Plasma diagnostics in large area plasma processing systemD. Leonhardt, S. G. Walton, D. D. Blackwell, W. E. Amatucci, D. P. Murphy, R. F. Fernsler, and R. A. Meger Citation: Journal of Vacuum Science & Technology A 19, 1367 (2001); doi: 10.1116/1.1359554 View online: http://dx.doi.org/10.1116/1.1359554 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/19/4?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Plasma diagnostics of low pressure high power impulse magnetron sputtering assisted by electron cyclotronwave resonance plasma J. Appl. Phys. 112, 093305 (2012); 10.1063/1.4764102 E-H mode transition in low-pressure inductively coupled nitrogen-argon and oxygen-argon plasmas J. Appl. Phys. 109, 113302 (2011); 10.1063/1.3587156 Pressure and input power dependence of Ar/N 2 H 2 inductively coupled plasma systems J. Vac. Sci. Technol. A 19, 2335 (2001); 10.1116/1.1385904 Diagnostics of chlorine inductively coupled plasmas. Measurement of electron temperatures and electron energydistribution functions J. Appl. Phys. 87, 1642 (2000); 10.1063/1.372072 Large-area ion source combining microwaves with inductively coupled plasma Rev. Sci. Instrum. 71, 716 (2000); 10.1063/1.1150271

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Plasma diagnostics in large area plasma processing systemD. Leonhardt,a) S. G. Walton, D. D. Blackwell, W. E. Amatucci, D. P. Murphy,R. F. Fernsler, and R. A. MegerU.S. Naval Research Laboratory, Plasma Physics Division, Washington, DC 20375

~Received 9 October 2000; accepted 5 February 2001!

A series of plasma diagnostic have been carried out in our large area plasma processing systemwhich is based on a modulated electron-beam produced plasma. These discharges were created inboth electrically conducting and insulated vacuum chambers operated in 30–120 mTorr of puregases~argon, oxygen, and nitrogen!. Langmuir probes, microwave transmission, mass spectrometry,and external current sensors show robust discharges were made over fairly wide parameter rangesresulting in plasma densities of 1 – 431011 cm23 and temperature ranging from 0.2 eV for themolecular gases and 1.4 eV for argon. The effects of various experimental techniques, parameters,and contamination issues are discussed in detail. ©2001 American Vacuum Society.@DOI: 10.1116/1.1359554#

I. INTRODUCTION

Electron beam~e-beam! generated discharges have beenshown to be efficient at producing high density plasmas1 andpossess characteristics favorable for materials’ processingapplications.2 The previous ‘‘large area’’ work1 emphasizedreflection ofX-band microwaves~requiring electron densities.1012 cm23! at fairly high pressures~up to 300 mTorr!using various operating modes of hollow-cathode dischargesto generate ane-beam. Although thee-beam was collimatedby an external magnetic field, the mode operation was notalways stable, typically having narrow operating windows.Materials’ processing work2 applied a freely expandinge-beam plasma as a secondary source of ionization in a reac-tive ion etching reactor. In this configuration, enhancedplasma densities of 109– 1010 cm23 were seen in the beamregion but etch uniformity was low since the beam did notpropagate the entire substrate length. The authors noted thataccurately defining the beam geometry and extent was nec-essary due to the rapid variation of plasma characteristicswith distance to the substrate.

In this article, we studied highly stable modes of opera-tion of magnetically collimatede-beam plasmas with globalmeasurements~microwave transmission, current sensors! aswell as highly localized plasma diagnostics~Langmuirprobes!. Surface interactions and relative cation fluxes werealso investigated in a high vacuum system via a quadrupolemass spectrometer to further diagnose such dischargesin situfor future materials’ processing applications.

