S1-3

Present Status of Extreme Ultraviolet Source Development
by Laser Produced Plasma
レーザープラズマEUV光源開発研究

H. Nishimura, S. Fujioka, Y. Tao, M. Yamaura*. Y.Shimada*, S. Uchida*, A. Sunahara*,
H. Furukawa* K. Nagai, T. Norimatsu, M. Murakami, K. Nishihara, N. Miyanaga, and Y. Izawa
西村博明，陶業争、藤岡慎介，山浦道照*，島田義則*、内田成明*、砂原淳*、古河裕之*、
ଥ井圭治、乗松孝好、村上匡且、西原功修、宮永憲明、井澤๓和Institute of Laser Engineering, Osaka University,2-6 Yamada-oka, Suita, Osaka 565-0871 Japan大‫ޥ‬大学レーザーエネルギー学研究センター、〒565-0871 吹田市山田丘2-6Institute for Laser Technology, 2-6 Yamada-oka, Suita, Osaka 565-0871 Japan
レーザー技術総合研究所、〒565-0871 吹田市山田丘2-6
Properties of laser-produced plasmas (LPP) are discussed to generate extreme ultra violet (EUV) light,
particularly at 13.5 nm in 2% bandwidth (BW), for use in the next-generation lithography. We present
some topics of the LLP-EUV source; dependence of conversion efficiency to the EUV light on laser
conditions for tin target, present understanding of EUV emission, and relevant experimental results

1. Introduction
Intense radiation emanating from laser-producedplasma has a wide range of applications including
radiography [1], a short-pulse probe for condensed
matter research [2], and lithography [3]. Amongthem, extreme ultraviolet (EUV) radiation has
recently attracted particular attention for use inproduction of the next generation semiconductor
devices. It is expected to generate over 115 W at
13.5 nm within 2% bandwidth at the repetition rateof 7-10 kHz [4]. In 2003 Leading Project (LP), of
the Ministry of Education, Culture, Sports, Scienceand Technology (MEXT) of Japan was started
dedicated for the EUV source development. The
main tasks of LP are(1) Acquisition of databases:
For practical application of EUV source, it is ofgreat importance to optimize plasma conditions.
Thus comprehensive experimental databases must
be provided for a wide range of parameters
including laser intensity, wavelength, pulse
waveform, target materials, structures, and so on.
These experimental data will be utilized tobenchmark a radiation hydrodynamic code
including equation-of-state solvers and advanced
atomic kinetic models suitable for EUV plasmas.
Energy levels and transition probabilities of multiionized state of high-Z material such as xenon andtin will be solved by these codes.
(2) Target design and fabrication:
Target technology is another key issue for EUV
lithography system. Various types of targets havebeen proposed including gas, liquid, solid orclusters. In addition, innovative targets are desiredin order to attain acceptably high conversion
efficiency and to mitigate debris from targets.
(3) Establishment of laser technology forcommercial use in EUV lithography system:
In order to supply sufficient EUV output, lasersystem of several-ns in duration, a few Joule inenergy, one- or sub-mm in wavelength at a highrepetition rate over 10 kHz will be necessary. Thus,
establishment of advance technologies needed forsuch a high average power laser system is anotherimportant task. A 5 kW laser system will be
constructed to demonstrate feasibility of efficient
EUV generation
under optimized
plasmaconditions.
2. Optimum plasma and laser conditions
To satisfy the EUV power demanded, emission
energy EEUV of 30-40 mJ per pulse should be
attained at repetition rate of 10 kHz. Suppose EUV
pulse duration of tEUV=1-10 ns, EUV flux will beIEUV=EEUV/StEUV, here S is the source area-size
defined by the demanded etendue (product ofsource area-size and available solid-angle) This willbe that of a circle of 600 mm in diameter assuming
the etendue of 1 mm2sr.. Using typical EUV
conversion efficiency of 1%, optimum laser
intensity will be 1011-1012 W/cm2.
Ion density can be estimated using EUV photon
number flux : fEUV = IEUV /hn = 7x1025~7x1026
photons/scm2, and this should be balanced with(nEUV lEUV)OD=1A. Here nEUV is the fractional ion
density contributing to the EUV emission. Since† A coefficient for Sn or Xe plasma ofEinsteinconcern is typically 10 ps, product of density andFig. 1 Optimum plasma parameters and relation withlaser conditions.

4. Electron density and EUV emission profiles of
Sn planer plasma
We investigated electron density profile of laserproduced planar Sn plasmas with combination ofgreen and UV interferometers. Good coincidence
between the two sets data confirmed their reliability.
Comparison
withone-dimension
radiationhydrodynamic simulation showed a reasonableagreement and the discrepancy could be attributedtomulti-dimensional
plasmaexpansion.
Comparison with 13.5 nm extreme ultraviolet(EUV) emission profile denoted that the combined
profile covers the dominant region of EUVemission. It is found that most of the EUV lightcomes from well under-dense plasma region, due toopacity effect [7]. This result validates theprediction described above.

electron density (cm- -3)

length (nEUV lEUV)OD=1 will be 1015-1016 cm-3. Wecan then obtain lEUV as a product of plasma soundspeed and laser pulse duration. Figure 1 shows the
optimum ion density and plasma length for EUV
emission, and relation with laser parameters. Laserconditions were obtained assuming kLlEUV = 1~2
where kL is the laser absorption coefficient
supposing plasma temperature of 30 eV and inverse
Bremsstrahlung for laser absorption. For longer
wavelength laser, optimum pulse duration tends tobe longer. In fact, strong self-absorption is observedin Sn plasma generated even with 0.53 mm laser
light [5].

1x1021

1x1020simulation

1x10190con. effi. of 13.5 nm at 2% BW

3. EUV emission from spherical Sn plasma
Spherical solid tin targets were illuminateduniformly with twelve beams from the Gekko XII
laser system to create spherical plasmas, and theextreme ultraviolet (EUV) emission spectra fromthe plasmas were absolutely measured. As shown in
Fig. 2, the highest conversion efficiency of 3% to13.5 nm EUV light in 2% BW was attained for anirradiance of around 5x1010 W/cm2 of 1.05 mmwavelength laser. The experimental results werereproduced fairly well using a theoretical model
taking the power balance in the plasma into
consideration [6].
4E-MON
3TGS
model

total

2

1
from high densiy
01010

1011
laser intensity (W/cm2)

1012

Fig. 2 EUV conversion efficiency measured with a Sn
spherical plasma, and comparison with the power
balance model

100200300 400
distance (mm)500

Fig. 3 Electron density profile measured with a laserinterferometer. EUV emission profile measure with amonochromatic imager coupled with an EUV streak
camera is also shown for comparison.
REFERENCES[1] I.C.E. Turcu and J. B. Dance, “X-rays fromLaser Plasmas: Generations and Applications”,
Chap. 10, John Wiley & Sons, West Sussex,England (1999).
[2] C. Rischel, et al., Nature 390, 490 (1997): A. H.
Chin, at al., Phys. Rev. Lett., 83, 336 (1999).
[3] For example, F. Jin and M. Richardson, Applied
Optics 34, 5750 (1995) and references therein.
[4]V.
Banine,“Update EUVL
sourceRequirements”, Proceeding of the 2nd EUVL
Source Workshop, Sep. 29, 2003 Antwerp,
International SEMATECH (www.sematech.org).
[5] M. Yamaura, et al., Proceedings of SPIE 5448
pp.1159-1164 (2004).
[6] Y. Shimada, et al., to appear in Appl. Phys. Lett
[7] Y. Tao, et al., to be submitted.