ORIGINAL RESEARCH

Simulation of Nitrogen and Oxygen Spectra Emitted from HighDensity Hot Plasma

S. Alsheikh Salo • M. Akel • C. S. Wong

Published online: 19 June 2014

� Springer Science+Business Media New York 2014

Abstract The expected emission spectra of nitrogen and

oxygen high density plasma have been studied for different

conditions. Expected nitrogen and oxygen plasma spectra

at certain electron temperature range have been plotted.

Suitable electron temperature ranges for nitrogen and

oxygen plasma soft X-ray emission and extreme ultraviolet

emission have been investigated. Numerical experiments

confirm the possibility of developing nitrogen and oxygen

plasma focus as a powerful X-ray radiation source for

water-window X-ray microscopy, by selecting the working

gas pressure, choosing corresponding design and operating

parameters of the device. We have illustrated that the

results obtained from XRAYFIL simulation could be used

to provide spectroscopic information of the plasma focus

simulated by Lee model.

Keywords Nitrogen and oxygen plasma � Soft X-ray �EUV emission � X-rays ratio method

Introduction

High density plasma focus (1020 cm-3) of nitrogen [1–5]

and oxygen [6] gases has been used as an emitter of soft

X-rays suitable for water-window X-ray microscopy [7–9].

The emission spectra of the plasma depend on the operat-

ing parameters [10, 11], therefore the plasma focus could

also be considered as a possible light source for extreme

ultraviolet lithography (EUVL) at certain operating con-

ditions [11]. This is of interest to the semiconductor

manufacturing industry due to the expectation that the next

generation lithography (NGL) will be using the wavelength

of 135 A [12, 13]. Various types of EUV radiation sources,

including the laser produced plasma and pulsed discharge

sources, such as the capillary discharge [14, 15], vacuum

spark [16, 17] and plasma focus [18, 19], are being con-

sidered. These radiation sources, especially the pulsed

discharge sources are favorable as X-ray and EUV radia-

tion sources because of their lower cost and simplicity in

operation, when compared to other radiation sources.

Generally, it is hardly possible to get the detailed

accurate knowledge of the states of the plasma. Approxi-

mate estimations, by calculations based on simplified

plasma models, may be carried out. The methods of fuzzy

scaling and genetic algorithms for obtaining the possibility

of description of X-ray emission, scattering and applica-

tions on fusion energy have been used [20–22]. On the

other hand, the most tractable plasma models are the local

thermodynamic equilibrium (LTE), the non-local thermo-

dynamic equilibrium (NLTE) or the corona equilibrium

(CE), and the collisional-radiative equilibrium (CRE) [23–

26]. One of the best known of these is the suite of three

codes (POPULATE, SPECTRA, and RATIO) called

RATION [27–30]. The POPULATE code uses the princi-

ple of detailed balance to calculate the rate of inverse

processes. The SPECTRA code computes the expected

emission spectrum of the plasma, while the RATIO code

allows the user to view the results of POPULATE in

graphical forms. The graphical outputs include the popu-

lation, ratio of the intensities of selected transitions and the

optical depths of transitions, as a function of temperature or

density. Since the code provides details of the populations

S. Alsheikh Salo (&) � M. Akel

Department of Physics, Atomic Energy Commission,

6091 Damascus, Syria

e-mail: pscientific2@aec.org.sy

C. S. Wong

Physics Department, Plasma Technology Research Center,

University of Malaya, 50603 Kuala Lumpur, Malaysia

123

J Fusion Energ (2014) 33:677–683

DOI 10.1007/s10894-014-9725-2

in Lithium-like through fully stripped ions, an X-ray filter

analysis code, XRAYFIL, [31] has been developed to allow

more accurate non-dispersive X-ray plasma diagnosis with

absorption filters by using a series of trial spectra of the

emitting species. The code calculates a set of emission

spectra for a given plasma using RATION, and convolves

it with the transmission characteristics of the filter set used,

as well as the response function of the detector chosen.