II. EXPERIMENT

Electron beam produced discharges were created in a cy-lindrical Plexiglas chamber~38 cm in diameter and 50 cmtall! and a cylindrical aluminum vessel~50 cfm in diameterand 65 cm tall!. For the Plexiglas chamber, a~45 cm! rotaryvane pump provided an ultimate base pressure of;4 mTorrand manual variable leak valve permitted good control of gasflow to vary chamber operating pressure. The aluminum high

vacuum system was pumped by a 150 l/s tubomolecularpump, and provide based pressures in the 831026 to 1.531025 Torr range. Gas flow in this chamber was controlledby throttling the turbo pump via a low-conductance tube andintroducing gases through mass flow controller units~MKS1179 Series!. The purity of gases used in these experimentswere 99.999% argon and.99.8% nitrogen and oxygen.Convectron® and/or Pirani-type vacuum gauges were usedto determine operating pressures and the reported readingshave been corrected for the particular gas type.

The electron beam was generated by a hollow cathode~asin Ref. 1! consisting of a 1 cmwide channel in 2.5 cm diambrass rod stock. The channel was approximately 1.5 cm deepand 22 cm long. A 1.6 mm thick teflon sheet and groundshield cover the inactive cathode surfaces to suppress un-wanted electrical breakdowns. The electron beam generatedby the cathode is physically skimmed by a stainless steelshim stock anode containing a 1320 cm slot aligned withthe cathode hollow and placed;5 cm downstream. A sec-ond anode, placed;25 cm downstream of the first acts asthe beam dump. Both anodes are grounded outside of thechamber so that the currents collected on them can be moni-tored with Pearson coils. The beam between the anodes iscentered in a pair of 50 cm diam. Helmholtz coils, whichprovides an axial magnetic field with uniformity better than2% over the discharge area. Figure 1 shows the geometriclayout of the cathode and anodes. The resultante-beam sheetis therefore defined by the slotted anode~1 cm320 cm! andthe distance~;25 cm!3 to the beam dump anode. A largeimpedance voltage divider is connected in parallel with thecathode/anode on the vacuum chamber to monitor the volt-age drop across the cathode and beam dump anode and toinfer a relative plasma impedance. High voltage pulses~2.5kV! are generated via a homemade unit4 based on insulatedgate bipolar transistor technology. Pulses used here are 500–1000ms in length at repetition rates of 1–50 Hz. While muchlarger duty factors~.50%! are possible without loss ofplasma stability, data collection in these experiments wasa!Electronic mail: darrin@ccf.nrl.navy.mil

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dictated by human response times.Microwave transmission measurements were carried out

in the Plexiglas chamber using 20 dB horns straddling theplasma sheet outside the chamber. A Hewlett-Packard fre-quency synthesizer~1–25 GHz! was used as the microwavesignal source. The microwave signal is amplitude modulatedat ;20 kHz to clearly identify baseline levels of the signalduring the plasma modulation.

Due to the rough vacuum environment in the Plexiglaschamber, contamination of the Langmuir probe resulted inirreproducible data. We found it necessary to heat the probeexternally5 to achieve consistent data in this environment.Thus, single Langmuir probe construction consisted of a 0.25mm ~10 mil! thoriated tungsten wire protruding 2 mm out ofan alumina tube. The aluminum tube was 0.156 cm in diam-eter with four 0.381 mm~15 mil! bores. A 4 cm ‘‘hairpin’’of nichrome wire was wound around the alumina tube andwas used to heat the probe tip locally. Ceramic paste coatedthe heater element and contact wires to shield them from theplasma. A typeK thermocouple was also inserted throughtwo of the bores to monitor the probe’s temperature. For allbut the last 4 in. of the probe, all wires and connections wereenclosed in a grounded tube. It was found that if the probetemperature was kept at around 500 °C, reproducible resultscould be obtained with this system. The probe bias voltagewas provided by a potentiometer connected to gel cell bat-teries, which were checked for better than 10ms time re-sponse and extremely low noise. Probe currents during andafter the pulse were recorded on a digital oscilloscope.

