Plasma and Fusion Research: Regular Articles

Volume 14, 3401069 (2019)

Generation of Spiral Shape Nitrogen Recombining Plasma for
Atomic Nitrogen Source∗)
Koji ASAOKA, Noriyasu OHNO, Yuki HAYASHI1) , Shin KAJITA2) and Hirohiko TANAKA
Graduate School of Engineering, Nagoya University, Nagoya 466-8603, Japan
National Institute for Fusion Science, National Institutes of Natural Sciences, Toki 509-5292, Japan
2)
Institute of Materials and Systems for Sustainability, Nagoya University, Nagoya 466-8603, Japan

1)(Received 9 January 2019 / Accepted 27 February 2019)
We propose a new method using a spiral shape nitrogen plasma with a long magnetic connection length toproduce high density nitrogen (N) atoms for nitriding application. A high density N molecular ion plasma was
generated by DC discharge and is transported along a long spiral magnetic field line. As a result, the electron
temperature drops along the magnetic field to be low enough to produce dissociate recombining plasma, where
dissociative recombination would occur to generate N atoms. 2D Langmuir probe measurement showed that theelectron temperature and density decreased along the magnetic field. Optical emission spectroscopy showed theratio of the atomic N and N molecular emission intensities increased with a discharge power. The ground state N
atom density was estimated by analyzing the atomic line intensity.
c 2019 The Japan Society of Plasma Science and Nuclear Fusion ResearchKeywords: nitrogen atom source, dissociative recombination, spiral shape plasma, nitriding method, NAGDIS-T
DOI: 10.1585/pfr.14.3401069

1. Introduction
It is commonly known that nitriding metal can improve abrasion, fatigue, corrosion, and heat resistance. Because it could make the lifetime of materials longer, various nitriding methods have been actively studied. Oneof the methods is gas nitriding with ammonia. However,this method requires to heat up materials over 50 hours
and produces fragile compound layer on the surface [1].
Another method is ion nitriding, which is carried out in anitrogen-hydrogen gas mixture at several Torr pressure. A
glow discharge is operated with the sample as the cathodeto be hardened [2]. This method also has several disadvantages: materials are damaged by ions accelerated by
the electric field and the surface treatment area depends on
plasma size.
In order to overcome the disadvantages mentioned
above, the radical nitriding method was proposed. In thismethod, the materials are irradiated with nitrogen atom (N)radicals to form a nitrided surface without damage by ion
bombardment. The most important task in this method
is to produce high density N atoms. However, with conventional plasma discharges, it is diﬃcult to generate highdensity N atoms because dissociation of N molecule byelectron impact collision hardly occurs due to the highbinding energy of the N molecule at 8.80 eV [3].
The electron-beam-excited plasma (EBEP) source has
been developed to generate the N atoms with the dissoauthor’s e-mail: asaoka.koji@f.mbox.nagoya-u.ac.jp∗) This article is based on the presentation at the 27th International Toki
Conference (ITC27) & the 13th Asia Pacific Plasma Theory Conference(APPTC2018).

Fig. 1 Rate coeﬃcient of electron impact ionization and dissociative recombination processes.

ciation process of N molecules induced by high energetic
electron beam, and the generated atomic N density is of
the order of 1017 m−3 [4]. After 6.5 hours treatment by thismethod, The nitride layer of 100 µm was obtained and thehardness of the surface reached 1000 HV(0.1) [5]. Figure 1
shows the rate coeﬃcient of ionization and dissociative
recombination between 0.01 and 100 eV. The rate coeﬃcient of ionization was calculated from the cross-section
data reported by Halas et al. [6]. Peterson et al. reported
that the rate coeﬃcient of dissociative recombination data
were well fitted between 0.01 and 0.1 eV with the formula;σv = 1.70 × 10−13 (eT e /300kB )−0.30 [m3 s−1 ], where e isthe elementary charge and kB is the Boltzmann constant[7]. The EBEP method aims to generate the N atoms byelectron impact dissociation (N2 + e− → N + N+ + 2e− ).
However, the figure indicates that molecular N ions are
dominant ion species in the conventional discharge with an