Comparison of the ratio of the signal through the different

filters from these calculated values to that recorded in the

experiment allows us to obtain a measurement of the

plasma temperature and density. The code incorporates a

number of options for various emission scenarios and

detectors. The main purpose of XRAYFIL code is to cal-

culate the emission spectra of nitrogen and oxygen plasma

and then to work out the number of photons passing

through the chosen composite filter (BPX65 PIN diode

detector, in our case). So, the emission spectra (full,

Bremsstrahlung, recombination, and line) are computed

using XRAYFIL code (in unit of number of photons/cm3/

Angstrom/Sec./Sterad.) [31].

In this work, the XRAYFIL code coupled with the

POPULATE code is used to study the EUV and X-ray

emissions of nitrogen and oxygen plasma focus. The

spectra of radiation emissions (full, Bremsstrahlung,

recombination, and line) from the nitrogen and oxygen

plasma focus have been simulated for different plasma

conditions. The calibrated X-ray ratio curves for electron

temperature measurements of nitrogen and oxygen plasma

focus have been deduced.

Results and Discussion of Numerical Experiments

Spectrum of the Nitrogen and Oxygen Plasma

in the Extreme Ultraviolet Range

Many numerical experiments have been carried out using

POPULATE and XRAYFIL codes for calculations of

nitrogen and oxygen plasma spectra in the extreme ultra-

violet range at different temperatures for NLTE (for

nitrogen 10–100 eV and for oxygen 10–200 eV). Figure 1a

presents the expected radiative emissions (full, Brems-

strahlung, recombination, and line) of nitrogen plasma

at Te = 100 eV for NLTE model, Ne = 1019 cm-3,

Ni = 1018 cm-3 for a wide range of the wavelength

(10–180 A). From this figure it can be seen that the rect-

angle and its sharp edge obtained in the full spectra are

related to the variation of the recombination emission

versus wavelength. The obtained results in the extreme

a

b

Fig. 1 a Calculated full, Brem. and Recomb. spectra for nitrogen

plasma with Te = 100 eV. b Computed full EUV nitrogen plasma

focus spectra at Te = 100 eV for NLTE model

Fig. 2 Computed EUV oxygen plasma focus spectra at different

temperatures for NLTE model

678 J Fusion Energ (2014) 33:677–683

123

ultraviolet range showed that Te = 100 eV is the optimum

value for the strong emission at or near wavelength of

135 A (see Fig. 1b).

While for oxygen plasma Te = 125 eV is the suitable

temperature for higher EUV emission (see Figs. 2, 3).

Nitrogen and Oxygen Plasma as Soft X-ray Source

The ion distribution in the plasma has been investigated for

various conditions assuming the NLTE model. Then the

plasma radiations in the X-ray range have been simulated

using POPULATE and XRAYFIL codes. The computed

spectral lines emitted from a plasma are broadened by

Doppler broadening [27, 32].

The nitrogen plasma spectrum has been computed at dif-

ferent temperatures (Te in the range of 50–500 eV), electron

density (Ne in the range of 1018–1020 cm-3) and ion density

(Ni in the range of 1017–1019 cm-3). Figure 4 presents the

expected radiative X-ray emissions of nitrogen plasma focus

at Te = 100 eV for NLTE model, Ne = 1019 cm-3, Ni =

1018 cm-3. The effects of electron temperature and density on

the plasma emissions have been studied. The electron plasma

temperature influence on the radiative emission has been found

to be more dominant than electron density. Figure 5 shows the

variations of the expected full emission spectra of plasma focus

at various temperatures. As expected from theoretical con-

sideration of plasma emission [32], the continuum of the X-ray

emission spectrum is observed to shift towards shorter wave-

length (higher photon energy) with increasing electron tem-

perature. The relative population of the ionic species present is

also affected by the temperature.

