Hot electron emission from composite metal-insulator

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Hot electron emission from composite metal-insulator micropoint cathodes

This content has been downloaded from IOPscience. Please scroll down to see the full text. 1986 J. Phys. D: Appl. Phys. 19 699 (http://iopscience.iop.org/0022-3727/19/4/021) View the table of contents for this issue, or go to the journal homepage for more

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J. Phys. D: Appl. Phys.19 (1986) 699-713. Printed in Great Britain

Hot electron emission from composite metal-insulator micropoint cathodes R V Latham and MS Mousa Department of Mathematics and Physics. Universityof Aston. Birmingham B17ET. LK

Received 1 July 1985.in final form 20 September 1985

Abstract. Field electron emission measurements have been made on composite emitters consisting of electrolytically etched tungsten micropoint cathodes overlayed by a 10-200nm thick layer of epoxy resin. Their emission properties include( a ) an initial switch-on effect at threshold fields of -lo9 V m". ( b ) a subsequent reversible Z-Vcharacteristic thatgives a linear FN plot at low fields (6lo8 V m - i ) and saturated emission at high fields 3 4 X IOs V m-'. (c) electron spectra whoseF\\'H!d and energyshift is strongly field dependent. ( d )singlespot emission images. This unusual patternof behaviour has been interpretedin terms of a hot electron emission mechanism resulting from field penetration in the dielectric overlayer. Consideration is also given to the technological significance of such composite microemitters.

1. Introduction

From recent studies of the microscopically localised electron emission process that is responsible for the prebreakdown currents flowing between vacuum-insulated high voltage electrodes (Latham 1981.1982.1983, Athwal and Latham 1984). it has become evident that electrons can be 'cold-emitted' from dielectric surface microinclusions at fields that are often over two orders of magnitude lower than the threshold value of -3 X l o 9V m-1 required for the well known Fowler-Nordheim 'metallic' field electron emission FEE tunnelling mechanism (see for example Young 1959). An explanation of this anomalous field emission has recently beengiven by Latham (1981.1982.1983) and co-workers (Allen et a1 1979, Athwal and Latham1984) in terms of a field-induced hotelectron FIHEE mechanism involvinga compositemetal-insulator-vacuum MIV emission regime. According to this model, the applied field penetrates the sub-micron thick dielectric region. and. at fields in the range of 10-20 MV m", initiates a 'switching' process whereby a conducting channel or filament is formed in the dielectric: i.e. by a similar mechanism to that proposed by Dearnaley et a1 (1970) and Adler et a1 (1978) to explain the behaviour of metal-insulator-metal MIM switching devices. However, the absence of a 'top' metal electrode in the MIV regime excludes the possibility of hole injection, and as a result there exists an ambient population of hot electrons at the insulator vacuum interface, someof which are able tobe 'thermionically' emitted over the surface potential barrier (Latham 1981, 1982, 1983). Whilst this model has been shown to provide a satisfactory explanation of typical experimental data (Latham 1982) e.g. the current-voltage characteristic, and the field it has not been subjected and temperature-dependence ofthe electron spectral halfwidth, 0022-3727/86/040699

+ 15 $02.50 @ 1986 The Institute of Physics

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R V Latham andM S Mousa

to directverification. Accordingly, the present simulation study was undertaken to study the field-inducedemission of electrons from anMIV regime under controlled laboratory conditions. Thus, techniques were developedforfabricating and testing composite microemitters consisting of a metal substrate of known microscopic profile, and covered by a submicron thick layer of a suitable insulating material. Aswill be described, the emission characteristics of these control emitters showed several striking similarities to those obtained from the naturally occurring processes: however, they alsoshowed some significant and physically interesting differences. A parallel objectiveof this study was to determine whether such composite emitters had any technological potential as a 'cold' electron source for possible use in such commercial devices as cathode ray tubes (Latham and Wilson 1983). Thus, their performance has been characterisedin terms of such practical parameters as beam brightness, energy spread, short and long-term current stability and emitter lifetime. From this data. it has also been possible to make a quantitative comparison of the emission characteristics of these composite emitters with other comparable types of cold emission electron sources such as tungsten (Muller 1937, Creweetal 1968,1969) and carbonfibre (Baker etal 1972, Braun etal 1975, Latham andWilson 1982.1983, Prohaska and Fisher 1982. Erickson and Mace 1983).

