Improving the soft X‐ray reflectivity of Cr/Ti multilayers by co‐deposition of B4C.

The 'water window', covering 2.4–4.4 nm, is an important wavelength range particularly essential to biology research. Cr/Ti multilayers are one of the promising reflecting elements in this region because the near‐normal‐incidence reflectivity is theoretically as high as 64% at 2.73 nm. However, due to multilayer imperfections, the reported reflectivity is lower than 3% for near‐normal incidence. Here, B and C were intentionally incorporated into ultra‐thin Cr/Ti soft X‐ray multilayers by co‐deposition of B4C at the interfaces. The effect on the multilayer structure and composition has been investigated using X‐ray reflectometry, X‐ray photoelectron spectroscopy, and cross‐section electron microscopy. It is shown that B and C are mainly bonded to Ti sites, forming a nonstoichiometric TiBxCy composition, which hinders the interface diffusion, supresses the crystallization of the Cr/Ti multilayer and dramatically improves the interface quality of Cr/TiBxCy multilayers. As a result, the near‐normal‐incidence reflectivity of soft X‐rays increases from 4.48% to 15.75% at a wavelength of 2.73 nm. [ABSTRACT FROM AUTHOR]