SURFACE CORONA-BAR DISCHARGES FOR PRODUCTION OF PRE-IONIZATION UV LIGHT FOR HIGH PRESSURE PLASMAS * Zhongmin Xiong and Mark J. Kushner University of Michigan Department of Electrical Engineering and Computer Science Ann Arbor, MI 48109 USA http://uigelz.eecs.umich.edu zxiong@umich.edu, mjkush@umich.edu AVS 57th International Symposium & Exhibition Albuquerque, New Mexico, USA October 17-22, 2010  * Work supported by Cymer Inc., DOE Office of Fusion Energy Science

DISCHARGE-PUMPED EXCIMER LASERS Excimer lasers oscillate on a bound free-transition of rare-gas halogen molecules. The UV wave lengths make them attractive for photolithography. For example, the ArF excimer laser (193 nm) is excited by harpoon and ion-ion reactions. excitation laser Ar* + F2 Ar, F ArF* r E ArF e + Ar  Ar* + e e + Ar  Ar+ + e + e e + F2  F- + F Ar* + F2  ArF* + F Ar+ + F- + M  ArF* + M ArF*  Ar + F + h (www.spie.org Electric discharge excited lasers are typically 3-4 atm with Ne/Ar/F2/Xe gas mixtures and pulse lengths of 10s to 100s ns. University of Michigan Institute for Plasma Science & Engr. AVS2010_CORONA_1

CORONA BAR PREIONIZED DISCHARGE Insulator For uniform glow-like excitation, UV or X-ray preionization is required. UV-VUV fluxes are typically produced by a corona bar discharge. Photo-electrons trigger the main avalanche. 0.2 mm Metal Corona Bar (grounded) Dielectric e = 8. Insulator Cathode Ne/Ar/F2/Xe 2600 Torr, 350K 1.25 cm Anode Metal Insulator University of Michigan Institute for Plasma Science & Engr. Geometry provided by Cymer, Inc. AVS2010_CORONA_2

SURFACE CORONA BAR DISCHARGE Investigate the fundamentals of UV-VUV production from corona bar discharges. Simplification: Geometry (only corona bar and electrode), gas composition (Ne/Xe=99/1, 3.5 atm) and pulsing scheme (step -25kV). Photo-shield to isolate top-bottom. Ne/Xe (99:1) Max = 6ev Te  Te in absence of photoshield University of Michigan Institute for Plasma Science & Engr. AVS2010_CORONA_3

MODELING PLATFORM: nonPDPSIM Poisson’s equation: Transport of charged and neutral species: Charged Species:  = Sharfetter Gummel Neutral Species:  = Diffusion Surface Charge: Electron Temperature (transport coefficients obtained from Boltzmann’s equation) University of Michigan Institute for Plasma Science & Engr. AVS2010_CORONA_4 5 5

MODELING PLATFORM: nonPDPSIM Radiation transport and photoionization: Electron Monte Carlo Simulation tracks sheath accelerated secondary electrons produced by ion and UV bombardment of surfaces. EMCS is performed on a structured grid overlayed on unstructured grid of fluid simulation. E-fields from fluid and ionization sources from EMCS are interpolated between grids. University of Michigan Institute for Plasma Science & Engr. AVS2010_CORONA_5 6 6

PHOTOIONIZATION MECHANISM Photo-ionization from the corona bar discharge triggers the main avalanche by providing seed electrons in regions of high E/N. We selected emission from Ne-dimers as model photoionization source– no radiation trapping and able to ionize all impurities and additives, such as Xe. Emission Ne2*  Ne + Ne + h 800 Å, 15.5 eV Absorption Xe + h  Xe+ + e Ionization potential 12.13 eV University of Michigan Institute for Plasma Science & Engr. AVS2010_CORONA_6

Te, POTENTIAL: SINGLE IONIZATION FRONT Ionization front driven by charge separation, similar to streamer (E/N=50-70Td). Average speed of the ionization front about 2  108 cm-s-1 Electrons negatively charge the surface of the dielectric, trapping potential in dielectric. 0-20ns Potential Te, Max. 7.5ev, University of Michigan Institute for Plasma Science & Engr. AVS2010_CORONA_7 Ne/Xe = 99/1, 3.5 atm, -25kV

S-EIMPACT, [e] : SINGLE IONIZATION FRONT S-EIMPACT (electron impact ionization source) with an extended leading edge generates plasma in a thin sheet about 200mm thick Maximum electron density is around 5x1015 cm-3 Electrons deposition on the surface leads to charge separation S-EIMPACT (cm-3 s-1) [e] (cm-3 ) Max.= 7x1023 Max.= 5x1015 Ne/Xe = 99/1, 3.5 atm, -25kV University of Michigan Institute for Plasma Science & Engr. AVS2010_CORONA_8 9

Ne2* , S-PHOTO: SINGLE IONIZATION FRONT Ne2* generated within the thin plasma sheet with a max.=9x1012 cm-3 following that of electrons, but with a time delay S-photo generated by the radiation of Ne2* that are distributed along the whole circumference (within the view angle), integral effects Ne2* S-PHOTO 0-20ns [e] Max. 1x1012 Max. 5x1020 Te Ne/Xe = 99/1, 3.5 atm, -25kV University of Michigan Institute for Plasma Science & Engr. AVS2010_CORONA_9 10