2. Experimental

2.1. Emitter fabrication The tungsten microtips used for the metal substrate of the composite emitters studied in this investigation were prepared by electrolytically etching a 0.1 mm diameter wire at the meniscus of a 2mol 1" NaOH solution, i.e. by the same techniques used for tungsten emitters (Muller 1937). After ultrasonic cleaning, the tips were mounted in a chuck attached to a micrometer-operated vertical carriage which provided a controlled means of dipping the tips into the coating medium of epoxylite resin (Clark Electromedical Instruments). The actual coating procedure, as recommended by the manufacturers, involved firstly dipping a tip into the resin and then carefully withdrawingit to ensure that only a thinfilm (i.e. rather than a droplet) remained on the tip. stabilise To thisfilm on the substrate surface. the coated istip inverted, transferred to an oven and subjected to a curingcycle of thirty minutes at 100 "C to drive off the solvents, followed by thirty minutes at 175-190 "C to complete thecuring of the resin. The thickness of such a resin film was measured in an 80 kV transmission electron microscope (Philips EM200) using a specially designed specimen holder (Mousa 1984). Typically, a single-dipfilm would be -0.04 ,pm thick: however, by successively dipping the tip into theresin before curing,it was possible to vary the final film thickness within the range 0.04 to 0.2 pm. Thus figure 1is an electron micrographof a composite emitter having a thick film produced by twelve dips in the resin, where the tip radius of the the of the substrate tungsten micropointis -30 nm. The sharp contrast between images metallic core and non-metallic overlayer is a consequenceof the widely differing densities of these materials, typically by a factor of -15.

2.2. Current-voltage characteristics These measurements, andalso the emission image data tobe presented in the following

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Figure 1. A transmission electron micrograph of a compositefield emitting sourceconsisting of a tungsten micro-pointof tip radius-30 nm that has been coveredby an insulating epoxy resin layer of thickness -150 nm.

section, were made in a simple point-to-plane geometry field emission microscope, where the axial tip-screen separation was standardised at -10 mm. The typical prebake-out operating pressureof the device was -1O-8 mbar, which fell to -10"" mbar after bakingto -200 "C for -12 hours. In order to concisely present the collated current-voltage (I-V) data froma rangeof emitters having different insulating layer thicknesses, and thermal processing treatof the I-Vcharacteristicobtained ments, it wasfirst necessary toidentify thegeneral form from composite emitters. Thus, referring to theschematic plotof Figure 2, it is seen that as the applied voltage is slowly increased across virgin a emitter, a pointis reached, V,,, where the emission current suddenly 'switches-on' from an effective zero-value to a i,,,. This saturatedregion 3 extends to an upper voltage limit Vmax, stable saturated value marked by the onset of current instabilities and possible tip explosion, and down to a lower voltage limit Vsat,beyond which the emission current falls smoothly to zero as the applied vo!tage is decreased to a threshold value Vth. However, this part of the a transition region2, limited by V,, and characteristic can conveniently be divided into a low-field region 1 where the I-V data gives a linear Fowler-Nordheim (FN) plot. On subsequently re-cycling the voltage, the characteristicfollows the double-arrowed broken curves,i.e. exhibiting a hysteresis effect.

Figure 2. The generalised form of the current-voltage (I-V) characteristic of a composite emitter.

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R V Latham and M S Mousa

Figure 3. A comparison of the l-V characteristic of an uncoated W-emitter of tip radius -30 nrn (curve A ) . and that obtained after coating with 200 a nm thick layer of epoxy resin (curve B) (See figure 1).

A practical exampleof this typeof behaviour is presented by figure 3, which compares the I-Vcharacteristics of an uncoated tungsten emitterof tip radius -30 nm (curve A). with that obtained afterit had been coatedwith a 150 mm thick layer of resin (curveB). From apractical point of view, these plots highlighthow the resin-coating resultsin ( a ) a current-stabilised emitter, and ( b ) a significantly lower threshold voltage V,, with a consequently lower operating voltage to obtain an emission current equal to Isat. For completeness. figure 4 compares the FN plots of the uncoated emitter (curve A) and that obtained from the low-field (region 1)I-Vdata of the composite emitter (curve B). Thus. it will be seen that the resin coating resultsin a decreasein the slopeof these plots from m,+= 9870 to m, + r = 2420 the physical significance of which will be discussed in a 3. -21r

-231 -251-271

-39

0

1

1

10

20

,

30

40

50

IO"+/ v Figure 4. Fowler-Nordheim plots of the uncoated (curve A) and resin coated (curke B) emitter whose I-Vcharacteristics are presented in figure3.

