Post on 26-Jun-2020
A KINETIC MODEL FOR EXCIMER UV AND VUVRADIATION IN DIELECTRIC BARRIER
DISCHARGES*
Xudong “Peter” Xu and Mark J. KushnerUniversity of Illinois
Department of Electrical and Computer EngineeringUrbana, IL 61801
October 1998
*Work supported by the National Science Foundation
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
AGENDA
GEC98-01
• Introduction • Plasma Chemistry for Xe/Cl 2
• Description of the Model for Microdischarges • Results for Microdischarge Mechanics and UV Radiation • Concluding Remarks
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
DIELECTRIC BARRIER DISCHARGES
GEC98-02
• Dielectric barrier discharges (DBDs) have shown promise as high intensity sources of UV and VUV radiation emitted by dimer and trimer excimers.
• These UV photons have a number of applications- Biological sterilization- Photochemical degradation of organic compounds in flue gases- Photo-induced surface modification and material deposition- Lithography
• DBDs are usually operated at high pressure on the order of 1 bar, so a number of microdischarge or breakdown channels are generated.
• 2 mm - 5 mm gap • 10s Hz - a few kHz
Dielectric
UV
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
EXCIMER GENERATION IN Xe/Cl2 DISCHARGE
GEC98-03
XeCl*
Xe+ + Cl -Xe* + Cl2
Xe** + Cl2
Xe2* + Cl2
Xe2+ + Cl -
Cl2* + Xe
hυ 308 nm
Xe + Cl
e, Xe, Cl, Cl 2
Xe2*
Xe* + Xe + M Xe** + Xe + M
hυ 172 nm
Xe + Xe
Cl2
XeCl*
Xe2Cl*
XeCl* + Xe + Xe XeCl* + Xe + Cl 2
hυ 490 nm
Xe + Xe + Cl
e, Xe, Cl2
Cl2*
Cl2 + e Cl+ + Cl -
hυ 259 nm
Xe + Xe
Xe
XeCl*
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
SCHEMATIC OF PLASMA CHEMISTRY ANDHYDRODYNAMICS MODELS
GEC98-04
• 1-d plasma chemistry and hydrodynamics models have been developed to investigate excimer radiation from dielectric barrier discharges in mixtures of xenon and chlorine.
CircuitModule
BoltzmannModule
Neutral / IonChemistryDatabasek, Te,
µe
N ( r, t )σ (r, t )
E/N
e-ImpactCross-Section
Database
Navier-StokesModule
PlasmaChemistry
Module
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
MICRODISCHARGE IN Xe/Cl2 = 99/1
GEC98-05
• Cl 2 has a large rate of dissociative electron attachment at low E/N. The electron density shows an expanding avalanching shell for 1% Cl 2.• An absolute low voltage region is produced in the core of the microdischarge during the voltage pulse, and around the edge after the pulse.
During Voltage Pulse
0
0.5
1.0
1.5
2.0
2.5
40.4
40.7
44.6
55.1
40.1
0 50 100 150 200
40.140.4
40.7
-12
-8
-4
0
0
0.4
0.8
1.2
0.5
1.1 10.7 36.020.4
0 50 100 150 200
0.5
1.1 10.7 36.0
0
4
8
12
20.4
After Voltage Pulse
• Xe/Cl 2 = 95/1,
Pressure: 0.6 atmosphere, Gas Temperature: 300 K, V o = 10 kV,
τ = 40 ns,
ε = 5 ε0
RADIUS (µm) RADIUS (µm)
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
XeCl* AND 308 nm GENERATION IN Xe/Cl 2 = 99/1
GEC98-06
• The formation of the XeCl* excimer is delayed compared to electron generation.
• The total generation 308 nm photons is ultimately uniform across the microdischarge.
RADIUS (µm)(a) (b) RADIUS (µm)
Integration of emission
4.8
10.7 20.4
40.1
50.0
83.2
0.0
0.4
0.8
1.2
0 50 100 150 200
61.1
1.1
4.8 20.1
40.1
44.6
55.1
83.20
1
2
3
0 50 100 150 200
• Xe/Cl 2= 99/1, 0.6 atm, 300 K,
Vo = 10 kV, τ = 40 ns, ε = 5
Intantaneous density
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
OTHER PHOTONS IN Xe/Cl2 = 99/1
GEC98-07
Vo = 12 kV, τ = 40 ns, ε = 5• 1 atm, 400 K,
• Xe 2* (172 nm) has a similar lifetime as XeCl*.
