Benefits of mask 3D effects in high-NA and hyper-NA EUV lithography

Mask 3D (M3D) effects, arising from light scattering by 3D absorbers atop reflective multilayers, strongly influence the imaging performance of extreme ultraviolet (EUV) lithography. This study shows that M3D effects—traditionally considered detrimental—can be harnessed to improve imaging fidelity in high-NA and hyper-NA EUV, under certain conditions. We performed fully rigorous source-mask optimizations (SMO) to compare the achievable performance of 3D masks with that of Kirchhoff-type flat masks and to identify the best imaging solutions for both. In-depth analyses of near-fields, diffraction, and aerial images explain the advantages of 3D dark-field masks: tighter, higher near-field intensity peaks due to light guiding in low-n absorbers and more efficient coupling into diffraction orders within the projection NA. Realizing these benefits requires split-pupil exposure or other techniques to offset contrast loss. Our simulation results indicate that advances in 3D-mask-aware SMO/ILT and mask technology can further unlock M3D benefits.