Efficient Generation of Extreme Ultraviolet Light From $\mathrm{Nd}$:YAG-Driven Microdroplet-Tin Plasma

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

R. Schupp, F. Torretti, R.A. Meijer, M. Bayraktar, J. Scheers, D. Kurilovich, A. Bayerle, K.S.E. Eikema, S. Witte, W. Ubachs, R. Hoekstra, and O.O. Versolato

Phys. Rev. Applied 12, 014010 – Published 8 July 2019

Abstract

We experimentally investigate the emission of EUV light from a mass-limited laser-produced plasma over a wide parameter range by varying the diameter of the targeted tin microdroplets and the pulse duration and energy of the 1- $μ m$ -wavelength $Nd$ :YAG drive laser. Combining spectroscopic data with absolute measurements of the emission into the 2% bandwidth around 13.5 nm relevant for nanolithographic applications, the plasma’s efficiency in radiating EUV light is quantified. All observed dependencies of this radiative efficiency on the experimental parameters are successfully captured in a geometrical model featuring the plasma absorption length as the primary parameter. It is found that laser intensity is the pertinent parameter setting the plasma temperature and the tin-ion charge-state distribution when varying laser pulse energy and duration over almost 2 orders of magnitude. These insights enabled us to obtain a record-high 3.2% conversion efficiency of laser light into 13.5-nm radiation and to identify paths towards obtaining even higher efficiencies with 1- $μ m$ solid-state lasers that may rival those of current state-of-the-art ${CO}_{2}$ -laser-driven sources.

Received 18 December 2018
Revised 20 February 2019

DOI:https://doi.org/10.1103/PhysRevApplied.12.014010

Physics Subject Headings (PhySH)

Atomic spectra Plasma production & heating by laser beams, laser-foil, laser-cluster Plasma sources

Liquid metals Nanostructures Optical sources & detectors

Lithography

Atomic, Molecular & OpticalPlasma Physics

Authors & Affiliations

R. Schupp ¹, F. Torretti^1,2, R.A. Meijer^1,2, M. Bayraktar³, J. Scheers^1,2, D. Kurilovich^1,2, A. Bayerle¹, K.S.E. Eikema^1,2, S. Witte^1,2, W. Ubachs^1,2, R. Hoekstra^1,4, and O.O. Versolato^1,*

¹Advanced Research Center for Nanolithography, Science Park 106, 1098 XG Amsterdam, Netherlands
²Department of Physics and Astronomy and LaserLaB, Vrije Universiteit, De Boelelaan 1081, 1081 HV Amsterdam, Netherlands
³Industrial Focus Group XUV Optics, MESA+ Institute for Nanotechnology, University of Twente, Drienerlolaan 5, 7522 NB Enschede, Netherlands
⁴Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, Netherlands

^*o.versolato@arcnl.nl

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Vol. 12, Iss. 1 — July 2019

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Figure 1
Schematic representation of the experimental setup. The laser beam, depicted in red, illuminates the falling tin microdroplets (gray spheres). The produced plasma emission is observed using a transmission-grating spectrometer (gray box) as well as four EUV photodiode assemblies (gray cylinders) measuring the absolute amount of produced in-band radiation. All angles $θ$ are indicated with respect to the laser-beam propagation axis, with $\cos θ = \cos ϕ \cos φ$ .
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Figure 2
Angular dependency $(θ)$ of the in-band emission with respect to the laser-beam axis. Depicted are measurements for the 96- $μ m$ laser-beam size. For each pulse duration, the data from the smallest and biggest droplet sizes are shown normalized to their maximum values. The dashed blue line is the result of a global fit of Eq. (2).
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Figure 3
Normalized emission spectra for various $Nd$ :YAG laser intensities resulting from a 46- $μ m$ -diameter tin droplet illuminated with a 96- $μ m$ -sized laser beam of 15-ns duration. The gray-shaded area shows the 2% bandwidth around 13.5 nm, relevant for nanolithographic applications. The emission features attributed to the various $Sn$ ions are labeled with the respective charge-state number. Also labeled are the most relevant transition arrays contributing to the in-band emission and the short-wavelength features between 7 and 12 nm, where $m$ is an integer between 0 and 6 corresponding to ${Sn}^{14 +}$ and ${Sn}^{8 +}$ , respectively [4, 6, 61]. The inset figure shows the relative peak-emission intensities as a function of laser pulse intensity.
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Figure 4
Top row: spectra for tin microdroplets irradiated by (a) various $Nd$ :YAG laser pulse intensities, (b) laser pulse durations, and (c) droplet sizes. The gray vertical bar depicts a 2% bandwidth around 13.5 nm. Center row: SPs (here divided by a factor 2 for convenience) and CE corresponding to the spectra above. Bottom row: radiative efficiencies ( $η_{rad} \equiv CE / SP$ ) and the result of a global fit of our plasma expansion model as described in Sec. 5 (solid lines). The gray dashed lines in (g)–(i) depict the geometrical overlap ( $R_{0}^{2} / 2 w^{2}$ ) of laser beam and droplet for isotropic emission.
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Figure 5
Schematic representation of the laser-produced plasma system. The laser pulse (orange shaded area) propagating in positive $z$ direction is absorbed by the liquid $Sn$ droplet of radius $R_{0}$ (gray sphere). During laser irradiation plasma is created around the droplet (light gray shaded area) increasing the absorbing area. The latter expands with a constant velocity $v$ laterally to the laser-beam propagation direction, increasing the effective radius of absorption to $R_{eff} = R_{0} + v t$ .
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Figure 6
(a) Spectra for two droplet sizes and two laser-beam sizes (see main text) at a laser intensity of $1.4 \times 10^{11} W / {cm}^{2}$ ; (b) values for SP/2 and CE versus droplet diameter for the two laser-beam sizes; (c) radiative efficiency versus droplet diameter for the two laser-beam sizes. The dash-dotted and solid lines represent the model curves. For the 56- $μ m$ laser beam, the regimes corresponding to the three model cases (i)–(iii) of Eq. (3) are highlighted by the vertical dashed lines (see main text).
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Physical Review Applied

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

R. Schupp, F. Torretti, R.A. Meijer, M. Bayraktar, J. Scheers, D. Kurilovich, A. Bayerle, K.S.E. Eikema, S. Witte, W. Ubachs, R. Hoekstra, and O.O. Versolato

Phys. Rev. Applied 12, 014010 – Published 8 July 2019

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