An analysis procedure was developed that enables studying diffusion in ultrathin films by utilizing the depth-resolved information that is contained in the background of low energy ion scattering (LEIS) spectra. Using a high-sensitivity analyzer/detector combination allows for such a low ion dose that the ion-induced perturbation caused by this technique is negligible and not measurable with LEIS. The developed analysis procedure provides a unique opportunity to study diffusion processes in nanoscaled systems. It was applied to the Mo/Si system, a system that is relevant for extreme ultraviolet optics.

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The effect of an amorphous-to-nanocrystalline phase transition on the diffusion across an interface layer of subnanometer thickness has been investigated in real-time. The diffusion in the Mo/ B 4 C / Si thin film structure studied was found to instantaneously enhance by an order of magnitude upon the formation of nanocrystals inducing the atomic-scale onset of grain boundary diffusion.

To enhance the thermal stability, B 4 C diffusion barrier layers are often added to Mo/Si multilayer structures for extreme ultraviolet optics. Knowledge about the chemical interaction between B 4 C and Mo or Si, however is largely lacking. Therefore, the chemical processes during annealing up to 600°C of a Mo/ B 4 C / Si layered structure have been investigated in situ with hard x-ray photoelectron spectroscopy and ex situ with depth profiling x-ray photoelectron spectroscopy. Mo/ B/Si and Mo/C/Si structures have also been analyzed as reference systems. The chemical processes in these systems have been identified, with two stages being distinguished. In the first stage, B and C diffuse and react predominantly with Mo. MoSi x forms in the second stage. If the diffusion barrier consists of C or B 4 C, a compound forms that is stable up to the maximum probed temperature and annealing time. We suggest that the diffusion barrier function of B 4 C interlayers as reported in literature can be caused by the stability of the formed compound, rather than by the stability of B 4 C itself.

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