A novel, nonequilibrium, high-pressure, direct current discharge, the microhollow cathode discharge, has been found to be an intense source of xenon and argon excimer radiation peaking at wavelengths of 170 and 130 nm, respectively. In argon discharges with a 100 μm diam hollow cathode, the intensity of the excimer radiation increased by a factor of 5 over the pressure range from 100 to 800 mbar. In xenon discharges, the intensity at 170 nm increased by two orders of magnitude when the pressure was raised from 250 mbar to 1 bar. Sustaining voltages were 200 V for argon and 400 V for xenon discharges, at current levels on the order of mA. The resistive current–voltage characteristics of the microdischarges indicate the possibility to form arrays for direct current, flat panel excimer lamps

El-Habachi, Ahmed

Schoenbach, Karl H.

English

Old Dominion University

Old Dominion UniversityODU Digital CommonsBioelectrics Publications Frank Reidy Research Center for Bioelectrics1998Emission of Excimer Radiation From DirectCurrent, High-Pressure Hollow Cathode DischargeAhmed El-HabachiOld Dominion UniversityKarl H. SchoenbachOld Dominion UniversityFollow this and additional works at: https://digitalcommons.odu.edu/bioelectrics_pubsPart of the Atomic, Molecular and Optical Physics Commons, and the Electrical and ElectronicsCommonsThis Article is brought to you for free and open access by the Frank Reidy Research Center for Bioelectrics at ODU Digital Commons. It has beenaccepted for inclusion in Bioelectrics Publications by an authorized administrator of ODU Digital Commons. For more information, please contactdigitalcommons@odu.edu.Repository CitationEl-Habachi, Ahmed and Schoenbach, Karl H., "Emission of Excimer Radiation From Direct Current, High-Pressure Hollow CathodeDischarge" (1998). Bioelectrics Publications. 250.https://digitalcommons.odu.edu/bioelectrics_pubs/250Original Publication CitationEl-Habachi, A., & Schoenbach, K. H. (1998). Emission of excimer radiation from direct current, high-pressure hollow cathodedischarges. Applied Physics Letters, 72(1), 22-24. doi:10.1063/1.120634Emission of excimer radiation from direct current, high-pressure hollowcathode dischargesAhmed El-Habachi and Karl H. Schoenbacha)Electrical and Computer Engineering Department, Physical Electronics Research Institute, Old DominionUniversity, Norfolk, Virginia 23529~Received 15 July 1997; accepted for publication 4 November 1997!A novel, nonequilibrium, high-pressure, direct current discharge, the microhollow cathodedischarge, has been found to be an intense source of xenon and argon excimer radiation peaking atwavelengths of 170 and 130 nm, respectively. In argon discharges with a 100mm diam hollowcathode, the intensity of the excimer radiation increased by a factor of 5 over the pressure rangefrom 100 to 800 mbar. In xenon discharges, the intensity at 170 nm increased by two orders ofmagnitude when the pressure was raised from 250 mbar to 1 bar. Sustaining voltages were 200 Vfor argon and 400 V for xenon discharges, at current levels on the order of mA. The resistivecurrent–voltage characteristics of the microdischarges indicate the possibility to form arrays fordirect current, flat panel excimer lamps. ©1998 American Institute of Physics.@S0003-6951~98!03501-3#Excimer lamps are quasimonochromatic light sourcesthat can be operated over a wide range of wavelengths in theultraviolet ~UV! and vacuum ultraviolet~VUV!. The opera-tion of excimer lamps is based on the formation of excitedmolecular complexes~excimers!and the transition from thebound excimer state to a repulsive ground state. In order togenerate excimer radiation at high efficiency two conditionsneed to be satisfied:1 the electron-energy distributions needto contain a sufficient concentration of electrons with ener-gies required for the generation of the precursors of the ex-cimers, which for rare-gas dimers are excited and ionizedrare-gas atoms. Second, since the formation of the excimersis a three-body process, the pressure needs to be high, pos-sibly on the order of one atmosphere or more. Both condi-tions can only be satisfied simultaneously in nonequilibriumdischarges.There are two ways to generate nonequilibrium plasmas:either by operating on a short enough time scale such thatthermalization