Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 993–996

Evaluation of a microstructured field-emitter deviceas a source of electrons in an angle- and TOF-resolving

electron spectrometer

Michael Lange∗, Jun Matsumoto, Andy Setiawan,Julian C.A. Lower, Stephen J. Buckman

Atomic and Molecular Physics Laboratories, Research School of Physical Sciences and Engineering,The Australian National University, Canberra, ACT 0200, Australia

Available online 7 March 2005

Abstract

We report on new developments on a ns-pulsed electron gun, which is to be used in a new TOF electron spectrometer. To possibly reacht as a novelt e emissionc©

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he desired energy resolution without using an electron monochromator, we have investigated a microstructured field emitterype of electron source for electron spectrometry. After giving an overview over the spectrometer, we present measurements of thharacteristic, and also of the energy distribution of the electron beam produced by the field-emission device.2005 Elsevier B.V. All rights reserved.

ACS:34.80.Bm; 34.80.Dp; 34.80.Gs; 39.90.+d; 41.75.Fr

eywords:Electron spectrometers; Electron beams; Electron field emission

. Introduction

At the Australian National University, a time-of-flightTOF) electron spectrometer (Fig. 1) is presently under con-truction, for measuring absolute, differential cross-sectionsf electron-impact excitation of atoms and molecules close

o the thresholds of inelastic channels, at projectile ener-ies of 3–30 eV. It consists of an electron gun, mountedn a turntable which can be rotated about a gaseous targeteam, and a large (80-mm diameter) microchannel plate de-

ector (MCP) at a fixed position. At present, free electronsre created by thermionic emission from a directly heated

horiated tungsten filament, which are then sent through aemispherical monochromator to reduce their large initialnergy spread. The resulting beam is passed onto a pulsingnit, where it is chopped into pulses of a few ns durationt 1 MHz repetition rate, by means of a pulsed deflectioneld which moves the beam over an aperture. This pulsed

∗ Corresponding author.E-mail address:michael.lange@anu.edu.au (M. Lange).

beam is then crossed with an atomic or molecular beathe target gas of interest, which effuses from a single captube.

The pulsing provides a zero point for the time-of-fliof the scattered electrons: After being scattered by theget, the electrons drift in a field-free region for≈22 cm, before they are post-accelerated from a metal mesh towthe MCP to achieve uniform detection efficiency. Both tarrival time and position are measured with a delay-lineode (DLA), and then mapped to the scattering energyangle (at an angular resolution of 0.2◦). The use of a largearea detector accepting electrons scattered into a wiof ±10.6◦ around a preset median angle between 40◦ and120◦ guarantees high count rates and measurement efficIn addition, the TOF technique allows the measuremeseveral scattering channels at once. Since this includeelastic channel, for which the cross-section is usuallyknown, the measured inelastic cross-sections can immately be normalized to an absolute scale, without neeany absolute knowledge of the electron beam currentnumber density of target atoms/molecules or, most im

368-2048/$ – see front matter © 2005 Elsevier B.V. All rights reserved.oi:10.1016/j.elspec.2005.01.237

994 M. Lange et al. / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 993–996

Fig. 1. Schematic drawing of the spectrometer. D:xy-deflector sets, L: elec-trostatic lenses.

tantly, the transmission of the analyser for the different energyelectrons.

Compared to a spectrometer with a conventional elec-trostatic analyzer, the high count rate and the capability ofmeasuring all scattering channels at once, together with theuniform detection efficiency of the TOF technique and thelarge angular range make this spectrometer an ideal toolfor investigating electron-impact excitation close to thres-hold.

2. The pulsed electron gun

To minimize influence on the electron energy distribution,the deflectors of the pulsing unit are operated with a voltagepulse of a symmetric triangular shape (peak valueUp), in-stead of a fast voltage transient[1]. In addition, a d.c. offsetvoltage of−Up/2 is applied between the plates so that whenthe deflection pulse reaches its peak value, the total voltagebetween the plates has exactly been inverted from its initialvalue. If the duration of the voltage pulse is matched to theelectron’s drift time between the 30-mm long plates (≈16 nsat 10 eV), an electron which enters the pulsing unit at thestart of a deflection pulse will arrive at the exit without ex-periencing any lateral displacement or momentum change.H rliero osew dif-fi thise hasb nvent ergyo inedf idtho ientfe samew s notd rfor-m thep elec-

trons with energies below 2 eV, the pulse duration does notexceed the time-of-flight broadening caused by the velocitydispersion.

