Interface modification in Co/Cu multilayers prepared by ion-assisted deposition
The interfacial structure of Co/Cu multilayers deposited under energetic ion bombardment has been investigated using X-ray reflectivity techniques. An ionassisted deposition system, based on unbalanced magnetron sputtering principles, has been developed in order to provide a method for the independent control of ion flux and energy. Using this system ion current densities measured at the substrate could be varied from ~30 µAcm2 to ~100 µAcm2 and average ion energies of ~50-200 eV could be selected. It was therefore possible to prepare Co/Cu multilayers under a variety of ion bombardment conditions and thus study the effect of this energetic particle bombardment on the evolution of interface structure. Samples were characterised by measuring the specular and diffuse X-ray reflectivity, using anomalous scattering effects to enhance the contrast between the cobalt and copper layers. Quantitative analysis of the specular reflection revealed the existence of variations of the interface roughness in multilayers deposited under ~200 eV ion bombardment, such that the interfaces became smoother towards the free surface. Films deposited with the highest available levels of ion flux were found to display the greatest interface smoothing effect. However, no such variation was observed when ion energies of ~50 eV were applied, although differences in the large lateral scale correlated roughness were observed between ~50 eV ion-assisted and non ion-assisted samples. The sensitivity of the interface structure to energetic bombardment during growth is thus demonstrated and the results are discussed in terms of resputtering and ion bombardment induced reordering of the growing surface. It is shown that these effects can be exploited to obtain multilayers with atomically smooth interfaces.