of the electrons is prevented, or by operatingon a small enough spatial scale, e.g., in the cathode fall of agas discharge. The first concept utilizes barrier discharges orsilent discharges for excimer lamps.1 The second type ofnonequilibrium discharges are of such a size that they do notallow the development of a positive column, but only theelectrode fall regions and the negative glow as, e.g., hollowcathode discharges.2 The electrode geometry consists of acathode, which contains some kind of a hollow and an arbi-trarily shaped anode. The hole diameter,D, scales inverselywith pressure,p, up to pressures of approximately 5–10Torr/D~cm! for noble gases.3,4 Atmospheric pressure opera-tion in argon was, therefore, expected to be possible by re-ducing the hole diameter to values on the order of 100mmand below.The electrode geometry for the atmospheric pressure dis-charge is shown in Fig. 1, together with an end-on photo-graph of a microhollow cathode discharge in argon at 1.05bar. The electrodes consist of molybdenum foils of 100mmthickness, separated by a mica spacer of 200mm thickness,and a cathode opening of 100mm diam. Spectral measure-ments have been performed using a 0.5 m McPherson scan-ning monochromator, model 219, with a grating of 600G/mm blazed at 150 nm. The discharge chamber with aMgF2 window was mounted directly at the inlet of the mono-chromator. The spectrally resolved radiation at the exit slitwas detected with a photomultiplier tube~Hamamatsu modelR375!after conversion to visible light, centered around 425nm, by a sodium salicylate scintillator. The entire system,including the monochromator, the space between the sourceand the entrance slit, and the space between the exit slit andscintillator was evacuated to a pressure of 60mbar. The in-strument resolution was;2 nm full width at half maximum.The spectra were not corrected for detector response or forwindow transmittance.We have concentrated on the VUV emission from mi-crohollow cathode discharges in argon and xenon. Excimeremission from such discharges has been observedpreviously.4,5 However, the discharges tended to become un-stable at high currents and the emission changed from dc topulsed.4 By reducing the cathode hole diameter to 100mmwe were able to extend the current up to 7 mA~a valuewhich was not exceeded because of concerns about thermala!Electronic mail: schoenb@rolls-royce.eng.odu.eduFIG. 1. Cross section of the hollow cathode geometry and an end-on pho-tograph of a microhollow cathode discharge in argon at 1.05 bar, and acurrent of 1 mA. The circular cathode opening has a diameter ofD5100mm. The mica spacer has a thickness of 200mm.22 Appl. Phys. Lett. 72 (1), 5 January 1998 0003-6951/98/72(1)/22/3/$15.00 © 1998 American Institute of Physics----->< ><- Cathode :J [= ::::= I Dielectric Anode damage of the electrodes! and still operate the discharge in adc mode for both argon and xenon. Measurements in neonand nitrogen in a discharge geometry similar to ours, butwith hole diameters of 200–400mm have shown strongemission in the UV range,6 however, the limited spectralrange of the optical system did not allow the observation ofneon excimer radiation.The argon spectrum for discharge operation with hollowcathodes of 100mm diam at pressures of 100 and 800 mbaris shown in Fig. 2. At low pressure~100 mbar!, the spectrumover the range of 100–200 nm is dominated by Ar II lines,mostly transitions between states having a 3s23p4(3P) ioniccore.7 At high pressure the intensity of these lines is stronglyreduced and the main spectral feature is the excimer line.The peak of the excimer line was observed at 130 nm. Due toa rapid change of the transmittance of the MgF2 windowbetween 125 and 130 nm~53% and 60%, respectively!, anda similar reflectivity change of the MgF2-coated mirrors in-side the monochromator, the actual peak could be at slightlyshorter wavelength. The argon discharge was operated inflowing gas, at 380 sccm. Operating the discharge withoutflow caused a reduction of the intensity by more than anorder of magnitude. A possible reason for this decay is theincreased contamination of the gas by electrode vapor.The emission of the argon excimer radiation is, besideson the gas pressure, also dependent on the discharge current.The sustaining voltage,V, of the