Acta Polytechnica


Acta Polytechnica 53(2):117–122, 2013 © Czech Technical University in Prague, 2013
available online at http://ctn.cvut.cz/ap/

THE FORMATION OF A POWER MULTI-PULSE EXTREME
ULTRAVIOLET RADIATION IN THE PULSE PLASMA DIODE

OF LOW PRESSURE

Ievgeniia V. Borguna,∗, Mykola O. Azarenkova, Ahmed Hassaneinb,
Oleksandr F. Tseluykoa, Vasyl I. Maslova, Dmitro L. Ryabchikovka

a Karazin Kharkiv National University, pl. Svobody 4, Kharkov 61022, Ukraine
b School of Nuclear Engineering, Purdue University, 400 Central Drive, West Lafayette, IN 47907–2017, USA
∗ corresponding author: ievgeniia.borgun@mail.ru

Abstract. In this paper results are presented on experimental studies of the temporal characteristics
of spike extreme ultraviolet (EUV) radiation in the spectral range of 12.2 ÷ 15.8 nm from the anode
region of high-current (I = 40 kA) pulsed discharges in tin vapor. It is observed that the intense
multi-spike radiation in this range arises at an inductive stage of the discharge. It has been shown that
the radiation spikes correlate with the sharp increase of active resistance and of pumped power, due
to plasma heating by an electron beam, formed in the double layer of charged particles. It has been
observed that for large number of spikes the conversion efficiency of pumped energy into radiation
at double layer formation is essentially higher in comparison with collisional heating.

Keywords: EUV Sources, plasma discharge, Z–pinch, EUV radiation, conversion efficiency.

1. Introduction
This study is devoted to the investigation of phenom-
ena, occurring in a high-current pulse plasma diode,
where the plasma with tin multi-charged ions gener-
ates extreme ultraviolet (EUV). One of the methods
for the creation of high-power EUV sources for nano-
lithography is an application of high-current dis-
charges in tin vapor [3, 11]. An emission source based
on the dense plasma of multi-charged tin ions is advan-
tageous against gas-filled systems because of the ex-
pected higher conversion efficiency [7] and is capable
of operating at ultralow pressures. The latter is impor-
tant for probability reduction of parasitic breakdowns
and for reduction of losses of radiation in optical paths.
Since such a source is required to have a rather high
output power [9], increasing the conversion efficiency
of the supplied electric power into radiation is an im-
portant goal. Large conversion efficiency of the source
is predicted for the tin [9]. For production of high-
contrast nonlinear photoresists, operating in EUV
range, for nanolithography above-threshold pulse in-
tensities are required [9]. It can be provided by the nar-
row peak pulses (spikes) of radiation.

This work is aimed to investigation of the processes
affecting the efficiency of radiation generation in high-
current tin-vapor discharges. The results are pre-
sented on studies of the generation of intense radia-
tion in the range of 12.2 ÷ 15.8 nm wavelength from
an extended plasma diode, operating in the regime
of self-sustained plasma-beam discharge [1]. The dense
high-temperature plasma of multi-charged tin ions
is produced by pulsed evaporation of the anode mate-
rial, fast ionization of the vapor, and heating of the
resulting plasma by the current and by the electron

beam, formed by the double electric layer in the anode
region of the diode.

2. Experiments
The experimental setup dedicated to studies of the
EUV yield from the plasma of multi-charged tin ions
is presented in Fig. 1. The setup consists of the pulsed
high-current plasma diode with the igniting electrode,
the photo-detector for measurement of the integral
radiation intensity, and the semicoductor detector
AXUV-20 for the radiation intensity measurement
of selected wavelength range. AXUV-20 detector (fab-
ricated by International Radiation Detectors Incor-
poration, California) with input Mo−Si optical filter
on 12.2 ÷ 15.8 nm wavelength range was calibrated
by intensity with help of the synchrotron radiation.
The damped alternating current in the diode is excited
between the cylindrical electrodes due to the discharge
of the low-inductance capacitor bank C0 of capac-
ity 2.0 µF at starting pressure 2 × 10−6 Torr. The fea-
tures of the applied scheme of discharge gap are the use
of electrodes with a working surface of various sizes
and initially the applying a positive voltage to the
electrode with a small effective surface (the anode),
the negative voltage – to the electrode with a larger
effective surface (the cathode). At the discharge
current 10 ÷ 40 kA and the effective anode surface
2 ÷ 20 mm2 the current density reaches value about
of 0.2 ÷ 2.0 MA cm−2. This leads to dense plasma
formation near anode and to the necessary position
stabilization of this plasma. In addition the small an-
ode effective area provides suitable conditions of the
electrical double layer formation exactly in near an-
ode region. The double layer formation has been

