Multilayer mirror technology for soft-x-ray projection lithography

TLDR

Recent advances in multilayer mirror technology meet many of the stringent demands of soft‑x‑ray projection lithography. The study aims to develop uniform multilayer coatings for SXPL imaging optics that preserve surface figure for diffraction‑limited performance while grading condenser optics for varying incidence angles. Graded multilayer coatings are applied to modify optical substrate figure without increasing surface roughness, enabling precise figure control. Mo/Si multilayers achieve 66 % normal‑incidence reflectivity at 13.4 nm with ~0.5 % layer‑thickness control, sufficient for SXPL throughput and enabling precise fabrication of aspheric imaging optics.

Abstract

Recent advances in multilayer mirror technology meet many of the stringent demands of soft-x-ray projection lithography (SXPL). The maximum normal-incidence reflectivity achieved to date is 66% for Mo/Si multilayers at a soft-x-ray wavelength of 13.4 nm, which is sufficient to satisfy the x-ray throughput requirements of SXPL. These high-performance coatings can be deposited on figured optics with layer thickness control of ~ 0.5%. Uniform multilayer coatings are required for SXPL imaging optics, for which maintaining the surface figure is critical to achieving diffraction-limited performance.In contrast the coatings on the condenser optics will be graded to accommodate a large range of angles of incidence. Graded multilayer coatings can also be used to modify the figure of optical substrates without increasing the surface roughness. This offers a potential method for precise fabrication of aspheric imaging optics.