argon microhollow cathodedischarge and the intensity of the excimer radiation is plottedversus the discharge current,I , in Fig. 3. Typical sustainingvoltages are 200 V. TheI –V characteristic has a positiveslope over most of the current range, similar to an abnormalglow discharge in a plane–plane geometry. This is expectedfor a hollow cathode discharge at high current.8 In the cur-rent range of zero and positive differential resistivity the in-tensity of the excimer line, measured at 130 nm, increaseslinearly with current. Regions of negative differential resis-tivity, as the one seen in Fig. 3 at a current of 1.3 mA, arecorrelated with a nonlinear increase in excimer emission.This effect, which was found to be even more pronounced inxenon, however at higher current, is assumed to be caused bythe transition from a glow discharge into a hollow cathodedischarge with an increased rate of ionization due to pendu-lum electrons.2,8 Another possible cause for the observednegative differential resistivity is the increased ionization ofmetastable states with increasing current. Such a multistageionization process causes discharge instability, the so-calledionization instability,9 which is associated with a negativevoltage–current characteristic.10 The increase in ionizationcauses a drastic increase in the concentration of molecularions generated through three-body collisions of the atomicions with atoms. This process in turn leads to an increasedformation of excimers, with a consequent increase in theintensity of excimer radiation.Experiments in xenon were only performed in a staticgas environment. The spectra of xenon at pressures of 100and 1000 mbar are shown in Fig. 4. Most of the lines seen inFIG. 2. VUV spectra of discharges in argon at 100 and 800 mbar. Thesustaining voltage for the lower-pressure discharge was 221 V and the cur-rent was 5 mA. For the higher-pressure discharge the voltage was 200 V,and the current was 5.4 mA.FIG. 3. Voltage–current characteristics of microhollow discharges in argon~800 mbar!and corresponding current-dependent intensities of the argonexcimer radiation at 130 nm.FIG. 4. VUV spectra of discharges in xenon at 100 and 1000 mbar. Thesustaining voltage for the lower-pressure discharge was 427 V and the cur-rent was 4.75 mA. For the higher-pressure discharge the voltage was 395 V,and the current was 4.8 mA.23Appl. Phys. Lett., Vol. 72, No. 1, 5 January 1998 A. El-Habachi and K. H. Schoenbach120 230 100 Ar • Intensity (A= 130 nm) 90 100 0 Voltage • -- 100mbar 220 ., 80 -- 800mbar ,· 70 ~ 80 ~ 210 ., ~ ::::i 60 ::::i ~ 0 •• ~ • Q) 0 •• • ocr,ca:x:Fo9° 50 ~ ~ 60 Cl 0 "' .l!! 200 c9 o:9~0000 "iii C ~ C .l!l 40 .l!l -= 40 •• -= • 30 190 ·-· .. Ar 20 20 180 • 800 mbar 10 0 0 100 150 200 250 300 0 2 3 4 5 6 7 Wavelength (nm) Current (mA) 120 Xe -- 100 mbar 100 -- 1000mbar ~ ::::i 80 ~ ~ "iii 60 C .l!l -= 40 20 0 150 200 250 300 350 Wavelength (nm) the xenon spectrum at low pressure are Xe II lines. The lineat 315 nm was identified as a molybdenum~electrode mate-rial! line. It is also visible in the argon discharge spectrum.Its reduced intensity at higher pressure indicates less sputter-ing of the electrode material. At atmospheric pressure themain feature in the xenon spectrum is the excimer line witha measured maximum emission at 170 nm. The sustainingvoltage for xenon discharges is twice as high as for argondischarges, 400 V for xenon versus 200 V for argon.The observed excimer emission points to the presence ofa large concentration of electrons with energies on the orderof the metastable argon atom~12.5 eV!. The Ar II lines seenat low pressure indicate even higher electron energies, on theorder of 30 eV. This is not too surprising since electron-energy measurements in hollow cathode discharges at lowpressure11 and in the negative glow of low-pressuredischarges12 have shown the presence of electrons with en-ergies even greater than 100 eV. Extending the dischargeoperation to high pressure by reducing the electrode dimen-sions seems to only reduce the density of electrons in thehigh-energy tail of the electron-energy distribution but noteliminate them.High-pressure operation, however, favors the generationof excimers, a three-body process. This is obvious from thepressure dependence of the excimer emission. In