117

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Ievgeniia V. Borgun et al. Acta Polytechnica

Figure 1. Scheme of the experimental setup: VC –
vacuum chamber, SD – detector AXUV-20, PD – pho-
toelectron detector, A – anode, C – cathode, Id –
Rogowski coil, Vd – voltage divider, Vig – igniting
electrode.

observed in [10, 12]. In these articles at an achieve-
ment by the discharge current of some critical value
all voltage is concentrated in a narrow region and
a high-current electron beam is formed, which car-
ries all discharge current. This narrow region divides
the discharge on two parts. Further study of condi-
tions of facilitation of double layer and high-current
electron beam formation in such type of discharge
studied by the authors of the articles [5, 4, 6]. In our
article the double layer formation and its dynamics
are not investigated in detail. Based on the knowl-
edge of methods for forming the double layer and its
stabilization, we have created such experiment con-
ditions to create the double layer near one electrode
with a small effective surface. The using of double
layer is an effective method for local plasma heating.
Thus the presence in the same location of the dense
plasma and its source of heating makes it possible
to form dense plasma with multi-charged ions in near
anode region.
The length of the discharge space can be varied

from 3 to 10 cm, the diameter of the cathode is 10 mm
and the anode diameter equals 1.5 mm or 2.5 mm,
or 5 mm. The working surface of the electrodes are
covered with 0.5 mm thick tin layer. The side surface
of a rod anode is enveloped by tubular ceramic in-
sulator to increase the current density to the anode.
The discharge voltage is from 4 to 15 kV, the cur-
rent amplitude from 10 to 40 kA, the current density
on the anode reaches to 0.2 ÷ 2.0 MA cm−2, the half-
cycle of current oscillations is 1.7 µs. The discharge
current and voltage are measured with Rogowski
coil Id and balanced voltage divider Vd, respectively.
The current through the diode is excited after filling
of the discharge interval by preliminary plasma due

to surface discharge on the cathode, using a ignit-
ing electrode. The pulsed voltage 0.2 ÷ 5.0 kV sup-
plies on a igniting electrode from the capacity con-
denser 0.025 µF through thyratron and inductance
of value 400 µH.
The integral radiation by plasma is monitored by

the current measurement of photoelectrons. The pho-
toelectrons are collected by fine-mesh grid placed
at a distance of 0.5 mm from the photocathode.
The grid is grounded, and the photocathode is at a neg-
ative potential of −50 V, supplied from an autonomous
voltage source.

At the radiarion intensity measuring of the selected
wavelength range it is used the special measures.
To protect the AXUV-20 detector from the plasma and
charged particle beams the inlet diafragmatic channel
is located in transverse magnetic field (strength 0.2 T,
length 25 cm). To exclude the effect of photoelectron
current on the detector signal, we use a set of shading
diaphragms and the detector is biased by +20 V with
respect to the channel.
The diode discharge develops in two stages.

The first stage begins with a surface breakdown
at the cathode and finished when the primary plasma
reaches the anode. In the first stage, which lasts
for 2 ÷ 6 µs (for the electrode gap length of 5 cm),
the discharge is operated in the regime of a vac-
uum diode with a plasma emitter. In this case,
the discharge current is carried by the electron beam.
Thus the working surface of the anode is preheated
by the beam and the initial vapor envelope is formed.
In the second stage, when the dense plasma occu-
pied the discharge gap, the discharge switches to the
plasma diode regime, in which the discharge current
is determined by the parameters of the plasma and
discharge circuit.
Probe measurements have shown that beetwen