Based on the corona model [33–35] the prominent spe-

cies present in nitrogen plasma at electron temperature

of 90 eV are N?5 and N?6, while at 150 eV, N?5, N?6 are

present with small fraction of N?7. Finally at electron

temperature higher than 200 eV, N?7 become prominent,

together with N?6. This will affect the recombination and

line radiations. At electron temperature much higher than

300 keV, when the plasma becomes fully ionized, Brems-

strahlung is expected to dominate. Numerical experiments

have also shown that the shapes of spectra for different

electron densities (1018–1020 cm-3) are similar, but differ-

ent in amplitude, due to the Ne2-dependence.

Figure 6 shows variation of the soft X-ray intensity (H-

like ions) versus electron temperature. From Fig. 6 it can

be seen that the most suitable temperatures for H-like is

found to be about 100–150 eV.

Fig. 3 Computed EUV oxygen plasma focus spectra at Te = 125 eV

for NLTE model, Ne = 1019 cm-3

23.0 23.5 24.0 24.5 25.0 25.5 26.0

1E20

1E21

1E22

1E23

1E24

1E25

1E26

1E27

1E28

1E29

λ = 24.9 Ao

λ = 24.781 Ao

Num

ber

of p

hoto

ns/c

m3 /s

ec./S

tera

d./A

o

Wavelength, Ao

Soft X-ray nitrogen spectrum at Te = 100 eV

Fig. 4 Computed spectrum for nitrogen plasma with Te = 100 eV

Fig. 5 Computed spectra of nitrogen plasma at three different

temperatures for Ne = 1019 cm-3

J Fusion Energ (2014) 33:677–683 679

123

Numerical experiments have also been performed on the

oxygen plasma to find the emission spectra at different

conditions. Figure 7 presents the expected radiative X-ray

emissions of oxygen plasma focus at Te = 200 eV for

NLTE model, Ne = 1019 cm-3, Ni = 1018 cm-3. The

electron temperature and density effects on the plasma

emissions have also been studied. Based on the corona

model [34–36] the prominent species present in oxygen

plasma at electron temperature of 150–200 eV are O?6 and

O?7, while at electron temperature higher than 300 eV,

O?8 become prominent, together with O?7. Figures 8 and 9

show variations of the soft X-ray intensity (He-like and

H-like ions) versus electron temperature. From Fig. 8 it can

be seen that the most suitable temperature for He-like is

found to be about 125 eV. While the most suitable tem-

perature for H-like ions is found to be about 200 eV (see

Fig. 9).

The plasma focus device is the simplest low-cost high

density hot plasma, and so many numerical experiments

simulating X-ray radiations emitted from plasma focus

devices and optimizing the plasma focus devices operating

with nitrogen and oxygen gases using Lee model for gen-

erating a maximum soft X-ray yield have been conducted

[33–39]. Numerical experiments using Lee model on

plasma focus operated with nitrogen and oxygen gases

showed that the suitable temperature windows for gener-

ating soft X-ray from nitrogen and oxygen are 74–173 and

119–260 eV for nitrogen and oxygen, respectively. And it

Fig. 6 Variation of H-like ion emission line intensity of nitrogen

plasma with Te for NLTE model

Fig. 7 Computed spectrum for oxygen plasma with Te = 200 eV

Fig. 8 Variation of He-like ion emission line intensity of oxygen

plasma with Te for NLTE model

Fig. 9 Variation of H-like ion emission line intensity of oxygen

plasma with Te for NLTE model

680 J Fusion Energ (2014) 33:677–683

123

has been shown that optimized operational and geometrical

conditions are required for improvement of the X-ray yield.

