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Stochasticity is increasingly recognized as the critical factor affecting EUV lithography performances, while it’s also been recognized that the usable k1-factor is lower for EUV than for DUV exposures. This paper explores the problems in EUV from a statistical viewpoint. Relationships between probability density functions (PDF) for chemo-physical events in image-exposed resist films and practical lithography performances, such as the RLS tradeoff relationship are explored based on stochastic simulations. The developments of PDFs for a series of chemo-physical events in lithography processes (from radio-chemical reactions to dissolutions) show the contrast enhancement as the event step proceeds, showing the ability of the resist process to achieve rectangular resist profiles from sinusoidal image profiles regardless of the differences in photon energies and reaction mechanisms. This is because the number of photons contributing to each event generation increases with proceeding the event steps through the multi-photon-induced molecular-solubility-switching and the collaborative dissolution. PDF distributions near pattern edges are blurry in EUV; however, this is because of the increased inhomogeneities in event distributions due to localized secondary electron generation and the interaction among molecules in dissolution processes. Both degrade the dissolution-rate slope near edges and increase the LER. Further, the situation worsens when they are combined. Statistical variations in reaction-dissolution event densities in resist films and molecular interactions in dissolution dominate the ultimate resolution of EUV. We discuss the effectiveness of increasing contributing photons through increasing required reaction density and molecular interaction for suppressing intrinsic inhomogeneity and enhancing the contrast at the expense of resist sensitivity.
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