stages of the vacuum diode and the plasma diode
the transition regime exists in which the double elec-
tric layer near the anode surface is formated. The in-
tense electron beam are accelerated into the layer
and affects on the anode surface. A high-current
double electric layer exists for 0.5 µs. Under these
conditions, the material is intensely evaporated from
the anode surface, the vapor is quickly ionized and
the plasma is rapidly heated due to the beam–plasma
interaction. The total energy pumped in the anode
plasma and the anode itself during the first half-period
of the discharge current reaches 80 % of the energy
stored in the capacitor bank. The formation of the
double layer in the transition regime is determined
by inability of the discharge gap plasma to provide
the high discharge current density [1, 5]. As soon
as the dense plasma increases, the current is no longer
limited and the double layer is disappeared. Then,
the discharge operates in the inductive phase.

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vol. 53 no. 2/2013 The Formation of a Poser Multi-Pulse Extreme UV Radiation

3. Results and Discussion
From the results of the experiments it follows that
the intensive radiation in the range of wavelengths
12.2 ÷ 15.8 nm arises both at the transition regime
and at the inductive stage of the discharge. The pecu-
liarity of this radiation is that there are the powerful
(up to 1MW) short (100 ÷ 200 ns) spikes against of the
background of wide radiation pulses with duration
about half-period of discharge current oscillations.
The transversal dimension of the region of generation
is comparable with the diameter of the anode, and
its length changes depending on a discharge voltage
and equals 4 ÷ 7 mm. Typical waveforms of discharge
current and voltage, the radiation intensity in the se-
lected wavelength range and integral radiation versus
time are shown in Fig. 2. It is seen that in the selected
wavelength range there are several radiation spikes,
correlating with the corresponding half-periods of the
discharge current. In the first half-period there are
as the wide pulse with the relatively small amplitude
and a powerful spike. Whereas the pulses emitted
during the second and third half-periods have only
the form of the powerful shorter narrow spikes.

It is clearly seen that the radiation spikes of the se-
lected wavelength range coincide with spike pulses
of integral radiation (see Fig. 2c and 2d). Note
that during first half-cycle the narrow spike of du-
ration 200 ns is observed only at discharge voltage
more than 7 kV. Its intensity grows with increase of the
discharge voltage. More than 70 % of energy, radiated
during first half-cycle, is concentrated in this peak
pulse. The time of occurrence of the narrow peak
pulse depends on an ignition voltage. (For higher
voltage the peak pulse appears earlier.)

In second and third half-cycles these radiation spikes
are always observed near maximum of the current.
The radiation spikes are registered at the current am-
plitude more than 10 kA. Their intensities grow with
growth of the discharge voltage. Note that in sec-
ond half-cycle of the current oscillations an additional
spike-satellite of duration 200 ns is observed at dis-
charge voltage 5 ÷ 8 kV (Fig. 2c). This pulse follows
after the basic spike through 200 ns. The intensity
of the spike-satellite also grows with increase of the dis-
charge voltage. At voltage more than 8 kV the spike-
satellite disappears. Similar two spikes during the one
half-period have been observed in [8].
For effective generation of the radiation in the se-

lected wavelength range it is required an additional
energy pumping. In our case, additional energy
pumping is provided by the electron beam, generated
by the electric double layer which is formed period-
ically. In Fig. 3 the dependence of power, pumped
in discharge, and corresponding pulses of radiation
on time are shown. One can see that the spikes
of radiation coincide with spikes of power, pumped
in discharge. The different time behavior of radiation
in this wavelength range in the first and subsequent
half-periods can be explained as follows. In the first

Figure 2. Waveforms of the (a) discharge cur-
rent, (b) discharge voltage, (c) radiation intensity
in the 12.2 ÷ 15.8 nm wavelength range, and (d) in-
tegral plasma radiation versus time, da = 2.5 mm,
Vdis = 8 kV, ldis = 5 cm.

half-period, there is still an influx of neutral atoms into
the anode region due to intense evaporation of the an-
ode material. Moreover, since the discharge current
is high, the energy distribution of plasma electrons
is fairly wide. This leads to a significant collisional
energy pumping; the broadening of the distribution
function of ions over charge states; and, accordingly,
the generation of ordinary recombination radiation
within a wide spectral range with a relatively low in-
tensity. By the end of the first half-period, the number
of ions in the charge states corresponding to radiation
in the 12.2 ÷ 15.8 nm wavelength range decreases.