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c 2019 The Japan Society of Plasma
Science and Nuclear Fusion Research

Plasma and Fusion Research: Regular Articles

Volume 14, 3401069 (2019)

Fig. 2 Schematic of our proposed method to produce high density nitrogen atom.

electron temperature less than 10 eV due to the ionization
process (N2 + e− → N+2 + 2e− ).
On the other hand, when the electron temperature
is decreased to be less than 1 eV, the rate coeﬃcient forthe dissociative recombination of the molecular N ions
(N+2 + e− → N + N) becomes significantly large to produce
N atoms from molecular N ions. Based on this process,the alternative method, an expanding supersonic nitrogenarc-jet plasma has been extensively researched for understanding about the reentry of a shuttle into the Earth’s atmosphere and for applications of engineering, such as thepolymerization and the metal surface modification [8–10].
In the arc-jet plasma, N molecules are highly ionized andflow to expansion region. Because of the diameter of thechamber is much larger than the diameter of the nozzle
electrode, electrons with residual gas molecules in the expanding region are rapidly cooled. In this process, dissociative recombination occurs and N atoms are generated.
The expanding nitrogen arc-jet method is expected to become high density atomic N source and creates a high quality nitride layer on the surface of the material with economic excellence. Akatsuka et al. reported the generated
N atom density is 1024 m−3 maximum at the arc discharge
region. The loss of nitrogen atoms to the wall is small
because of supersonic arc-jet [11]. Ichiki et al. reported
about a new nitriding method in which pulsed-arc plasma
jet with N and hydrogen mixture gas is used. This method
can be used in the atmospheric pressure. Further, materials are not biased, contributing to the low ion damage on
the material surfaces. However, in this method, thin oxide
layer is formed because of residual oxygen and it interrupts
nitriding. In addition, the surface treatment area depends
on the pulsed-arc plasma size [12].
In this paper, we propose an alternative new method togenerate a higher density atomic N source based on dissociate recombining process. Figure 2 shows the schematicof our proposed method to produce high-density nitrogenatom. First, a high density and large size plasma in whichmolecular N ions are dominating is generated in the upstream region by utilizing DC discharge. Next, the produced high density and large size plasma is transported
along the long magnetic field and cooled down to ∼1 eV.
As a result, the plasma condition is changed from ionizing to recombining phase, where dissociative recombina-

Fig. 3 (a) The top view of the NAGDIS-T. (b) The schematic
diagram of NAGDIS-T with two types of the magnetic
fields.

tion would occur to generate high density N atoms. It is
expected that high-density N atoms could realize a faster
nitriding process without the ion injection damages. Further, a high-quality diﬀusion layer which has large surface
area thickness uniformity is expected to be formed on thesurface without forming the compound layer.
It is an important issue how to generate high density
N plasma with long magnetic connection length with a relatively small apparatus size.

2. Experimental Setup
Figure 3 (a) shows a top view of NAGDIS-T (Nagoya
University Divertor Simulator with Toroidal magnetic
configuration) [13]. A toroidal angle (ϕ) is defined as theangle from the cathode in the clockwise direction. The
major radius is 340 mm and poloidal cross-section is 280
× 180 mm2 . This device has two magnetic fields, toroidal
magnetic field (BT ) and vertical magnetic field (Bv ), as
shown in Fig. 3 (b). The maximum BT was 0.10 T and the
maximum Bv was 3.0 × 10−3 T. By using the two magnetic
field, we can generate spiral shape magnetic field with thelong connection length (LC ) between the top and bottom
walls.
Figure 4 (a) show a picture of the plasma production
region. The cathode was located at the bottom of thevacuum vessel. A circular-shape lanthanum hexaboride(LaB6 ) with a diameter of 108 mm, which was heated bya carbon heater. The anode was a rectangle plate made3401069-2

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Fig. 4 (a) The NAGDIS-T DC discharge system. (b) The
schematic diagram of the 2D Langmuir probe measurement.