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Table 1. Collated emission data from six typical composite micro-emitters

27 30 30 33 66 100

10 200 150 100 l50 80

1.1 0.8 0.7 2.4 1.1 2.5

1.2 1.5 4.5 7 5 10

1.7 1.1 0.9 2.9 0.9 1.0

0.9 0.6 0.5 1.8 0.9 2.1

1.6 l.? 1.1 3.1 2.0

5.0

0.15 0.1 0.05 0.2 0.25 0.1

0.11 0.13 0.07 0.08 0.18 0.16

6.2 5.7 1.1 1.6 5,l 1.7

1.7 2.0 2.2 1.7 2.2 2.3

To give some perspective on the variabilityin the behaviour of this type of emitter. table 1 presents the collated data obtained from the I-V plots of six emitters having substrate radii r,\ varying from 30-100 nm and resin coatings varying in thickness Ad from 40 to 200 nm. Although this sampleof data is small, it is nevertheless possible to identify the following general trends: (i) Except for the thinnest resin coatings (Ad 6 0 . 0 5 ,pm). the switch-on field F,, and saturated emission current I,,, typically have values of 1 X 10' V m-' and 5 pA respectively. (ii) The threshold emission voltage V,, of a switched-on emitter is typically one tenth of the initial switch-on voltageV,, . (iii) T h e FN slope of a coated emitter is typically -$ that of the substrate tungsten emitter. (iv) The maximum voltage that may be applied for stable emission is typically twice the voltage required for saturated emission. Apart from the .typical' emission behaviour illustrated in figure 2 . some emitters were found to have I-V characteristics that deviated significantly from this simple form (Mousa 1984). Thus. referring to the exampleof figure S. they exhibit an initial switchon effect. but on subsequently cycling the applied voltage, several additional switching processes are observed involving step-like changes in the saturated emission current. As the emission image data of the following section will show. this type of behaviour

/ / / /

l

I

100 &h

K,

950

1150

K,

K,

1350

2050

v (VI Figure 5 . The l-V characteristics of a composite emitter having several independent subemission centres.

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R V Latham and

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is indicative of a composite emitter that has contributing to thetotal emission current.

several independent emission centres

2.3. Emission images These were photographed directly from the phosphor screen of the field emission microscope. Typically,the emission was concentrated into single a very bright spot that showed no apparent structurewithin the image; i.e. in marked contrast to the typeof image obtained froma metallic emitter.To highlight this difference,figure 6 compares the images obtained ata standardemission current of 4.5 pA from the uncoated emitters whose I-Vcharacteristics are presented respectively as curvesA and B of figure 3. From these images, it is immediately seen that, for a given emission current, the composite emitter is a very much brighter electron source: in quantitative terms, the sources shown in figure 6(a) and ( b ) have brightness values in the approximate ratioof 1 to 6 . If the

Figure 6. Projection images obtained from (a) a clean tungsten emitter and ( b )after coating with a 150nm layer of resin. Both images were recorded with the same tip-to-screen separation and the same emission current of 4.5 PA.

voltage appliedto the composite emitter was progressively lowered, such that it followed the I-Vcharacteristic of figure 3, the image spot-size was found to decrease stronglywith the emission current as illustrated in figure 7. This finding has important physical implications which will be discussed in a following section. Whilst the majority of emitters exhibited the type of single-spot image described

Figure 7. A sequence of projection images reproduced at actual size showing how the spot size changes with emission current.