• Xe 2Cl* (490 nm) has a long lifetime since its quenching rate is independent of the Xe pressure and only dependent on the Cl 2 density .• Cl 2* (259 nm) can be depleted by conversion to XeCl*.
RADIUS (µm) RADIUS (µm)
Integration of emission
4.8 20.4
40.1
50.0
92.1
61.1
0
1
2
3
0 50 100 150 200
4
5
10.7 20.4
40.1
55.1
92.1
67.8
0.0
0.4
0.8
1.2
0 50 100 150 200
1.1 20.4
40.1
> 44.6.1
0.0
0.5
1.0
1.5
0 50 100 150 200
41.0
RADIUS (µm)
Integration of emission Integration of emission
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
MICRODISCHARGE IN Xe/Cl2 = 95/5
GEC98-08
• Xe/Cl 2 = 95/5,
Pressure: 0.6 atm, Gas Temperature: 300 K, V o = 10 kV,
τ = 40 ns,
ε = 5 ε0
• Xe/Cl 2 = 95/5 drives the electron density to such low values in the core of the microdischarge during the pulse that there is not an immediate second avalanche in the core at the end ofpulse. • After the pulse, the electron density shell propagates towards the center of the microdischarge due to high gap voltages in the core.
During Voltage Pulse After Voltage Pulse
40.140.744.661.2
0 50 100 150 200-12
0
-4
-8
0 50 100 150 200
0.5
1.1 20.4 36.0
0
12
4
8
0.5
1.1 10.7 36.020.4
2
4
6
8
10
0
40.144.6
55.1
67.8
40.7
41.3
20
10
0
RADIUS (µm) RADIUS (µm)
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
XeCl* AND 308 nm GENERATION IN Xe/Cl 2 = 95/5
GEC98-09
Vo = 12 kV, τ = 40 ns, ε = 5• 1 atm, 400 K,
• XeCl* in the core of the microdischarge is regenerated as the second avalanche moves towards the center after the voltage pulse.
• The total number of 308 nm photons is similar to the case of Xe/Cl = 99/1.
RADIUS (µm) RADIUS (µm)
1.1
36.020.44.7
0 50 100 150 200
4
0
2
3
140.1
41.6
44.661.2
83.2
0 50 100 150 200
4
0
2
6
4.8
10.7 20.4
40.1
44.6
83.2
61.1
0.0
0.4
0.8
1.2
0 50 100 150 200
Integration of Emission
RADIUS (µm)
During Voltage Pulse After Voltage Pulse
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
OTHER PHOTONS IN Xe/Cl2 = 95/5
GEC98-10
• Xe/Cl 2= 95/5, 0.6 atm, 300 K,
Vo = 10 kV, τ = 40 ns, ε = 5
• For the 5% Cl 2 gas mixture, the generation of 172 nm and 490 photons dramatically decreases and the generation of 259 nm photons increases, compared to Xe/Cl 2 = 99/1.
• This trend is due to the fact that Cl 2 is very efficient at quenching Xe 2* and
Xe 2Cl*, and the higher Cl 2 mole fraction is beneficial to generating Cl 2*.
RADIUS (µm) RADIUS (µm)RADIUS (µm)
4.8 20.4
40.1
44.6
83.2
55.1
0 50 100 150 2000
1
2
3
4.8 20.4
40.1
49.6
92.1
61.2
0 50 100 150 2000
1
2
3
10.7 4.8
40.1
44.6
75.4
55.1
0 50 100 150 2000
2
4
6
10.7 20.4
40.7
Integration of emission Integration of emission Integration of emission
0
5
10
15
20
0 10 20 30 40 50
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
DIELECTRIC CAPACITANCE
GEC98-11
• With increasing dielectric capacitance, the energy deposition and generated photons linearly increase.
Energy Deposition for One Microdischarge (x10 -6 J)
Dielectric Capacitance (pF/cm 2)
0
2
4
6
8
10
12
0 10 20 30 40 50
Total Generated 308 nm Photons PerUnit Gas Volume in Microdischarge
Region (x10 14 cm-3)
Dielectric Capacitance (pF/cm 2)
0.68
0.70
0.72
0.74
0.76
0 10 20 30 40 502.90
3.00
3.10
3.20
3.30
0 10 20 30 40 50
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
EFFICIENCY VS DIELECTRIC CAPACITANCE
GEC98-12
• With increasing dielectric capacitance, the efficiencies of photon generation decrease.
• This is because large dielectric capacitance leads to high plasma density which has a stronger quenching effect.
