Capillary discharge sources of hard UV radiation

INSTITUTE OF PHYSICS PUBLISHING
PLASMA SOURCES SCIENCE AND TECHNOLOGY
Plasma Sources Sci. Technol. 11 (2002) A64–A68
PII: S0963-0252(02)39384-8
Capillary discharge sources of hard UV
radiation
C Cachoncinlle, R Dussart, E Robert, S G¨otze, J Pons,
S R Mohanty, R Viladrosa, C Fleurier and J M Pouvesle
Gremi-espeo, Universit´e d’Orl´eans, 14 Rue D’Issoudun, BP 6744, 45067 Orl´eans Cedex 2,
France
E-mail: [email protected]
Received 26 September 2001, in final form 2 July 2002
Published 19 August 2002
Online at stacks.iop.org/PSST/11/A64
Abstract
We developed and studied three different extreme ultraviolet (EUV)
capillary discharge sources either dedicated to the generation of coherent or
incoherent EUV radiation. The CAPELLA source has been developed
especially as an EUV source for the metrology at 13.4 nm. With one of these
sources, we were able to produce gain on the Balmer-Hα (18.22 nm) and Hβ
(13.46 nm) spectral lines in carbon plasma. By injecting 70 GW cm−3 we
measured gain-length products up to 1.62 and 3.02 for the Hα and Hβ,
respectively. Optimization of the EUV capillary source CAPELLA led to
the development of an EUV lamp which emits 2 mJ in the bandwidth of the
MoSi mirror, per joule stored, per shot and in full solid angle. The wall-plug
efficiency is 0.2%. Stability of this lamp is better than 4% and the lamp can
operate at a repetition rate of 50 Hz.
1. Introduction
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0963-0252/02/SA0064+05$30.00
© 2002 IOP Publishing Ltd
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Two fields of scientific and technical applications mainly drive
the research on sources capable of emitting a significant level of
photon fluxes in the spectral domain of the extreme ultraviolet
(EUV). The first one is the biological and living science
research for which the generation of photon in the so-called
‘water window’ is of crucial interest. A compact EUV source
[1], driven by an electrical discharge, could easily be applied
at very low cost to biological imaging in living cells. As
illustrated in figure 1, the wavelength range from these sources
spans over the ‘water window’ region of EUV or ‘soft-x-ray
region’ where water is less absorbing than carbon. The second
field of application is EUV microlithography.
In the area of microlithography and materials processing,
the mercury vapour discharge lamp has been providing very
low cost illuminating sources for the last two decades.
More recently, UV laser sources, at wavelengths as short
as 157 nm, have been tested for sub-micrometre lithography
for large volume manufacturing of integrated circuits. As a
consequence of the well-known ‘Moore’s law’, new generation
lithography (NGL) will require shorter wavelengths in the
EUV region for sub-0.1 µm features.
During the last
few years tremendous efforts have been made to realize
LαCVI 3.4 nm
CV 4.0 nm
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CV 3.5 nm
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1000
800
600
400
2.0
2.5
3.0
3.5
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4.0
4.5
Figure 1. Carbon spectra in the ‘water window’ obtained with the
ablative capillary discharge developed in this experiment.
and develop new EUV sources for the illumination system.
Europe, Japan and the United States have already initiated
efforts to solve intricate problems of EUV lithography like
mask, metrology, photoresists and ‘alpha’ tool. Significant
progress has been made in all areas of technological issues
concerning EUV lithography. In contrast, none of the
presently available EUV sources really matches the technical
and financial requirements of the illuminating device for
commercial tools. Much effort is still needed through both
Printed in the UK
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Capillary sources of UV radiation
research and development programmes on the various EUV
sources to fulfil the specifications required, especially in terms
of lifetime and average power at reasonable cost.
Various types of EUV sources already exist. One of
them is a laser-produced plasma EUV source [1–3] in which a
powerful laser is focused on a target to convert energy into EUV
radiation. The efforts, for many years, towards developing
a powerful EUV source based on this technique have led
now to the production of high fluxes in the EUV spectral
range. The brightest EUV source for scientific applications
is obtained from the synchrotron machines. Nevertheless, a
synchrotron machine cannot be suitable for commercial usage
due to its huge size and cost. On the other hand, EUV sources
based on electrical discharges are of great interest from the
commercial viewpoint since they are simple, compact, costeffective, and they can operate with a very good ‘wall-plug’
efficiency. Depending on geometric and physical properties,
these sources can emit coherent [4–7] as well as incoherent
[1–3, 9, 10] radiation. A few years ago, compact EUV lasers
driven by electrical discharges were demonstrated [11].