An emissive probe was inserted into the aluminum cham-ber to more accurately identify the plasma potential andeliminate any systematic errors within the single probe mea-

surements. Similar ceramic and tungsten materials were usedfor this probe as well. The tungsten wire6 was bent into ahairpin geometry with a radius of curvature of;1 mm. Thiswire was press fit into ceramic tubing with 18 gauge copperwire which ran to a high current vacuum feedthrough. Emis-sion of the probe was controlled by a~floating! dc powersupply operated in constant current mode. Typical probecurrent–voltage~I–V! traces were obtained by applying abias voltage to the probe and monitoring the collection cur-rent though the probe by measuring the voltage across a stan-dard carbon resistor, as in the single probe measurements.Data were recorded using boxcar averagers~Stanford Re-search Systems SR250! with 1–10 ms sample gates posi-tioned during thee-beam, then 50 and 100ms after the beamhad been terminated.

Mass spectrometry was carried out in the aluminum highvacuum chamber using a quadrupole mass analyzer~HidenEQP300!. The mass spectrometer was differentially pumpedand sampled species through a 100mm orifice. The orificewas grounded and positioned parallel to thee-beam, simulat-ing a wafer chuck for materials’ processing applications 1cm away from the edge of thee-beam~see Fig. 1 detail!. Thebase pressure was;131028 Torr which increased to<231026 Torr when the plasma chamber pressure was 100mTorr. The body of the mass spectrometer was constantlybaked to minimize background gas interferences. Further de-tails are available in Ref. 7.

III. DATA

A. Oxygen and nitrogen

With a 2.5 keV e-beam, electron beam currents8 of;5 mA/cm2 were readily produced by the cathode, over alarge pressure range~30–120 mTorr! of oxygen or nitrogenand magnetic field~120–300 G!. Figure 2 shows N2 dis-charge parameters as measured by the external sensors~Pear-son coils and microwave transmission! in the Plexiglaschamber. Corresponding raw data from the time-resolvedLangmuir probe measurements are shown in Fig. 3 for theanalogous O2 discharge. The probe is heated to 500 °C andpositioned in the center of the discharge. Emissive probedata taken in the high vacuum chamber are shown in Fig. 4for the same N2 discharge. All nonspatially resolved probemeasurements shown are taken at the center of the discharge.Emissive probe measurements were not achievable in an O2

working gas.

B. Argon

The 2.5 kV e-beam produced beam currents of;20mA/cm2 as measured by the external sensors similar to N2

and O2. Under the same operating conditions, the argon dis-charge had a higher plasma density, attenuating 7.5 GHzmicrowaves completely. In certain pressure and magneticfield regimes, much more beam current was produced by thecathode, which led to a significant drop in anode–cathodevoltage. These higher current/lower impedance modes pos-sessed significantly higher electron densities but were un-

FIG. 1. Cross-sectional layout of hollow cathode and anodes used to gener-ate electron beam and subsequent electron beam generated discharge. Thehollow cathode is drawn in solid black, teflon insulation by dashed line. A1and A2 are the slotted and beam dump anodes, respectively. Pearson coilsare labeled PC. Position of mass spectrometer orifice with respect to theplasma is shown separately in detail for clarity.

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stable in time and therefore not studied extensively. Charac-teristic probesI–V traces from the Plexiglas chamber areshown in Fig. 5.

IV. RESULTS

A. Oxygen and nitrogen

Oxygen and nitrogene-beam generated discharges showsimilar characteristics at the conditions investigated here.Both gases formed plasmas with densities from 131010 to531011 cm23 according to probe and microwave cutoff9

measurements. Microwave cutoff measurements are pro-vided as a lower bound and sanity check on the probe data.Probe measurements in the Plexiglas chamber were analyzedusing a two temperature fitting routine10 which assumesMaxwellian electron distributions. The current in the ionsaturation region is assumed to be proportional to(2V)1/2, then coupled to two electron saturation regionswith exp(2V/Te) dependencies. Variables of the fit are ad-justed to minimize the discrepancy between the fit and thedata. Two-temperature distributions down to 0.10 eV havebeen clearly seen using this particular algorithm, thus webelieve it to be well suited for this application. Spatial pro-files for the resultant densities and electron temperatures forN2 discharges are given in Figs. 6 and 7. Figure 6 showsdensities and temperatures along the electron beam axis. Fig-ure 7~a! shows the ion saturation current measured with aprobe bias of220 V, and Fig. 7~b! shows the electron tem-

perature across the plasma sheet. Oxygen data showed verysimilar results, giving slightly lower plasma densities (2.531011 cm23) and temperatures~;0.32 eV!. Temporally re-solved probe data are shown in Fig. 8 for the same N2 dis-charge conditions. Oxygen discharges also show similarrapid stabilization.