3. Microstructured field emitters

In order to simplify the pulsed electron gun, we have re-cently investigated replacing the heated filament with a fieldemission type source. Since this emission mechanism re-quires no heating of the electron source, we aimed at find-ing out whether the intrinsic width of the energy distribu-tion of the emitted electrons would be narrow enough (below100 meV FWHM) so that the monochromator would not benecessary at all.

Field emission electron sources have become consider-ably smaller and easier to use over the last decade, whichis almost entirely due to miniaturization of the emitting tip.We have obtained several such field emission sources as apre-production sample from Extreme Devices, Inc. Theseelectron sources consist of an array of many, microscopi-cally small diamond tips with corresponding gate anodes,which are all located within a circular area of 250�m diam-eter [3]. Common to most of these types of sources is thatbecause of the small dimensions of the individual field emit-ters, a voltage ofU = 30–50 V between the emitter sub-s vinga uriw ource( entn ert3r pon-t thed esh-o dme selfit ess opens ilet addi-tl hold.T siont on.B d the“ rementag

ndedf ail-

owever, the effects on electrons which enter slightly ear later than the beginning of a voltage pulse, or on thith an energy slightly detuned from the mean value arecult to assess, and are best found experimentally. Tond, a preliminary version of the pulsed electron sourceeen built and tested on He as a target gas, using a co

ional single-channel TOF detector and a projectile enf 23.45 eV. The pulse duration of 5 ns which was obta

rom the TOF spectra met expectations, but the energy wf the pulsed beam of 350 meV (FWHM) was not suffic

or fully resolving the fine structure of the He (n = 2) in-lastic scattering channel. However, since almost theidth was already present in the unpulsed beam, this waue to the pulsing, but rather to problems with the peance of the monochromator. It is worth noting that atlanned energy resolution of 100 meV, and for scattered

-

getrate and the gate anode is usually sufficient for achien emission currentIe > 1 �A [3]. This threshold behavio

s evident in the emission characteristicIe versusUge, whiche have measured for both a new and a “seasoned” s

Fig. 2). For the new source,Ie rose above the measuremoise level of 1 nA as soon asUge exceeded 36 V. Howev

he normalized standard deviationσ(Ie)/Ie plotted in Fig.shows thatIe still exhibits strong variations, untilUge is

aised by another 10 V. This effect is probably due to saneous switching of single tips or larger domains onevice, as long as only the few tips with the lowest thrld voltages are emitting. WithUge further increasing, anore and more tips reaching emission threshold,σ(Ie)/Ie

ventually stabilizes at roughly 10%, while the current itncreases nearly exponentially withUge, as expected fromhe Fowler–Nordheim theory[2] of field emission. Once thource had been operated at currents over 100�A, its emis-ion characteristic changed, as can be seen from theymbols inFig. 2: The threshold increased to 41 V, whhe exponential region became slightly less steep. Inion,σ(Ie)/Ie (the open symbols inFig. 3) was also typicallyower, especially at voltages just above emission threshis suggests that those tips with particularly low emis

hreshold or high dI/dV have been destroyed by operatiut since these are small changes, for both the new an

seasoned” sources, the agreement between our measund the emission characteristic from the datasheet[3] is fairlyood.

