Feasibility study of fabricating 20 nm resolution dielectric Fresnel zone plates with ultrahigh aspect ratio for EUV optics

Abstract

EUV light optics are either reflective or diffractive due to the substantial absorption characteristics by almost all materials. Despite great successes in manufacturing integrated circuit chips, reflective EUV optics are still unfriendly to small-to-medium enterprise (SME) because of the enormous costs. Recently, diffractive EUV optics has come to the light in hopes to be able to establish manufacturing nanoscale products and inspecting nanoscale structures. Diffractive zone plates with high resolution in EUV wavelengths are urgently needed. This paper reports our latest success in developing 20 nm resolution zone plates for focusing and imaging in the EUV and soft X-ray regions. It firstly discusses the diffraction efficiency of FZPs tailored for 13.5 nm wavelength to decide the essential height of the zone plate. Then, Monte Carlo simulation method was used to figure out the achievable zone plate parameters by high-resolution electron beam lithography (EBL). Finally, this work systematically explored the viability of nanofabricating top-tier 20 nm resolution hydrogen silsesquioxane (HSQ) zone plates with the duty cycle ratio nearing 1:1 and the aspect ratio approaching 13:1 on 50 nm thick Si₃N₄ membranes.