HIGH REMOVAL RATE OF CF4 USING DC PLASMA
WITHIN BUBBLES AND TRAPPING OF FLUORINE
Summary
<Background ; CF4 is widely used in semiconductor manufacturing processes; however, CF4 emissions must be reduced because of their large global-warming potential. >
CF4 was removed at the rate of 96.7%, using an atmospheric-pressure DC plasma within gas bubbles. Emission spectra showed that the
mean electron energy might be 3 eV and the gas temperature might be 3000～3400 K. Electron energy distribution function shows that the
electrons in this plasma might have sufficient energy to decompose CF4. Thermodynamic equilibrium compositions indicate that the gas
temperature of this plasma might be sufficient for CF4 decomposition. We consider that the water around bubbles is effective in humidifying
the discharged gas, lowering the temperature required to decompose CF4 and capturing the fluorine to achieve high removal rate.
Experimental Setup
Results
CF4 removal rate
as a function of current
F- concentration
as a function of plasma treatment time
100
2000
Voltage
10
1000
5
2
3
4
Time [min]
Experimental condition
200
400 600
Time [ms]
1
80
60
40
20
0
5
0
800 1000
0
5
10
15
Current [mA]
20
The fluorine atoms detached from CF4
are captured in the water.
The maximum removal rate of 96.7%
was obtained.
Gas: 1%CF4, 99%N2
Gas flow rate: 10 sccm
Solution: Tap water 50 mL
Current: 3, 5, 7, 10, 13, 15 mA
Current
0
10
0
500
0
20
0
Current [mA]
1500
Voltage [V]
15
3mA
5mA
7mA
10mA
13mA
15mA
CF4 removal rate [%]
Waveforms
(Current : 5 mA)
F－ concentration [mg/L]
30
Discussions
Deduction of electron energy
by the intensity ratio of N2 spectra
Electron energy distribution function （by BOLSIG+）
Emission spectrum of 1%CF4/N2base
N2
(0,2)
+
N2
(0,0)
0.5
Gas : 1%CF4, 96%N2, 3%H2O
The mean electron energy : 3 eV
E/N : 126 Td
0.5
The electron energy
was estimated at 3 eV.
0.4
f()
Intensity [a.u.]
1
BOLSIG+ condition
0.6
CF4 can be decomposed by electron collision.
0.3
Reaction
0.2
0.1
0
0
300 320 340 360 380 400 420
Wavelength [nm]
0
10
20
energy [eV]
CF4 + e → CF3 + F–
CF3 + e → CF2 + F + e
CF2 + e → CF + F + e
CF + e → C + F + e
30
Electrons with the energy of 2~5 eV exist.
Deduction of the gas temperature of the plasma
Dissociation
energy
[eV]
4.8
3.0
4.6
2.7
Thermodynamic equilibrium composition(by MALT)
Emission spectrum
of 1%CF4/N2base at 15 mA
1%CF4, 98.9%N2, 0.1%H2O
0.04
1%CF4, 98%N2, 1%H2O
0.04
1.2
The gas temperature was
estimated at 3000~3400 K at
the current range of 5~15 mA.
0.6
0.4
0.2
0
360
370
380
Wavelength [nm]
Current
[mA]
Gas temperature
[K]
5
10
15
3000
3200
3400
F
0.03
390
CF4 can be thermally decomposed.
N
0.02
CF4
0.01
HF
0
0
1000
CF2
F
0.03
Mole fraction
0.8
Experiment
Simulation
Tr=3400 K
Tv=3200 K
Mole fraction
intensity [a.u.]
1
CN
CF
2000 3000 4000
Temperature [K]
N
HF
0.02
H
CO
0.01
CF4
C
H
5000
CF4 is decomposed at 2700 K.
0
COF2
O
CO2
0
1000
2000 3000 4000
Temperature [K]
5000
CF4 is decomposed at 2300 K.
 Increasing H2O concentration decreases CF4 decomposition
temperature. Thus, H2O contributes effectively to CF4 decomposition.
 At the experimental current, H2O concentration is estimated at
several percent.[1] Therefore, CF4 is decomposed at 2300 K at least
in the state of thermal equilibrium.
[1] N Takeuchi, Y Ishii and K Yasuoka, Plasma Sources Sci. Technol. 21, 015006 (2012).
(Y. Matsuya)