S15-6
Efficient plasma EUV source and its application
プラズマEUV光源の高効率化とその応用
Takeshi Higashiguchi
東口武史
Utsunomiya University, Yoto 7-1-2, Utsunomiya, Tochigi 321-8585 Japan
宇都宮大学 〒321-8585	  宇都宮市陽東7-1-2
We demonstrate EUV and soft x-ray sources in the 2 to 7 nm spectral region related to the beyond EUV(BEUV) question at 6.x nm and a water window source based on laser-produced high-Z plasmas.
Resonance emission from multiply charged ions merges to produce intense unresolved transition arrays(UTAs), extending below the carbon K edge (4.37 nm). An outline of a microscope design for single-shotlive cell imaging is proposed based on a high-Z plasma UTA source, coupled to x-ray optics. We will
discuss the progress and Z-scaling of UTA emission spectra to achieve lab-scale table-top, efficient,high-brightness high-Z plasma EUV-soft x-ray sources for in vivo bio-imaging applications.

1. Introduction
Development of shorter wavelength sources inthe extreme ultraviolet (EUV) and soft x-ray
spectral regions has been motivated by theirapplication in a number of high profile areas ofscience and technology. Efficient, high-power
extreme-ultraviolet(EUV)
sourcesforsemiconductor lithography at 13.5 and 6.7 nmbased on laser-produced plasmas have been
demonstrated in high-volume manufacturing ofintegrated circuits (IC) having node sizes of 22nm or less [1]. Plasmas of the high-Z elements Snand Gd produce strong resonant emission due to4d−4f and 4p−4d transitions at 13.5 nm and 6.7nm, respectively, which overlap in adjacent ion
stages to yield an intense unresolved transition
array (UTA) in their spectra. The in-band
high-energy emission is thus attributable tohundreds of thousands of near-degenerateresonance lines lying within a narrow wavelength
range [2].
All are based on n = 4−n = 4 (4d−4f and 4p−4d)transitions that overlap to generate an intense UTA.
For efficient 13.5-nm operation, it is important toproduce an optimum plasma electron temperatureof 30-50 eV. The rare-earth elements of gadolinium(64Gd) and terbium (65Tb) produce strong emission
near 6.7 nm, which is maximized at electron
temperatures in the 100-120 eV range depending on
initial focusing conditions [3]. Because it moves toshorter wavelength with increasing atomic number,
Z, the n = 4−n = 4 UTA is expected to lie in thewater window if higher Z elements from 79Au to
83Bi are used [4].
In this presentation, we report the efficient
EUV-water window soft x-ray source by strong
UTA band emission in laser-produced high-Z

plasmas. Our proposed procedure for producing thewater window soft x-ray emission is expected to beefficient and scalable in output yield. We haveinitiated a number of experiments to explore how
this emission may be optimized in practice.
2. Z-scaling of efficient UTA emission:
Quasi-Moseley’s law
We show that the strong resonance UTAs ofNd:YAG LPPs for elements with Z = 50−83 obey aquasi-Moseley’s law [5]. Figures 1(a)−1(k) show
LPP emission spectra from high-Z metal targets.
The main UTA peak at 8.17 nm in the case of Ndclearly shifts to shorter wavelength with increasing
atomic number, 3.95 nm in the case of Bi. This
movement indicates the availability of a wide
wavelength range for a LPP light source. Opticallythinner LHD plasma spectra are shown in Figs. 1(l)−1(q). It should be noted that the electron
temperatures of LHD plasma were relatively low, ≤1 keV, but higher than in ps-LPPs. Comparing LPPand LHD spectra, the UTA widths in LHD spectra
are relatively narrower than in LPPs especially forlighter elements. Self-absorption effects are clearlyobserved in the case of ns-LPP for Nd due tooptical thickness. Although the n = 4−n = 4 UTA
transition peak was observed at 8.05nm in the LHDspectrum, the strongest 4d−4f transitions essentiallydisappear in the ns-LPP owing to self-absorption.
Because of their large transition probabilities,
resonant lines that are strong in emission alsostrongly absorb in underdense (ne < nc, where nc is
the critical electron density) or optically thickplasma conditions. An optically thinner plasma
reduces the self-absorption effects and increases thespectral efficiency of n = 4−n = 4 UTA emissions.

