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 o f t he d iffracted 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 a nd t heir i mpact o n t he o ptical r esolution l imit o f h igh N A a nd h yper N A EUV lithography, this paper investigates the imaging of lines/spaces (L/S) with a pitch of 9 nm using an ideal fictive diffraction-limited p rojection s ystem w ith a N A o f 0 .85. T he r esults o f o ur s imulations s uggest t hat 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 i maging. 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 p erformance. The opportunities for such innovative solutions depend on the limitations of mask fabrication, which are not considered in this discussion.