2. The use of capillary discharges for the production
of coherent and incoherent EUV photon sources
To produce significant fluxes of high-energy photons, the
electrical discharge has to be confined in a very small active
volume in order to reach a very high power density and a
high plasma temperature. Two methods are mainly used to
obtain the power density required for emitting plasma. The first
one is based on the z-pinch effect and converts the electrical
energy into EUV radiation. The second one uses a capillary
tube. The latter technique can be achieved with or without
gas to produce a very high temperature plasma. When used
under vacuum operation conditions, the emitting plasma is
created from the ablation of the capillary wall. This technique
using recombination schemes in carbon plasma has already
demonstrated a gain in the EUV spectral domain. But the
lifetime of vacuum operating discharge is limited since the
diameter of the capillary rapidly increases after long operation
times and thus leads to a significant reduction in the input power
density. In contrast, when the capillary is filled with different
gases, various spectral emissions resulting from transitions
between high lying multi-charged ion states could be obtained
in the whole EUV spectral domain with significant output
power from shot to shot up to kilohertz operation.
In all cases, the current flowing through the capillary
channel should be strong enough to produce high a electronic
temperature ranging from a few tens of electron volts up
to several hundreds of electron volts. Since the current is
proportional to the square root of the capacitance which stores
the energy and varies with the inverse of the square root of
the inductance, one has, to reach a current of tens of kiloamperes, to work with pulse forming line as fast as possible,
i.e. of lowest inductance. Usually, capillary discharges work
with small capacitances but charge to high voltage to store a
few joules.
Once the breakdown occurs inside a capillary channel,
in our case of the capillary filled with low-pressure gas, the
current begins to flow mostly along the axis whereas in the case
of operation under sufficient vacuum condition, the current
flows along the wall causing ablation of the capillary material.
But in both cases, hot and dense plasma rapidly fills the whole
volume of the channel.
As the EUV light emitted by the plasma has to be collected,
a more convenient means in this geometry is to use hollow
electrodes. By doing so, one can easily view the plasma
along the capillary channel. The geometry of cathode and
anode is important for convenient flow of the charged particles,
because hot spots can severely damage electrodes by reducing
drastically the lifetime of the source. In fact, the whole set-up
is, in principle, very close to a pseudo-spark discharge in
which a dielectric capillary would have been added for better
confinement of the plasma.
The high degree of ionization of the plasma is obtained
as a result of the very high power density injected into the
gas. Tens of gigawatts per cubic-centimetre can easily be
reached in such capillary discharge sources. Amplification
of spontaneous emission (ASE) can be expected from such
fast and energetic discharges. EUV ASE, the so-called soft
x-ray laser, can occur following different pumping schemes:
collisional [11], recombination [12] and charge exchange
[13, 14] process. Using capillary discharge drivers, higher
gain-length products have been achieved in the collisional
scheme [6, 11]. According to this scheme, the electronic
temperature has to be high enough to ensure population
inversion between states of highly ionized atoms. In the
recombination scheme, inversion of population is realized
only when the electrons are cold enough to encounter
recombination. According to this scheme, the proximity of
the wall in small diameter capillaries can help fast cooling of
the electronic population. Finally, charged exchange process
involves ions of different ionization degrees in collision to lead
to the required inversion of population.
3. The ablative capillary discharge in carbon plasma
The configuration of the capillary discharge used in our
experiment for probing the recombination scheme is shown
in figure 2. We used a classic RLC circuit as the electrical
driver [10]. A high voltage pulse triggered the discharge. The
energy stored ranged from 40 to 80 J. Two different materials
were used for the capillary of 1 mm in diameter: a polyethylene
one and a polyacetal one. Only the polyethylene capillary can
be used at very high power density up to 70 GW cm−3 since
Faraday cage
Optical path of the beam
Capillary
polyimide filter
Pinhole
Ø=50 µm
Detection part
Instrumentation box
– 40 kV
Figure 2. Experimental set-up.
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C Cachoncinlle et al
the polyacetal capillary breaks more easily. The capillary
was evacuated using a turbo molecular pumping unit. A
Rogowski coil is used to monitor the current. An EUV fast
photodiode was used to record time evolution of the photon
pulse. Using appropriate spectral filters, a reliable estimation
of the pulse energy can be derived from the photodiode signal.
End-on images of plasma were obtained by putting a 50 µm
diameter pinhole between the source and the image plane. A
flat field spectrometer (Jobin & Yvon PGMPG500) coupled to
a gated micro-channel plate connected to an intensified CCD
camera was employed to record time resolved (10 ns) spectral
measurements of the carbon plasma from the water window
region (3 nm) up to 40 nm. Three gratings having different
spectral resolutions were used.
