Reflective multilayer optics for 6.7 nm wavelength radiation sources and next generation lithography

Reported is a computational and chemical analysis of near normal incidence reflective multilayer optics for 6.7 nm wavelength applications in e.g. the Free Electron Laser FLASH and next generations of EUV lithography. We model that combinations of B or B4C with La offer a reflectivity of ~ 70%. The small reflectivity bandwidth poses problems in applications, but it can be significantly improved by replacing La with Th or U. Grazing incidence X-ray reflectometry, cross-section TEM, and in-depth XPS analysis of B/La and B4C/La multilayers reveal chemical reactivity at the interfaces. Significant LaBx interlayer formation is observed in especially B/La multilayers, stressing the relevance of interface passivation. We propose nitridation of the interfaces, which mitigates interlayer formation and simultaneously increases optical contrast.