S1-2 High Power EUV Sources Based on Discharge Produced Plasma
放電プラズマEUV光源開発研究

Sunao Katsuki, Takashi Sakugawa, Hidenori Akiyama
勝木 淳，佐久川貴志，秋山秀典
Graduate School of Science and Technology, Kumamoto University,
Kurokami 2-39-1, Kumamoto 860-8555, Japan
熊本大学大学院，〒860-8555 熊本市黒髪2丁目39番1号
We describe the basic character of xenon and tin plasmas generated by the z-pinch device developed at
Kumamoto University, using a time-resolved plasma imaging system and an interferometer. Also the EUV
emission of the plasma is evaluated with respect to the in-band EUV energy, spectrum, and angular
distribution of the emission. In addition, the experimental result on controlling the z-pinch plasmas byusing the external magnetic field is presented.

1. Introduction
Presently, the extreme ultraviolet (EUV)
emission of xenon, tin or lithium ions areintensively being investigated for its use in future
32 nm or less rule photolithography process. The"usable" in-band EUV power (13.5 nm 2%BW)
required for the practical lithography tool is 115 W.
which is really challenging task to obtain it. Highenergy density plasmas that emit EUV light are
produced by two concepts, namely laser-produced
plasmas (LPPs) and pinched discharge produced
plasmas (DPPs). So far, the DPP scheme has moreprogressed in terms of EUV output, and the value isgoing to be close to the requirement.
In DPP EUV sources with its highly repetitive
operation, a huge electrical power in the range oftens of kW is dissipated in the discharge chamber ofwhich volume is in the range of tens of cm3. The
power density at the inner surface facing to theplasma amounts to the order of 10 kW/cm2. The
thermal load at the chamber causes problems which
degrade the lifetime of the components including
electrodes, insulator wall as well as the lightcndenser optics. Most of the problems in DPP
sources attribute the thermal load. In order toreduce the thermal load, it is important to minimize
the energy input to the discharge and to improve theconversion efficiency (CE) from the discharge toin-band EUV energy, in addition to cooling thesystem. The experimentally demonstrated CE in thecase of tin target discharges is at most 2% so far.
The optimization of the plasma temperature anddensity for the 13.5 nm emission seems the mostpromising way to improve the CE if new target
materials and their combination for the emission are
not found.
The generation process of the high energy
density plasmas in the DPP scheme is relatively
flexible and dependent on the current waveforms,

target gas pressure, electrode geometry and initial
ionization state of the target material. Also, an
external magnetic field parallel to z axis has astrong influence on the process. The correctunderstanding of their roles in the plasmageneration process to control the plasma. Inaddition, in order to optimize the EUV output, amore specific understanding of the plasma characteris needed. There are a number of spectroscopic
investigations of the plasma. However, only a few
experiments on plasma diagnostics were performed
despite the plasma is the core of the EUV
technology.
Here, we describe the basic character of xenon
and tin plasmas generated by the z-pinch device
developed at Kumamoto University, using a laser
imaging system and an interferometer. Also theEUV emission of the plasma is evaluated withrespect to the in-band EUV energy, spectrum, andangular distribution of the emission. In addition, theexperimental result on controlling the z-pinch
plasmas by using the external magnetic field ispresented.
2. Z-pinch device

Z axisZ axisFig. 1 Z-pinch discharge based EUV light source installed at
Kumamoto University.

5 mmφFig. 2 Still photograph of the Z-pinched EUV plasma. View
from 30˚ for the z axis. The diameter of the discharge tube
is 5 mmφ.
5EUV energy per shot (mJ/sr)

3. Xenon and tin target plasmas
Tin target has significant potential for high CE at
13.5 nm. Several theoretical calculations[2] showthat predominantly 4d-4f transitions in a number ofadjacent ion stages (Sn8+-Sn13+) produce unresolved
transition arrays (UTAs) that are localized near 13.5
nm. We introduced Tin target to the z-pinch
discharge. Tin vapor including atoms and ions isproduced by the ablation of the solid tin rod (6 mm),which is due to the energy flux from the plasma.
Xenon or argon is used as a background gas. Figure
3 shows the EUV energy as a function of therepetition rate of the discharge. The repetition rate
influence the surface temperature of the tin rod
placed 15 mm far from the plasma. The EUVemission also depends on the distance between theplasma and the rod surface (~ 10s mm).

Needle-likepinched plasma

Xe 40 ccm + Sn rod
43
Xe 40 ccm
21plasmatin rodd = 15 mm

0050

100

150200

Repetition rate (pps)Fig. 3 Contribution of Tin emission on the EUV yield as afunction of pulse repetition rate.
3.5

EUV energy per shot (mJ/sr)

Figure 1 shows the cross sectional view of thez-pinch discharge device assembled together withthe final pulse compression circuit, which delivers
the pulsed current with amplitude of 35 kA andduration of 110 ns to the short circuit. Xenon gas
flowing into the chamber is controlled.
Figure 2 shows a still photograph of the pinched
Xenon plasma operating in 100 Hz. The plasma
appears a needle and spatially stable. The diameterand the length along z-axis are approximately 500µm and 5 mm, respectively in this condition.
The EUV emission from z-pinch discharge ischaracterized using a transmission spectrometer, acalibrated in-band EUV energy detector, and a fast
EUV detector. A time-resolved imaging based on
Schlieren method and interferogram, and Thomson
scattering system are employed to characterize theZ-pinch plasma.

50 Hz
32.5
2
1.5
1 Hz

10.5
0
0

100

200

300

400

500

600

700

Bz (gauss)

4. Effects of an external magnetic field
An external longitudinal magnetic field applied
to the z-pinch plasma makes the plasma implosion
softly, so that the plasma density and temperature,
which are strongly associated with the EUVspectrum, might be reduced. Therefore, the EUV
spectrum can be controlled by adjusting themagnetic field strength, in addition to lengthening
the duration of EUV emission. Besides, the soft
implosion helps suppressing plasma instabilities
and possibly changing the property of high energy
ions generated during the implosion process[2].
5. Prospect
Until the beginning of 2004, the most important
issue for the EUV source development was thesource output. However, the 3rd EUVL conference
held at Miyazaki in November 2004, some of major

Fig. 4 In-band EUV energy per shot as a function of amplitudeof the external magnetic field.

developers announced the output is no longer the topissue. The top issue is the lifetime of condenser optics.
The development of EUV sources is surely being
progressed.
Acknowledgments
This research is supported by NEDO, EUVA and21st COE program on Pulsed Power Science.
References[1] Gerard O’Sullivan, EUVL Source Workshop,
Antwerp 2003; available at www.sematech.org.
[2] Information obtained at 3rd EUVL Symposium,Miyazaki, Nov. 1~4, 2004.