As an example, numerical experiments for optimizing the

UNU/ICTP PFF plasma focus device using Lee model

showed that the peak axial speeds suitable for maximum

nitrogen and oxygen soft X-ray yields are about 2.5 cm/ls

[37] and 4 cm/ls [39], respectively. Practically, these

needed axial speeds, with other optimized parameters to

reach ranges of temperatures suitable for generating soft

X-ray from low atomic number gases (nitrogen and oxy-

gen), could be achieved in the plasma focus devices. While

for plasma focus operated with high atomic number gases

like krypton and xenon, the required axial speeds suitable

for soft X-ray generation are higher than 11 cm/ls [40],

and it is not clear whether plasma focus devices will

operate in such high speed regimes for Kr and Xe. How-

ever, based on our obtained X-ray spectroscopic results

using the POPULATE and XRAYFIL codes, it can be said

that the suitable Te ranges for soft X-ray from nitrogen and

oxygen plasma correspond to the Te windows used in the

numerical experiments performed using Lee model.

Electron Temperature Measurements of Nitrogen

and Oxygen Plasma Based on Ratio Method

The electron temperature of the plasma can be determined

from the analysis of radiation in the X-ray region [41–43]

using the five channels BPX65 PIN diodes with foils of

different thicknesses [44–48]. The attenuated radiative

emissions of plasma through different channels of BPX65

PIN diodes with varying absorption filters have been cal-

culated using the Ratio-BPX65 code [49, 50]. Briefly, the

code has been written in FORTRAN 77 for studying the

effect of the response of BPX65 photodiode, with Mylar

and aluminium foils. The attenuated plasma spectrum

through aluminized mylar foils and detected by the BPX65

photodiode can be determined by using the following for-

mula [31]:

I0 ¼Z

allk

Pðk;TeÞ � SðkÞ � exp � lmylarðkÞxmylar dk ð1Þ

Similarly, the X-ray emission detected by diodes with

additional aluminium foils of various thicknesses can be

expressed as:

I¼Z

allk

Pðk;TeÞ�SðkÞ�exp� lmylarðkÞxmylarþlAlðkÞxAl

� �dk

ð2Þ

Finally, the ratio of the X-ray signals obtained by diodes

with additional aluminium foils against that with alumi-

nized mylar only can then be calculated as R = I/I0:

R¼ I

I0

¼

Rallk

Pðk;TeÞ �SðkÞ �exp� lmylarðkÞxmylarþlAlðkÞxAl

� �dk

Rallk

Pðk;TeÞ �SðkÞ �exp�lmylarðkÞxmylar dk

ð3Þ

In Eqs. (1), (2) and (3) S (k) is the BPX65 sensitivity, l is

mass absorption coefficient of material, and x is the

absorption foil thickness.

As an example, the radiative emission from the nitrogen

and oxygen plasma focus, actually detected by the BPX65

PIN diode with 12 lm aluminized Mylar and with addi-

tional aluminum foils of varying thicknesses (10–30 lm),

have been calculated. Figures 10 and 11 show the detected

nitrogen and oxygen plasma emission, respectively, which

show the attenuated X-ray intensities recorded after pass-

ing through different filters at Te = 100 eV for nitrogen

and Te = 200 eV for oxygen.

The signals recorded by the BPX65 detector provide

information about the time evolution of the X-rays pro-

duced by the plasma focus and these can be used to

determine the electron temperature of the plasma focus

by the X-ray foil absorption technique. For this purpose,

the sets of nitrogen and oxygen plasma spectra for dif-

ferent temperatures in the range of Te = 50–200 eV for

nitrogen and Te = 100–200 eV for oxygen have been

calculated to get the X-ray signal ratio R = I/I0 (see

Figs. 12, 13).

1011E8

1E10

1E12

1E14

1E16

1E18

1E20

1E22

1E24

1E26

1E28

Wavelength, Ao

Num

ber

of p

hoto

ns/c

m3 /s

ec./S

tera

d./A

o

Nitrogen specta Te = 100 eVNLTE, Ne = 1e19,

Full spectra Spectra + BPX65 + 12 m Mylar Spectra + BPX65 + 12 m Mylar + 10 m Al Spectra + BPX65 + 12 m Mylar + 30 m Al

