Exploring the crucial role of mask 3D-induced imaging mechanisms in high- and hyper-NA EUV lithography: a study of the near- and far-field of the diffracted light

Mask3D-induced effects, including orientation-dependent image asymmetries, non-telecentricity, pitch-dependent best focus, and image blur, are increasingly important for EUV imaging. To improve the fundamental understanding of these effects and their impact on the optical resolution limit of high NA and hyper NA EUV lithography, this paper investigates the imaging of lines/spaces (L/S) with a pitch of 9 nm using an ideal fictive diffraction-limited projection system with a NA of 0.85. The results of our simulations suggest that mask3D effects will not limit the achievable imaging performance of high NA and hyper NA EUV systems. Comparisons of rigorous mask simulations with results obtained by a Kirchhoff (flat) mask model indicate that mask3D effects do not necessarily negatively impact EUV imaging. Absorber patterns for the smallest pitches behave like volume gratings. Such volume gratings exhibit a significant dependency of the diffracted light on the illumination direction. In contrast to thin gratings, volume gratings enable a more flexible distribution of light between diffraction orders. Based on the improved understanding of the involved imaging mechanisms, one could take advantage of mask3D effects to enhance the imaging performance. The opportunities for such innovative solutions depend on the limitations of mask fabrication, which are not considered in this discussion.