24G11
Analysis of dissociative excitation reactions of CH3CN
with the ECR plasmas of Ar and He
ArとHeのECRプラズマを用いたCH3CNの解離励起反応解析
Saori Onitsuka, Haruhiko Ito
鬼束さおり, 伊藤治彦
Nagaoka University of Technology
1603-1, Kamitomioka, Nagaoka, Niigata 940-2188, Japan
長岡技術科学大学 〒940-2188 長岡市上富岡町1603-1
The dissociative excitation reactions of CH3CN with the electron-cyclotron resonance (ECR) plasmas of Arand He were studied based on the electrostatic-probe measurements and on the optical emission spectra ofthe CN(B2Σ+−X2Σ+) transition. The density and the temperature of free electrons and the CN(B2Σ+−X2Σ+)emission intensity in these plasmas are varied by adding trace amount of H2O. Based on a kinetic analysis,
the decomposition of CH3CN proceeds, predominantly, via the electron impact in the Ar plasma and via
the electron impact and/or the ion-electron recombination in the He plasma. These results are found to beconsistent with the evaluation of the number densities of CN(B2Σ+) using the steady-state method. In
addition, the compositional analysis of hydrogenated amorphous carbon nitride (a-CNx:H) films preparedby the decomposition of CH3CN in the ECR plasmas of Ar and He were carried out, yielding the[N]/([N]+[C]) ratios of 0.22 and 0.25, respectively.

1. Introduction
The dissociative excitation reactions of CH3CN
with microwave discharge of rare gass flows havebeen known to produce CN radicals with highefficiency, and strong emission spectra of theCN(B2Σ+−X2Σ+) transition have been observed [1].
We have applied these reactions to the synthesis ofhydrogenated amorphous carbon nitride (a-CNx:H)
thin films [2].
In the present study, the dissociative excitation
mechanisms of CH3CN in the electron-cyclotronresonance (ECR) plasmas of Ar and He werestudiedbasedontheelectrostatic-probe
measurements and on the optical emission spectraof the CN(B2Σ+−X2Σ+) transition.
First, the dependencies of the CN(B2Σ+−X2Σ+)emission intensity and the electron-energy
distribution on the pressure of H2O added to theECR plasma flows of Ar and He were analyzed.
Second, the formation rates of CN(B2Σ+) state wereevaluated by using the rate constants for therelevant reactions of the other molecules. Basedon these results, the dissociative excitation
processes of CH3CN to produce the CN(B2Σ+) state
were clarified. In addition, a-CNx:H films weredeposited, and a compositional analysis of thesefilms was carried out.
2. Experiments
The ECR plasma CVD apparatus was used. Afterthe chamber was evacuated <10-4 Torr, Ar or He (2mTorr) was introduced through a desiccant (P2O5).
Then, microwaves (2.45 GHz, 70 W) wereintroduced into the chamber. H2O molecules
adsorbed on the wall of the chamber were removed
by discharging Ar or He for 2 h. CH3CN (1 mTorr)
was introduced through P2O5. H2O (0-0.6 mTorr)
was introduced into the flow of CH3CN.
The I-V characteristics were measured using theelectrostatic-probe method to evaluate the density,ne, and the temperature, Te, of electrons. The opticalemission spectra of the CN(B2Σ+−X2Σ+) transition
were measured.
Films of a-CNx:H were formed for the Ar and He
plasmas. In this experiment, the partial pressuresof Ar or He and CH3CN were 3 and 1 mTorr,
respectively, where H2O was not introduced. Theatomic composition was obtained using X-ray
photoelectron spectroscopy (XPS). Films deposited
onto Si substrates of ≈7 mm2 were used as thesamples, for which the XPS measurements werecarried out under the condition of 2.0×10−7 Pa(1.5×10−9 Torr) using a MgKα source.
3. Results and discussion
The relative value of the emission intensity, Iobs,of the CN(B2Σ+−X2Σ+) transition was plottedagainst PH2O as shown by circles in Figs.1(a) and1(b) for Ar and He, respectively. The electron
density with the specific kinetic energy, Ne(Ee),Ne (Ee ) = 2ne (Ee / π )1/ 2 (kTe )−3/ 2 exp(− Ee / kTe )(1)

was evaluated from ne and Te. The emission
intensity of the CN(B2Σ+−X2Σ+) transition, IEI andIRec, assuming the impact of high energy electrons
and the charge transfer followed by the ion-electron
recombination are expressed, respectively, as∫ E N (E )dE

I EI ∝

12eeeobs.
ICN(B-X) [arb.units]

30 eV

1.0(2)

e8.9 eV12.1 eVI Rec ∝∫ E N (E )dE .
1e2eee0.8calc.(E.I.)

0.6

calc.(Rec.)

0.4
0.2
0.0(3)

0.0

0.20 eV

4. Conclusion
In the present study, the mechanism of
dissociative excitation of CH3CN to produce
CN(B2Σ+) radicals in the ECR plasmas of Ar andHe was studied. In conclusion, the dissociation
proceeds, predominantly, via the electron impact in
the Ar plasma, and via the electron impact and/or
the ion-electron recombination in the He plasma.
The method employed in this study is possible toelucidate the decomposition mechanism of starting
materials from the results of the optical emission
measurements.

ICN(B-X) [arb.units]

Eqs. (2) and (3) were evaluated numerically andnormalized at PH2O=0 mTorr, and the results areshown by triangles and squares, respectively, inFigs.1(a) and 1(b). As shown in Fig.1(a), theobserved values of the Ar plasma are in good
agreement with the evaluations assuming electronimpact. Therefore, the dominant process ofproduction of CN(B2Σ+) radicals is the electron
impact in the ECR plasma of Ar. On the other hand,
the two processes can not be distinguished in thecase of He. In order to confirm the above findings,
the number densities of CN(B2Σ+) assuming
electron impact and the ion-electron recombination,nEI and nRec, were evaluated by the steady-statemethod. Table 1 lists the nEI and nRec values for theAr and He plasmas. The nEI value for Ar was about
three times larger than nRec. In the case of the He
plasma, nEI and nRec were almost identical. Theseevaluations are consistent with the conclusion
derived from Figs. 1(a) and 1(b).
Table 2 shows the compositional analysis ofa-CNx:H films prepared by the decomposition ofCH3CN in the ECR plasmas of Ar and He, fromwhich the [N]/([N]+[C]) ratios yielded to be 0.22
and 0.25, respectively. C and N atoms may
originate in the starting material as expected. Oatoms are presumably due to the atmospheric H2O
molecules adsorbed during the waiting time
(several days) of the XPS measurement.

0.4

0.6

(a) Arobs.

1.0

calc.(E.I.)
calc.(Rec.)
0.5

0.0
0.0

0.2

0.4

0.6(b) He
Fig.1 PH2O dependencies of emission intensities
Table 1 The number densities of CN(B)(in units of 109 m-3)
ArHenEI
7.41
6.39

nRec
2.58
5.96

Table 2 Compositional analysis of a-CNx:H film
prepared in the Ar and He plasmas.

ArHe

C [at%]
72.7
64.4

N [at%]
20.1
21.0

O [at%] [N]/([N]+[C])
7.3
0.22
14.6
0.25

References[1] H. Ito, N. Ito, T. Takahashi, D. Tanaka, H.
Takamatsu, and H. Saitoh, Jpn. J. Appl. Phys. 40(2001) 332.
[2] H. Saitoh, H. Takamatsu, D. Tanaka, N. Ito, S.
Ohshio, and H. Ito, Jpn. J. Appl. Phys. 39 (2000)
1258.