Atomic structure considerations for the low-temperature opacity of Sn

Abstract

We have begun a preliminary investigation into the opacity of Sn at low temperatures (< 50 eV). The emissivity and opacity of Sn is a crucial factor in determining the utility of Sn in EUV lithography, with numerous industrial implications. To this end, we have been exploring the accuracy of some approximations used in opacity models for the relevant ion stages of Sn (neutral through ∼ 18 times ionized). We find that the use of intermediate-coupling, as compared to full configuration-interaction, is not adequate to obtain accurate line positions of the important bound-bound transitions in Sn. One requires full configuration-interaction to properly describe the strong mixing between the various n = 4 sub-shells that give rise to the Δn = 0 transitions that dominate the opacity spectrum at low temperatures. Since calculations that include full configuration-interaction for large numbers of configurations quickly become computationally prohibitive, we have explored hybrid calculations, in which full configuration-interaction is retained for the most important transitions, while intermediate-coupling is employed for all other transitions. After extensive exploration of the atomic structure properties, local-thermodynamicequilibrium (LTE) opacities are generated using the ATOMIC code at selected temperatures and densities.

Efficient Generation of Extreme Ultraviolet Light From Nd :YAG-Driven Microdroplet-Tin Plasma

“…3 is the shift of the main emission feature from 15 nm toward the desired 13.5 nm with increasing laser intensity. This shift is related to the creation of tin ions of higher charge states (Sn 8+ to Sn 15+ ) relevant for in-band emission [1,4,6,12]. Additionally, the main emission feature narrows towards higher intensity, reaching a FWHM of 0.9 nm at 1.4 × 10 11 W/cm 2 .…”

“…Highly charged tin ions in laser-produced transient plasmas are the atomic sources of EUV light at 13.5-nm wavelength for nanolithography [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. In state-of-the-art EUV sources such plasmas are produced by irradiation of micrometer-sized tin droplets with high-power CO 2 -gaslaser pulses [16][17][18] in a two-step process where first a laser prepulse [16][17][18][19][20][21][22][23] shapes the droplet into a target optimized for EUV production with a second, more energetic pulse.…”

“…In turn, more light is emitted outside of the accepted bandwidth. Modeling these opacity effects challenges even the most advanced atomic structure calculations [12] due to the strongly correlated electronic structure of the multielectron open-4d-shell tin ions raising the need for experimental investigations.…”

Prominent radiative contributions from multiply-excited states in laser-produced tin plasma for nanolithography

“…This shortwavelength radiation is used to imprint smaller features on commercial microchips. The aptness of Sn ions to this application stems from their open-4d-subshell structures [10][11][12][13][14][15][16][17][18][19][20] . Within these structures, Δn = 0 one-electron-excited configurations are very well documented in the literature to decay to the ground state manifold via a multitude of transitions clustered together in unresolved transition arrays (UTAs) 21 , centered in the industrially relevant band around 13.5 nm.…”

New Los Alamos Opacity Calculations

“…Although such tables are not yet finalized, below we show various calculations and comparisons to experimental data, where available. We also note that we have embarked on some preliminary studies of the opacity of Sn at temperatures relevant to lithography applications [16], where the importance of accurate atomic structure to the resulting opacity is crucial.…”