Short-wavelength out-of-band EUV emission from Sn laser-produced plasma

摘要

We present the results of spectroscopic measurements in the extremeultraviolet (EUV) regime (7-17 nm) of molten tin microdroplets illuminated by ahigh-intensity 3-J, 60-ns Nd:YAG laser pulse. The strong 13.5 nm emission fromthis laser-produced plasma is of relevance for next-generation nanolithographymachines. Here, we focus on the shorter wavelength features between 7 and 12 nmwhich have so far remained poorly investigated despite their diagnosticrelevance. Using flexible atomic code calculations and local thermodynamicequilibrium arguments, we show that the line features in this region of thespectrum can be explained by transitions from high-lying configurations withinthe Sn$^{8+}$-Sn$^{15+}$ ions. The dominant transitions for all ions butSn$^{8+}$ are found to be electric-dipole transitions towards the $n$=4 groundstate from the core-excited configuration in which a 4$p$ electron is promotedto the 5$s$ sub-shell. Our results resolve some long-standing spectroscopicissues and provide reliable charge state identification for Sn laser-producedplasma, which could be employed as a useful tool for diagnostic purposes.