Refining Atomic Data for 13.5 nm EUV Emission from Laser-Produced Tin Plasma via Multiply-Excited States and Spectral Validation

Chun-Tse Wu^1*, Shih-Hung Chen¹, Shivam Gupta², Yao-Li Liu²

¹Physics, National Central University, Taoyuan, Taiwan
²Institute of Space and Plasma Sciences, National Cheng Kung University, Tainan, Taiwan

* Presenter:Chun-Tse Wu, email:112282002@cc.ncu.edu.tw

Extreme ultraviolet (EUV) lithography relies on a 13.5 nm source with a 2% bandwidth to enable sub-10 nm patterning. Laser-produced plasma (LPP) of tin is the leading approach, where a high-power laser drives tin targets to hot, radiative conditions populated by many ionic states. We developed a self-consistent simulation framework that formulates atomic, ionization, and fluid dynamics as independent but interacting modules to predict in-band emission. Accurate atomic data—transition probabilities and wavelengths—are essential for modeling the 13.5 nm output. While singly excited states are often assumed to dominate, recent work has demonstrated that multiply excited configurations can contribute significantly to the in-band emissivity of LPP Sn plasmas [1]. Motivated by this, we computed detailed level structures and radiative rates using the Flexible Atomic Code (FAC) [2], explicitly including multiply excited states, and validated the predicted wavelengths by comparison with charge-exchange spectra for highly charged tin ions reported by Ohashi et al. [3]. Our calculations indicate that for Sn¹⁵⁺, inclusion of multiply excited states yields pronounced enhancement of in-band emission and produces a red-shift of ≈ 0.85 nm in the synthetic spectrum relative to models limited to singly excited states. These results underscore the importance of comprehensive configuration sets in atomic databases for LPP EUV source modeling and provide refined inputs to improve predictive accuracy for source optimization in EUV lithography.

References
[1] Torretti, F., Sheil, J., Schupp, R., Basko, M. M., Bayraktar, M., Meijer, R. A., ... & Colgan, J. (2020). Prominent radiative contributions from multiply-excited states in laser-produced tin plasma for nanolithography. Nature communications, 11(1), 2334.
[2] Gu, M. F. (2008). The flexible atomic code. Canadian Journal of Physics, 86(5), 675-689.
[3] Ohashi, H., Suda, S., Tanuma, H., Fujioka, S., Nishimura, H., Sasaki, A., & Nishihara, K. (2010). EUV emission spectra in collisions of multiply charged Sn ions with He and Xe. Journal of Physics B: Atomic, Molecular and Optical Physics, 43(6), 065204.

Keywords: Laser-Produced Plasma, Extreme Ultraviolet, Atomic Physics, Tin Spectrum