High-energy ions from Nd:YAG laser ablation of tin microdroplets: comparison between experiment and a single-fluid hydrodynamic model

Abstract

We present the results of a joint experimental and theoretical study of plasma expansion arising from Nd:YAG laser ablation (laser wavelength λ = 1.064 μm) of tin microdroplets in the context of extreme ultraviolet lithography. Measurements of the ion energy distribution reveal a near-plateau in the distribution for kinetic energies in the range 0.03-1 keV and a peak near 2 keV followed by a sharp fall-off in the distribution for energies above 2 keV. Charge-state resolved measurements attribute this peak to the existence of peaks centered near 2 keV in the Sn 3+ -Sn 8+ ion energy distributions. To better understand the physical processes governing the shape of the ion energy distribution, we have modelled the laser-droplet interaction and subsequent plasma expansion using two-dimensional radiation hydrodynamic simulations. We find excellent agreement between the simulated ion energy distribution and the measurements both in terms of the shape of the distribution and the absolute number of detected ions. We attribute a peak in the distribution near 2 keV to a quasi-spherical expanding shell formed at early times in the expansion.

Energy- and charge-state-resolved spectrometry of tin laser-produced plasma using a retarding field energy analyzer

“…Ions with higher charge may still be present but at negligible quantities. The individual charge-energy spectra follow trends similar to those seen previously using an electrostatic time-of-flight analyzer [11,22], with in particular the higher charge states bunching up to form the aforementioned high-energy peak.…”

“…The FC-like spectrum exhibits a high-energy peak between 1 and 2 keV. This high-energy peak has recently been attributed to a quasi-spherical expanding shell formed at early times in the plasma expansion into which subsequent hotter plasma is bunched-an effect that is characteristic to temporally Gaussian-shaped laser pulses [22].…”

Microdroplet-tin plasma sources of EUV radiation driven by solid-state-lasers (Topical Review)

“…Charge-state resolved measurements, performed with a cross-calibrated electrostatic analyzer in time-of-flight mode (ESA-ToF) [107], attributed this peaked feature at 2 keV to the existence of peaks (centered near 2 keV) in the Sn 3+ − Sn 8+ ion energy distributions. To understand the physical origin of this peaked feature, Hemminga et al [106] performed two-dimensional simulations of the plasma initiation, growth and subsequent expansion using the radiation hydrodynamic code RALEF-2D. As is evident from figure 7, excellent agreement was found between the simulated ion energy distribution and the measurements both in terms of the shape of the distribution and the absolute number of detected ions.…”

“…The experimental ion energy distribution is shown in red (solid curve) and the RALEF-2D ion energy distribution is shown in black (solid curve). Also illustrated in green are the predictions of analytic models of plasma expansion (see discussion in [106]). Reproduced from [106].…”

“…Also illustrated in green are the predictions of analytic models of plasma expansion (see discussion in [106]). Reproduced from [106]. © The Author(s).…”