富山大学水素同位体科学研究センター研究報告34：21－30，2014.

論文
グロー放電発光分析法による炭素堆積層の分析
波多野雄治 1, 信太祐二 21

富山大学水素同位体科学研究センター〒930-8555 富山市五福 3190
2
北海道大学大学院工学研究院量子理工学部門プラズマ理工学分野〒060-8628 札幌市北区北 13 条西 8 丁目

Measurements of Carbon Deposition Layers using Glow-Discharge
Optical Emission Spectroscopy (GDOES)
Yuji Hatano1, Yuji Nobuta2
1

Hydrogen Isotope Research Center, University of Toyama,
Toyama 930-8555, Japan
2
Division of Quantum Science and Engineering, Graduate School of Engineering,
Hokkaido University
Sapporo 060-8628, Japan(Received April 13, 2015; accepted July 5, 2015)
AbstractIn fusion devices with plasma-facing components manufactured fromcarbon-based materials, co-deposition of hydrogen isotopes and carbon takes place inregions where sputtering rate is relatively small. Fuel retention and recycling depend on
properties of those deposits. Glow-discharge optical emission spectroscopy (GDOES) is
one of attractive techniques to analyze chemical composition and thickness of these
deposition layers. However, influence of hydrogen content on sputtering of these layers
by glow-discharge plasma is not fully understood. In this study, two different types ofcarbon layer with quite different hydrogen content were deposited on stainless steel

21波多野雄治・信太祐二

substrate, and sputtering rate during the GDOES measurement was examined for betterinterpretation of results obtained.

1. Introduction
Carbon materials such as isotropic graphite and carbon fiber composite havebeen used as plasma-facing materials in fusion device because of various preferable
properties of carbon that includes high thermal conductivity, high thermal shock
resistance and low efficiency of radiation cooling of plasma due to small atomic numberof carbon. However, interaction of hydrogen isotope plasma with carbon-based
plasma-facing materials results in sputtering erosion. Eroded particles emitted from theplasma facing materials will form re-deposited carbon layers in areas where deposition
rate is larger than erosion rate. These deposition layers in general contain a large
amount of hydrogen isotopes and consequently have strong influences on tritium
inventory and fuel recycling. In addition, the deposition layers degrade reflectivity ofmirrors for plasma diagnostics. It is therefore important to reveal properties of thecarbon deposition layers such as thickness, density, contents of hydrogen isotopes, etc.
Glow discharge optical emission spectroscopy (GDOES) is a technique tomeasure depth profiles of constituent elements in a solid specimen by detecting
emissions from atoms accommodated in plasma by sputtering of a specimen surface[1,2]. This technique allows very quick measurements with relatively high depth
resolution (a few nanometers) because ultra-high vacuum is not necessary. Besides,
large-sized specimens are acceptable in the GDOES measurements. In addition, isotopic
analysis of hydrogen and He detection are also possible [3]. These features of theGDOES technique are in general suitable for analysis of carbon deposition layers

22グロー放電発光分析法による炭素堆積層の分析

formed in fusion devices. Indeed, Akiyama et al. [4] analysed the corner cube mirrors
used in the Large Helical Device (LHD) and found that a thick carbon deposition layer
was formed during the hydrogen glow discharge cleaning before the main plasma
discharge.
There are, however, still several issues to be solved for detailed analysis ofcarbon deposition layers. One of those is influence of hydrogen content on sputteringrate. Sputtering rate is an essential parameter in depth profile measurements to convert
sputtering time to the depth. In addition, signal intensity in GDOES measurements is
proportional to sputtering rate. Carbon deposition layers formed in fusion devices
contains hydrogen isotopes in various concentration levels, but the correlation between
sputtering rate of the carbon deposition layers and hydrogen isotope content has notbeen examined. Another issue is change in sputtering rate at interface between thedeposition layer and substrate material. In the depth profile reported by Akiyama et al.
[4], intensity of carbon signal increases at the interface between the deposition layer andsubstrate stainless steel and then decreases with increase in depth in the substrate. Oneof the possible explanations for the observed increase in carbon intensity at the interface
is the increase in sputtering rate at the interface. However, this idea has not beenconfirmed.
In this study, two types of diamond-like carbon films with different hydrogencontents were deposited on the stainless steel substrate and analyzed using the GDOES
technique. Additionally, sputtering rates of these films and substrates were examinedusing a surface profiler. The influence of hydrogen content on the sputtering rate wasdiscussed as well as the mechanisms underlying the increase in carbon intensity at theinterface.