In the second and third half-periods the plasma al-
ready exists. It is necessary only to increase the charge
states (i.e., plasma temperature) of ions up to neces-
sary ones. Moreover the plasma is relatively dense
and focused during previous half-period(s). In the sec-
ond and third half-periods the neutral atom flux into
the anode region is much less than one in the first

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Ievgeniia V. Borgun et al. Acta Polytechnica

Figure 3. (a) The temporal dependences of power,
pumped in discharge, (b) radiation intensity in
the 12.2 ÷ 15.8 nm wavelength range; da = 2.5 mm,
Vdis = 8 kV, ldis = 5 cm.

half-period, because existing plasma shades anode
from electron beam bombardment. Thus in the sub-
sequent half-periods the energy expenditure on for-
mation of dense plasma with necessary charge states
of ions is essentially smaller than in first half-period.
After formation of plasma with necessary charge states
of ions in the second and third half-periods the dis-
tribution function of ions on charge states is more
narrow than in the first half-period.
We examined how the radiation intensity and

the conversion efficiency of the stored energy into
radiation depend on the external conditions. Fig. 4a
shows how the total (within the 2π solid angle) en-
ergy of the radiation pulses (all pulses: spike and
wide) in the 12.2 ÷ 15.8 nm wavelength range depends
on the energy stored in the capacitor bank. The de-
pendence is seen to reach maximum at stored energies
larger than 140 J. It is necessary to note that this max-
imum in dependence of the radiation energy on the en-
ergy stored in the capacitor bank is observed at use
of anode of different diameters (from 1.5 to 5 mm).
As it is shown in Fig. 5 for larger diameter of the an-
ode this maximum shifts to region of larger energy
stored in the capacitor bank. For smaller diameter
of the anode this maximum shifts to region of smaller
energy stored in the capacitor bank.

In Fig. 4 the diameter of the anode equals 2.5 mm.
Figure 4b also shows the energies of the radiation
pulses emitted during the first, second, and third
half-periods of the discharge current. At low stored
energies, radiation is mainly emitted during the first
half-period. As the stored energy increases, the en-
ergies of radiation pulses emitted in different half-
periods approach one another. At stored energies,

Figure 4. (a) Radiation energies (spike and wide
pulse energies) in the 12.2 ÷ 15.8 nm wavelength range
during first W1r, second W2r, third W3r half-periods,
(b) fractions of the energy radiated in the selected
wavelength range in different half-periods of the dis-
charge current as functions of the stored energy W0;
here W0r is the total radiation energy, emitted in the se-
lected wavelength range during a discharge into the 2π
solid angle; W1r/W0r, W2r/W0r, W3r/W0r are the en-
ergy fractions emitted in the first, second, and third
half-periods, respectively, da = 2.5 mm, ldis = 5 cm.

exceeding 120 J, they become nearly equal. Figure 4b
shows the relative energies of radiation pulses emitted
in each half-period of the discharge current as func-
tions of the stored energy.

Figure 6a shows the relative energy, pumped in each
half-period of the discharge current, as a function
of the stored energy. It is seen that the energy is
mainly pumped in the first half-period, during which
the anode material is intensely evaporated and neu-
tral atoms are multiply ionized. In other words,
the stored energy is mainly spent on the production
of dense plasma. Very small energy in comparison
with the plasma formation energy is spent on plasma
support during next half-periods. Therefore, when
the total stored energy is low, the rest of the en-
ergy pumped in the subsequent half-periods is low.
However, the radiation intensity in those half-periods
is relatively high. As the stored energy increases,
the current in the second and third half-periods in-
creases. The energy fraction pumped in the subse-
quent half-periods increases, thereby leading to an
increase in the radiation energy.
The equalization of the absolute and relative radi-

ation energies emitted over one half-period of the
discharge current with increasing energy, as well
as the fact that the radiation intensity in the second

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vol. 53 no. 2/2013 The Formation of a Poser Multi-Pulse Extreme UV Radiation

Figure 5. Radiation energies (spike and wide pulse en-
ergies) in the 12.2 ÷ 15.8 nm wavelength range as func-
tions of the stored energy for the different anode di-
ametres: da = 1.5, 2.5, 5.0 mm.

