Abstract
We present the results of spectroscopic measurements in the extreme ultraviolet regime (7–17 nm) of molten tin microdroplets illuminated by a high-intensity 3 J, 60 ns Nd:YAG laser pulse. The strong 13.5 nm emission from this laser-produced plasma (LPP) is of relevance for next-generation nanolithography machines. Here, we focus on the shorter wavelength features between 7 and 12 nm which have so far remained poorly investigated despite their diagnostic relevance. Using flexible atomic code calculations and local thermodynamic equilibrium arguments, we show that the line features in this region of the spectrum can be explained by transitions from high-lying configurations within the Sn–Sn ions. The dominant transitions for all ions but Sn are found to be electric-dipole transitions towards the n = 4 ground state from the core-excited configuration in which a 4p electron is promoted to the 5s subshell. Our results resolve some long-standing spectroscopic issues and provide reliable charge state identification for Sn LPP, which could be employed as a useful tool for diagnostic purposes.
Highlights
We present the results of spectroscopic measurements in the extreme ultraviolet regime (7–17 nm) of molten tin microdroplets illuminated by a high-intensity 3 J, 60 ns Nd:YAG laser pulse
Sn and its highly charged ions are of undoubtable technological importance, as these are the emitters of extreme ultraviolet (EUV) radiation around 13.5 nm used in nanolithographic applications [1, 2]
Photons with wavelengths close to 13.5 nm. This is advantageous as it corresponds to the peak reflectivity of the mirrors composing the projection optics in nanolithography machines. These molybdenum–silicon multi-layer mirrors [5, 6] are characterised by an ‘in-band’, 2% reflectivity bandwidth centred around 13.5 nm, which conveniently overlaps with the strong EUV emission of Sn laser-produced plasma (LPP)
Summary
Sn and its highly charged ions are of undoubtable technological importance, as these are the emitters of extreme ultraviolet (EUV) radiation around 13.5 nm used in nanolithographic applications [1, 2] In such state-of-the-art lithography machines, EUV light is generated using pulsed, droplet-based, laser-produced plasma (LPP) [3, 4]. In dense and hot plasmas, a significant amount of energy can be radiated at shorter wavelengths arising from configurations that remain poorly investigated with, to the best of our knowledge, but a single study [12] dedicated to the corresponding transition arrays This short-wavelength, ‘out-of-band’ radiation could very well negatively affect the optics lifetime [6, 23], whilst obviously hindering the conversion efficiency from laser energy to 13.5 nm photons. Using the Bauche–Arnoult UTA formalism [8, 9, 25] as well as Gaussian fits to our results, we provide simplified outcome of our calculations which can be used to straightforwardly interpret Sn LPP spectra
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