22P022-O
Measurement of radical densities in microwave plasma CVD employing
carbon-containing gas mixture
炭素含有ガス混合を用いたマイクロ波プラズマCVD中のラジカル計測
Kenichi Murata1, Masateru Naito1, Mineo Hiramatsu1, Seigo Takashima2
Hiroki Kondo3 and Masaru Hori3
村田健一1, 内藤全晃1，平松美根男1，高島成剛2，近藤博基3，堀 勝3
1

Department of Electrical and Electronic Engineering, Meijo University,
1-501 Shiogamaguchi, Tempaku, Nagoya, 468-8502, Japan,
2
Plasma Center for Industrial Applications, Nagoya Industries Promotion Corporation,
2268-1 Anagahora, Shimoshidami, Moriyama, Nagoya 463-0003, Japan3
Department of Electrical Engineering and Computer Science, Nagoya University,Furo-cho, Chikusa, Nagoya, 464-8603, Japan1
名城大学大学院理工学研究科電気電子工学専攻 〒468-8502 名古屋市天白区塩釜口1-501
2
プラズマ技術産業応用センター 〒463-0003 名古屋市守山区穴ヶ洞2268-1
3
名古屋大学大学院工学研究科電子情報システム専攻 〒464-8603 名古屋市千種区不老町
To realize high-performance plasma process, it is important to elucidate the specific species that contributeto the deposition. And then, it is necessary to control the process plasma to obtain carbon nanostructureswith structure and morphology customized for a specific application on the basis of the knowledge of thespecies. In this study, carbon (C) atom density was measured using vacuum ultraviolet absorption
spectroscopy (VUVAS) with micro-discharge hollow-cathode lamp (MHCL). It was found C atom density
was to be 1.2×1012 cm-3 at CH4/(CH4+H2) flow rate of 2% below, while was below the detection limit(~1011 cm-3) over CH4 flow rate of 3%. This suggests that background absorption by source gas moleculesand species produced in plasma was dominant and the determination of C atom density could not be
attained when the CH4 concentration increased.

1. Introduction
Microwave plasma, which is one ofhigh-density plasmas and is suitable fordecomposing H2 molecules to generate H atom
effectively, has been used to fabricate carbon
allotropes including diamond, nanocrystalline
diamond (NCDs), and carbon nanotubes (CNTs).
The main reaction mechanisms of deposition
using plasmas are determined by the species
produced in the plasma. To realize highperformance plasma process, it is important tofirst elucidate the specific species that contributeto the deposition. Second, on the basis of theknowledge of the species, it is necessary tocontrol the process plasma to obtain carbon
nanostructures with structure and morphology
customized for a specific application.
Thus far, we measured C 2 radical density at thelowest excited state using white-light absorption
spectroscopy with Xe lamp emitting a continuous
spectrum as a light source and to be on the orderof 1012 cm-3 at CH4/(CH4+H2) flow rate of 25-30%[1]. In this study, carbon (C) atom density wasmeasured using vacuum ultraviolet absorption
spectroscopy (VUVAS).

2. Experimental
Figure 1 shows a schematic of experimentalset-up for C atom density measurement using
VUVAS with high pressure CO2 micro-discharge
hollow-cathode lamp (C-MHCL) as a VUV lightsource for absorption spectroscopy [2,3]. The
transition lines used for C atom density
measurements were 2s22p2 3P1 – 2s22p3s 3P2o at
165.626 nm, 2s22p2 3P0 – 2s22p3s 3P1o at 165.692nm, 2s22p2 3P2 – 2s22p3s 3P2o at 165.700 nm, 2s22p23P1 – 2s22p3s 3P1o at 165.737 nm, 2s22p2 3P3 –2s22p3s 3P1o at 165.789 nm, and 2s22p2 3P5 –2s22p3s 3P3o at 165.811 nm for the C atom. Since
the wavelength resolution of the VUVmonochromator was 0.4 nm, the total absorption
intensity of six transition lines 2s22p2 3PJ – 2s22p3s
3PoJ (J=0, 1, 2) at 165.7 nm was measured for theabsolute C atom density. He line at 164.0 nm andXe lamp emitting a continuous spectrum around165.7 nm were used for the evaluation ofbackground absorption.
Measurement was carried out employing CH4/H2
mixture at a total pressure of 55 Torr and a
microwave power of 800 W. Total gas flow rate
was kept at 200 sccm, and CH4/(CH4+H2) flow rate
was changed.