Fig. 5 Pictures of spiral shape plasmas at (a) pn = 3.1 mTorr,
Id = 10 A, Vd = 130 V, (b) pn = 6.7 mTorr, Id = 10 A, Vd= 60 V, and (c) pn = 1.1 mTorr, Id = 1 A, Vd = 190 V.

of tungsten. The width was 170 mm, and the height was
35 mm.
A 2D Langmuir probe was installed at ϕ = 90◦ . 2D
distribution of ion saturation current Isat was obtained inside the green square region, shown in Fig. 4 (b). The electron temperature, T e , and density, ne , were also evaluatedby single probe method.
A 0.75-m Czerny-Turner spectrograph ( f = 750 mm)
with a 1800 grooves/mm grating was used at ϕ = 240◦ .
The measurement resolution was 0.013 nm, and the slit
width was 50 µm.

3. Experimental Results
3.1

2D distribution of ion saturation current
at various discharge conditions

Figures 5 (a - c) show pictures of the plasmas at BT =8.3 mT and Bv = 0.60 mT taken at ϕ = 60◦ with diﬀerent
gas pressures. The pitch angle against the horizontal planewas θ = 4.1◦ , and LC = 3.88 m. The neutral pressures,
discharge currents and discharge voltages of plasmas were,respectively, (a) pn = 3.1 mTorr, Id = 10 A, and Vd =130 V, (b) pn = 6.7 mTorr, Id = 10 A, and Vd = 60 V, and(c) pn = 1.1 mTorr, Id = 1 A, and Vd = 190 V. Figures 5 (a c) show that spiral shape plasmas were generated from thevicinity of the cathode.
Figure 6 shows the 2D distribution of Isat at ϕ = 90◦ .
Poloidal cross-section of the cathode was mapped as red
squares on the poloidal cross section. The first turn plasma
in all pn was located at radially outer-side edge of the redsquares for all the cases.
At pn = 3.1 mTorr (Fig. 6 (a)), the full widths at half
maximum of Isat distribution of the first and second turns
were 62 and 77 mm, respectively. Therefore, the broadening ratio of Isat due to plasma transport along the magnetic field line (2 m) was 24%. Further, the second turnplasma was drifted to the lower left. At higher pressure, pn= 6.7 mTorr (Fig. 6 (b)), the Isat at the peak position of thesecond turn was very small; Isat distribution was re-plottedin log scale (Fig. 6 (d)). The full widths at half maximum

Fig. 6 (a) 2D distribution of ion saturation current at 3.1 mTorr,(b) 6.7 mTorr and (c) 1.1 mTorr. (d) 2D distribution ofion saturation current at 6.7 mTorr in log scale.

of Isat distribution of the first and second turns were 41 and
74 mm, respectively. At a lower pressure, pn = 1.1 mTorr(Fig. 6 (c)), DC discharge was unstable, leading to the scattered Isat distribution.

3.2 Power dependences of electron temperature and density
We investigated discharge power dependences of T e
and ne at Isat peak positions in Fig. 6. At pn = 3.1 mTorr, theplasma parameters were measured in the first and secondturns, whereas the parameters was measured only in thesecond turn at pn = 6.7 mTorr.
Figure 7 (a) shows the power dependances of T e . In
the first and second turns, T e increased with the discharge power at pn = 3.1 mTorr. The T e decreased to3401069-3

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Fig. 8 Spiral shape DC nitrogen discharge emission spectrum
around (a) 745 nm, and (b) 821 nm.