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above, some 10% of those tested switched-on to give a multi-spot image such as the of example shownin figure 8. (NB: this was photographed from the phosphored anode the Van Oostrom retarding potential electron spectrometer to be described in the following section and illustrated in figure 9. Also note that the 1 mm diameter probe hole can be seen centred over ofone the spots). An important characteristic of this type of image is that the numberof spots is generally field dependent: thus, as the voltage is cycled, the individual spots switch on andoff independently at characteristic voltages. It is in fact thisphenomenon thatis responsible for thestep-like discontinuities exhibited by the typeof I-Vcharacteristic shown in figure 5 .

Figure 8. A typical multi-spot emission image.

Figure 9. A schematic diagram of the retarding potentialanalyser facility used for recording the energy spectraof composite emitters.

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R V Latham and M S Mowa

Image stability wasfound to be very sensitiveto theinitial thermal processingof the emitters. Thus, without a prolonged( > l 2 h) low-temperatureinsiru bakingat -200 "C, emission images were very unstable, with individual spots both flickering in intensity, and randomlyswitching on andoff at a constant appliedfield. Even after baking, there was generally someresidual instability, although this could usually be further reduced several hours at constant by the self-conditioning procedure of running the emitter for current. 2.4. Electron spectra

These were extremely important measurements since they provided valuable information about the emiss'ion mechanism associated with these composite regimes. The instrumental facility,which is shown in figure 9, was based uponthe well known retarding potential analyser design of Van Oostrom (1966), but incorporating fully-automated 1984). In particular, locka electronically controlled drive and detection systems (Mousa in amplifier technique was used to obtain a direct differential spectral output, i.e. comparable to thatgiven by the morewidely used hemispherical typeof analyser. The instrument was calibrated against tungsten thermionic field and emission spectra (Young 1959, Braun et a1 1978), so that theposition of the Fermilevel of the metallic substrate of composite emitterscould subsequently be identified on their emission spectra. Thus figure 10 compares the spectraof the clean tungsten and resin-coated emitters

Figure 10. A comparison of the energy spectra obtained from a tungsten emitter ( a ) before, and ( b )after coating with a 150 nm thick layerof resin at identical emission currents of4 PA. (FL= Fermi level of the substratecathode).

whose I-V characteristics and emission images are shown respectively in figures 3 and 6. From this result,it is clear that the spectrum obtained from the composite emitter is (a) displaced towards lower energies by -0.7 eV with respect to theFermi level of the metallic cathode; ( b )has a significantly larger full width at half-maximum (FWHM), and (c) is more symmetrical than the tungsten spectrum.In fact, as shown in the collated data (Mousa 1984) of figure 11, the spectralshift and, toa lesser extent, theFWHM are both functions of the emission current; i.e. in sharp contrast to the behaviour of the metallic emitter. In the case of multi-spot images such as that shown in figure 8, the

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\

i

Emlsslon current I P A )

Figure 11. Plots illustrating how the experimentally measured spectral shift and half-width (FWHM) vary with emission current.

sampling anode probe hole could be used to show that each spot its own hadindependent single-peak spectrumwhich was uniquely positioned with respect to the cathode Fermi level at anygiven applied voltage (Mousa 1984). was It also found that theshift from the Fermi level of each peak varied independently with the applied voltage so that the relative separations of the peaks did not generally remain constant as the voltage was cycled through the &"characteristic. The physical significances of this observation will be discussed in a later section. 3. Discussion

3.l , Physical implications It will be recalled that theinitial objective of this investigation was to simulate the hotelectron mechanism thatis thought to operate localised at dielectric inclusionson broadarea (BA) electrodes (Latham 1982). To evaluate how closely this objective has been realised. it is necessary to compare the emission characteristics of the present metalinsulator composite micropoint emitterswith those of naturally occurring sites (Latham 1981, 1982. 1983. Athwal and Latham 1984, Bayliss and Latham 1985, 1986). Considering therefore the typical current-voltage I-Vcharacteristic shown variously in figures 2-4. the first evident similarity is the initial switch-on phenomenon. With BA emission sites this process typically occurs at macroscopic gap fields of 2-3 X lo7 V m-' which. when account is taken of local field enhancement at the vacuum-dielectric

