Extreme ultraviolet (EUV) lithography is a critical technology for advanced semiconductor manufacturing that requires nearly defect-free EUV masks, necessitating high-resolution actinic (at-wavelength) imaging analysis for mask defect inspection.

This study aims to explore the potential for achieving higher resolution in actinic mask review by developing an elliptical zone plate design with anisotropic numerical aperture.

First, a simulation platform was established based on the EUV light generated by undulator equipment at the Shanghai Synchrotron Radiation Facility (SSRF). Second, simulations based on partial coherent light imaging modelwere performed to evaluate the imaging performance of three different collimator configurations under Fourier-synthesis illumination conditions. Finally, an elliptical off-axis zone plate with anisotropic numerical aperture was designed and optimized through systematic computational studies.

The simulation results demonstrate that the elliptical zone plate design achieves a numerical aperture of 4×NA=1.15 in the horizontal direction, significantly higher than the conventional circular zone plate's 4×NA=0.625. Under extreme dipole illumination conditions, a theoretical resolution of 12 nm half-pitch in the horizontal direction is achieved while maintaining 22 nm resolution in the vertical direction. The anisotropic design effectively overcame the reflectivity limitations of multilayer mirrors at large incident angles (<19°).

The proposed elliptical zone plate design with anisotropic numerical aperture in this study improves the resolution limit by 45.5% compared to conventional circular designs, achieving a 12 nm half-pitch resolution that surpasses the current international benchmark of 22 nm. This advancement provides a promising pathway for next-generation high-resolution EUV mask defect review systems.