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Advanced multilayer mirror design to mitigate EUV shadowing
Author(s): Stuart Sherwin; Andrew R. Neureuther; Laura Waller; Patrick Naulleau
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Paper Abstract

Two central challenges in EUV patterning are mask 3D effects and shadowing, which arise from the relatively thick absorber and the tilted optical axis, respectively. Together, these effects result in an asymmetrical reflection function in the shadowing orientation that depends heavily on the angle of illumination. In particular, the reflection function for a shadowing orientation grating contains one bright edge and one dark (shadowed) edge; the asymmetry increases the further the illumination in the shadowing direction is from on-axis. This asymmetry significantly complicates source-mask optimization compared to DUV lithography. Others have explored using a thinner absorber made of a more absorbing material, and have shown significant reductions in mask 3D effects, but this approach still does not entirely eliminate shadowing. However, much less attention has been paid to the role of the multilayer mirror, particularly in terms of addressing the complex nature of the reflection function, which includes both amplitude and more subtle phase effects. Indeed, the phase curvature of the reflection function vs angle can be interpreted as encoding an effective reflection plane buried within the multilayer. From this interpretation, we see that the downward transmission through the absorber gets translated by an amount roughly proportional to the illumination angle in the shadowing orientation, before transmitting upward through the absorber. This effect is directly responsible for creating the shadowed edge, demonstrating the link between the phase of the multilayer mirror reflection function and shadowing. In this work we present a novel approach of optimizing the multilayer mirror design to mitigate these shadowing effects. We introduce a method to estimate the optimal complex reflection function for the multilayer mirror based on a low-rank decomposition of the absorber transmission function. We then go on to quantify the benefits of our approach versus traditional multilayer designs. Finally, we explore the feasibility of manufacturing a multilayer mirror with such a reflection function.

Paper Details

Date Published: 14 March 2019
Proc. SPIE 10957, Extreme Ultraviolet (EUV) Lithography X, 1095715 (14 March 2019); doi: 10.1117/12.2515095
Show Author Affiliations
Stuart Sherwin, Univ. of California, Berkeley (United States)
Andrew R. Neureuther, Univ. of California, Berkeley (United States)
Laura Waller, Univ. of California, Berkeley (United States)
Patrick Naulleau, Lawrence Berkeley National Lab. (United States)

Published in SPIE Proceedings Vol. 10957:
Extreme Ultraviolet (EUV) Lithography X
Kenneth A. Goldberg, Editor(s)

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