![(left) Stroboscopic side-view shadowgraph images of tin microdroplets obtained before and after the interaction with a laser pulse. The laser pulse arrives from the left at t = 0 µs. The plot below shows the time-dependent position of center-of-pixels of images along the laser light propagation axis. (right) Measured propulsion velocity of metallic microdroplets as a function of the impinging laser energy. The dashed lines represents power-law fits to the data. Figures reproduced from Ref. [2].](https://www.researchgate.net/profile/Wim-Ubachs/publication/324076327/figure/fig1/AS:610954186326017@1522674237060/left-Stroboscopic-side-view-shadowgraph-images-of-tin-microdroplets-obtained-before-and_Q320.jpg)
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ET3B.4.pdf High-brightness Sources and Light-driven Interactions
Congress 2018 (HILAS, MICS, EUVXRAY) © OSA 2018
Physics of Laser-Produced Plasma Sources of
Extreme Ultraviolet Radiation
O.O. Versolato1, D. Kurilovich1,2, F. Torretti1,2, R. Schupp1, J. Scheers1,2,
M. J. Deuzeman1,3, A. Bayerle1, R. Hoekstra1,3, W. Ubachs1,2
1Advanced Research Center for Nanolithography, Science Park 110, 1098 XG Amsterdam, The Netherlands
2Department of Physics and Astronomy, and LaserLaB, Vrije Universiteit, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands
3Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands
Author e-mail address: o.versolato@arcnl.nl
Abstract: Laser-produced tin plasmas are used for the generation of extreme ultraviolet light for
nanolithography. We use an extensive diagnostic toolset to characterize and understand the
physics of these plasma light sources at the atomic level.
OCIS codes: (300.0300) Spectroscopy; (020.0020) Atomic and molecular physics; (350.5400) Plasmas
1. Introduction
Laser-produced tin plasmas are used for the generation of extreme ultraviolet (EUV) light for nanolithography in a
two-step process. In the first step, a laser “prepulse” deforms a microscopic liquid tin droplet into an optimal target
shape. Plasma and fluid physics together determine the droplet response to such laser pulse impact. In the second
step, an energetic “main pulse” produces a bright EUV-emitting plasma. Light is generated from highly charged tin
ions. Thousands of atomic lines contribute to the emission from the hot and very dense, rapidly expanding plasma in
a narrow band around the required 13.5 nm wavelength.
2. Droplet propulsion and deformation
We studied the propulsion of liquid tin (and several of its alloys) microdroplets by nanosecond-pulse laser impact by
carefully comparing experimental data to simulations and analytical theory [1,2]. We captured the physics of the
observed droplet propulsion in a scaling law that accurately describes the plasma-imparted momentum over nearly
three decades of laser pulse energy (see Fig. 1), enabling the optimization of the laser-droplet coupling. Having
found good agreement between the experiment and the simulations, we investigated the analytic origins of the
obtained power law. Interestingly, we found that none of the available analytic theories agree with the observed
scaling law. The subsequent deformation of the droplet, set in motion by the impact of the laser pulse, is well-
described by an analytical model that accounts for the droplet’s propulsion velocity and its liquid properties.
Fig. 1. (left) Stroboscopic side-view shadowgraph images of tin microdroplets obtained before and after the interaction with a laser pulse. The
laser pulse arrives from the left at t = 0 µs. The plot below shows the time-dependent position of center-of-pixels of images along the laser light
propagation axis. (right) Measured propulsion velocity of metallic microdroplets as a function of the impinging laser energy. The dashed lines
represents power-law fits to the data. Figures reproduced from Ref. [2].
© 2018 The Author(s)
ET3B.4.pdf High-brightness Sources and Light-driven Interactions
Congress 2018 (HILAS, MICS, EUVXRAY) © OSA 2018
3. Spectroscopy of highly charged Sn ions
To obtain a better understanding of the myriad processes within the laser-generated plasma we performed charge-
state-resolved measurements of the relevant tin ions using an electron beam ion trap (EBIT) as well as spectroscopic
investigations of actual laser-produced plasma at ARCNL. In the EBIT measurements [3,4], we experimentally re-
evaluated the fine structure of Sn7+–Sn14+ ions, combining optical spectroscopy of magnetic dipole M1 transitions
and EUV spectra with charge-state selective ionization in an EBIT. Our measurements confirm the predictive power
of ab initio calculations based on Fock space coupled cluster theory and on configuration-interaction many-body
perturbation theory (CI+MBPT). We validated our line identification using semi-empirical COWAN calculations
with adjustable wave-function parameters. Comparison with previous work suggests that line identifications in the
EUV need to be revisited.
In the laser-produced plasma measurements [5], we focused on relatively short-wavelength features
between 7 and 12 nm which have so far remained relatively poorly investigated despite their diagnostic relevance.
Using flexible atomic code calculations and local thermodynamic equilibrium arguments, we show that the line
features in this wavelength region can be explained by transitions from high-lying configurations within the Sn8+–
Sn15+ ions towards the respective ground states. 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.
4. Sn ion energy distributions
When the laser pulse ends, the unconfined plasma rapidly cools down and no longer emits any EUV radiation.
Nevertheless, it contains significant energy and the plasma rapidly expands with ion velocities reaching ~10 km/s.
These impinging particles may affect the performance of any light collecting optics. We studied the Sn ion energy
distributions of both nanosecond- and picosecond-laser produced plasmas [6,7] on solid as well as on droplet targets.
Measured ion energy distributions are compared to two self-similar solutions assuming isothermal expansion of the
plasma plume into the vacuum. For planar and droplet targets exposed to ps-long pulses we find a good agreement
between the experimental results and the self-similar solution of a semi-infinite simple planar plasma configuration
with an exponential density profile. The ion energy distributions resulting from solid Sn exposed to ns-pulses agrees
with solutions of a limited-mass model that assumes a Gaussian-shaped initial density profile.
5. References
[1] D. Kurilovich, et al, “Plasma propulsion of a metallic microdroplet and its deformation upon laser impact,” Phys. Rev. Appl. 6,
014018 (2016).
[2] D. Kurilovich, et al, “Power-law scaling of plasma pressure on laser-ablated tin microdroplets,” submitted, arXiv:1710.11426 (2017).
[3] F. Torretti, et al, “Optical spectroscopy of complex open-4d-shell ions Sn7+–Sn10+,” Phys. Rev. A. 95, 042503 (2017).
[4] A. Windberger, et al, “Analysis of the fine structure of Sn11+–Sn14+ ions by optical spectroscopy in an electron-beam ion trap,” Phys. Rev. A.
94, 012506 (2016).
[5] F. Torretti, et al, “Short-wavelength out-of-band EUV emission from Sn laser-produced plasma,” submitted, arXiv:1709.02626 (2017).
[6] M.J. Deuzeman, et al, “Ion distribution and ablation depth measurements of a fs-ps laser-irradiated solid tin target,” J. Appl. Phys. 121,
103301 (2017).
[7] A. Bayerle, et al, “Sn ion energy distributions of ns-and ps-laser produced plasmas,” submitted, arXiv:1711.02342 (2017).