Translation of the cathode from the Plexiglas~insulating!chamber to the aluminum~conducting! chamber showed noappreciable effect on the discharge conditions as measuredby the external sensors and the two probe techniques.~Asmentioned earlier, emissive probe measurements were only

FIG. 2. External plasma diagnostics~voltage, cathode current, anode current,and microwave transmission! for N2 discharge at 75 mTorr and 150 G. Theanode current is offset for clarity. Complete attenuation of the 1.5 GHzmicrowaves implies~see Ref. 9! a plasmas density of.531010 cm23.Electron beam pulsewidth is 1 ms.

FIG. 3. Single Langmuir probe currents of various probe bias voltages be-fore, during, and after thee-beam produced O2 plasma. Probe is heated to500 °C via the external heater and the plasma conditions are the same as inFig. 2, at a 800ms pulsewidth.

FIG. 4. Emissive probe measurements from the high vacuum chamber forthe same N2 discharge. Emission is labeled by the dc current flowingthrough the probe. Plasma potential as identified by strong emission curve isgiven by dotted line.

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carried out in N2.! The impetus for carrying out the emissiveprobe measurements under higher vacuum was to substanti-ate the data analysis in the poorer based pressure chamber.These details are discussed in a subsequent section. Thefluxes of ions detected by the mass spectrometer showed O1

to be the dominant ion near the electron beam, with aO1/O2

1 flux ratio of 2:1.

B. Argon

Probe data in the Plexiglas chamber shows electron tem-peratures of;1.2 eV during thee-beam and a rapid coolingonce thee-beam is turned off. Plasma electron densities were;331011 cm23 and also dropped rapidly once thee-beamwas turned off, which agrees well with the microwave cutoffdata. The plasma potential from these measurements was;3–5 V. A time-resolved data plot ofne andTe is shown inFig. 9. Plasma potential and temperature measurements fromthe emissive probe in aluminum chamber were both slightly

lower, typically ;2–3 V and 1 eV during thee-beam andcooled more slowly once thee-beam was turned off. Duringthe beam, the dominant ion flux in the mass spectrometerwas atomic ions (Ar1).

FIG. 5. CharacteisticI–V plots from argon discharge in Plexiglas chamberusing heated Langmuir probe. Postpulse curves are labeled according totheir time after the plasma pulse.

FIG. 6. Electron densities and temperatures along the electron beam axis forthe N2 discharge shown in Fig. 2. Dotted lines show the positions of the twoanodes~the e-beam travels from left to right!.

FIG. 7. ~a! Ion saturation current measured with a probe bias of220 V and~b! electron density measurements while the probe traverses the plasmasheet for the N2 discharge shown in Fig. 2.

FIG. 8. Temporally resolved single probe data for the N2 discharge shown inFig. 2.