3. Results and discussion

Microwave power(2.45 GHz)
Quartz window

PlasmaVUV
Monochromator

MHCL

MgF2 window

MgF2 window

Fig. 1 Schematic of experimental set-up used for C atom
density measurement with MHCL (micro-dischargehollow-cathode lamp) as a VUV light source.
CH4流量比に対する吸収率（C、He）の変化
10080

Absorption
rate [%]
吸収率[%]

Figure 2 shows the absorption intensities
measured at C atom resonance line and backgroundabsorption as a function of CH4 flow rate ratio. Catom density was estimated to be 1.2×1012 cm-3 at
CH4/(CH4+H2) flow rate below 2%. As the flowrate ratio increased above 2%, on the other hand,the behavior measured at C atom resonance line
was almost the same as those of backgroundabsorption measured at 164.0 nm He line and 165.7
nm using Xe lamp emitting a continuous spectrum.
This suggests that background absorption by sourcegas molecules and species produced in plasma wasdominant and the determination of C atom density
could not be attained when the CH4 concentration
increased. In the case of microwave CH4/H2 plasma
sustained at relatively high pressures under thetypical condition for the growth of aligned CNT andNCD films, C2 radical density was on the order of1012 cm-3, while the C atom density was below thedetection limit (~1011 cm-3) over CH4 flow rate of
3%. Moreover, emission from C resonance line at
165.7 nm was hardly observed in the microwave
CH4/H2 plasma. In this case, a lot of by-productmolecules such as acetylene were generated in theplasma and absorbed dominantly the VUV lightincluding C resonance line. This backgroundabsorption deteriorated the sensitivity of C atom
detection at large flow rate of CH4 seriously.
It is likely in CH4 plasmas that C atoms aregenerated in stepwise processes via CH 3, CH2, andCH. On the other hand, C atoms would be removed
due to the reactions with CHx radicals to produce
acetylene and higher-order hydrocarbons. It wasreported that acetylene molecules on the order of1013 cm-3 were generated in hot filament CVD
employing CH4/H2 mixture at 20 Torr [4]. Moreover,
C atoms can react with acetylene [5]. Therefore, inthe case of microwave plasma employing CH4/H2
mixture at relatively high pressures, it is consideredthat the direct effect of C atoms by way of thesurface reaction might be negligible for the growthof NCD and CNT films.
Now, we are going to investigate dominantmolecular species in CH4/H2 plasma by mass
spectrometry and discuss the growth mechanism ofcarbon nanostructures in more detail.

60

C atom resonanceline (165.7nm)

40

He emission line(164.0nm)

20Xe eximer line
(165.7nm)

00246810CH4 flowrate [%]CH4流量比[%]Fig. 2 Absorption rate of C atom line and backgroundabsorption as a function of CH4 flow rate

References[1] M. Hiramatsu, K. Kato, C.H. Lau, J.S. Foord,
M. Hori, Diam. Relat. Mater. 12, 365 (2003).
[2] S. Takashima, M. Hori, T. Goto, Appl. Phys.
Lett. 75, 3929 (1999).
[3]W. Takeuchi, H. Sasaki, S. Kato, S. Takashima,M. Hiramatsu, M. Hori, J. Appl. Phys. 105,113305 (2009).
[4] J. B. Wills, M. N. R. Ashfold, A. J. Orr-Ewing,Y.A. Mankelevich, and N. V. Suetin, Diamond
Relat. Mater. 12, 1346 (2003).
[5] J. Benedikt, M. Wisse, R. V. Woen, R. Engeln,and M. C. M. van de Sanden, J. Appl. Phys. 94,6932 (2003).