  A set of Mo/Si periodic multilayers is studied by non destructive analysis
methods. The thickness of the Si layers is 5 nm while the thickness of the Mo
layers changes from one multilayer to another, from 2 to 4 nm. This enables us
to probe the effect of the transition between the amorphous to crystalline
state of the Mo layers near the interfaces with Si on the optical performances
of the multilayers. This transition results in the variation of the refractive
index (density variation) of the Mo layers, as observed by x-ray reflectivity
(XRR) at a wavelength of 0.154 nm. Combining x-ray emission spectroscopy and
XRR, the parameters (composition, thickness and roughness) of the interfacial
layers formed by the interaction between the Mo and Si layers are determined.
However, these parameters do not evolve significantly as a function of the Mo
thickness. It is observed by diffuse scattering at 1.33 nm that the lateral
correlation length of the roughness strongly decreases when the Mo thickness
goes from 2 to 3 nm. This is due to the development of Mo crystallites parallel
to the multilayer surface.
