High-NA EUV Has Astigmatism

This aberration needs to be corrected before serious consideration of High-NA EUV imaging.

Astigmatism refers to focusing horizontal (x-oriented) and vertical (y-oriented) lines at different z-planes [1]. In lithography, this is a forbidden aberration, as both features must be simultaneously in focus in the plane of the same layer.

Recently, it has been possible to evaluate the imaging of horizontal lines vs. vertical lines on both 0.33 NA and 0.55 NA systems [2]. Horizontal lines and vertical lines have different image shifts on 0.55 NA vs. 0.33 NA systems; this can be translated into phase offsets from the image at a best focus condition (Figure 1).

Figure 1. Relative difference of phase offsets (from best focus) between vertical and horizontal half-pitch lines, for standard Ta-based EUV mask stack [2].

This essentially means that horizontal and vertical lines cannot be focused simultaneously on 0.55 NA systems as they practically can be on 0.33 NA systems. For the 0.55 NA system there is a more substantial pitch-dependent phase offset between horizontal and vertical lines. This is aggravated by the fact that the depth of focus is much lower on 0.55 NA systems compared to 0.33 NA systems.

The fundamental reason for this difference is that in 0.55 NA systems, the incident angles for forming horizontal lines are closer to the center ray at the mask, due to the larger demagnification factore (8x instead of 4x). Moreover, the center ray is slightly less than the 6 degrees used in 0.33 NA systems [3].

Just as we use corrective lenses to compensate for astigmatism in human vision, the EUV mask stack may need to be modified [4] to compensate, at least partly, the newly revealed astigmatism of High-NA EUV systems.

References

[1] https://en.wikipedia.org/wiki/Astigmatism_(optical_systems)

[2] I. Lee, J-H. Franke, V. Philipsen, K. Ronse, S. De Gendt, E. Hendrickx, J. Micro/Nanopattern. Mater. Metrol. 22, 043202 (2023).

[3] E. van Setten, G. Bottiglieri, L. de Winter, J. McNamara, P. Rusu, J. Lubkoll, G. Rispens, J. van Schoot, Proc. SPIE 10450, 104500W (2017).

[4] I. Lee, J-H. Franke, V. Philipsen, K. Ronse, S. De Gendt, E. Hendrickx, Proc. SPIE 12593, 125930O (2024).