22P079-P
Kinetic Modeling of the Decomposition of PFCs
in Electron-Beam-Generated Atmospheric-Pressure Plasmas
電子ビーム生成大気圧プラズマによる PFC 分解のモデル
Susumu Kato and Isao. Okuda
加藤 進，奥田 功
National Institute of Advanced Industrial Science and Technology (AIST),
Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan
産業技術総合研究所 〒 305-8568 つくば市梅園 1-1-1 中央第 2

A kinetic model demonstrating the decomposition of CF4 by using atmospheric-pressure lowtemperature argon plasma generated by pulsed electron beam is developed. The validity of the model isverified by comparison of the removal efficiencies of CF4 using the kinetic simulation with the recent
experiment [Jpn. J. Appl. Phys. 47, 5681 (2008)]. The result shows that the principal processes of CF4
decomposition are the reactions of ion and metastable argon not impacting electron.
1. Introduction
The electron-beam (e-beam) generated plasmas
have been widely used for control of the fuel gases
and volatile organic compounds [1, 2, 3]. Recently,
the removal of perfluoro compounds (PFCs) such as
CF4 and C2 F6 becomes important for the abatement ofgreenhouse gases from semiconductor facilities. Nonthermal plasma technology by discharges has been
applied for the decomposition of PFCs [4]. Atmospheric plasma abatement system for PFCs using microwave has been used commercially. In addition, various methods have been investigated, for example hightemperature plasmas such as water plasmas [5] and additional fuel hybrid plasmas [6, 7].
The decomposition of CF4 , which is the mostdifficult compounds in PFCs, by using atmosphericpressure, non-thermal plasmas generated by pulsed
electron beam was experimentally demonstrated [8, 9].
The results show that the removal efficiencies of thepulsed-electron-beam method are comparable to or
higher than those of corona discharge methods.
The reactions of ions and excited atoms with PFCs
is the principal decomposition process in e-beamgenerated plasmas because most energetic electrons interact with the buffer gas due to dilute concentration ofPFC. The reaction of CF4 with metastable argon is also
important in e-beam generated argon plasmas [10, 11].
Based on the assumption of the above decomposition reactions, we have developed a kinetic model
demonstrating the decomposition of CF4 in argon gas[12, 13]. The simple kinetic models are commonlyused for the treatment of the excimer lasers [14] andflue gas cleaning by irradiation with e-beams [1]. The
validity of the kinetic model of the decomposition of
CF4 has not yet been verified by the experimental data.
It is impossible to consider every chemical reaction inkinetic model. In addition, all rate coefficients generally include uncertainties. Therefore, the verificationof the model is important by experimental data. It isalso important to optimize the concentration of additives such as H2 O, O2 in gas phase reaction.
In this paper, we apply the kinetic model to thepreviously reported experimental CF4 decomposition
using atmospheric-pressure non-thermal argon plasma
generated by pulsed electron beam [8]. We compare
the removal efficiencies of CF4 obtained in this experiment with those using the kinetic model simulation.
2. Verification of simulation model
A computational method based on the kinetic
model of CF4 in argon gas, using H2 O as an additive
gas under pulsed-e-beam irradiation, was used. Thismethod employs a zero-dimensional kinetic model ofa gas phase reaction. The details of the kinetic model
are provided in our previous papers [12, 13]. However,the validity of the kinetic model has not yet been verified by the experimental data. The decomposition of
CF4 by using atmospheric-pressure, non-thermal argon
plasma generated by pulsed e-beams was experimentally reported [8]. In this section, we verify the modelby comparing the CF4 removal efficiencies of the resent experiment with those of the model simulation.
First, our experiment [8] is briefly reviewed. Decomposition experiment using argon gas under satu-Second, the kinetic model simulation condition
for the experiment is shown. The total gas density, which corresponds to a total pressure of 130
kPa at 293 K, was [Ar] + [CF4 ] + [H2 O] = 3.2 × 1019
cm−3 , where [X] is the number density of X,[CF4 ] = 3.2 × 1016 cm−3 . The H2 O concentration ofsaturated water vapor depends strongly on the gas temperature. The saturated vapor pressures are 2.3 kPa at293 K, corresponding to H2 O concentrations of 1.8%,
namely, [H2 O] = 5.8 × 1017 cm−3 . We assumed thatthe input e-beam had a square pulse with a pulse duration of 80 ns and an input energy density per pulse of
25 mJ/cm3 . To model the gas circulation, we removed
the gases, except for CF4 and H2 O, after 23 shots of irradiation. The density of CF4 was maintained and thedensity of H2 O was restored to the initial density. We
assumed a 1-ms interval between pulses to save computational time, because the reactions terminate within
0.1 ms when H2 O is used as the additive gas [12].
Figure 1 shows the experimental and simulated
removal efficiencies of CF4 . The experimentally obtained removal efficiency values are denoted by closed
circles and its error is approximately ±10%. The solidline shows the simulation results. The removal efficiencies obtained by the simulation agree reasonably
well with those obtained experimentally. It shows thatmain decomposition of CF4 in the experiment can be
explained by reactions with the ions and metastableatoms of argon.
3. Conclusion
A kinetic model of CF4 decomposition in an argon
gas using e-beam was proposed. We verified the modelby comparison of the removal efficiencies of CF4 withthe recent experiment [8].

1Removal efficiency

rated water vapor conditions was conducted at a totalpressure of 130 kPa and an initial CF4 concentrationof 1000 ppm. The e-beam pulse duration was 80 ns
and the input energy density in the irradiated volume
per pulse was 25 mJ/cm3 which is estimated from thepressure-jump method. Therefore, an error is approximately ±10% considering radiation loss. [8] And anerror of the concentration of CF4 is less than 10 ppm.
The interval between shots was 5.3 s and the total shot
number of the e-beam irradiation was 2671. The totalgas volume was 58 L and the gas volume irradiated by
the e-beam was approximately 10 L. The gas was circulated through a water reservoir at a gas flow rate of
5 L/min; thus, the gas flow between shots was 0.44 L.
Therefore, the gas remained in the irradiated volumefor 23 shots. The products are removed by gas circulation through a water reservoir with calcium hydroxide.

0.80.60.40.200510153Input energy density (J/cm )Fig. 1 Removal efficiency as a function of input energy density. Lines represent the simulation results. Closed
circles show the experimental data and an error ofinput energy is approximately ±10%.

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