Development and Interfacial Characterization of Co/Mg Periodic Multilayers for the EUV Range

We propose a new system, namely the periodic Co/Mg multilayer system, for optics applications in the EUV range. Close to the Mg L edge, i.e., around a wavelength of 25 nm or a photon energy of 50 eV, a reflectivity of about 43% is measured at 45° for s-polarized radiation. Moreover, it appears that this system is stable over a period of time of three months. The introduction of thin boron carbide interfacial layers proves disastrous contrary to simulations that show this could be beneficial. We combine X-ray reflectivity in the hard X-ray range, X-ray emission spectroscopy, and nuclear magnetic resonance to determine the thickness and roughness of the Co and Mg layers as well as the chemical state of the Co and Mg atoms at the interfaces. This reveals that in the Co/Mg system the interfaces are abrupt and there is no interdiffusion between the Co and Mg layers. Then the difference between the experimental and simulated reflectivities is ascribed to the interfacial roughness of the order of 0.4 nm. In the Co/Mg/B4C system, evidence of a large mixing of the Co and B4C layers is presented and explains the poor reflectance of this system.