Abstract
A recent study (2020 Nat. Commun. 11 2334) has found that transitions between multiply-excited configurations in open 4d-subshell tin ions are the dominant contributors to intense EUV emission from dense, Nd:YAG-driven (laser wavelength λ = 1.064 μm) tin plasmas. In the present study, we employ the Los Alamos Atomic code to investigate the spectral contribution from these transitions under industrially-relevant, CO2 laser-driven (λ = 10.6 μm) tin plasma conditions. First, we employ Busquet’s ionisation temperature method to match the average charge state ⟨Z⟩ of a non-local-thermodynamic equilibrium (non-LTE) plasma with an LTE one. This is done by varying the temperature of the LTE calculations until a so-called ionisation temperature T Z is established. Importantly, this approach generates LTE-computed configuration populations in excellent agreement with the non-LTE populations. A corollary of this observation is that the non-LTE populations are well-described by Boltzmann-type exponential distributions having effective temperatures T eff ≈ T Z . In the second part of this work, we perform extensive level-resolved LTE opacity calculations at T Z . It is found that 66% of the opacity in the industrially-relevant 2% bandwidth centred at 13.5 nm arises from transitions between multiply-excited states. These results reinforce the need for the consideration of complex, multiply-excited states in modelling the radiative properties of laser-driven plasma sources of EUV light.
Highlights
It is found that 66% of the opacity in the industrially-relevant 2% bandwidth centered at 13.5 nm arises from transitions between multiply-excited states
The key technology driving feature-size miniaturisation in the semiconductor industry today is extreme ultraviolet lithography (EUVL) [10,11,12,13]. This technology uses EUV light to print features on integrated circuits (ICs). This EUV light is generated in a laser-produced plasma (LPP) formed by irradiating tin microdroplets with CO2 laser light (λ = 10.6 μm) [14, 15]
II, we describe the atomic structure and opacity calculations performed with the Los Alamos suite of atomic physics and population kinetics codes
Summary
We employ the Los Alamos Atomic code to investigate the spectral contribution from these transitions under industrially-relevant, CO2 laser-driven (λ = 10.6 μm) tin plasma conditions. The well-known 4p6 4dm −(4p5 4dm+1 +4p6 4dm−1 4f ) transitions play only a minor role; they comprise 11% of the opacity in the 5 − 20 nm region and 19% of the opacity in the 2% bandwidth It is transitions between complex, multiply-excited configurations 0.87/0.75 to the radial integrals associated with singly/multiply-excited energy levels These two scaling factors were adopted for all charge states considered in this work.
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