Characterization of 1- and $2\text{\ensuremath{-}}\ensuremath{\mu}\mathrm{m}$-wavelength laser-produced microdroplet-tin plasma for generating extreme-ultraviolet light

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Characterization of 1- and $2 − μ m$ -wavelength laser-produced microdroplet-tin plasma for generating extreme-ultraviolet light

R. Schupp, L. Behnke, J. Sheil, Z. Bouza, M. Bayraktar, W. Ubachs, R. Hoekstra, and O. O. Versolato

Phys. Rev. Research 3, 013294 – Published 31 March 2021

Abstract

Experimental spectroscopic studies are presented, in a 5.5–25.5 nm extreme-ultraviolet (EUV) wavelength range, of the light emitted from plasma produced by the irradiation of tin microdroplets by 5-ns-pulsed, $2 − μ m$ -wavelength laser light. Emission spectra are compared to those obtained from plasma driven by $1 − μ m$ -wavelength laser light over a range of laser intensities spanning approximately $(0.3 - 5) \times 10^{11} W / {cm}^{2}$ , under otherwise identical conditions. Over this range of drive laser intensities, we find that similar spectra and underlying plasma charge state distributions are obtained when keeping the ratio of 1- to $2 − μ m$ laser intensities fixed at a value of 2.1(6), which is in good agreement with ralef-2d radiation-hydrodynamic simulations. Our experimental findings, supported by the simulations, indicate an approximately inversely proportional scaling $\sim λ^{- 1}$ of the relevant plasma electron density, and of the aforementioned required drive laser intensities, with drive laser wavelength $λ$ . This scaling also extends to the optical depth that is captured in the observed changes in spectra over a range of droplet diameters spanning 16–51 $μ m$ at a constant laser intensity that maximizes the emission in a 2% bandwidth around $13.5 nm$ relative to the total spectral energy, the bandwidth relevant for EUV lithography. The significant improvement of the spectral performance of the $2 − μ m$ - versus $1 − μ m$ driven plasma provides strong motivation for the development of high-power, high-energy near-infrared lasers to enable the development of more efficient and powerful sources of EUV light.

Received 23 July 2020
Accepted 26 February 2021

DOI:https://doi.org/10.1103/PhysRevResearch.3.013294

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

High-energy-density plasmas Plasma production & heating by laser beams, laser-foil, laser-cluster Plasma sources

Plasma Physics

Authors & Affiliations

R. Schupp ¹, L. Behnke^1,3, J. Sheil¹, Z. Bouza^1,3, M. Bayraktar ², W. Ubachs^1,3, R. Hoekstra^1,4, and O. O. Versolato^1,3,*

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

^*o.versolato@arcnl.nl

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Vol. 3, Iss. 1 — March - May 2021

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Plasma Physics

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Figure 1
Schematic representation of the experimental setup. A master oscillator power amplifier (MOPA) setup, comprising an optical parametric oscillator (OPO) and an optical parametric amplifier (OPA), is pumped by a Nd:YAG laser (blue line). The signal beam is separated via polarization optics, and the idler beam ( $λ = 2.17$ $μ m$ ) is focused onto tin microdroplets within a vacuum chamber. EUV emission is captured by a transmission grating spectrometer positioned at $60^{\circ}$ with respect to the laser axis. An additional, third KTP crystal (dashed outline) was used in the OPA in a subset of the experiments.
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Figure 2
Spectra from tin droplet plasma observed at various intensities of the (a) $2 − μ m$ beam (4.7 ns pulse duration, 106 $μ m$ FWHM, 3 KTP crystals) and the (b) $1 − μ m$ Nd:YAG beam (10 ns pulse duration, 86 $μ m$ FWHM). The droplet size in both cases is 30 $μ m$ . (c) Ratio of the intensities of 1- and $2 − μ m$ driven laser beams needed to obtain spectra with similar spectral features. The data points represent the average intensity ratios from data taken with four different laser spot sizes of the 2 $μ m$ laser beam of $65 \times 88$ , 106, 152, and 194 $μ m$ (FWHM), respectively. The error bars indicate the standard deviation per measurement. The red line represents the average over all data points, and the shaded band represents the standard deviation of the average.
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Figure 3
Top: maximum temperature of a tin plasma for various laser intensities calculated with the two-dimensional-radiation transport code ralef-2d. A $30 − μ m$ -diameter droplet is illuminated with temporally and spatially Gaussian-shaped laser pulses of wavelengths 1 and 2 $μ m$ . Center: temperature and electron density lineout along the axis of the incoming laser beam. Bottom: frequency-integrated local radiation field intensity $I_{rad}$ of the plasma and its normalized derivative $d I_{rad}$ . The radiation field intensity is calculated from Eq. (3) using the density and temperature lineouts depicted in the center panel. For more details, see the text.
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Figure 4
Spectral emission from tin plasmas produced with 1- and $2 − μ m$ laser wavelength for small and large droplet diameters at laser intensities of 2.4 and $1.1 \times 10^{11} W / {cm}^{2}$ , respectively.
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Figure 5
Spectrum produced with $2 − μ m$ laser light (red line) compared to the radiation-transported reference spectrum for a peak optical depth value of $τ_{p} = 2.2$ (gray line, barely distinguishable from the red line). Reference and $2 − μ m$ driven spectra were both obtained using a droplet diameter of 30 $μ m$ . Also shown is a spectrum obtained using a $10 − μ m$ ${CO}_{2}$ laser that represents the case of small optical depth (reproduced from Ref. [41]).
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Figure 6
(a) Dependency of peak optical depth $τ_{i, p}$ on droplet diameter for 5- and 10-ns laser pulse duration at $1 − μ m$ wavelength and for 4.3-ns pulse duration at $2 − μ m$ wavelength. Circles indicate the Gaussian spatial laser profile and boxes indicate a homogeneous “flattop” laser beam profile. Peak optical depth is fitted with respect to the spectrum obtained at $1 − μ m$ wavelength, 10-ns pulse duration, and $30 − μ m$ droplet diameter with an optical depth of $τ_{0, p} = 4.5$ . (b) Experimental values for spectral purity (SP) vs peak optical depth. The dashed line represents SP as calculated from the radiation-transported reference spectrum. The diamond symbol indicates the SP value of the radiation-transported reference spectrum for a peak optical depth value $τ_{i, p} = 0.4$ (close to the optically thin case), obtained from a comparison of the reference spectrum with the emission of the ${CO}_{2}$ -laser-driven plasma spectrum illustrated in Fig. 5.
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Characterization of 1- and 2−μm-wavelength laser-produced microdroplet-tin plasma for generating extreme-ultraviolet light

R. Schupp, L. Behnke, J. Sheil, Z. Bouza, M. Bayraktar, W. Ubachs, R. Hoekstra, and O. O. Versolato

Phys. Rev. Research 3, 013294 – Published 31 March 2021

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Characterization of 1- and $2 − μ m$ -wavelength laser-produced microdroplet-tin plasma for generating extreme-ultraviolet light