九州大学大学院総合理工学府報告

Engineering Sciences Reports, Kyushu University

第 40 巻 第 2 号 11-15 頁 平成 31 年 2 月

Vol. 40, No. 2, pp. 11-15, FEB, 2019

Observation of the whole Thomson scattering spectrum oflaser-produced plasmaｓ for EUV light sources
Yuta SATO*1,† Raimu FUKADA*1 Fumitaka ITO*1
Kentaero TOMITA*2 and Kiichiro UCHINO*2†E-mail of corresponding author:3ES16011G@s.kyushu-u.ac.jp(Received December 29, 2018, accepted December 30, 2018)
The whole spectrum including the ion feature and the electron feature of collective Thomsonscattering from laser-produced tin plasmas for EUV light sources has been observed for the first time.
The purpose is to realize the evaluation of electron density, electron temperature, ion temperature
and average ionic charge of the plasmas at the same time. Since the intensity of the electron feature isvery weak compared with self-emissions from the plasma, efforts were concentrated to detect the peakwavelength of the electron feature. By applying the spectrometer system appropriately designed andoptimizing the scattering geometry, it was achieved to detect the peak of the electron feature with thesignal-to-noise ratio larger than one.
Key words: EUV lithography, laser Thomson scattering

1.

Introduction

Extreme ultraviolet (EUV) lithography
is a promising technology for high-volume
manufacturing
ofnext-generation
semiconductor
devices.1-4 Laser-producedtin (Sn) plasma is being considered as astrong candidate of EUV light sources. A
carbon dioxide (CO2) drive laser and a Sn
droplet target are employed as an efficient
EUV light source. In order to put it into
practical use, the primary challenge
involves the improvement of the conversionefficiency (CE) of EUV light sources withrespect to the output of CO2 lasers. Plasma
parameters, such as electron density (ne),electron temperature (Te), and average
ionic charge (Z) 5,6 are important because
the EUV emissivity strongly depends onthem. In order to measure theseparameters, a collective laser Thomson
scattering (LTS) system, which is tuned todetect the ion feature spectra, has beendeveloped.7,8Thissystem
revealed
time-resolved two-dimensional profiles of ne,
Ti, and Z of Sn plasmas.9,10 Here, we
assumed Ti = Te. The ion feature is notenough to fix the four parameters, because
the ion feature has only three information,*1 Department of Applied Science for Electronics andMaterials, Graduate Student*2 Department of Electrical and Material Sciencei.e., absolute signal intensity, a width
between two peaks and a dip between them.
In order to evaluate the four parameters
without any assumption, simultaneous
detections of the ion feature and theelectron feature are required. Therefore, wetried to detect both the ion feature and theelectron feature of the Thomson scattering
spectrum from the plasma which has theparameters of the EUV light source plasma.
To detect both features, there are twothings which should be considered. One isdifferences in spectral widths of the twofeatures. An estimated spectral width of theion feature from the typical EUV lightsource plasma (ne, Te, Ti, and Z are 1024 m-3,
30 eV, 30 eV, and 10, respectively) is 200 pm.
On the other hand, the width of the electronfeature is estimated to be 10 nm. To realize
simultaneous measurements, we decided tofabricate two different spectrometers which
are suitable for the ion feature and theelectron feature. The other problem is avery small intensity of the electron featurecompared to the noise level of thebremsstrahlung
radiation
fromtheplasma.11-13 Because the peak wavelength ofthe electron feature of collective Thomson
scattering gives information about theelectrondensity
andtheelectrontemperature, we concentrated to detect the12

Observation of the whole Thomson scattering spectrum of laser-produced plasmas

peak wavelength of the electron feature inthis study. By optimizing the scatteringgeometry, we finally succeeded to detect thepeak of the electron feature with thesignal-to-noise ratio larger than one.

2.

Collective Thomson scattering

3.