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V. DISCUSSION

A. Oxygen and nitrogen

The strong similarities in the O2 and N2 discharges showthat e-beam ionization is fairly independent of backgroundgas. Erratic probe analyses in the~rough vacuum! Plexiglaschamber~especially in argon, see below! prompted the inde-pendent measurement of the plasma potentials via the emis-sive probe in the cleaner high vacuum aluminum chamber.As seen in Figs. 3 and 5, the probeI–V traces obtained in thePlexiglas chamber did not show a clear leveling out of theelectron saturation region and thus provided poor identifica-tion of the plasma potential. Best fits to the Plexiglas cham-ber experimental data were achieved by forcing the plasmapotential to be lower than indicated by typical first derivative(dIprobe/dVbias) techniques. Excellent fits were achievedwhen the data were analyzed in this manner, however anindependent test was still necessary to corroborate these re-sults. The emissive probe data in the high vacuum systemindeed showed similar results~see Fig. 4!. A nonemissiveprobe trace~‘‘no emission’’ curve in Fig. 4! showed a shiftedI–V curve, whereas when the probe was pushed even slightlyemissive~9 A curve!, the probeI–V trace shifted to a lowerplasma potential and remained there through more emissivestates~9.7 A and 10.3 A curves!. Plasma potentials identifiedeither from analyzing the slightly emissive curve or takingVfloat5Vplasma~dotted line in Fig. 4! for the extreme emissioncurve agreed well with the plasma potentials~and electrontemperatures! from the cruder probe measurements in thePlexiglas chamber. These trends seen in the emissive probewere consistent regardless of the previous probeoperation—a probe ‘‘cleaned’’ by running high currentsthrough it gave the same ‘‘no emission’’ curve as a probewhich had been cycled through 100 emission cycles. Hence,the probe surface ‘‘contamination’’ does not depend oninsitu or ex situsurface preparations; the chamber base pres-sure of;131025 Torr deposits 10 monolayers of oxygen,

water, etc. every second, which must be constantly burnedoff to get accurate plasma potential identification.

The fact that both types of probe measurements fit well totwo-temperature distributions demonstrates that the actualbulk electron distribution is non-Maxwellian. We expectcold distributions frome-beam generated plasmas created inmolecular gases because ionization of the plasma comesfrom the beam and ionization by the plasma electrons isnegligible. Thus, heating of plasma electrons is generallyabsent, except as byproducts of the ionizing collisions.@Intypical inductively coupled plasma~ICP! or glow discharges,electron temperatures are high because the plasma electronsmust be hot enough to ionize the background gas.# In mo-lecular gases, the primary plasma electron temperature frome-beam produced plasma is especially low because of thelarge sea of inelastic levels that rapidly cools the plasmaelectrons.

From the data fitting routine, very cold~;0.35 eV in N2

and;0.25 eV in O2! electrons were measured with a secondpopulation consistently lower in density~by a factor of 2–10and higher temperature~factor of 5–10!. The dependence ofthis second population could be very erratic. Initially, itsexistence was attributed to the probe’s response to the highenergy electron beam. However, the beam current in theprobe is well below all of the probe current measured andthere was no spatial dependence associated with thegeometric profile of the e-beam. Actually the two-temperature Maxwellian fit provides a mathematicalbreakdown of the non-Maxwellian bulk electron distribution.The mean electron temperature from two Maxwellians is ac-tually give by Te5n1Te1 /(n11n2)1n2Te2 /(n11n2)'Te1

1(n2 /n1)Te2 . Considering a worse case wheren1;2n2 andTe1;Te2/10, Te;4Te1 . This mean temperature for thebulk distribution is indeed still very low, 1–2 eV, especiallywhen compared to the other@non-Maxwellian# high densityplasma discharges. While determining the electron energydistribution function~eedf! is not possible with the presentdata11 the well defined, consistently low plasma potentials~;1 V! also indicate that the bulkTe is indeed low.

Secondary electrons produced frome-beam ionizationscan indeed be highly energetic initially, with a mean energyroughly one-half the ionization energy of the gas. If this highenergy contribution to the actual eedf was substantial, thelower energy portion would compensate to keep thermal ion-ization small, driving the eedf even more non-Maxwellian.Thus, the second population generated by the two tempera-ture fitting routine provides more of a degree of perturbationon an ideal Maxwellian eedf.

