Periodic and Depth-Graded Cu/Si Multilayer Films for Hard X-ray Optics

We describe the growth and characterization of periodic and depth-graded Cu/Si multilayer structures for use as coatings in hard X-ray optics. Films have been grown by DC magnetron sputtering, and were characterized by grazing incidence X-ray (8 keV) reflectance analysis. The X-ray reflectance of both periodic and depth-graded structures was found to be high, and stable at room temperature over a period of at least several months. The interface widths determined from fits to the X-ray reflectance data were found to lie in the range 0.23 - 0.3 nm. X-ray diffraction revealed no crystalline phases present in any of the films, only broad peaks indicating either an amorphous Cu or possibly copper-silicide phase. As a result of the small measured interface widths and the low X-ray absorption of Cu just below, and also far above the Cu K-edge (~9 keV,) highly-efficient depth-graded Cu/Si multilayer coatings can now be used for broadband, grazing-incidence X-ray mirrors, for X-ray energies greater than at least 100 keV.