J. Plasma Fusion Res. SERIES, Vol.7 (2006) 73-76

Observation of Xenon EUV Spectra in Compact Helical System
SUZUKI Chihiro, NISHIMURA Hiroaki1 , SONG Mi-Young, KATO Takako, OKAMURA Shoichi,MORE Richard M., NISHIHARA Katsunobu1 , NAKAI Mitsuo1 , SHIGEMORI Keisuke1 ,FUJIOKA Shinsuke1 , OGAWA Hiroaki2 and SASAKI Akira3
National Institute for Fusion Science, Toki 509-5292, Japan
1 Institute of Laser Engineering, Osaka University, Suita 565-0871, Japan
2 Naka Fusion Research Establishment, Japan Atomic Energy Research Institute, Naka 311-0193, Japan
3 Kansai Research Establishment, Japan Atomic Energy Research Institute, Kizu 619-0215, Japan（Received: 4 October 2004 / Accepted: 28 April 2005）

Abstract
We have carried out spectroscopic measurements of xenon (Xe) extreme ultraviolet (EUV) emissions inplasmas produced by a medium-size fusion-oriented torus device, Compact Helical System (CHS), for thedevelopment of reliable spectroscopic data of Xe ions. The EUV spectra have been measured in low density(<1019 m−3 ) hydrogen plasma with Xe gas injection by an existing grazing incidence spectrometer. Electrondensity and temperature profiles are measured by Thomson scattering diagnostic. Observed spectral lines arecompared with the results of HULLAC code calculation and charge exchanging collisions experiment. Thecharge states of Xe ions and transitions are inferred for several prominent peaks.
Keywords:
EUV light source, lithography, xenon, CHS, grazing incidence spectrometer, HULLAC
1．Introduction
Optical lithography is one of the essential processes
in the fabrication of semiconductor devices. According to the technological target for the next generation
lithography process, bright extreme ultraviolet (EUV)
light source for the wavelength of 2 % bandwidtharound 13.5 nm is indispensable for processing patterns
less than 50 nm. Though laser-produced xenon (Xe)plasma is one of the promising candidates for the EUV
light source [1,2], construction of theoretical model for
detailed atomic processes and benchmarking with various experimental data of Xe ion spectra are required
to optimize the eﬃciency of the light source. However, the laser-produced plasmas are unsuitable for thebenchmarking since detailed measurements of plasma
density and temperature profiles are generally diﬃcultdue to their sharp spatial and temporal variations. On
the other hand, magnetically confined plasmas utilized
for fusion research have relatively mild temperature anddensity gradients measured by reliable diagnostic tools.
In this study, we have carried out spectroscopic measurements of Xe EUV emissions in discharge plasmas
produced in a medium-size fusion-oriented torus device, Compact Helical System (CHS) [3], to contributeto the development of reliable spectroscopic data of Xeions. Though electron densities in magnetically con-fined plasmas are generally much lower than those inlaser produced ones, they are more suitable for spectroscopic data analyses because of easier control of plasma
parameters and dominance of line spectra. In this article
we concentrate on the experimental setup and preliminary results of the spectroscopic measurements by anexisting spectrometer, and detailed discussions on physical processes will be reported elsewhere.
2．Experimental details
Compact Helical System (CHS) is a medium-size
torus plasma device utilized for fusion research at National Institute for Fusion Science (NIFS) [3]. A top
view of the standard boundary configuration of CHS
plasma is shown in Fig. 1(a). The shape of the plasma
cross section is like an ellipse which is twisted 8 times
during one toroidal rotation. The average major and minor radii of the torus plasma are 1 m and 0.2 m, respectively. In this study, the plasmas are produced by electron cyclotron resonance heating (ECH) using a 53 GHz
gyrotron with the maximum duration of 100 ms. The
gyrotron power is injected vertically from the upper port
indicated in Fig. 1(a), and the plasmas are confined by amagnetic field of 0.88 T (at the plasma center).
Electron density and temperature profiles are meaC 2006 by The Japan Society of Plasma

Science and Nuclear Fusion Research

Corresponding author’s e-mail: csuzuki@nifs.ac.jp73Observation of Xenon EUV Spectra in Compact Helical System(a)vacuumSpectrometer
Viewline

Xe gas feedair
CCD Sensor+ Detector Head

Incoming lightfrom grating

SerialController

FOPSCSI

torus centerPC

Chevron MCP+ Phosphor screen
Vacuum Flange

ECH powerinjection(b)

PlasmaFig. 2 Schematic diagram of the detector system of thespectrometer.