Experimental setup

The theory of Thomson scattering has
been extensively described in Ref. 14. Here
we only describe the principle to the extent
necessary to understand the situation ofLTS for this study. When a visible laser isused as a probing laser, the predicted LTS
spectra from laser produced Sn plasmas lie
in the collective regime, in which thescattering parameter α is larger than 1 [α =(kλD) −1, where λD is the Debye length and kis the wavenumber of the scattered light].
In the collective regime, a LTS spectrum
consists of an ion feature and an electronfeature.
For the case of ion feature, the shapereflects the ion-acoustic-wave frequency
𝜔𝑎𝑐 = 𝑘√(𝑎2 /1 + 𝑎2 )(𝑍𝜅𝑇𝑒 + 3𝜅𝑇𝑖 /𝑚𝑖 ),where  is the Boltzmann constant, mi is
the ion mass. The spectrum also exhibits
two peaks with a dip between them. Thewidth between the two peaks is related tothe probing laser wavelength λ0, the speedof light c, and ωac, and is expressed as
2Δλpeak-i [𝛥𝜆peak−i = (𝜆20 /2𝜋𝑐)𝜔𝑎𝑐 ] .
Bydetermining Δλpeak-i and the spectral shape,
which is characterized by ion-acoustic wave
damping, a product of Z and Te (ZTe), and Ti
can be obtained. In addition, it is possible toestimate ne as a zeroth order value by
performing the absolute calibration of
signal intensity, because the intensity of
the ion feature is strongly related to ne andis proportional a dynamic form factors Si =𝑍𝑎4 /[(1 + 𝑎2 ){1 + 𝑎2 (1 + 𝑍𝑇𝑒 /𝑇𝑖 )}] . For thecase of the electron feature, the spectrum isalso characterized by two peaks, and theangular frequency at which the peakappears
isexpressedas∆ω =√𝑛𝑒 𝑒 2 /𝑚𝑒 𝜀0 + 3𝜅𝑇𝑒 𝑘 2 /𝑚𝑒 , where ε0 is thedielectric constant of vacuum. The
wavelength difference (Δλpeak-e) from λ0 is
expressed as 𝛥𝜆peak−e = (𝜆20 /2𝜋𝑐)𝛥𝜔 . The
intensity of the electron feature is alsostrongly related to ne and is proportional a
dynamic form factors𝑆𝑒 = 1/(1 + 𝑎2 ).

Figure 1 shows a schematic diagram ofthe experimental setup. The x, y, and z axes
are defined in Fig. 1. Two different lasers
were used to generate plasma. A first laser
was a Nd:YAG laser (Continuum Surelite II,pulse width: 10 ns full width at halfmaximum (FWHM), laser energy: 320 mJ,wavelength: 1064 nm, typical spot
diameter: 200 µm) and propagated in the zaxis. Sn plasma was produced by the firstlaser. A second laser was also a Nd:YAGlaser (Continuum Surelite II, pulse width:
10 ns full width at half maximum (FWHM),
laser energy: 320 mJ, wavelength: 1064 nm,
typical spot diameter: 80 µm), which wasused to heat the plasmas. In this paper, werefer to the first and the second lasers as
the driving laser and heating laser,respectively. After the Sn plasma washeated by the heating laser, a probing laser
was injected into the plasma for LTS
measurements. The probing laser was asecond-harmonic Nd:YAG laser withinjection seeding (Continuum Powerlite
9010 with an injection seeder; spectral
spread <0.1 pm, pulse width: 8 ns FWHM,
laser energy: 10 mJ, wavelength λ0 = 532
nm, typical spot diameter: 50 µm). The time
zero (t = 0 ns) for the LTS measurements
was the peak of the driving laser. The
heating and the probing lasers wereinjected at t = 97 ns and t = 100 ns,respectively. The two lasers were injectedco-axially along the x axis and were focusedat the same position above the target
surface by an achromatic lens with thesame focal length (f = 150 mm). A tin platetarget was set in the vacuum chamber(<10-2 Torr) at z = -150 µm and a thin edge
face (thickness 100 m) was used as theFig. 1

Schematic diagram of experimental setup平成 30 年度九州大学大学院総合理工学報告

laser-irradiated surface. This thickness wasselected so as to reduce the amount of theplasma self-emission.
The ion feature and the electron feature
were collected by two different achromaticlenses at angles of 135° and 55° withrespect to the incidence angle of the probing
laser, respectively, as shown in Fig.1. Then,the LTS signals were focused on theentrance slit of the two spectrometers forthe ion feature and the electron feature.
The spectrometer for the ion feature wassame as that described in Ref. 9. The
spectrometer having 6 gratings achieved a12 pm spectral resolution and a sufficient
stray-light rejection (10-4) with a very
narrow wavelength block range [within ±14
pm from λ0 (= 532 nm)]. The gate width of adetector, which was an intensified
charge-coupled device (ICCD) camera(quantum efficiency: 35% at 532 nm), wasset to 3 ns. A conventional triple-grating
spectrometer (TGS) was utilized forelectron feature measurements. The TGS
mainly consisted of six achromatic lenses(L1: f = 250 mm, L2-L5: f = 170 mm, L6: f =220 mm effective diameter: 46 mm), threediffraction gratings (1200 grooves/mm,effective area: 56 mm × 56 mm), an
entrance slit (width: 60 μm), anintermediate slit (width: 200 μm), and athin tungsten wire (diameter: 500 μm) toblock the stray light. Another ICCD camera(quantum efficiency: 45 % at 532 nm, gate
width: 2ns) was used as a detector. Withthe TGS, a spectral resolution of 0.37 nm
and a sufficient stray-light rejection at ±1.5nm from λ0 were achieved. We note that theabsolute calibration of both the ion featureand electron feature was performed byRayleigh scattering measurements from
nitrogen gas.