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R V Latham and M S Mousa

interface(Bayliss1984a),correspondsto typicalmicroscopicswitch-on fields of -1 - 8 X lo8 V m-’. This range is somewhat lower than that typically found for the present control emitters, viz 0.9-1.5 X lo9V m-l, and presumablyreflects the inevitable differences that must exist between electronic properties of the dielectric materials involved in the two cases. In this context it has to be remembered that no precise information is available aboutthematerialnature of the micron-sizeddielectric inclusions responsible forBA emission sites. A secondimportant similarity is the reversible low-field Z-Vcharacteristic that gives a linearlog Z/V2 versus Z/VFowler-Nordheim (FN) plot: however, the slope values of such plots are typically -2500, which is a factor of -4 lower than those found for BA sites (Latham1982). In marked contrast, the saturation effect observed with the control emitters at higher fields (see figures 2 and 3) is not observed with BA sites, although the FN plots of the latter do exhibit high-field a fall-off (Latham 1981) which, aswill be discussed later, may well be a less obvious manifestation of the same physical phenomenon. Another striking similarity is to be found between the spectral characteristics of the two types of emission regime. Thus the spectral shape, and the field-dependence of the half-width and shift from the cathode Fermi level, illustrated in figures 10and 11closely resemble the behaviour of BA emission sites (Athwal and Latham 1981, Bayliss and Latham 1985). Lastly there are broad parallels between the structure-less spot-like appearance of the emission images shown in figures 6 to 8 and those recorded fromBA sites (Bayliss and Latham 1985, 1986). In both cases, individual spots are found to give a single-peak spectrum with a characteristic field-dependence. Furthermore, the individual spots tend toswitch on and off randomly with time, asin the case of BA sites (Bayliss and Latham 1985). with a mean frequency that depends upon the emitted currents; for example, at 1 ,uA this ‘flicker’ frequency is typically -1 Hz. However, unlike theimages of BA sites (Baylissand Latham 1985), the diameter of individual spots shows a marked dependence on theemission current (see figure 7). On the basis of this evidence it can be qualitatively concluded that electrons are emitted from the present composite micropoint sources by essentially the same hotelectron emission mechanism as that operating at BA sites. According to this model, which has been fully detailed elsewhere (Latham 1981,1982,1983, Athwal and Latham 1984, Bayliss and Latham 1986), theswitch-on event corresponds to the formationof a conducting channel within the dielectric, as illustrated schematically in figure 12(a). This process is assumed to be very similar to that occurring in MIM switching devices, as discussed. for example, by Dearnaley er a1 (1970) and Adleret a1 (1978). For the present system, the initial switch-on process, and the subsequent low-current phase. will be determined by the electronic properties of the complex interface existing between the metal substrate and the insulatinglayer:athigher currents however, the emission characteristics will be determined by the bulk electronic properties of the insulating medium (Bayliss and Latham 1986). The band diagram of the ‘on-state’is shown infigure 12(b) where a population of hot electrons are createdin a high field zone close to the dielectric-vacuum interface, with some of them being emitted over the surface potential barrier in a quasi-thermionic manner. According to this model, the current Zemitted from a channel of cross-sectional area a( = m f ) is given by Latham (1982)

Z = aA T: exp( - eX/kT,)

(1)

where T, is the enhanced temperature of the hot-electron population (i.e. above ambient conditions),theelectron affinity of the dielectric material, and A theRichardson

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I R

I

d

Figure 12. ( a ) A schematic representation of electrons being emitted from a conducting ( b ) The corresponding band channel ‘formed’ in the sub-micron thick insulating layer. diagram representationof an emitting channel.

constant. Neglecting the energy losses resulting from electron-phonon scattering processes, the enhanced temperature of the hot-electron population behind the surface potential barrier will be given by Latham (1982),

T , = 2eAV/3k

(2)

where AVis the effective potential drop appearing across the conducting channel formed in the dielectric layer. This will however be considerably less than the ‘static’ potential for drop AVo appearing across thedielectric layerunder non-emitting conditions. Thus, the emitter geometryshown in figure 1, it can be shown from elementary electrostatic field considerations that AVo = ( d / X ) ( V / e , ) which predicts a valueof AVo

(3)

- 40 V for a tip radiusrw = 30 nm, afilm thickness of d =

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R V Latham and

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150 nm and relative dielectric constant E, = 3, and V = 150 V. By comparison. it is informative to note that a similar thickness of dielectric material is a uniform field geometry would support a 'static' potential drop of only -3 V (Latham 1982). To account for this effect, equation(2) may be re-expressed in the form

