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In recent years, attenuated phase shift masks (AttPSM) for EUV lithography have gained attention as a contrast-enhancing and throughput-improving mask architecture. However, the relatively transparent nature of the absorber structure, combined with the inherently reflective nature of EUV photomasks, means that multilayer roughness effects must be carefully understood. Previous studies on tantalum masks have shown that, relative to target feature size, multilayer roughness has an increased anisotropic effect on aerial image roughness in High (0.55) NA EUV lithography as compared to 0.33NA in the shadowing direction despite the anamorphic demagnification1. These effects are presumably more severe for AttPSMs given the increased amount of light transmitted through the absorber-coated regions of the mask. More recent studies on AttPSM have shown that phase shifter thickness can be tuned to co-optimize NILS, MEEF, and throughput to mitigate local critical dimension uniformity effects arising from the mask absorber structure patterned atop the ideal multilayer2. In this paper, we utilize the methods detailed in Reference 1 to examine the impact of multilayer roughness on pattern roughness for AttPSM EUV masks, focusing on the mask stacks and patterns detailed in Reference 2. In particular, we seek to understand how multilayer roughness influences the choice of absorber stack material and thickness, both within the AttPSM family of designs, as well as relative to the traditional tantalum-based photomask design.
[1] Naulleau, P., Wang, Y.-G. and Pistor, T., “Extreme ultraviolet mask roughness effects in high numerical aperture lithography,” Appl Opt (2018).
[2] Ahn, C.-N., Nam, D.-S., Seong, N. and Yen, A., “Optical design of EUV attenuated PSM for contact-hole applications,” 19 February 2021, 12, SPIE-Intl Soc Optical Eng.
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