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

TL;DR

This study investigates the short-wavelength EUV emission from laser-produced tin plasma, revealing new spectral features and ion transitions that enhance plasma diagnostics for nanolithography applications.

Contribution

It provides the first detailed analysis of 7-12 nm EUV features in tin plasma, identifying specific ion transitions and resolving longstanding spectroscopic issues.

Findings

Identified dominant electric-dipole transitions in Sn$^{8+}$-Sn$^{15+}$ ions.

Resolved previous spectroscopic ambiguities in the EUV spectrum.

Provided reliable charge state identification for plasma diagnostics.

Abstract

We present the results of spectroscopic measurements in the extreme ultraviolet (EUV) 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 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$^{8+}$-Sn$^{15+}$ ions. The dominant transitions for all ions but Sn$^{8+}$ are found to be electric-dipole transitions towards the $n$=4 ground state from the core-excited configuration in which a 4$p$ electron is promoted to the 5$s$ sub-shell. Our results resolve some long-standing spectroscopic issues and provide reliable charge state identification for Sn laser-produced plasma, which could be employed as a useful tool for diagnostic purposes.