4.

第 40 巻第2号

13

the electron feature was divided into 2
optical paths using a polarization beamsplitter. Both signal beams were focused on
an ICCD camera. The vertically polarized
beam contained the plasma self-emission
spectrum including the electron feature andits image is shown in Fig. 3(a). The
horizontally polarized beam contained theplasma self-emission spectrum alone andits image is shown in Fig. 3(b). It must be
noticed that the transmission efficiency ofthe spectrometer for the vertically polarized
lights are about 2 times larger than that for
the horizontally polarized lights. Therefore,
the intensity spectrum for Fig. 3(b) wascorrected by this factor and compared withthat for Fig. 3(a) in Fig. 3(c). From Fig. 3(c),it can be seen that the intensity of thespectrum from Fig. 3(a) is slightly larger
than that from Fig. 3(b) at around Δλ =13nm. Figure 3(d) shows a spectrum obtainedby the subtraction of the two spectra. The
Te was decided from the peak wavelength ofFig. 3(d) and the ne estimated by the ion
feature as the first step. Since the scatteredFig. 2 (a) Ion feature image, (b) ion feature spectrum
extracted from (a) and the theoretical curve fit

Results

Figure 2(a) shows the ion feature image
detected by the ICCD camera. Figure 2(b)shows the LTS spectrum extracted from theFig. 2(a). The spectrum was successfully
fitted by a theoretical curve, as shown in
Fig. 2(b). From the fitting curve and theabsolute calibration, ne, ZTe, and Ti were
fixed to be 1.0×1024 m-3, 271, and 25 eV,
respectively.
At the final stage of the spectrometer forFig. 3
Images of (a) the electron feature and theself-emission, and (b) the self-emission. (c) Spectra extracted
from (a) and (b). (d) Spectrum of the electron feature.

14

Observation of the whole Thomson scattering spectrum of laser-produced plasmas

light intensity of the ion feature weakly
depends on Te, several iterations wererequired to decide the values of ne and Te.
Finally, ne, Te, Ti, and Z were decided to be
1.3×1024 m-3, 33 eV, 25 eV, and 8.2,respectively.

5.

Discussion

First, it was confirmed whether theelectron feature intensity shown in Fig. 3(d)is consistent with the intensity of the ionfeature. As described before, each intensity
is proportional the dynamic form factors,
therefore a ratio of the calibrated signal
intensity must be equal to the ratio
between Si and Se. Since the ion feature andthe electron feature were detected bydifferent detectors, absolute calibration wasperformed by Rayleigh scattering. As aresult of the comparison, the signal
intensity of the electron feature was about
half of what was anticipated. We found that
this is due to the vignetting reduction in thespectrometer and confirmed that the signal
intensity at the Δλ = 12 nm becomes about
half of the signal at the center wavelength.
Therefore, we conclude that the observed
electron feature is plausible.
Second, the signal-to-noise ratio (SNR) of
the electron feature is considered. Whenthe present ICCD camera is used as thedetector, a portion of the photons
illuminating the photo-cathode is converted
to photo-electrons (pe), which areaccelerated toward the micro-channel-plate(MCP). These multiplied electrons impingeon a phosphor screen and they emit photons.
These multiplied photons are converted toelectrons by the charge-coupled-device(CCD). The electronic gain through theMCP is estimated to be as high as 10,000.
Finally, a single photo-electron is output as
400 analog-to-digital units (ADU). Here, we
calculate the number of photo-electrons
detected under the measurements of theelectron component of Thomson scattering.
As shown in Fig. 3, the peak intensity of theelectron component observed at around Δ λ= 12 nm was 6000 ADU. This signalintensity is equivalent to 15 pe. The
dominant noise, in this case, is the shotnoise which is due to the statisticalfluctuation of single photo-electrons and isdetermined by the square root of the penumber. As shown in Fig. 3(c), the total
signal intensity of the plasma radiation at
Δλ = 12 nm is the order of 30,000 ADU. The
shot noise based on both the Thomson
scattering and the plasma radiation iscalculated to be 11 pe. Therefore, the SNR
is higher than one.
Third, we checked the value of Te
obtained by this experiment. The results
show that Te was 1.3 times higher than Ti.
To discuss the validity of the difference, theelectron heating by absorbing the probing
laser (λ = 532 nm) is estimated. The method
to evaluate the increment of Te has been
already discussed in previous papers15-18, in
which the processes of energy absorption by
the inverse-bremsstrahlung and the heatdiffusion to the volume surrounding thelaser beam. Based on the calculations
shown in Ref. 15, 16, it is found that anincrease in Te is at most 0.3 eV, which isless than 1% of the measured Te. Then, weconcluded that the effect of electron heatingvia the probing laser can be negligible.
Therefore, we finally concluded thesignal observed here is really that ofelectron feature and gives a proper value ofTe to be 33 eV.