In these molecular gases, it is believed that the plasmadecay or afterglow is dominated by electron–ion recombina-tion. In that case, after the electron beam turns off, theplasma density decays from its density at the end of the pulsen(0) according todne /dt52bne

2, whereb is the recombi-nation coefficient and electrons are the only negative chargecarriers.12 Solving this equation, we see that the ion densityvaries asn(t)5n(0)/@11bn(0)t#, wheret is the time afterthe pulse. As a check of this decay mechanism, 1/ne from

FIG. 9. Temporally resolved argon plasma electron densities and tempera-tures measured in Plexiglas chamber at different times during and after theelectron beam, for the argon discharge shown in Fig. 5.

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Fig. 8 in the N2 afterglow was plotted and fit to a straightline to derive an experimental recombination coefficient.This fit yieldsb58.131028 cm3/s for the initial decay. InN2 the recombination coefficient varies as13 b54.331028

3@Te#20.39, which when equated to our experimental value,

gives an afterglow plasma temperature of 0.19 eV, in goodagreement with the data in Fig. 8. Because the plasma tem-perature decreases with time, a perfect fit is not obtainedover the entire range of the decay. While this result is some-what fortuitous, given the weak temperature dependence ofthe recombination coefficient, the low electron temperatures~<1 eV! measured nevertheless support the plasmas decaymechanism. A similar analysis in the O2 discharge~using14

b52.4310283@Te#20.5) results in the same afterglow tem-

perature, which is slightly higher than the measured 0.15 eVtemperature derived from probe measurements.

B. Argon

Again, the strong similarities in the molecular and noblegas discharges show thate-beam ionization is fairly indepen-dent of background gas. As alluded to earlier, the erraticbehavior seen in the probe data from the Plexiglas chamberprompted independent measurements of plasma potentials inthe cleaner aluminum vacuum chamber with an emissiveprobe. The emissive probe gave a more distinct identificationof the plasma potential (Vp51.5– 2.5 V!. This potential ishigher than that in the molecular gases as is the electrontemperature (Te51 – 1.5 eV!; see Fig. 9. Higher electrontemperature during the discharge is reasonable, since argonhas no excited states below 11.5 eV. Hence, plasma coolingis slower relative to the cooling rate in molecular gases. Evenhigher electron temperatures are expected in pure argon thanthose measured here. Moreover, since recombination is not aviable decay mechanism for Ar1 ~given that it is a three-body process!, the discharge should be diffusion dominated.Thus the plasma should show a slowly decaying afterglowbut instead shows rapid quenching of plasma species as inthe molecular gas case. In the Plexiglas chamber, this rapidquenching was attributed to the poor base pressure, whichresulted in 5%–10% of the plasma operating pressure beingair. In the aluminum chamber, the base pressure was at least2 orders of magnitude better and yet similar results wereseen. This result is surprising until we look at the plasmafluxes as seen by the mass spectrometer during and after thee-beam. Once thee-beam source is off, we see that the domi-nant ion quickly becomes H2O1. The trace amounts of H2Oin the high vacuum environment were still enough~and thecross section large enough! to charge exchange the Ar1 toH2O1 very rapidly. See Ref. 7 for additional treatments ofthis and other mass spectrometry measurements ine-beamproduced plasmas.

VI. SUMMARY

We have used electron beam produced plasmas in mo-lecular and noble gases resulting in plasma densities in1 – 531011 cm23 in conducting~aluminum! and insulating~Plexiglas! vessels as measured by Langmuir probes and mi-

crowave transmission. While completely different probetechniques were used in each system, the discharge charac-teristics were transferable across chambers. probe contami-nation issues are very different but were quantified and dis-cussed in detail. A two-temperature fitting routine showedprimary electron temperatures in these discharges were 0.3–0.5 eV for the molecular gasses and;1.2 eV for argon. Dueto the e-beam ionization mechanism, nontraditional heatingof the plasma electrons can result in extremely non-Maxwellian distributions. The consistently low~1–3 V!plasma potentials, however, demonstrate that the electrontemperature must be low as well, regardless of the distribu-tion. Mass spectrometry shows relative ion species in anoxygen discharge to be 2(O1):1(O2

1) and predominantlyAr1 in an argon discharge, at operating pressures of 40–100mTorr.

