Periodic multilayers of nanometric period are widely used as optical components for the X-ray and extreme UV
(EUV) ranges, in X-ray space telescopes, X-ray microscopes, EUV photolithography or synchrotron beamlines for
example. Their optical performances depend on the quality of the interfaces between the various layers: chemical
interdiffusion or mechanical roughness shifts the application wavelength and can drastically decrease the reflectance.
Since under high thermal charge interdiffusion is known to get enhanced, the study of the thermal stability of such
structures is essential to understand how interfacial compounds develop. We have characterized X-ray and EUV siliconcontaining
multilayers (Mo/Si, Sc/Si and Mg/SiC) as a function of the annealing temperature (up to 600°C) using two
non-destructive methods. X-ray emission from the silicon atoms, describing the Si valence states, is used to determine
the chemical nature of the compounds present in the interphases while X-ray reflectivity in the hard and soft X-ray
ranges can be related to the optical properties. In the three cases, interfacial metallic (Mo, Sc, Mg) silicides are evidenced
and the thickness of the interphase increases with the annealing temperature. For Mo/Si and Sc/Si multilayers, silicides
are even present in the as-prepared multilayers. Characteristic parameters of the stacks are determined: composition of
the interphases, thickness and roughness of the layers and interphases if any. Finally, we have evidenced the maximum
temperature of application of these multilayers to minimize interdiffusion.