and third half-periods is high in spite of a much lower
(as compared to the first half-period) energy pump,
indicates that the number of particles in the dense
radiating plasma remains nearly constant for a fairly
long time (approximately 6 µs). This is confirmed
by the results obtained in [2], where long-lived dense
plasma was also observed. Thus, after the plasma
has formed in the first half-period, it is supplied
with energy in the subsequent half-periods, a fraction
of the supplied energy being instantaneously converted
into radiation.
Detailed analysis of the energy pumped in the dis-

charge during one half-period and the energy radiated
over the same time permits to find the energy conver-
sion efficiency for each half-period. Figure 6b shows
the conversion efficiency of the energy pumped in the
discharge during each half-period into the radiation
energy in the 12.2 ÷ 15.8 nm wavelength range, and
the total energy conversion efficiency ns0 = W0 r/W0,
as a function of the total stored energy. It is seen
that the conversion efficiency in the second and third
half-periods reaches a few percent, the energy pumped
in the discharge being 10 J. For a 2.5 mm diameter
anode, there is an optimum at a stored energy of 80 J
(see Fig. 6b).

A comparison of the radiation intensity in the se-
lected wavelength range and the integral intensity
shows that, when the stored energy is more above a cer-
tain value, the integral radiation intensity increases
sharply, whereas the radiation intensity in the selected
wavelength range of increases insignificantly. Presum-
ably, this is related to the increasing of tin ion with
the higher charge states, which leads to generation
of the harder radiation and, therefore, to extra energy
consumption.

Figure 6. Energy fractions, pumped during one half-
period (W1, W2, and W3 are the pumped energies and
W1/W0, W2/W0, and W3/W0 are the energy fractions
pumped in the first, second, and third half-periods
of the discharge current, respectively); (b) conversion
efficiency ηi = Wi r/Wi (i = 1, 2, 3) of the energy,
pumped during one half-period Wi, into the radiation
energy Wi r in the 12.2 ÷ 15.8 nm wavelength range
as functions of the stored energy W0; W0 r is the radi-
ation energy in the selected wavelength range during
all half-periods, da = 2.5 mm, ldis = 5 cm.

4. Conclusions
Multi-spike radiation, important for use of high-
contrast nonlinear photoresists for nanolithography,
in the wavelength range 12.2 ÷ 15.8 nm from a tin
vapor discharge has been observed. The duration
of radiation spikes is found to be much shorter than
the half-period of the discharge current.
In a narrow energy range in second half-cycle

the spike-satellite is observed.
An analysis of the experimental data indicates

the presence of long-lived dense plasma. When this
plasma is optimally supplied with energy, radiation
spikes are generated in the selected wavelength range.
Analysis of the experimental data indicates that
the intensity of these spikes is mainly determined
by the discharge current and by the energy pumped
in the discharge.

For an efficient radiation source development it is ex-
pedient that the radiating plasma be used repeatedly,
because most of the stored energy is spent on plasma
formation. In order to achieve quasi-steady emission,
the discharge should be supplied with optimal por-
tions of energy.

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Ievgeniia V. Borgun et al. Acta Polytechnica

The possibility of the power (several MW) ex-
treme ultraviolet generation in kind of train of spikes
of duration 200 ns in the high-current pulse plasma
diode in the inductive stage of discharge development
at the current density to the anode 0.2 ÷ 2.0 MA cm−2
has been shown. The power spikes of radiation are
generated in conditions of plasma heating by elec-
tron beam. The electron beam is formed by double
electrical layer.
The conversion efficiency of pumped energy into

radiation energy at the double layer formation (beam
mechanism of plasma heating) is essentially higher
in comparison with ordinary heating by current, be-
cause in first case approximately all energy is pumped
in very small volume.

The use of plasma diode with anode of small dimen-
sion (in comparison with cathode dimension) helps
to control and stabilize in space dense plasma local-
ization and the position of double layer formation.
At this condition the space of dense plasma localiza-
tion coincides with the source of its heating.

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	Acta Polytechnica 53(2):117–122, 2013
	1 Introduction
	2 Experiments
	3 Results and Discussion
	4 Conclusions
	References