Extreme ultraviolet scanner with high numerical aperture: obscuration and wavefront description

Background: A unique extreme ultraviolet (EUV-) scanner with a high numerical aperture (NA) of 0.55 was designed to enable printing of resolution lines with 8 nm half-pitch in a single exposure. The introduction of a central obscuration in the optics design reduces the angular load on the multilayer mirrors, enabling a high transmission and throughput. The central obscuration area has been minimized for best imaging, overlay, and transmission. Aim: The wavefront is only available in the non-obscured area. This raises the question of how to describe such a wavefront. Approach: We discuss the choice of fringe-Tatian basis functions to represent the wavefront for an obscured pupil. To make this choice, one needs to balance mathematical correctness while maintaining a simple and intuitive description. Results: We provide a detailed analysis for selecting basis functions that are adequate to describe measured wavefronts on the non-obscured part of the pupil. This statement is supported by imaging simulations. A fast and stable evaluation of the chosen basis functions is presented. An adapted definition of the wavefront root-mean-square deviation for these functions is proposed; it has the advantage of being simple and independent of the number of basis functions used. Conclusions: Because of the benefits of the proposed representation, the community is encouraged to use the same formalism.