6.

Conclusion

1.

U. Stamm, I. Ahmad, I. Balogh, H. Birner, D.
Bolshukhin, J. Brudermann, S. Enke, F. Flohrer,
K. Gäbel, S. Götze, G. Hergenhan, J.
Kleinschmidt, D. Klöpfel, V. Korobotchko, J.
Ringling, G. Schriever, C. D. Tran, and C. Ziener,
Proc. SPIE 5037, 119 (2003).
T. Higashiguchi, K. Kawasaki, W. Sasaki, and S.
Kubodera, Appl. Phys.Lett. 88, 161502 (2006).
K. Koshelev, V. Krivtsun, V. Ivanov, O.
Yakushev, A. Chekmarev, V. Koloshnikov, E.
Snegirev, and V. Medvedev, J. icro/Nanolithogr.
MEMSMOEMS 11, 021103 (2012).
H. Mizoguchi, H. Nakarai, T. Abe, K. M. Nowak,

In this study, the ion feature and theelectron feature of the laser-produced Sn
plasma having parameters necessary for
EUV
light
sources
weredetected
simultaneously for the first time. The
electron feature was observed with a SN
ratio more than one, and found to beconsistent with the ion feature giving aproper value of Te.

References

2.
3.

4.

平成 30 年度

5.

6.

7.

8.

9.

10.

11.

12.

13.
14.

15.

16.
17.
18.

九州大学大学院総合理工学報告

Y. Kawasuji, H. Tanaka, Y. Watanabe, T. Hori, T.
Kodama, Y. Shiraishi, T. Yanagida, G.
Soumagne, T. Yamada, T. Yamazaki, S. Okazaki,and T. Saitou, Proc. SPIE 9422, 94220C (2015).
K. Nishihara, A. Sunahara, A. Sasaki, M.
Nunami, H. Tanuma, S. Fujioka, Y. Shimada, K.
Fujima, H. Furukawa, T. Kato, F. Koike, R. More,M. Murakami, T. Nishikawa, V. Zhakhovskii, K.
Gamata, A. Takata, H. Ueda, H. Nishimura, Y.
Izawa, N. Miyanaga, and K. Mima, Phys.
Plasmas 15, 056708 (2008).
A. Sasaki, A. Sunahara, H. Furukawa, K.
Nishihara, S. Fujioka, T. Nishikawa, F. Koike, H.
Ohashi, and H. Tanuma, J. Appl. Phys. 107,113303 (2010).
K. Tomita, Y. Sato, K. Nishikawa, K. Uchino, T.
Yanagida, H. Tomuro, Y. Wada, M. Kunishima,T. Kodama, and H. Mizoguchi, Appl. Phys.
Express 8, 126101 (2015).
K. Tomita, K. Nakayama, K. Inoue, A. Sunahara,and K. Uchino, Appl. Phys. Express 6, 076101(2013).
Y. Sato, K. Tomita, S. Tsukiyama, T. Eguchi, K.
Uchino, K. Kouge, H. Tomuro, T. Yanagida, Y.
wada, M. Kunishima, T. Kodama, and H.
Mizoguchi, Jan. J. Appl. Phys. 56, 036201 (2017)
K. Tomita, Y. Sato, S. Tsukiyama, T. Eguchi, K.
Uchino, K. Kouge, H. Tomuro, T. Yanagida, Y.
wada, M. Kunishima, T. Kodama, H. Mizoguchi,A. Sunahara, and K. Nishihara, Sci. Rep. 7,12328 (2017)
T. Morita, Y. Sakawa, K. Tomita, T. Ide, Y.
Kuramitsu, K. Nishio, K. Nakayama, K. Inoue, T.
Moritaka, H. Ide, M. Kuwada, K. Tsubouchi, K.
Uchino, and H. Takabe, Phys. Plasmas 20,092115 (2013).
E. R. Kieft, J. J. A. M. van der Mullen, G. M. W.
Kroesen, V. Banine, and K. N. Koshelev, Phys.
Rev. E 70, 056413 (2004)
E. R. Kieft, J. J. A. M. van der Mullen, and V.
Banine, Phys. Rev. E 72, 026415 (2005).
D. H. Froula, S. H. Glenzer, N. C. Luhmann, Jr.,and J. Sheffield, Plasma Scattering ofElectromagnetic Radiation (Academic Press,New York, 2011) 2nd ed., Chap. 5.
K. Tomita, S. Hassaballa, and K. Uchino: Denki
Gakkai Ronbunshi A 130 (2010) 1099 [inJapanese].
H. J. Kunze: Z. Naturforsch. A 20, 801 (1965).
Bentley R E, J. Phys. D: Appl. Phys. 30, 2880
(1997).
Murphy A B, Phys. Rev. Lett. 89, 025002 (2002).

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