Time-resolved pinhole imaging [15] of the plasma reveals
that hotter plasma, which is highly emissive in the soft
x-ray spectral domain, is concentrated on the axis of the
capillary channel. But no magnetic compression had been
observed. The visible light, corresponding to colder plasma
is, as expected, localized in an annular sheet at the periphery,
close to the capillary wall.
Time-resolved spectroscopy of the carbon plasma shows
a strong emission of the Balmer-Hα (18.22 nm) and Hβ
(13.46 nm) lines in the initial stages of the discharge before
the current reaches the maximum.
Then carbon lines
from lower ionization degrees dominate the spectrum. The
electronic density has been estimated by Stark broadening
measurement on a CIV atomic line at 580.29 nm. The
values obtained in the plasma column range from 2 × 1018 to
1 × 1019 cm−3 . Estimation of the time evolution of electronic
temperature was performed by measuring the ratio of the
intensity of atomic lines in the spectral domain 17–19 nm.
The temperature reaches a maximum value of 80 eV. Time
integrated measurement of gain has been performed on
different lines. As illustrated in figure 3, the spectral lines of
CV at 16.7 nm increases linearly with the length of the plasma.
Only the two CVI lines Hα (18.22 nm) and Hβ (13.46 nm)
exhibit a non-linear behaviour with respect to the capillary
channel length. If we interpret this behaviour as an indication
of ASE, the light intensity, I (l), should increase following the
classical law
I (l) = S(exp(gl) − 1)
(1)
where l is the length of the capillary, g the gain value, and S the
source function.
Numerical optimization of the experimental data sets
according to equation (1) gave a gain-length product of 1.62
and 3.02 for the Hα and Hβ, respectively
Interpretation in terms of ASE is strongly supported by
experimental data [14] and plasma modelling results. Two
different numerical codes have been used to simulate the time
evolution of the population of different energy levels. Both
codes, FLY and CADILAC, predicted inversion of population
of the H-like level of carbon. According to the results of these
codes the inversion of population occurred only 100 ns after
the current began to increase. This value agrees completely
with our experimental observations.
4. The CAPELLA source
Under a French national project PREUVE that focuses on
the realization of a EUV lithography at 13.4 nm, we have
developed an EUV source CAPELLA (Capillary EUV Lamp
for Lithography Approach). CAPELLA is based on gas-filled
capillary pulsed electric drivers, i.e. a fast high voltage pulsed
forming line has been used to produce the gas breakdown
instead of direct capacitive discharge. A high voltage pulse
(15 kV) of tens of nanoseconds was applied to water-cooled
electrodes. The diagnostics surrounding the CAPELLA were
the same as those mentioned in the preceding section for the
ablative capillary discharge.
Most of the investigations have been carried out on a small
capillary channel of 10 mm length and 1 mm diameter. Such a
geometry is suitable for incoherent sources that are generally
required for NGL applications.
Such incoherent photon sources are obviously simpler
to operate than the coherent ones. Flowing gas to fill the
capillary at pressures from 0.2 to 2 mbar assures sufficient
density for the production of a high flux of energetic EUV
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c
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0
0.00 0.18 0.36 0.54 0.72 0.90 1.08 1.26 1.44 1.62 1.80
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d
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e
0
0.00 0.18 0.36 0.54 0.72 0.90 1.08 1.26 1.44 1.62 1.80
Capillary length (cm)
Figure 3. The spectral intensity from carbon plasma versus capillary length. (a) Hα line: exponential fit; (b) Hα line: linear fit; (c) CV
16.7 nm line: linear fit; (d) Hβ line: exponential fit; (e) Hβ line: linear fit.
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Capillary sources of UV radiation
photons. Time resolved spectroscopic measurements were
carried out for different gases (nitrogen, oxygen, neon, argon,
krypton and xenon) and a mixture of gases with helium. In
the EUV wavelength spectral domain, the brighter emissions
were obtained in krypton (8–11 nm) and xenon (10–15 nm)
[16]. The xenon filled capillary is recognized as the best
candidate for sources dedicated to NGL applications because
of the presence of a wide fluorescence around 13.5 nm that
matches the bandwidth of the classical Mo–Si mirror. This
mirror is known to reach 0.69 in reflectivity at 13.4 nm in a 2%
of maximum bandwidth.
