Interface formation and optical design of EUV reflective multilayer mirrors

Abstract Reflective multilayer mirrors are indispensable optical components in the extreme ultraviolet (EUV) wavelength region, where conventional refractive and single-layer reflective optics are ineffective. This review summarizes the design, fabrication, and performance limitations of EUV reflective multilayer films, with emphasis on material selection, optical design, deposition processes, and interface formation. Established systems such as Mo/Si multilayers for 13.5 nm lithography demonstrate that EUV performance is approaching intrinsic limits imposed by absorption and unavoidable interfacial mixing. When extending these design concepts toward shorter wavelengths in the beyond-EUV (BEUV) region around 6. x nm, additional challenges emerge, including increased absorption, reduced optical contrast, and enhanced sensitivity to interfacial reactions and non-equilibrium growth processes. This review clarifies the fundamental differences between BEUV and EUV multilayers and emphasizes the importance of interface control for improving performance, providing general guidelines for next-generation EUV and BEUV reflective optics.