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

Abstract

The scratch adhesion test was applied to determine the adhesion strength of copper films deposited by sputtering onto nickel substrates. Before deposition, the substrates were mechanically polished and received a surface pretreatment which consisted of a chemical or an ion bombardment etching. Spreading in the critical load values due to local variations in the adhesion strength were taken into account by using a mean critical load derived from a Weibull-like statistical analysis. The mean critical load is very weak as long as the amorphous layer produced by mechanical polishing on the substrate surface has not been eliminated. After removal of this layer, the mean critical load increases with increasing etching time, but the results obtained by ion bombardment reach levels substantially larger than those obtained by chemical etching. For a sufficiently long ion bombardment etching time, the adhesion strength is so high that it is impossible to provide evidence of an adhesion loss. In order to understand the origin of the mean critical load enhancement, observations were made by scanning electron microscopy and Auger electron spectroscopy. In the case of chemical etching, it was found that, as a general rule, the mean critical load increases when the roughness of the surface is enhanced. A similar observation had been already made in an earlier study carried out on aluminium substrates. The roughness arising from an ion bombardment etching pretreatment is substantially lower than that resulting from chemical etching. Therefore the very high values of the mean critical load obtained in that case cannot be accounted for by a roughness effect. The analyses performed by Auger electron spectroscopy suggest that elimination of surface contamination by ion bombardment prior to deposition could be responsible for this significant improvement. However, the mechanism might be complex because of the possibility that the ion beam creates vacancies and microcavities.