From time-resolved pinhole images of the xenon plasma,
there clearly appears to be a z-pinch effect at the time of
the high level of EUV radiation production. The observed
characteristic timescale of the colliding plasma is in complete
agreement with our calculation. We first used the very simple
and well-known ‘snowploughs’ model, which only needs
three parameters: the current, the pressure and the radius of
the capillary channel. It is predicted that the maximum of
compression of the plasma should occur between 40 and 60 ns
in our experimental conditions. A more sophisticated code,
based on the conservation of energy, has been developed [17]
to gain more physical understanding of the phenomenon, since
the compression of the plasma is consequently stopped by the
increased pressure. This code predicts a maximal compression
at round 45 ns followed by another compression (85 ns) since
the time of current flow through the capillary is longer than
the characteristic time of the z-pinch effect. The luminous
spike observed in figure 4 corresponds to the first maximum
compression of the plasma column. It is probably shorter
than 5 ns which is the limit of the time resolution of the gated
detector.
It should be pointed out that, in contrast to other similar
observations [18], this ultra-fast luminous spike observed on
the photodiode signal should not be attributed to a laser
effect since the spectroscopic measurements, run in optimal
experimental conditions, did not show any specific increase in
intensity of one particular spectral line in the whole spectral
domain. Furthermore, the temporal behaviour of the resistance
of the plasma column, which should vary as the square of
the radius of the plasma, shows that the power transferred to
Xenon : 1 torr
Diam : 1.2 mm
Length : 10 mm
5000
4
Current
Intensity [a.u.]
3
4000
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Intensity
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2
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0
0
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40
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Figure 4. Discharge current waveform together with the time
evolution of the plasma diameter and intensity of radiation as
observed from pinhole image.
Plasma diameter [a.u.]
6000
plasma (in joules) presents the same fast picking behaviour.
It is obvious that the EUV light emitted by this plasma
is simply proportional to the electrical power injected at
each time.
Different experimental parameters were varied to optimize
the wall-plug efficiency of the xenon source at 13.4 nm.
The pressure, measured at the inlet of the capillary, is found
to be optimal at 2 mbar, but there exists a strong pressure
gradient along the capillary channel due to its very small
radius. The xenon flow was controlled by a mass flowmeter.
A turbomolecular pumping unit evacuated the capillary. A fast
photodiode evaluated the energy radiated. The light was
filtered through a EUV zirconium foil 1600 Å thick. From
the spectra recorded by the spectrometer, we estimated that no
more than 16% of the total energy measured by the photodiode
is in the bandwidth of a classical Mo–Si mirror (2% bandwidth
at 13.4 nm). Under these experimental conditions the source
emits about 2 mJ ‘in band’ per joule stored per shot and in full
solid angle. Thus, at the optimum of input power density, the
wall-plug efficiency of conversion into ‘in band’ EUV energy
is 0.2%. Such high wall-plug efficiency is one of the highest
reported in the literature for the discharge techniques [3] and
is of course much higher than the wall-plug efficiency of laser
plasma EUV sources [1]. The source has already been tested
at low repetition rate of 50 Hz leading to an average power
of 0.1 W in the bandwidth of the mirror. The shot to shot
fluctuation in EUV radiation from this source is less than 4%.
From these experiments we can expect to have sources of a
few watts at very low cost by using a proper switching element
and a small size cooling device.
5. Conclusion
From these studies, it appears that the capillary discharges
are good candidates for coherent and incoherent sources in
the soft x-ray spectral domain. They show exceptionally high
efficiency at a very low cost. We have already developed and
studied different types of capillary sources either dedicated to
the so-called ‘soft-x-ray laser’ or to EUV metrology.
In the radiation source based on the ablative capillary
discharge we are able to observe a gain in carbon plasma on the
Balmer-Hα (18.22 nm) and Hβ (13.46 nm) spectral lines. By
injecting 70 GW cm−3 power inside a polyethylene capillary
of 1 mm diameter, we measured the indication of ASE with
small gain-length products of 1.62 and 3.02 for Hα and Hβ,
respectively. The temperature inside the channel was estimated
to be 80 eV from spectroscopic measurements and the electron
density was of the order of 1019 cm−3 .
Optimization of the EUV capillary sources filled with
xenon led to the development of an EUV lamp (CAPELLA)
dedicated to the metrology for NGL. The lamp emits about
2 mJ ‘in band’ per joule stored per shot and in full solid angle.
The wall-plug efficiency of conversion into ‘in band’ EUV
energy is 0.2%. The shot to shot fluctuation of this lamp is less
than 4%. The low flux (0.1 W) lamp is operated at repetition
rate of 50 Hz. Upgrading of this lamp towards higher fluxes
is under progress. In the near future we would expect a low
cost, high flux (tens of watt) source with high repetition rate
(1 kHz) with improved wall-plug efficiency.
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C Cachoncinlle et al
Acknowledgments
This work was partially financially supported by the TMR
network contract No ERBFMRXCT 980186 and the ‘minist`ere
de l’industrie’ within the French Project PREUVE.
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