T , = 3e6AVo/3k where 6 = AV/AV,. The physical significance of the slope m,, - of the FN plot of figure 4 may be established by substituting from equations(3) and (4)in equation (1) to give

so that m u T T= 3 ~ , ~ ( + r ,d)/s6d. Thus, taking m w + r= 1200 from figure 4, and assuming E, = 3, X = 3 eV. r,, = 30 nm and d = 150 nm, one finds 6 0.01. This result indicates that the available potential energy eAV for heating the electronsis -0.4 eV which, from equation(2), would result in an enhanced electron temperature of T, 3000 K . Taking accountof the measured spectral shift (see figure l l ) , it can be concluded that the actual voltage drop across the conducting channel is -1 V. At higher energies, it can be assumed that there will be a rapid onset of electronphonon scattering processes, and this will lead to the well known situation where the hot electrons acquire a saturateddrift velocity. i.e. as manifested experimentally in figures 2 and 3 as a saturated emission current. It is also plausible to assume that the onset of these scattering processeswill lead to a field-dependent expansion of the cross-sectional area of the emitting channel,which in turn would lead to anassociated expansion of the emission image as found experimentally (seefigure 7).

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3.2. Technological implications The technological advantages of field emission electron sourcesare well known (see for example Swanson and Bell1973). Thus, (i) they are 'cold' electron sources, theoretically capable of operating at temperatures as low as T = 0 K; (ii) they can delivera large emission current density ( 3108Acm"); (iii) they have a small electron optical source size (2 to 3 nm) of high brightness (3109 A cm-* Sr-l); (iv) they have a highly non-linear L" characteristic; and (v) they have a relatively narrow energy distribution (S250 meV). Probably the mostsuccessful exploitation of such sources has beenby Dyke (1960) for the very-high-poweredx-ray tubes (Fexitrons) used forhigh-speed x-ray photography. However, field emission sources have also been successfully incorporated into highresolution scanning electron microscopes by Crewe et a1 (1968), cathode ray tubes devices.It is therefore (Latham and Wilson 1983) and many other electron beam appropriate to assess the practical potential of the present composite emittersin terms of the performance parameters (e.g. source brightness, electron energy spread. emitter lifetime and emission current stability) of the conventional tungsten micropoint emitter.

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3.2.1. Emission current. Under saturated emission conditions (see figures 2 and 3). the emission current is very stable. and certainly not sensitiveto fluctuations in the supply voltage, i.e. rather like a conventional thermionic emitter. At comparable pressures, the current from a composite emitter has a significantly higher signal-to-noise ratio than a conventional tungsten emitter. Furthermore. the field required to draw say1 pA of emission from a composite emitteris typically -0.5 of that required to draw the same current from the uncoated emitter. 3.2.2. Emission imagestructure andstability.A well known characteristicof the tungsten image is its fragmental appearance, which in the case of a clean emitter has the sym6 ( a ) . It is also proneto an instability in the formof a metrical structure shown in figure 'flickering' of individual spots (Latham and Wilson 1982. Hosoki et a1 1979. Adachiet a1 1983), which becomes very dramatic at pressures b 10"" mbar and results in the rapid destruction of the emitter from ion bombardment processes. Such sources are therefore onlysuitable for use under good UHV conditions.butevenhere it is necessary to periodically 'treat' the emitter by a flashing process (Prohaska and Fisher 1982). to remove adsorbed gas from the emitting surface. Similar instability problems are encountered with glassy carbon (Hosoki et a1 1979) and carbon fibre (Baker et a1 1974. Latham and Wilson 1982) emitters. althoughin the latter use, the noise has a different physical origin. In contrast. the typical emission image of the composite emitters reported in this paper generally consists of a single spot of uniform intensity as shown in figure 6 ( b ) . Furthermore. for the point-plane geometry used for these experiments, and at pressures mbar, the single-spot images. like the emission current, are verystable over a 100 hour periodof continuous emission. and certainly represents a better performance than the corresponding tungsten image. Multi-spot images. such as shown in figure 8, are thought to result from the formation of several independent channels distributed over the apex of the emitter (Mousa 1984). They are generally less stable than single-spot images. and are more likely to occur with the more divergent field conditions of a small tipscreen distance. 3.2.3. Source brightness.According to Borries and Ruska (1939). the average brightness B of an electron beam is defined as the emission current AZ per elemental area AA normal to the beam and per unit solid angle AS2, i.e. B = AZ/(AA. AQ). The beam brightness increases linearly with the emitted current density J from the cathode (i.e. J = AZ/AA) and with the accelerating voltage, but inversely with the lateral kinetic energy of the electrons. For tungsten micropoint emitters, a value forJ of 106 A cm-? was obtained by Dyke and Trolan (1953). and is now an accepted average value (Ranc et a1 1976), while the valueof B is usually taken as 10'' A cm-' W', although various othervalues have beengiven by different researchers, working undersomewhat different experimental conditions (Crewe et a1 1968, Lea 1973, Swanson 1975. Rancet a1 1976, Van der Mast 1983). In contrast, the composite emitters developed during this work achieved a higher B than that of clean tungsten for a given total emission current. This is clearly illustrated by figure 6, from which itis estimated that the brightnessof a resincoated emitter is-6 times higher than the uncoated emitter. 3.2.4. Energy spread of emitted electrons. It is well known from studies on conventional tungsten emitters that the spectral half-width is very sensitive to the presence of contaminating adatoms on the surface of the emitting tip. In fact, it is only by taking the

