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.
The high NA=0.55 EUV scanner has an obscuration in the pupil. This has led to the choice to expand the aberration wave-front not in Zernikes anymore, but in other, orthogonal, basis-functions instead. The reasons for this choice and the description of the basis-functions will be discussed.
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