  The synthesis and structural analysis of Mo/B periodical multilayer X-ray
mirrors (PMMs) for beyond extreme ultraviolet (BEUV) optics was performed. The
PMMs were deposited by a combination of pulsed DC and radio frequency (RF)
magnetron sputtering. The structure was analyzed by high-resolution
transmission electron microscopy, X-ray photoelectron spectroscopy, and grazing
incidence X-ray reflectometry. The formation of 0.35 nm-thick interlayers
comprised of a mixture of molybdenum borides was observed at the Mo/B
interfaces. Furthermore, a low interface roughness of 0.3-0.4 nm was reported.
The temperature of the substrate increased due to the increase in the
sputtering power. This resulted in an increase in the thickness of the
interlayers and the interface roughness; subsequently, the optical properties
of the PMM deteriorated. Theoretical calculations were performed based on the
real structure of the PMM to predict the reflectivity at a working wavelength
of 6.7 nm. The reflectivity of approximately 49% was two times higher than that
of the conventional B4C-based BEUV mirrors. Based on our study results, it can
be concluded that the as-synthesized PMMs will perform better than the
traditional mirrors and can be effectively used for the development of the next
generation of BEUV lithography.
