In the field of microelectronics, the Extreme Ultraviolet (EUV) lithography operating at λ=13,5 nm, appears, today, as the most promising future technology. The viability of this next generation technology has, however, to face several technical issues. Among them, the realisation of highly reflective EUV mirrors is one of the key issues and concerns the mirrors for light transport as well as the lithography masks. In this last case, the mirrors which consist in a periodic Mo/Si multi-layer coating deposited by ion beam sputtering suffer from reflectance limitations and thermal instabilities. In this paper, an experimental investigation on IBS deposited Mo/Si multi-layers is presented. The study is mainly focused on the formation of Mo/Si interfacial zones and their evolution with thermal annealing up to 600°C. Detailed investigations by high-resolution transmission electron microscopy (HRTEM) and energy filtering TEM have been performed in association with differential scanning calorimetric (DSC) analyses. For the purpose, dedicated samples were prepared enabling the identification of phase transformations occurring during thermal annealing. It will be shown, in particular, how this coupled characterisation approach (TEM+DSC) can help understanding the interface growth in such metal/silicon systems. The characterisation results are presented and discussed.