A KINETIC MODEL FOR EXCIMER UV AND VUV RADIATION IN DIELECTRIC BARRIER DISCHARGES*

Transcription

1 A KINETIC MODEL FOR EXCIMER UV AND VUV RADIATION IN DIELECTRIC BARRIER DISCHARGES* Xudong Peter Xu and Mark J. Kushner University of Illinois Department of Electrical and Computer Engineering Urbana, IL 6 October 99 *Work supported by the National Science Foundation

2 AGENDA Introduction Plasma Chemistry for Xe/Cl Description of the Model for Microdischarges Results for Microdischarge Mechanics and UV Radiation Concluding Remarks GEC9-

3 DIELECTRIC BARRIER DISCHARGES 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 bar, so a number of microdischarge or breakdown channels are generated. mm - mm gap s Hz - a few khz UV GEC9- Dielectric

4 EXCIMER GENERATION IN Xe/Cl DISCHARGE Xe** + Cl Xe* + Cl Xe+ + Cl - Xe + + Cl - Xe* + Xe + M Xe** + Xe + M Xe * + Cl Cl * + Xe Xe * hυ nm XeCl* e, Xe, Cl, Cl hυ 7 nm Xe + Xe Cl XeCl* Xe + Cl Cl + e Cl+ + Cl - XeCl* + Xe + Xe XeCl* + Xe + Cl GEC9- XeCl* hυ 9 nm e, Xe, Cl Xe + Xe + Cl hυ 9 nm Xe + Xe Cl * Xe XeCl*

5 SCHEMATIC OF PLASMA CHEMISTRY AND HYDRODYNAMICS MODELS -d plasma chemistry and hydrodynamics models have been developed to investigate excimer radiation from dielectric barrier discharges in mixtures of xenon and chlorine. Circuit Module E/N e-impact Cross- Section Database Boltzmann Module k, T e, µe Plasma Chemistry Module Neutral / Ion Chemistry Database Navier- Stokes Module σ (r, t ) N ( r, t ) GEC9-

6 MICRODISCHARGE IN Xe/Cl = 99/ Cl has a large rate of dissociative electron attachment at low E/N. The electron density shows an expanding avalanching shell for % Cl. 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. After Voltage Pulse Xe/Cl = 9/, Pressure:.6 atmosphere, Gas Temperature: K, V o = kv, τ = ns, ε = ε - - GEC9-

7 XeCl* AND nm GENERATION IN Xe/Cl = 99/ The formation of the XeCl* excimer is delayed compared to electron generation. The total generation nm photons is ultimately uniform across the microdischarge. Intantaneous density Integration of emission (a) (b) Xe/Cl = 99/,.6 atm, K, GEC9-6 Vo = kv, τ = ns, ε =

8 OTHER PHOTONS IN Xe/Cl = 99/ Xe * (7 nm) has a similar lifetime as XeCl*. Xe Cl* (9 nm) has a long lifetime since its quenching rate is independent of the Xe pressure and only dependent on the Cl density. Cl * (9 nm) can be depleted by conversion to XeCl*. Integration of emission. Integration of emission Integration of emission > GEC9-7 atm, K, Vo = kv, τ = ns, ε =

9 MICRODISCHARGE IN Xe/Cl = 9/ Xe/Cl = 9/ 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 Xe/Cl = 9/, Pressure:.6 atm, Gas Temperature: K, V o = kv, τ = ns,. ε = ε GEC9- -

10 XeCl* AND nm GENERATION IN Xe/Cl = 9/ 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 nm photons is similar to the case of Xe/Cl = 99/. During Voltage Pulse 6 After Voltage Pulse. Integration of Emission GEC9-9 atm, K, Vo = kv, τ = ns, ε =

11 OTHER PHOTONS IN Xe/Cl = 9/ For the % Cl gas mixture, the generation of 7 nm and 9 photons dramatically decreases and the generation of 9 nm photons increases, compared to Xe/Cl = 99/. This trend is due to the fact that Cl is very efficient at quenching Xe * and Xe Cl*, and the higher Cl mole fraction is beneficial to generating Cl *. Integration of emission Integration of emission Integration of emission GEC9- Xe/Cl = 9/,.6 atm, K, Vo = kv, τ = ns, ε =

12 DIELECTRIC CAPACITANCE With increasing dielectric capacitance, the energy deposition and generated photons linearly increase. Energy Deposition for One Microdischarge (x -6 J) Total Generated nm Photons Per Unit Gas Volume in Microdischarge Region (x cm- ) 6 Dielectric Capacitance (pf/cm ) Dielectric Capacitance (pf/cm ) GEC9-

13 EFFICIENCY VS DIELECTRIC CAPACITANCE 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 * nm photon generation (%).76 Efficiency of Xe Cl* 9 nm photon generation (%) Dielectric Capacitance (pf/cm ) Dielectric Capacitance (pf/cm ) GEC9-

14 DOE ANALYSIS: EFFICIENCY OF XeCl * nm PHOTON GENERATION The efficiency of XeCl * nm photon generation is influenced largely by gas pressure and slightly by Cl mole fraction and applied voltage. Efficiency of nm photon generation (%)... Cl (%) 9. Voltage (kv) Cl (%) Pressure (atm)...6 Pressure.7 (atm). 9 Voltage (kv) Pressure =.6 atmosphere Applied Voltage = kv Cl = % Tgas = K, τ = ns, ε = GEC9-

15 DOE ANALYSIS: EFFICIENCY OF Xe * 7 nm PHOTON GENERATION The efficiency of Xe * 7 nm photon generation is influenced largely by Cl mole fraction and slightly by gas pressure ratio and applied voltage. Efficiency of 7 nm photon generation (%) Cl (%) 9 Voltage (kv) Cl (%) Pressure (atm) 9 Voltage (kv) Pressure (atm) Pressure =.6 atmosphere Applied Voltage = kv Cl = % GEC9- Tgas = K, τ = ns, ε =

16 DOE ANALYSIS: EFFICIENCY OF XeCl*9 nm PHOTON GENERATION The efficiency of Xe Cl* 9 nm photon generation is influenced largely by Cl mole fraction and slightly by gas pressure ratio and applied voltage. Efficiency of 9 nm photon generation (%) Cl(%) 9 Voltage (kv).. Cl (%) Pressure (atm) Pressure.7 (atm). 9 Voltage (kv) Pressure =.6 atmosphere Applied Voltage = kv Cl = % GEC9- Tgas = K, τ = ns, ε =

17 DOE ANALYSIS: EFFICIENCY OF Cl * 9 nm PHOTON GENERATION The efficiency of Cl * 9 nm photon generation is influenced largely by Cl mole fraction and slightly by gas pressure ratio and applied voltage. Efficiency of 9 nm photon generation (%) Voltage (kv) 9 Cl (%) Pressure (atm).. Cl (%) 9 Voltage (kv) Pressure (atm) Pressure =.6 atmosphere Applied Voltage = kv Cl = % GEC9-6 Tgas = K, τ = ns, ε =

18 CONCLUDING REMARKS 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 gas mixture, the electron density show a shell expansion during the pulse due to dielectric charging and attachment of Cl at small radii at lower E/N. The strong attachment of Xe/Cl smaller radii after the pulse. = 9/ leads to shell propagation to Increasing capacitance of the dielectric increases the energy deposition and the total light generation with a slight loss in efficiency. The efficiency of nm photon generation is influenced mainly by gas pressure, decreasing with increasing pressure, and the efficiencies of 7, 9 and 9 nm photon generation are most sensitive to Cl density. GEC9-7