However, because these devices are primarily inteor use in computer displays, little information was av

M. Lange et al. / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 993–996 995

Fig. 2. Emission characteristic of the new field-emission device (filled sym-bols), and after 2 days of operation at over 100�A (“seasoned” source,open symbols). Between 65 and 90 V, the lower set of data points fromthe new source exhibits space charge limitation of the emission current.This was eliminated in the upper points, and for all measurements on theseasoned source, by raising the extraction voltage of the triode electrongun.

able on the energy distribution of the emitted beam. Wehave therefore built a retarding potential analyzer (RPA) tomeasure the energy width of this particular type of source.The design of our electron gun dictated that the RPA hadto be mounted after the first electrostatic lens, at a posi-tion where the beam would usually be focused. This ruledout the common type of RPA with plane parallel potentialsurfaces, because its transmission depends on the incidenangle of the electrons. The design of our RPA therefore fol-lows the outlines given in[4], where a retarding field al-most spherically symmetric about the location of the beamfocus makes transmission almost independent from the di-rection of the electron trajectories. The field is created be-hind a small nozzle, which acts as an entry aperture to thedevice, and which is inserted though a slightly larger hole

F mi ource),o nt overs

into the base of a metal cylinder kept at the retarding po-tential. Electrons enter the cylinder through the nozzle and,if not reflected by the retarding field, exit on its far sidethrough a fine metal mesh, to be collected by a faradaycup. Numeric modelling with the SIMION software packageshowed that the energy resolution of this very simple RPAcan be as small as 30 meV FWHM, at 10 eV average beamenergy.

Using this device, we have obtained several RPA spec-tra of the electron beam created by the “seasoned” fieldemitter, for gate-emitter voltages in the range of 50–75 V.These measurements were hampered by the fact that thepreferred direction of emission from the device seemedto change strongly, on the timescale of seconds, whichmade adjusting the electrostatic optics of our gun assem-bly (in particular the lateral deflectors) very difficult. How-ever, by averaging over many measurements we consistentlyfound a width of 980± 160 meV (FWHM) of the elec-tron energy distribution, with no perceivable dependence onUge.

4. Conclusions and outlook

We have presented the general design of our new time-o gun.I avei a re-p con-fi heet,b ergyd lsede thant . Ina irec-t ento avet rcei thes axi-m ointo thert oneo mp-t acem

d thei andh HMf thea hasa in-v in am ted tob

ig. 3. Relative standard deviationσ(Ie)/Ie of the emission current frots average (filled symbols: new source, open symbols: seasoned sbtained from a time-dependent measurement of the emission curreeveral minutes per data point, at a sample rate of≈1 Hz.

t

f-flight electron spectrometer and the pulsed electronn order to improve the electron energy distribution, we hnvestigated a microstructured field-emitting device aslacement for the directly heated filament. We havermed the emission characteristics given in the datasut have also found that the width of the electron enistribution is much larger than our demands on the pulectron gun (100 meV), and even considerably larger

he energy width of the heated filament itself (0.6 eV)ddition, the change, on a short timescale, of the d

ion of emission from the tested device made adjustmf the electrostatic optics very difficult. We therefore h

o conclude that at least this type of field-emitter sous not suited for our application. On the other hand,imple operation near room temperature, the large mum electron current and the good location of the pf emission could make these devices interesting for o

ypes of experiments, either where high electron flux isf the primary objectives, or where low power consu

ion and high mechanical stability are important (e.g. spissions).Since conducting these tests, we have also resolve

nitial problems with the hemispherical monochromator,ave recently confirmed an energy width of 102 meV FW

or the monochromatized but unchopped beam, usingbove RPA. The position- and time-sensitive detectorlso been mounted, at the time of this writing, we areestigating the energy distribution of the pulsed beameasurement on helium, and the spectrometer is expece fully operational in the near future.

996 M. Lange et al. / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 993–996

Acknowledgements

The authors would like to thank the staff of the RSPhysSEmechanical and electronics workshops for their continuingsupport and sharing of expertise while building the spec-trometer. This project has been supported by the AustralianResearch Council (ARC) and the Deutsche Forschungsge-meinschaft (DFG).

References

[1] L.R. LeClair, S. Trajmar, M.A. Khakoo, J.C. Nickel, Rev. Sci. Instrum.67 (1996) 1753.

[2] R.H. Fowler, L.W. Nordheim, Proc. R. Soc. London Ser. A 119 (1928)173.

[3] Datasheet for XD 138-250 Integrated Controllable Electron Source, Rev.3, Extreme Devices, Inc., 2003.

[4] C.L. Enloe, Rev. Sci. Instrum. 65 (1994) 507.