Optical study of beyond EUV mask and mirrors

<jats:p>Abstract: Beyond EUV (BEUV) lithography at 6.6 nm wavelength suffers from the small refractive-index contrast and requires multilayer (ML) mirrors of around 200 pairs. The large number of ML interfaces amplifies scattered light arising from surface roughness, and a greater effective reflection plane depth ( ) of ML drives mask 3D effect (M3D) in photomasks. Background: The problems originate from the same low-contrast constraint, and a systematic metric linking reactive surface growth and surface roughness has been lacking to improve reflectivity. In addition, nitride-based BEUV mask absorbers against M3D have not been optimized in our previous study. Aim: We investigate nitride-based BEUV optics to control kinetic roughness during gas-reactive deposition and identify a promising BEUV mask to control optical performance. Approach: A finite-mobility solid-on-solid model is introduced to describe reactive surface growth, in which chemisorption coverage governed by the Langmuir-Hinshelwood kinetics modifies the Ehrlich-Schwoebel (ES) and adatom diffusion barriers. As nitride-based components, we investigate TaN and CrN BEUV absorbers in terms of reflectivity, phase difference, and aerial images. Results: Our model predicts decreasing roughness with increasing chemisorption coverage, controlled by the ratio of the ES length to the adatom diffusion length. For photomask, CrN absorber gives 135° phase shifting at 62.6 nm thickness, at which the normalized image logarithmic slope (NILS) approaches saturation at 0.55 NA in our photomask model. Conclusions: Nitride interface kinetics suggests a route to roughness control for MLs and photomasks. CrN-based Att-PSMs are candidates for high-NA BEUV lithography at 0.55 NA with anamorphic (4x/8x) demagnification at an angle of incidence of around 6°.</jats:p>