23

波多野雄治・信太祐二

2. Experimental
Plates of type-304 stainless steel (SS304) (50×20×1 mm) were used as substrates.
These SS304 plates were mirror-finished and then heated in vacuum (10-5 Pa) at 1073 K
for 4 hours to remove hydrogen being present in the plates as an impurity. Two different
types of diamond-like carbon films were deposited on the steel plates in Hokunetsu Co.,
Ltd., Toyama, Japan. The first type was a conventional diamond-like carbon film
fabricated using a technique of unbalanced magnetron sputtering. The source of carbon
was a solid target and CH4 gas. In what follows, this film will be denoted as the DLCfilm. The second type was a tetrahedral amorphous carbon film prepared using an arc
ion plating technique with a solid carbon target, denoted hereafter as the DLC-i film.
According to the manufacturer, the mass densities of the DLC and DLC-i films are 2.2and 3.2 g cm-3, respectively.
The hydrogen contents in the DLC and DLC-i films were measured using thermal
desorption spectroscopy (TDS). The samples were heated up to 1273 K at a ramp rate of
0.5 K s-1. The films were analyzed using GDOES apparatus (Horiba GD-Profiler2)
operated with Ar plasma at 650 Pa and 35 W. The depth of craters formed by Ar
sputtering was measured using a surface profiler (Tokyo Seimitsu SURFCOM 1500DX).
All these measurements were performed at the Center for Research and Development in
Natural Sciences, University of Toyama.

3. Results
Thermal desorption spectra of H2 released from the DLC and DLC-i films areshown in Fig. 1. For the DLC film, the H2 desorption started at about 530 K, whereas
for the DLC-i film this temperature was shifted to about 830 K. The amount of H224

グロー放電発光分析法による炭素堆積層の分析

desorbed from the DLC film was 1.5×1022 m-2, whereas for the DLC-i film this value
was 2.3×1020 m-2. Note that the H content in the DLC film was higher by a factor of 65
than that in the DLC-i film because of use of CH4 gas in the fabrication process.
Depth profiles of C and Fe, the main constituent elements in the DLC specimen(i.e., in the DLC film deposited on the SS304 steel substrate), as measured by theGDOES technique, are shown in Fig. 2a. The interface between the deposited carbon
layer and the steel substrate appears after sputtering for about 100 s. To determine
sputtering rates of the diamond-like carbon deposition layer and the steel substrate,
several different areas on the DLC specimen surface were subjected to sputtering for 30,
60, 98 (the interface) and 210 s, and depths of sputtering-formed craters were measured
using the surface profiler.

Temp.

1400
1200

1019

DLC

1000

1018

800

1710600
DLC-i

1016
1015

0500

Temperature (K)Desorption flux (H2 m-2s-1)

1020

400

200
1000 1500 2000 2500 3000
Time (s)Fig. 1 Thermal desorption spectra of H2 released from a conventional
diamond-like carbon film fabricated using a technique of unbalanced magnetron
sputtering (DLC) and a tetrahedral amorphous carbon film prepared using an arc
ion plating technique (DLC-i).

25

波多野雄治・信太祐二

Fig. 2b shows the depth profiles of C and Fe in the DLC-i specimen, in which theinterface is observed at the sputtering time of 13 s. For this DLC-i specimen, the crater
depth was measured after the sputtering for 8, 13 (the interface) and 100 s. The

Intensity (arb. unit)

350

DLC

250
200

C

150
100

Fe500350
Intensity (arb. unit)

(a)

300

300

050
100
150
Sputtering time (s)

(b)

200

DLC-i

250200
150
100
500

C

05Fe

10 15 20 25 30 35 40
Sputtering time (s)Fig. 2. Depth profiles of C and Fe in the DLC (a) and DLC-i (b) films, as measuredby the GDOES technique. The crater depth was measured after sputtering for time
period indicated by arrows. Results of the measurements are summarized in Table 1.

26

グロー放電発光分析法による炭素堆積層の分析

sputtering for 4 s was also performed, but accurate measurement of the crater depth was
impossible due to too small depth.
Average sputtering rates and C signal intensities for the DLC and DLC-i
specimens are summarized in Table 1. According to data summarized in Table 1, athickness of the DLC and DLC-i films was 1600±60 and 240±40 nm, respectively.
From these values of the layer thickness and the above-mentioned mass density andhydrogen retention, the average hydrogen concentration in the DLC film was evaluatedto be [H]/[C]  0.17, whereas in the DLC-i film the value [H]/[C]  0.01; i.e., theaverage hydrogen concentration in the DLC-i film was lower by a factor of 17 than thatin the DLC film. In this evaluation, the hydrogen concentration in the substrate wasassumed to be negligibly small.
After beginning of sputtering, the C signal intensity (expressed in arbitrary units,a.u.) for the DLC film (~62 a.u.) was comparable with that for the DLC-i film (~65 a.u.).

Table 1 Results of depth profiling of the DLC (Fig. 2a) and DLC-i (Fig. 2b) films.

