The use of depth-graded multilayer coatings is foreseen in several future X-ray telescopes (e.g. Hexit-Sat/NHXT, NuSTAR, IXO, NeXT), in order to extend to higher energies (up to 80 keV) the focusing capabilities of present day telescopes, like Chandra and XMM/Newton, that are operative in the soft X-ray band (0.1-10 keV).
The deposition of multilayer coatings is a difficult process, in which a good control of the individual layer thickness is required. The process must also be stable for the long time needed (several hours).
The main issues are the accuracy of the film (stability of the process), interfacial and surface roughness, the lateral homogeneity and the long-term stability of the film. Physical properties of the film, like the density or the crystallicity are also important.
The analysis of reflectivity measurements is an important tool to study the structure of multilayer samples, investigating in a non-destructive way the whole properties of the film down to the deepest layers. Nevertheless the analysis of data is not easy, neither standardized.
Two samples were realized using different materials (Pt/C and W/Si). The film structures were designed to be representative of broad angular/energy band multilayer coatings foreseen in hard X-ray focusing telescopes. Their reflectivity was measured for several energies in the 40-130 keV range as a function of the reflection angle. The High energy reflectivity data are compared with the theoretical expectations derived from low-energy (8.04 keV) measures acquired after the deposition.