Fig. 7 (a) Power dependences of electron temperature at various
situations, and (b) Power dependences of electron density
at various situations.

approximately half while toroidally rotated from the firstto the second turn. At pn = 6.7 mTorr, T e in the second turn slightly decreased from T e at pn = 3.1 mTorr at
Pd ≥∼ 2 kW. Figure 7 (b) shows the power dependancesof ne . The first and second turns of ne at pn = 3.1 mTorr
and second turn of ne at pn = 6.7 mTorr increased with thedischarge power. The ne decreased to approximately 70%
while toroidally rotated from the first to the second turn.
At pn = 6.7 mTorr, ne in the second turn slightly decreased
from ne at pn = 3.1 mTorr.
From the above results, it was found that T e andne strongly decreased along the magnetic field at pn =3.1 mTorr. Because the diﬀusion eﬀect was not so large
as described in section 3.1, it was suggested that the second turn plasma at pn = 3.1 mTorr became recombining
plasma. Furthermore, it was suggested that the recombining process was facilitated at pn = 6.7 mTorr [14].

3.3

Optical emission spectroscopy (OES)

Vaudo et al. had performed optical measurement in an
ECR nitrogen plasma [15]; first positive series (1PS) emission is prominent around the wavelength of 745 nm. We

conducted optical emission spectroscopy in wavelengths
around 745 nm and 821 nm.
The discharge condition was as follows: pn = 3.1
mTorr, Id = 10 A, and Vd = 110 V. The measurement was
conducted at ϕ = 240◦ . Figures 8 (a) and (b) show emission spectra at the wavelength around 745 and 821 nm,
respectively. Spectra peaks of nitrogen atoms were seen
at 742.4 (4 P1/2 −4 S3/2 ), 744.2 (4 P3/2 −4 S3/2 ), and 746.8 nm(4 P5/2 −4 S3/2 ) in Fig. 8 (a). Also, the 1PS was observed in
the wavelength range of 739 - 752 nm. Around 845 nm,
as was suggested previously [15], many nitrogen atomic
emission were identified: 818.5 (4 P3/2 −4 P5/2 ), 818.8(4 P1/2 −4 P3/2 ), 820.0 (4 P1/2 −4 P1/2 ), 821.1 (4 P3/2 −4 P3/2 ),821.6 (4 P5/2 −4 P5/2 ), 822.3 (4 P3/2 −4 P1/2 ), and 824.2 nm(4 P5/2 −4 P3/2 ). It was suggested that both nitrogen atoms
and molecules existed in this plasma.
Figure 9 shows discharge power dependences of theratio of the atomic to molecular emission intensities defined askI =I746.8 [N],2I1PS [N2 ]

(1)where I746.8 [N] is the emission intensity of 746.8 nm,
which is the strongest emission from nitrogen atoms observed, and I1PS [N2 ] was the averaged 1PS emissions intensity. At pn = 6.7 mTorr, there was no clear emission
when with Pd = 0.89 and 1.6 kW. However, with increasing the discharge power, the emissions were observed at

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Fig. 10 Discharge power dependences of densities of groundstate nitrogen atom.

Fig. 9 Discharge power dependences of kI .

Pd = 2.7 kW, and kI increased slightly at Pd = 3.7 kW. At
pn = 3.1 mTorr, with increasing the discharge power to Pd= 4.6 kW from Pd = 1.1 kW, kI increased and was greaterthan at pn = 3.1 mTorr. As will be discussed later, this wasprobably caused by the variation in T e .
Since the population distribution was mainly determined by recombination component in recombining plasmas, which is not sensitive to the atomic density, it is difficult to assess the atomic density from the emission intensity. However, here, since the electron temperature could
be higher than 2 eV at pn = 3.1 mTorr, it was likely that the
population was mainly determined by the electron impact
excitation, though the process to generate atomic nitrogen
could be mainly produced by dissociative recombination,
as was described previously. Thus, for the first trial, we estimated the atomic nitrogen density with applying coronalequilibrium to the excited states. The densities of 3p 4 S2/3
state nitrogen atom can calculate as∗
I746.8[N]= n(3p 4 S3/2 ),4A(3p S3/2 , 3s 4 P5/2 )(2)∗[N] [sr−1 nm−1 m−2 s−1 ] is the calibrated intenwhere I746.8
sity and A(3p 4 S3/2 , 3s 4 P5/2 ) is the transition probability of deexcitation to 3s 4 P5/2 from 3p 4 S3/2 . Assumingthat electron impact excitation from the ground state is thedominant production process, the density of ground state(2p 4 S3/2 ) nitrogen atom can be calculated using the following relation [16]:n(3p 4 S3/2 ) =C(2p 4 S3/2 , 3p 4 S3/2 ) nen(2p 4 S3/2 ),A<3p 4 S3/2

