A KINETIC MODEL FOR EXCIMER UV AND VUV RADIATION IN ... · university of illinois optical and...

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


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


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*


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


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)


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


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*.


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


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



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.


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


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


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


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


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 (%)


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



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


Tgas = 300 K, τ = 40 ns, ε = 5


• 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.


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


Tgas = 300 K, τ = 40 ns, ε = 5


• 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.


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.

A KINETIC MODEL FOR EXCIMER UV AND VUV RADIATION IN ... · university of illinois optical and...

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