Optical study of beyond EUV mask and mirrors

Abstract

Beyond EUV (BEUV) lithography at around 6.6 nm wavelength suffers from the small refractive-index contrast and requires around 200 pairs of multilayer (ML) mirrors. The large number of ML interfaces amplifies scattered light arising from surface roughness, and a greater effective reflection plane depth (Zeff) in ML drives mask 3D effect (M3D) in photomasks. Background: The problems originate from the same low-contrast constraint, but our previous study neither linked reactive surface growth to the former, nor quantified the nitride absorber against M3D in the latter. Aim: We investigate nitride-based BEUV optics to control kinetic roughness during gas-reactive deposition and identify a promising BEUV mask to improve optical performance. Approach: Reactive nitride growth is described by a finite-mobility solid-on-solid model in which chemisorbed nitrogen coverage governed by the Langmuir-Hinshelwood kinetics modifies the Ehrlich-Schwoebel (ES) and adatom diffusion barriers. Furthermore, we investigate TaN- and CrN-based BEUV absorbers and their dependences on reflectivity, phase difference, and aerial images. Results: Assuming coverage-dependent barriers, the model predicts that roughness decreases with increasing nitrogen chemisorption coverage. As nitride-based components, CrN gives 17.1 % at 62.6 nm thickness with 135° phase shifting, and Mo2N/B ML has larger Zeff of around 70 nm, which is greater than Mo/Si ML for EUV lithography. Conclusions: Nitride interface kinetics provides a route to roughness control for MLs and photomasks. CrN-based Att-PSMs are candidates for high-NA BEUV lithography, considering 0.55 NA with 4x/8x demagnification at an angle of incidence of around 6°.