ADHESION STRENGTH OF COPPER THIN-FILMS TO A E24 CARBON-STEEL - EFFECTS OF SUBSTRATE SURFACE ION-BOMBARDMENT ETCHING

The scratch test was applied to determine the adhesion strength of d.c. magnetron sputtered copper films onto E24 carbon steel substrates. Some of the substrates were mechanically polished and directly coated with copper; others were ion-etched with argon ions after mechanical polishing and before coating. The critical load which was very low on mechanically polished substrates, increased significantly with increasing ion etching time, reaching a maximum value about 10 times that observed after mechanical polishing alone. Auger electron spectroscopy showed that the oxygen present in the interface layer in the case of a mechanically polished substrate was completely eliminated by ion bombardment. Measurements with variable energy positrons did not indicate the presence, after deposition of open volume defects in the interfacial domain, though it was observed by scanning electron microscopy (SEM) that the surface roughness of the samples was strongly increased by etching. This result is significantly different from that obtained in the case of SiO2 thin films deposited onto ion-etched stainless steel substrates, in which prolonged ion etching resulted in a high concentration, on the substrate side of the interface, of large, open-volume defects. Therefore, it is suggested that the surface microcavities created by ion etching could be filled by the copper during deposition, leading to an enhanced adhesion strength by mechanical interlocking of the coating to its substrate. The fact that the relative increase in the critical load values is higher for copper coating than for SiO2 could be due to differences in the filling of surface microcavities.