VII. CONCLUSION

The strong similarities in the molecular and noble gasdischarges show thate-beam ionization is fairly independentof background gas. Unlike typical ICP or glow discharges,electron temperatures are low because there is no externalheating and the plasma electrons are simply byproducts ofthe e-beam collisions. Standard Langmuir probe techniquescan be employed, although at these operating pressures andconditions, contamination issues can seriously affect infor-mation collected from such probes.

ACKNOWLEDGMENTS

The authors greatly appreciate insightful conversationswith Dr. Martin Lampe and Dr. Wally Manheimer. NRL/NRC postdoctoral research associates. S.G.W. and D.D.B.gratefully acknowledge support from the National ResearchCouncil. This work was supported by the Office of NavalResearch.

1R. F. Fernsler, W. M. Manheimer, R. A. Meger, J. Mathew, D. P. Mur-phy, R. E. Pechacek, and J. A. Gregor, Phys. Plasmas5, 2137~1998!; V.Burdovitsin and E. Oks, Rev. Sci. Instrum.70, 2975~1999!; W. M. Man-heimer, R. F. Fernsler, M. Lampe, and R. A. Meger, Plasma Sources Sci.Technol.9, 370 ~2000!.

2M. A. Kushner, W. Z. Collison, and D. N. Ruzic, J. Vac. Sci. Technol. A14, 2094 ~1996!; K. D. Schatz and D. N. Ruzic, Plasma Sources Sci.Technol.2, 100 ~1993!.

3From R. F. Fernsleret al. in Ref. 1, a 2 kVelectron beam loses;3eV/cm @30 mTorr, thus a 2.5 keV beam in 100 mTorr has a range of 1.4m, much shorter than the dimensions of our chamber.

4M. C. Myers, D. D. Hinshelwood, R. A. Meger, and R. E. Pechacek,Proceedings of the 12th IEEE Pulsed Power Conference, Monterey, CA1999.

5W. E. Amatucci, M. E. Koepke, T. E. Sheridan, M. J. Alport, and J. J.Carroll III, Rev. Sci. Instrum.64, 1253~1993!.

6It was necessary to use the same 10 mil tungsten wire in the emissiveprobe to get a more rugged filament for our operating conditions.

7S. G. Waltonet al., J. Vac. Sci. Technol. A~these proceedings!.8Electron beam current is considered to be the current collected at thebeam dump anode. This may be an overestimate.

9For a thin plasma sheet, the evanescent wave can get through the sheet ifthe sheet thickness,microwave wavelength. Basic result is a factor of 2increase inn (cm23) from the simple equationn(cm23)51.2331012

3@ f (GHz)/10#2. D. P. Murphy, R. F. Fernsler, and R. A. Merger, NRLInternal Report No. NRL/MR/6754--99-8368, May 1999~unpublished!.

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10A single temperature version of this algorithm is discussed by J. J. CarrollIII, M. E. Koepke, W. E. Amatucci, T. E. Sheridan, and M. J. Alport,Rev. Sci. Instrum.65, 2991~1994!.

11Attempts to derive the energy distribution function from the probe datataken in these experiments proved to be noninformative, as the datasmoothing required completely overshadowed the underlying physical in-formation. We have been implementing a 12-bit resolution data acquisi-tion system to circumvent this problem. See D. D. Blackwellet al., J.Vac. Sci. Technol. A~these proceedings!.

12We saw no evidence of significant negative ion densities during or afterthe e-beam pulse as characterized by a symmetric probeI–V trace. Thefact that we have cold plasma electrons would imply that the electronattachment mechanism is non-negligible. However, for the most elec-tronegative gas studied here, oxygen, the attachment cross section is ex-tremely low at these electron energies.~See D. Rapp and D. D. Briglia, J.Chem. Phys.43, 1480~1965!.

13F. J. Mehr and M. A. Biondi, Phys. Rev.181, 264 ~1969!.14P. M. Mul and J. W. McGowan, J. Phys. B12, 1591~1979!.

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