0.5 m

the spectral resolution.
Schematic diagram of the detector system is illustrated in Fig. 2. The Micro Channel Plate (MCP) assembly (Burle, 3040FM) consists of a Chevron type MCP
of 40 mm diameter placed along the focal plane. Amplified electrons are transferred into a visible spectral
image by a P20 phosphor screen placed behind the exitof the MCP. The spectral image is subsequently reduced
to 25 mm diameter via a fiber optic plate (FOP), andthen detected by a Charge Coupled Device (CCD) linear
image sensor (Hamamatsu, S7010) with the minimum
readout time of 10 ms. The pixel number of the CCD
array is 1024×252, and pixel charges are summed up
vertically by line binning. The analog data output of theCCD are converted to digital data by a CCD controller(Hamamatsu, C7557) which is controlled remotely by apersonal computer. The width of the entrance slit of thespectrometer is fixed at 10 µm in this study. In spite ofthe resolving power of the grating, overall spectral resolution is about 0.3 nm determined mainly by the spreadof electron clouds at the exit of the MCP.

Vacuum vessel

Viewing area

1 m from thetorus center

0.2 m

Fig. 1 (a) Top view of the standard conﬁguration of Compact Helical System (CHS) plasma. ECH power is
injected vertically from the upper viewport.
(b) Crosssectional view of the plasma and theviewline of the spectrometer.

sured by an existing Nd:YAG laser Thomson scattering
diagnostic system [4]. Line-averaged electron density ismeasured by a millimeter wave interferometer. Typicalelectron density of the ECH plasma is below 1019 m−3 ,which means that the plasma is optically thin. Electron temperature is typically 1 keV at the plasma center,
while much lower near the plasma edge.
We have used a flat field grazing incidence spectrometer (Shinku-Kogaku, model JYF-306) installed in CHS
for impurity monitoring. The viewline of the spectrometer is along the equatorial plane within the horizontallyelongated cross section as shown in Fig. 1(b). Therefore the measured spectra correspond to line integrated
emissivity along the viewline passing through the center and the edge of the plasma. The grating of thespectrometer is made of platinum coated toroidal concave blank to obtain flat focal image of 40 mm length
within the spectral range of 10-110 nm. Groove density and focal length of the grating are 450 mm−1 and306 mm, respectively. Though the active area size ofthe grating is 8 mm×30 mm, the available area is limited to 3 mm×22 mm by a masking plate to maximize

3．Preliminary results and analyses
We have already measured VUV spectra of several discharge schemes of pure Xe plasma or hydrogen plasma with Xe gas injection. In this article we
only discuss on one representative discharge of the latter case in which relatively strong Thomson scattering
signals can be obtained. Temporal variations of plasma
parameters in this discharge are displayed in Fig. 3. The
ECH power of 193 kW is applied during the periodt = 20–120 ms in low pressure (<10−2 Pa) hydrogengas atmosphere. Xe gas whose density is equivalent to
4.8×1017 m−3 in the vacuum vessel is puﬀed during theperiod of t = 50–60 ms. Line averaged electron density
is kept constant in 50–80 ms, then gradually decreased
until the termination of the discharge. Temporal behaviors of central electron temperature and density mea-74Observation of Xenon EUV Spectra in Compact Helical System(a)1Xe XI(4d-4f)

Xe injection

70000

ne60000

ECH50000

050


2Time(ms)100

Counts0Te (keV), ne (1019 m-3)

Xe X
(4d-4f)0.5

e<n > (1019-3m )1.5150

(b)

1.540000

H2 ECH+ Xe puff @50ms
Xe XI(4d-5p)40-50ms
70-80ms
100-110ms

Xe X(4d-5p)

30000

Xe IX
(4d-5p)

20000

Te(0)

110000ne(0)0.50050Time (ms)

100

1015

202530

Wavelength (nm)150

Fig. 4 EUV spectra measured before the Xe injection(40 50 ms), after the injection (70 80 ms), and in
the decay phase (100 110 ms).