IONIZATION FRONT STRUCTURE (I) As the ionization front traverses the circumference, its structure remains largely unchanged. Plasma sheet is about 200 m thick.  Electron density and potential  Ionization source and Te University of Michigan Institute for Plasma Science & Engr. AVS2010_CORONA_10 Ne/Xe = 99/1, 3.5 atm, -25kV

IONIZATION FRONT STRUCTURE (II) The ionization front consists of: 1) bulk electron impact; 2) secondary electron ionization, 3) secondary electron emission from dielectric, 4) photo-ionization 1) and 2) dominate – Photoionization is important in providing ionization far from layer. University of Michigan Institute for Plasma Science & Engr. AVS2010_CORONA_11 Ne/Xe = 99/1, 3.5 atm, -25kV

EVOLUTION OF ELECTRICAL FIELDS Progressive decrease of the potential as the dielectric is charged. Non-monotonic variation of electric fields due to the combined effects of surface charging and the geometric curvature. Variations of electron temperature follow the field. Electron density increases with time. University of Michigan Institute for Plasma Science & Engr. AVS2010_CORONA_12 Ne/Xe = 99/1, 3.5 atm, -25kV

PROPAGATION MECHANISM Electric field peak is 40 kV/cm ( 47 Td) with a sharp leading edge and a blunt trailing edge. Electron density coincides with the peak of the electric field, Normal electric field En reverses sign across the front resulting from the charging of the surface Tangential electric field Et is negative and peaks at the leading edge of the electron density. Large Et enables electrons to drift across the ionization front, seeding electrons for further avalanche. University of Michigan Institute for Plasma Science & Engr. AVS2010_CORONA_13 Ne/Xe = 99/1, 3.5 atm, -25kV

IONS AND EXCITED STATES Similar profiles of [e], and Ne+ Xe+ shows ionization occurs first through both Ne and Xe. After front passes, [Ne+] decreases while the molecular ions Xe2+ and Ne2+ increase due to charge exchange and 3-body association reactions. Both Ne* [Ne(3s)] and Xe* [Xe(6s)] are produced by direct electron impact and so their densities first rise sharply but fall once Te decreases. Although the Ne2* has a radiative lifetime of 11 ns, it is formed by 3-body association reaction with Ne(3s), which has a longer persistence. University of Michigan Institute for Plasma Science & Engr. AVS2010_CORONA_14 Ne/Xe = 99/1, 3.5 atm, -25kV

University of Michigan Institute for Plasma Science & Engr. PHOTON FLUXES The non-resonant VUV light emitted from Ne2* experiences little absorption by other than Xe. The photon collector is progressively illuminated according to distribution of Ne2* and the view angle spanned by the collector surface. Due to the longer effective life time of Ne2* , the collector at any point integrates emission from nearly half the circumference of the corona-bar. Optical transition from Xe(6p) with an artificially short life time of 5 ns follows ionization front more closely. University of Michigan Institute for Plasma Science & Engr. AVS2010_CORONA_15 Ne/Xe = 99/1, 3.5 atm, -25kV

University of Michigan Institute for Plasma Science & Engr. CONCLUDING REMARKS Corona-bar, surface discharges are often used as sources of UV/VUV radiation. The ionization front propagates along the circumference within a thin layer close to dielectric surface. Ionization front resembles a blend of a gas phase streamer and dielectric barrier discharge. Ionization front is sustained by charge separation. Charging of corona bar removes voltage from “gap” The propagation speed decreases with traversal distance. Depending on  and applied voltage, the maximum speed is about 3.5  108 cm-s-1. The ionization is produced primarily through bulk electron impact, with significant contributions from sheath accelerated secondary electrons. E-field and the Te vary in a non-monotonic way as the ionization front traverses the circumference. The VUV light fluxes are correlated with moving ionization front but with a time delay due to lifetime of emitters. University of Michigan Institute for Plasma Science & Engr. AVS2010_CORONA_16

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University of Michigan Institute for Plasma Science & Engr. EFFECTS OF DIELECTRIC CONSTANT Propagation speed decreases with time/distance, particularly so for e=2. Speed generally decreases with increasing dielectric constant – longer “dwell time” to charge capacitance. Maximum speed about 3.0 x108 cm-s-1. Propagation speed dependence on e not necessarily monotonic due to the complex electric field structure and surface curvature. The corresponding Te at the moving ionization front shows the same features. University of Michigan Institute for Plasma Science & Engr. AVS2010_CORONA Ne/Xe = 99/1, 3.5 atm, -25kV 19

University of Michigan Institute for Plasma Science & Engr. VOLTAGE AMPLITUDE The structure and propagation characteristics of the ionization front at different voltages are qualitatively similar The propagation speed increases with increasing voltage. The peak Te (after the initial decay) also increases with increasing voltage. The non-monotoneous variation of Te is present at all the voltages. Similar behavior as gas phase streamer. University of Michigan Institute for Plasma Science & Engr. AVS2010_CORONA Ne/Xe = 99/1, 3.5 atm, e=8 20