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maximum precautions to obtain anatomically clean tip that it is possible to approach the theoretical value of -230 meV (Young 1959). In contrast, the electron energy typically distribution measurements reportedin § 2.4, indicated that composite emitters have a spectral half-width of 5 300 meV for emission currents 5 5 PA, i.e. as a matter of routine without anyspecial precaution. Inthis respect therefore, composite emitters have a significant advantage over conventional emitters.

3.2.5 Life time. The time forwhich an electron source can deliver aconstant and stable emission current andimage is a vital practical considerationin determining whethersuch a source has potential technological applications. Thus,typical a life-time test of more than 100 h carried out on a resin-coated W-emitter generally revealed perfect stability in both emission current and image,with negligible recorded noise. Correspondingly,it was also found that profile the of compositeemitters remainedconstant duringoperation, even after running for periods exceeding 100 h. In contrast, published work on the behaviour of W-emitters, even under UHV conditions, indicates that current fluctuations and flicker noise appear after only a few hours of operation, when they require the ‘flashing’ treatment described previously. However, the onset of these instabilities, and the consequent flashing requirement, has also been shown to be associated with the progressive degradation of the tip profiles. A similar behaviour pattern has also been reported by Futamoto eta1 (1979) for glassy carbon tips.The most probable explanation for thestability of composite emittersis that theemission is controlled by the ‘protected’ metal-insulator interface existing betweenthe tungsten and resin (see§ 3.2), rather than by the dynamic distribution of surface states as in the case of a conventional metallic emitter. 4. Conclusion

It has been demonstrated thatfield the emisssion characteristicsof a tungstenmicropoint electron source are radically changed by coating the tip with a sub-micron layer of insulating material. Thus, such a composite micro-emitter exhibits an initial switchon phenomenon with a subsequent smooth and reversible Z-V characteristic whose threshold field is 3 to 4 times lower than the uncoated W-tip, and which saturates at 6 10 ,uA:at lower current levels, a linear Fowler-Nordheim plot is obtained with a slope value 4 times lower than that given by the uncoated emitter. Furthermore, emission the image typically consists of a single bright spot, whilst the energy distribution of the emitted electronsis ‘non-metallic’ in character. These properties have been interpreted in terms of a hot-electron emission mechanism that involves the formation of a conducting channel in the dielectric coating; i.e. somewhat similar to the process involved in MIM switching devices. From thetechnological point of view, these compositemicroemitters have been shown to have several promising operating characteristics, e.g. a high source brightness,low threshold emission fields, stable emission currents with long lifetime under UHV conditions.

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Acknowledgments

The authorswish to thank their colleagues K H Bayliss and N S Xu for many helpful and stimulating discussions, and Mr A E Marriott-Reynolds forhis assistance in preparing the figures for this paper.

Electron emissionfrom composite emitters

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