Fig. 3 Temporal variations of plasma parameters analyzed in this study. (a) Line averaged electron
density measured by an interferometer, and (b)temporal variations of central electron temperature and density measured by Thomson scattering diagnostic. Gray regions are the time intervals
corresponding to the spectra shown in Fig. 4.

the results of HULLAC code and charge exchanging
collisions experiment [5]. It is widely believed that lineemissions from Xe XI (Xe10+ ) would be important inemissivity around 13.5 nm. Charge states and transitions of Xe ions for several prominent spectral peaks
inferred from the results of the comparisons are also
indicated in Fig. 4. The results of the HULLAC code
predicts that 4d–5p transitions of Xe XI would appearin 13.0-13.7 nm, and 4d–4f in 10.8-10.9 nm [6]. Thesetransitions correspond to the strongest two peaks of thespectra. In addition, lines from diﬀerent charge states(Xe X and IX) are also found in Fig. 4. According to theHULLAC code, a line at 11.2 nm blended with Xe XI
4d-4f peak is assigned to Xe X (4d-4f). If we compare
the last two spectra (70–80 and 90–100 ms), it is clearlyobserved that the intensity ratio between Xe XI and Xe
X changes temporally, which means that the principal
charge numbers of Xe ions in the plasma become lower
with the decrease in the electron temperature.

sured by the laser Thomson scattering system are shownin Fig. 3(b). The central electron temperature reaches
above 1 keV before the Xe injection. After that, thetemperature is rapidly decreased and is below 100 eV
at 100 ms since the plasma is cooled by radiation loss.
This implies that the plasma is in recombining phase after the Xe injection. Electron temperature profile is hill
shape, while the density profile is almost uniform until
80 ms.
EUV spectra integrated during each 10 ms interval
have been measured since the minimum readout time ofthe detector is 10 ms. Figure 4 shows the EUV spectra
around 13.5 nm obtained at three diﬀerent time periodsin this discharge. Spectra before the Xe injection (40–50 ms), after the injection (70–80 ms), and in the decayphase (100–110 ms) are displayed. The 2 % bandwidthof 13.5 nm is indicated by the narrow gray region inFig. 4. Before the Xe injection (40–50 ms), there are
no Xe lines and only several oxygen lines (O V) arefound as an impurity. In the recombining phase after theXe injection, the Xe spectra begins to build up around60 ms, and the brightness becomes maximum during
the period of 70–80 ms, in which the spectrum is indicated by a thick solid line in Fig. 4. The strongest peakaround 11 nm and a moderate peak around 13.3 nm areclearly found, which is known to be characteristic structure of Xe ion EUV emissions in other experiments.
When the temperature decreases further, Xe emission
becomes weak as shown by a thin solid line in Fig. 4(100–110 ms).
The measured spectral data has been compared with

4．Summary and future plans
We have observed Xe EUV spectra around 13.5 nm
in high temperature and low density CHS plasmas with
Xe gas injection by the grazing incidence spectrometer.
Characteristic spectral peaks of Xe ions in the EUV region were found in the recombining phase of the plasma
after the Xe injection. Based on the comparisons withthe results of HULLAC code calculation and charge exchanging collisions experiment, charge states and transitions of Xe ions were inferred for several peaks in thespectra.
Before proceeding to the next experimental phase,
there are several matters to be considered. The optimum electron temperature to maximize the emissivityof Xe XI is believed to be about 30 eV by several model75

Observation of Xenon EUV Spectra in Compact Helical System

calculations. Hence the Xe emissions in this discharge
are expected to come from the plasma edge since thecentral electron temperature is too high as shown inFig. 3. Since it is diﬃcult to measure such low temperature region by the existing Thomson scattering system optimized for high temperature plasma, other diagnostics such as probe measurements should be applied to lower density and temperature plasmas generated by low-power heating methods. Since the viewlineof the spectrometer includes both the center and edgeof the plasma, we also plan to install another spectrometer with scannable viewline to clarify where the strong
emissions of Xe ions come from. Additionally, thepoor spectral resolution of the present system should be
improved to prevent uncertainties of the spectral lines
identification for the detailed analyses. We will employ
another spectrometer with narrower wavelength range
and better spectral resolution using soft X-ray CCD detector in the near future.
These improvements will provide more detailed in-formation on characteristics of Xe EUV spectra in CHS,
which make it easier to benchmark with the model calculation based on the line intensity analyses.
Acknowledgements
Authors would like to thank the members of CHS
experimental group, especially Dr. T. Minami for providing Thomson scattering diagnostic data.

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