An approximated curve for ps-LPPs with apower-law scaling of the peak wavelength given by
λ = aR 1(Z − s) b in nm where a = 21.86 ± 12.09, b =1.52 ± 0.12, s = 23.23 ± 2.87 is the screening constant
while Slater’s rule gives s = 36−39.15 for 4d electrons,
and R is the Rydberg constant. This empirical law issurprisingly similar to Moseley’s law where a = 4/3, b= 2 and s = 1 were used to give the transition
wavelength of the Kα-line of characteristic x-rays.
∞−−∞

the use of a single laser beam and small focal spot
diameters. Under multiple laser irradiation, it isexpected that the highest CE will be achieved as
plasma expansion loss can be neglected in plasmas
from targets irradiated by solid-state laser pulses.
We demonstrate a high conversion efficiency for
EUV emission around 6.5−6.7 nm from multiple
laser beam-produced 1D spherical plasmas.
Multiply charged-state ions produce strong
resonance emission lines, which combine to yield
intense unresolved transition arrays (UTAs) in theelements Gd, Tb, and Mo. The maximum in-band
EUV conversion efficiency (CE) was observed tobe 0.8%, which is one of the highest values everreported due to the reduction of the plasma
expansion loss, as shown in Fig. 2 [6].
GdTbMoPlanar GdFig. 1. Time-integrated EUV emission spectra of theNd:YAG LPPs for (a) 83Bi, (b) 82Pb, (c) 79Au, (d) 78Pt, (e)75Re, (f) 74W, (g) 73Ta, (h) 68Er, (i) 65Tb, (j) 64Gd and (k)60Nd targets with 150-ps laser (red, solid line) and 8-nslaser (blue, dotted line), respectively. The measured
LHD spectra (green, solid) for (l) Bi, (m) Pb, (n) Au, (o)W, (p) Gd and (q) Nd targets, respectively. An emission
line at 3.4 nm is from impurity carbon ions. Intensities
were normalized at each maximum of the n = 4−n = 4UTAs. Solid arrows indicate peak position of n = 4−n =4 UTAs of ps-LPP and LHD spectra. An open arrow
indicates structure due to self-absorption.

3. Efficient BEUV source at 6.7 nm
Lateral expansion of the plasma causes kinetic
energy losses which reduce the energy available forradiation and is particularly important for small
focal spot diameters. For practical light source
development, it is important to establish theoptimum plasma parameters related to laser
irradiation conditions and construct a database ofcharacteristics of the plasma EUV sources. Inaddition, to compare with one-dimensional (1D)
numerical simulation, it is important to produce 1D
expanding plasmas by use of multiple laser beams
based on the laser inertial confinement fusion (ICF)
geometry. Laboratory scale experiments have, todate, only been studied under 2D conditions due toFig. 2. Dependence of CEs on laser intensities and target
elements for Gd (circles), Tb (rectangles), and Mo(triangles) with planar Gd targets (rhombuses) in Ref. [4].
The maximum CEs of 0.8% from Gd at 6.7 nm and Tb at
6.5 nm.

Acknowledgments
This work was performed with the support andunder the auspices of the NIFS Collaboration
Research program (Nos. NIFS12KLPF025 andNIFS13KLPH020) and the joint research project of
the Institute of Laser Engineering, Osaka University(under contract subject “2013A1-20”).
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and references therein.
[4] T. Higashiguchi et al.: Appl. Phys. Lett. 100 (2012)
014103[5] H. Ohashi et al.: Appl. Phys. Lett. 104 (2014)
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[6] K. Yoshida et al.: Appl. Phys. Exp. 7 (2014) 086202.