xenon, ra-diation intensity at 170 nm increased by two orders of mag-nitude when the pressure was raised from 250 mbar to 1 bar.In argon, the intensity of the excimer radiation at 800 mbarwas five times that at 100 mbar. This pressure dependencecan be explained by considering the rates for the generationof excited rare-gas atomsR* , the precursors for the excitedrare-gas moleculesR2* :d@R* #dt5W82kR* R22R*ta,wherek is the rate coefficient for three-body collision andW8 is the source term for the generation of excited atoms.The apparent lifetime of the excited atoms,ta , is besidesbeing gas dependent also determined by radiationtrapping.13,14 For efficient excimer formation, the three-bodycollision term kR* R2 needs to exceed the radiative decayterm R* /ta . The value of the onset pressure for efficientexcimer radiation can, therefore, be estimated by setting thetwo terms equal. For xenon,k is 5310232 cm6/s,15 andta ,which scales with the square root of the plasma celldimensions,14 is estimated to be 1ms in the microhollowcathode geometry. Using these values, the onset pressure forexcimer radiation in xenon is 180 mbar. Since diffusion wasneglected as an additional loss process, the onset pressure isactually higher. This is in agreement with our experimentalresults, where excimer radiation in xenon becomes observ-able above 250 mbar.The fact that the current–voltage characteristic of micro-hollow discharges has a positive slope over a large range ofcurrent allows the operation of these discharges in parallel,16and consequently the construction of flat panel excimerlamps. A microhollow cathode discharge excimer lamp hasthe potential to operate dc, at voltages of several hundredvolts. The simplicity of the electrode geometry, which can begenerated by means of thin-film technology, may allow theformation of thin, almost two-dimensional excimer lamps,hich can easily be adapted to any load geometry.The authors like to thank U. Kogelschatz~ABB Corpo-rate Research! for stimulating discussions. This work is sup-ported by the U. S. Department of Energy, Advanced EnergyDivision.1U Kogelschatz, Appl. Surf. Sci.54, 410 ~1992!.2G. Schaefer and K. H. Schoenbach, inPhysics and Applications of Pseu-dosparks, edited by M. Gundersen and G. Schaefer~Plenum, New York,1990!.3J. W. Gewartowski and H. A. Watson,Principles of Electron Tubes~VanNostrand, Princeton, NJ, 1965!, p. 561.4K. H. Schoenbach, A. El-Habachi, W. Shi, and M. Ciocca, PlasmaSources Sci. Technol.6, 468 ~1997!.5K. H. Schoenbach, A. El-Habachi, W. Shi, and M. Ciocca, ConferenceRecord 1997 IEEE International Conference Plasma Science, San Diego,CA, Abstract 5A07, p. 239; K. H. Schoenbach, M. Ciocca, A. El-Habachi,W. Shi, F. Peterkin, and T. Tessnow, Proceedings of the 12th InternationalConference on Gas Discharges and Their Applications, Greifswald, Ger-many, 1997, Vol. 1, p. 280; K. H. Schoenbach, A. El-Habachi, W. Shi,and M. Ciocca, XXIII International Conference on Phenomena in IonizedGases~ICPIG!, Toulouse, France, 1997, Vol. V, paper V22.6J. W. Frame, D. J. Wheeler, T. A. DeTemple, and J. G. Eden, Appl. Phys.Lett. 71, 1165~1997!.7R. L. Kelly, Report No. ORNL-5922, Oak Ridge National Laboratory,Oak Ridge, TN~1982!.8A. Fiala, L. C. Pitchford, and J. P. Boeuf, XXII International Conferenceon Phemomena in Ionized Gases~ICPIG!, Hoboken, NJ, 1995, Contr.Papers 4, p. 191.9W. L. Nighan and W. J. Wiegand, Phys. Rev. A10, 922 ~1974!.10G. L. Rogoff, J. Appl. Phys.50, 6806~1979!.11K. Fujii, Jpn. J. Appl. Phys.16, 1081~1977!.12P. Gill and C. E. Webb, J. Phys. D10, 299 ~1977!.13T. Holstein, Phys. Rev.83, 1159~1951!; Y. Salamero, A. Birot, H. Bru-net, J. Galy, and P. Millet, J. Chem. Phys.80, 4774~1984!.14W. Wieme and P. Mortier, Physica~Utrecht! 65, 198 ~1973!.15J. K. Rice and A. W. Johnson, J. Chem. Phys.63, 5235~1975!.16K. H. Schoenbach, R. Verhappen, T. Tessnow, F. E. Peterkin, and W. W.Byszewski, Appl. Phys. Lett.68, 13 ~1996!.24 Appl. Phys. Lett., Vol. 72, No. 1, 5 January 1998 A. El-Habachi and K. H. Schoenbach

Emission of Excimer Radiation From Direct Current, High-Pressure Hollow Cathode Discharge

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Emission of Excimer Radiation From Direct Current, High-Pressure Hollow Cathode Discharge

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