μμμ

μμ

Fig. 10 Computed spectra of nitrogen plasma at electron temperature

Te = 100 eV through different sets of filters [BPX65 PIN diode with

12 lm aluminized Mylar (D.1), BPX65 PIN diode with 12 lm

aluminized mylar coupled to aluminum foil thickness of 10 lm (D.2),

and BPX65 PIN diode with 12 lm aluminized mylar coupled to

aluminum foil thickness of 30 lm (D.3))]

J Fusion Energ (2014) 33:677–683 681

123

These ratio curves can be used as calibration curves for

the measurement of electron temperatures for nitrogen and

oxygen plasmas. Finally, these numerical experiments

showed that for generation of EUV and soft X-ray from

plasma focus operated with nitrogen and oxygen optimized

operational and geometrical conditions are required. Then,

based on electron temperatures and ion densities of studied

plasma focus obtained by Lee model, we can proceed to

obtain spectroscopic information for EUV and X-ray

emissions of the plasma focus using XRAYFIL code.

Conclusions

The radiation emission spectra of nitrogen and oxygen

plasma at various plasma parameters have been computed

using the XRAYFIL code by assuming a NLTE model for

the plasma. Nitrogen and oxygen plasma focus spectra

have been calculated for plasma focus operation as soft

X-ray and EUV sources. The suitable electron temperature

ranges for soft X-ray and EUV emissions from studied hot

plasma were found. The calibration X-ray ratio curves for

electron temperature deduction of nitrogen and oxygen

plasma have been computed. These ratio curves could be

used as calibration curves for the measurement of electron

temperatures for nitrogen and oxygen plasma focus.

Finally, we believe that the simulation results presented

here may also be useful to set the condition for any pulsed

hot plasma source to be considered as EUV or soft X-ray

source.

Acknowledgments The authors would like to thank general director

of AECS for support, guidance and encouragement. C. S. Wong’s

participation in this work is supported by University of Malaya

research Grant RG204-11AFR.

1011E8

1E10

1E12

1E14

1E16

1E18

1E20

1E22

1E24

1E26

1E28

Wavelength, Ao

Num

ber

of p

hoto

ns/c

m3 /s

ec./S

tera

d./A

o

Nitrogen specta Te = 100 eVNLTE, Ne = 1e19,

Full spectra Spectra + BPX65 + 12 m Mylar Spectra + BPX65 + 12 m Mylar + 10 m Al Spectra + BPX65 + 12 m Mylar + 30 m Al

μμ

μμμ

Fig. 11 Computed spectra of oxygen plasma at electron temperature

Te = 200 eV through different sets of filters [BPX65 PIN diode with

12 lm aluminized Mylar (D.1), BPX65 PIN diode with 12 lm

aluminized mylar coupled to aluminum foil thickness of 10 lm (D.2),

and BPX65 PIN diode with 12 lm aluminized mylar coupled to

aluminum foil thickness of 30 lm (D.3)]

Fig. 12 Calculated X-ray ratio (R = I/I0) curves of BPX65 PIN

diode coupled to mylar (12 lm) and sets of BPX65 PIN diode

coupled to mylar (12 lm) with different aluminum foil thicknesses

(10, 20, 30, 40, and 90 lm) for X-rays of nitrogen plasma (NLTE,

Ne = 1019 cm-3) at various temperatures

0 10 20 30 401E-3

0.01

0.1

1

Rat

io

Al foil thickness ( m)

Spectra of oxygen plasmaNe = 1e19, NLTE

Te = 100 eV

Te = 150 eV

Te = 200 eV

μ

Fig. 13 Calculated X-ray ratio (R = I/I0) curves of BPX65 PIN

diode coupled to mylar (12 lm) and sets of BPX65 PIN diode

coupled to mylar (12 lm) with different aluminum foil thicknesses

(10, 20, 30, 40, and 50 lm) for X-rays of oxygen plasma (NLTE,

Ne = 1019 cm-3) at various temperatures

682 J Fusion Energ (2014) 33:677–683

123

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