination of the surface contamination could explain the significant adhesion improvement. However, the mechanism might be complex because of the possibility that the ion beam creates vacancies and microcavities on the substrate surface.