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

Abstract

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 TiB x C y composition, which hinders the interface diffusion, supresses the crystallization of the Cr/Ti multilayer and dramatically improves the interface quality of Cr/TiB x C y 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.

Multilayer Reflective Coatings for BEUV Lithography: A Review

“…Therefore, suppression or even full illumination of the intermixing is crucial for the improvement of the reflectivity of PMMs. In addition, the knowledge about intermixing mechanisms in Sc/Si and Co/C PMMs developed in early studies [37][38][39] allowed suppression of the intermixing by introducing diffusion barriers and improved the temperature stability of PMMs [41][42][43]. In addition to the intermixing, the interface roughness is another fac reflectivity.…”

Multilayer Reflective Coatings for BEUV Lithography: A Review

“…Therefore, suppression or even full illumination of the intermixing is crucial for the improvement of the reflectivity of PMMs. Besides, the knowledge about intermixing mechanisms in Sc/Si and Co/C PMMs developed in early studies [37] [38] [39] allowed suppression of the intermixing by introducing diffusion barriers and improve the temperature stability of PMMs [41] [42,43]. [44] with permission from Elsevier.…”