(3)
where <3p 4 S3/2 A is the sum of the all transition probability of deexcitation from 3p 4 S3/2 . The rate coeﬃcient C
can be obtained in [17]. We could not measure T e and neat the same position as the OES; we interpolated the value

from the electrostatic probe measurement performed at 150
and 210 degrees upstream and downstream from the OES
position, respectively.
Figure 10 shows the discharge power dependences ofdensities of ground state nitrogen atom. The error of thenitrogen atom density (nN ) was assessed here that interpolated T e and ne has errors of ±20% in the interpolated
angles. The nN was approximately 1 × 1017 m−3 .
It should be noted that the actinometry measurement
method which uses emission intensity of Ar atom andcollisional-radiative model is well known assessing the accurate density [18].

4. Summary
We have proposed a new method to generate high density nitrogen atoms based on dissociative recombination ofmolecular nitrogen ions. The usefulness of this method
was experimentally verified in the toroidal plasma device,
NAGDIS-T. Generation of high density nitrogen plasmaalong the spiral shape magnetic field was confirmed by 2D
distribution of Isat measurements. The electron temperature T e and density ne strongly decreased while the plasma
toroidally being rotated from the first to second turn andalso increased in neutral pressure pn . The optical emission spectroscopy suggested that both nitrogen atoms andmolecules existed in the plasma. The ratio of the atomic
to molecular emission intensities increased with the discharge power. The atomic nitrogen density nN estimatedwith applying coronal equilibrium was ∼1017 m−3 .

Acknowledgments
This research was supported by Grant-in-Aid for
Challenging Exploratory Research (17K18768) from theJapan Society for the Promotion of Science (JSPS). This
research was also supported by a grant from the Hibi Science Foundation and the NINS program of Promoting Research by Networking among Institutions (01411702). The
authors are deeply grateful to Mr. M. Takagi for technicalsupport.

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Volume 14, 3401069 (2019)[1] T. Takase, Tetsu-to-hagane 66, 9, 1423 (1980).
[2] M. Hudis, J. Appl. Phys. 44, 1489 (1973).
[3] J.M. Hendrie, J. Chem. Phys. 22, 1503 (1954).
[4] S. Tada et al., J. Appl. Phys. 88, 1756 (2000).
[5] H. Shoyama et al., J. Vac. Sci. Technol. 24, 1999 (2006).
[6] St. Halas et al., Int. J. Mass Spectram. Ion Phys. 10, 157(1972).
[7] J.R. Peterson et al., J. Chem. Phys. 108, 1978 (1998).
[8] H. Tahara, et al., IEEE Trans. Plasma Sci. 26, 1307 (1998).
[9] Q.S. Yu et al., Plasma Chem. Plasma Proc. 18, 461 (1998).
[10] L. Robin et al., Phys. Plasmas 1, 444 (1994).

[11] H. Akatsuka et al., IEEE Trans. Plasma Sci. 42, 3691
(2014).
[12] R. Ichiki et al., Mater. Lett. 71, 134 (2012).
[13] K. Yada et al., J. Nucl. Mater. 390-391, 290 (2009).
[14] N. Ohno, Plasma Phys. Control. Fusion 59, 3, 034007(2017).
[15] R.P. Vaudo et al., J. Vac. Sci. Technol. B: 12, 1232 (1994).
[16] M. Goto et al., J. Plasma Fusion Res. 79, 12, 1287 (2003).
[17] R.M. Frost et al., J. Appl. Phys. 84, 6 (1998).
[18] Y. Ichikawa et al., Jpn. J. Appl. Phys. 49, 106101 (2010).

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