Physico-chemical study of the interfaces of Mo/Si multilayer interferential mirrors: Correlation with the optical properties

We present the physico-chemical and optical study of Mo/Si multilayer interferential mirrors (MIM) with a Si thickness of 2 nm and a Mo thickness varying from 1 to 4 nm. In two other samples, barrier layers of B₄C have been added either at the Mo/Si interface or at the Si/Mo interface to enhance the optical index contrast in the soft X rays and to minimize the growth of molybdenum suicides. The physico-chemical analysis is carried out by X-ray emission spectroscopy (XES) induced by electrons. The Si 3p density of valence states of the silicon atoms that are present in the multilayers are obtained via the analysis of the band Si Kß (3p→1s transition) emission band. The emission band of the multilayer is fitted by a weighted sum of the compounds likely to be present at the interfaces: amorphous silicon, MoSi₂ and Mo₅Si₃. From the contribution of the suicides, the thickness of the transition layer is determined. The optical analysis is performed by X-ray reflectivity (XR) at two wavelengths (0.154 and 0.712 nm). The simulation of the results reveals the importance of considering a four-layer model to describe one period in the stack. Moreover, we evidence the good performance of the multilayer with a Mo thickness of 3 nm.