Optical considerations of EUVL wavelength, NA, and multilayers at large angles

As extreme-ultraviolet (EUV) lithography advances at 13.5 nm with numerical apertures (NA) of 0.55 and higher, the effects of large incidence angles on multilayer design, wavelength selection, and NA limits are examined. Much of this is driven by EUV multilayer (ML) design, which should be optimized for large-angle applications to achieve maximum angular and spectral bandwidth. Reflectance amplitude and phase-shift analyses of Mo/Si multilayers are performed for angles corresponding to 0.55–0.75 NA EUV lithography, demonstrating the importance of accounting for phase effects. Understanding higher-order phase contributions reveals that defocus and spherical aberration originate from the mask multilayer itself, as confirmed through transfer matrix method (TMM) design and rigorous coupled-wave (RCWA) analysis. A depth-grading approach to large-angle multilayer optimization is introduced, leveraging the EUV optical properties of the constituent materials to improve performance by adjusting the material ratios within the stack. Results are presented for solutions using Mo/Si and Ru/Si multilayers for 13.5 nm high-NA and hyper-NA EUVL. Considering polarization effects starting at high NA—both at the mask object plane and the resist image plane—alternative large-angle Ru/Be multilayer configurations operating at 11.3 nm are also evaluated. The introduction of an 11.3 nm/0.63 NA lithography option mitigates some polarization-induced degradation observed at 13.5 nm/0.75 NA and offers an additional increase in focal depth. Design strategies for depth-graded multilayers for optical coating and mask applications are detailed.