Specimen

DLC

DLC-i

Sputtering
time (s)Depth (nm)Average sputtering
rate, Sav (nm s-1)

C signal
intensity(arb. units)25

180±30

7±2

66

60

610±60

10±1

78

98

1,600±60

16.3±0.6288

210

10,000±800

48±4–

4–

–

67

8

90±20

11±3

77

13240±40

18±3

130

100

5,600±560

56±6–

27

波多野雄治・信太祐二

This C signal intensity increased with an increase in the crater depth and reached
310 a.u. for the DLC specimen and 140 a.u. for the DLC-i specimen at the interface, asshown in Figs. 2 (a) and (b). It should be noted that the average sputtering rate, Sav, also
increased with an increase in the depth (Table 1). In other words, the sputtering rate ofthe diamond-like carbon deposition layers was clearly smaller than that of the steel
substrate.

4. Discussion
In GDOES measurements, the signal intensity of element i, Ii, isIi = a·Wi + b,(1)where a is the sensitivity factor of element i, Wi is the mass of element i sputtered perunit area in unit time (g·cm-2·s-1), and b is the background. Here,
Wi = Ci · ·S,(2)

where Ci is the mass fraction of element i in a sample,  is the mass density of thesample (g·cm-3) and S is the sputtering rate (cm·s-1).
Therefore,
Ii = a·Ci · · S + b.

(3)

As described above, the C signal intensity at the interface was significantly
higher than that at the near-surface region of the DLC and DLC-i films. In addition, thesputtering rate increased with increase in the crater depth. As indicated by Eq. (3), the C
signal intensity is proportional to the sputtering rate. In the case of the DLC film, the C
signal intensity at the interface (after sputtering for 98 s) was 288 a.u., and larger thanthat in the deposition layer (66 a.u. after sputtering for 25 s) by a factor of 4.4. On theother hand, the average sputtering rate, Sav, during sputtering at 0–25 s was 7±2 nm·s-1

28

グロー放電発光分析法による炭素堆積層の分析(Table 1), whereas for sputtering at 60–98 s the value of Sav was evaluated to be
26±2 nm·s-1; the latter value was larger than the former one by a factor of 3.7. Thus, thedifferences in the C signal intensity and sputtering rate were comparable with each other.
Hence, it is appropriate to consider that the increase in the C signal intensity at theinterface was due to increase in the sputtering rate. It was impossible to perform thesimilar comparison for DLC-i film since the crater depth after sputtering for 4 s could
not be measured, as mentioned above. Further investigation on the interfacemicrostructure (mixing of C and metal atoms, carbide formation, etc) is necessary tounderstand the mechanisms underlying the enhanced sputtering at the interface.
The C signal intensities for the DLC and DLC-i films immediately after
beginning of sputtering were comparable. This observation suggests that, for the DLC
and DLC-i films, the masses of C sputtered per unit area in unit time were also
comparable (see Eq. (1)) despite the significantly different hydrogen concentration([H]/[C]  0.17 in the DLC film and [H]/[C]  0.01 in the DLC-i film). It means that theC signal intensity in the GDOES measurement was insensitive to hydrogen content indeposited diamond-like carbon film. On the other hand, because the mass density of theDLC-i film (3.2 g·cm-3) was higher than that of the DLC film (2.2 g·cm-3), thesputtering rate (nm·s-1 or cm·s-1) of the DLC-i film should be smaller than that of theDLC film (see Eq. (3)).

5. Summary
Two different types of diamond-like carbon films were deposited on type 304
stainless steel substrate and examined using the GDOES technique operated with Arplasma. The first carbon film was characterized by the mass density of 2.2 g·cm-3 and29

波多野雄治・信太祐二

hydrogen concentration of [H]/[C]~0.17. For the second carbon film, the corresponding
parameters were 3.2 g·cm-3 and [H]/[C]~0.01, respectively. In the GDOES
measurements, the C signal intensities for these two films were comparable in a value
despite the significant difference in the hydrogen concentration. This observationsuggested that the amount of C sputtered per unit area and unit time was insensitive tohydrogen content. The C signal intensity markedly increased at the interface between
the deposited film and stainless steel substrate. This increase in the C intensity wasascribed to an increase in the sputtering rate at the interface.

Acknowledgements
This study has been supported by JSPS KAKENHI Grant Number 23560998.
The authors express their sincere thanks to Prof. T. Yamazaki, Graduate School ofScience and Engineering, University of Toyama, for the use of surface profiler.

References[1]

R. Payling, D. Jones, A. Bengtson, Glow Discharge Optical EmissionSpectroscopy, John Wiley & Sons, Chichester, 1997.

[2]

T. Nelis, R. Payling, Glow Discharge Optical Emission Spectroscopy: A PracticalGuide, Royal Society of Chemistry, London, 2004.

[3]

Y. Hatano, J. Shi, N. Yoshida, N. Futagami, Y. Oya, H. Nakamura, Fusion Eng.
Design, 87 (2012) 1091.

[4]

T. Akiyama, N. Yoshida, M. Tokitani, S. Masuzaki, K. Kawahata, S. Okajima,
K. Nakayama, Plasma Fusion Res., 8 (2013) 1402092.

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