Modification by Ar and Kr ion bombardment of Mo/Si X-ray multilayers

We have investigated the details of the growth of electron-beam deposited molybdenum and silicon layers and the effect of ion-beam bombardment on the morphology and interface smoothness of those layers. Using in-situ X-ray reflectivity, q-2q reflectivity scans, Auger Electron Spectroscopy (AES) and Transmission Electron Microscopy (TEM) we find that a reduction in both Mo and Si surface roughness can occur as a result of the ion-beam bombardment. However, the overall smoothing effect is limited by interface mixing at the underlying interface and it is also dependent on the deposition morphology for Mo layers. When depositing multilayers with 10 periods of 6 nm we find a large (factor 4) improvement of the reflectivity as a result of the ion-beam bombardment. Our best results, obtained by polishing all Si layers after growth with 300 eV Kr⁺ ions, give a roughness of 0.3 nm for the Mo-on-Si interfaces and less than 0.5 nm for Si-on-Mo interfaces. Cross-section TEM pictures confirm the observations qualitatively.