Efficiency of XeCl * 308 nm photongeneration (%)
Dielectric Capacitance (pF/cm 2) Dielectric Capacitance (pF/cm 2)
Efficiency of Xe 2Cl* 490 nm photongeneration (%)
DOE ANALYSIS: EFFICIENCY OF XeCl* 308 nm PHOTONGENERATION
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
GEC98-13
Tgas = 300 K, τ = 40 ns, ε = 5
• The efficiency of XeCl * 308 nm photon generation is influenced largely by gas pressure and slightly by Cl 2 mole fraction and applied voltage.
Pressure = 0.6 atmosphere
23
45
3.3
3.1
1211
10
9
8
3.2
Cl2 (%)
3.0
Voltage (kV) 1
23
45
Cl2 (%)
0.40.5
0.60.7
0.8
0.42
3
4
5
Pressure (atm)
0.50.6
0.70.8
0.4
Pressure (atm) 12
1110
98
Voltage (kV)
2
3
4
5
Applied Voltage = 10 kV Cl2 = 3%
Efficiency of 308 nm photon generation (%)
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
GEC98-14
12
34
5
2.0
0.5
1211
109
8
1.0
Cl2 (%)
1.5
Voltage (kV)
12
34
5Cl2 (%) 0.5
0.60.7
0.8
0.4
1
2
Pressure (atm)
0.50.6
0.70.8
0.4Pressure
(atm)1211
109
8
Voltage (kV)
0.24
0.27
0.30
0.33
DOE ANALYSIS: EFFICIENCY OF Xe2* 172 nm PHOTONGENERATION
• The efficiency of Xe 2* 172 nm photon generation is influenced largely by Cl 2 mole fraction and slightly by gas pressure ratio and applied voltage.
Pressure = 0.6 atmosphere Applied Voltage = 10 kV Cl2 = 3%
Tgas = 300 K, τ = 40 ns, ε = 5
Efficiency of 172 nm photon generation (%)
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
GEC98-15
12
34
5
2.0
0.5
1211
109
8
1.0
Cl2(%)
1.5
Voltage (kV)
12
34
5Cl2 (%) 0.5
0.60.7
0.8
0.4
1.0
2.0
Pressure (atm)
1.5
0.5
0.50.6
0.70.8
0.4
Pressure (atm) 12
1110
98
Voltage (kV)
0.65
0.70
0.75
0.80
DOE ANALYSIS: EFFICIENCY OF Xe2Cl*490 nm PHOTONGENERATION
Pressure = 0.6 atmosphere Applied Voltage = 10 kV Cl2 = 3%
Tgas = 300 K, τ = 40 ns, ε = 5
Efficiency of 490 nm photon generation (%)
• The efficiency of Xe 2Cl* 490 nm photon generation is influenced largely by Cl 2 mole fraction and slightly by gas pressure ratio and applied voltage.
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
GEC98-16
0.15
12
34
5
Cl2 (%)0.50.6
0.70.8
0.4
0.10
Pressure (atm)
0.15
0.05
12
0.05
0.10
12
34
511
109
8Cl2 (%)Voltage
(kV)0.5
0.60.7
0.8
0.4Pressure
(atm)1211
109
8
Voltage (kV)
0.08
0.10
0.12
DOE ANALYSIS: EFFICIENCY OF Cl2* 259 nm PHOTONGENERATION
Pressure = 0.6 atmosphere Applied Voltage = 10 kV Cl2 = 3%
Tgas = 300 K, τ = 40 ns, ε = 5
Efficiency of 259 nm photon generation (%)
• The efficiency of Cl 2* 259 nm photon generation is influenced largely by Cl 2 mole fraction and slightly by gas pressure ratio and applied voltage.
UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
CONCLUDING REMARKS
GEC98-17
• Microdischarge dynamics in dielectric discharge have been investigated for high intensity sources of UV and VUV radiation emitted by dimer and trimer excimers.
• In Xe/Cl 2 gas mixture, the electron density show a shell expansion during the pulse due to dielectric charging and attachment of Cl 2 at small radii at
lower E/N.
• The strong attachment of Xe/Cl 2 = 95/5 leads to shell propagation to
smaller radii after the pulse.
• Increasing capacitance of the dielectric increases the energy deposition and the total light generation with a slight loss in efficiency.
• The efficiency of 308 nm photon generation is influenced mainly by gas pressure, decreasing with increasing pressure, and the efficiencies of 172, 259 and 490 nm photon generation are most sensitive to Cl 2 density.