EUV spectroscopy of highly chargedSn13+−Sn15+ions in an electron-beam ion trap

Abstract

Extreme-ultraviolet (EUV) spectra of ${\mathrm{Sn}}^{13+}\ensuremath{-}{\mathrm{Sn}}^{15+}$ ions have been measured in an electron-beam ion trap (EBIT). A matrix inversion method is employed to unravel convoluted spectra from a mixture of charge states typically present in an EBIT. The method is benchmarked against the spectral features of resonance transitions in ${\mathrm{Sn}}^{13+}$ and ${\mathrm{Sn}}^{14+}$ ions. Three new EUV lines in ${\mathrm{Sn}}^{14+}$ confirm its previously established level structure. This ion is relevant for EUV nanolithography plasma but no detailed experimental data currently exist. We used the Cowan code for first line identifications and assignments in ${\mathrm{Sn}}^{15+}$. The collisional-radiative modeling capabilities of the Flexible Atomic Code were used to include line intensities in the identification process. Using the 20 lines identified, we have established 17 level energies of the $4{p}^{4}4d$ configuration as well as the fine-structure splitting of the $4{p}^{5}$ ground-state configuration. Moreover, we provide state-of-the-art ab initio level structure calculations of ${\mathrm{Sn}}^{15+}$ using the configuration-interaction many-body perturbation code ambit. We find that the here-dominant emission features from the ${\mathrm{Sn}}^{15+}$ ion lie in the narrow 2% bandwidth around 13.5 nm that is relevant for plasma light sources for state-of-the-art nanolithography.