FIELD EMISSION FROM A NEW TYPE OF ELECTRON SOURCE

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FIELD EMISSION FROM A NEW TYPE OF ELECTRON SOURCE

M. Mousa

To cite this version:

M. Mousa. FIELD EMISSION FROM A NEW TYPE OF ELECTRON SOURCE. Journal de

Physique Colloques, 1987, 48 (C6), pp.C6-109-C6-114. �10.1051/jphyscol:1987618�. �jpa-00226821�

FIELD EMISSION FROM A NEW TYPE OF ELECTRON SOURCE

M.S. Mousa

Department of Natural Sciences, P.O. Box 7, Mu'tah University, A I - ~ a r a k , Jordan

Abstract: A new type of field emission electron source has been developed. In this paper, the construction, characteristics and behviour of tungsten micropoint emitters coated with a s u h i c r o n layer of hyd-rbon using a TEN with poor (- tom) vacuum conditions are described. The hydrocarbon coating has been verified using the X-Ray energy dispersive analysis technique of a SEM. The technical capabilities and potential of the new type of electron source are compared with those of other comparable composite micropoint field emitters and other types of electrun sources currently in use.

The emission properties presented here include I-V characteristics, emission images and electron energy spectra of this type of composite micropoint emitters. The effect on the behadour and characteristics of baking the coated emitters a t temperatures ranging between 140" C and 350" C i s also s t u d i d The behaviour of the emitter has been interpreted in terms of a field-indud hot-electron emission mechanism associated with metal-insulsator- vacuum (M-I-V) regime.

1. Introduction

For producing a high electric field (> 10' ~m-l) a t the surface of a metallic field emission cathode with a reasonably low extraction voltage (say

<

The interest in field emission soumes stems from their well known properties (eg. see Swanson and Bell 1973) [ 101. The applications of field emission cathodes requires UW conditions, to obtain current stability and long emitter life-times. A review of types of emitters, their applications and recent advances were given by Mousa (1984) [7] and Mulvey (1980) [ 9 ] .

The composite microemitters developed here consist of a tungsten substrate micropoint emitter coated (using Phillips EM 200 TEM) with a sub-micron layer of hydrocarbon. The practical potential of these microemitters was assessed by comparing i t s characteristics with those of other comparable types (eg

.

In the present paper, the experimental data on hydrocarbon coated tungsten microemitters have been analysed. The analysis shows that the emission process comes from a field-induced hot electron mechanism associated with the M-I-V microstructures occurring a t conducting channels.

2. kperimental Details

2.1 b i t t e r Fabrication Techiques

The main concern of this work is to develop e i q u e s for obtaining composite micropoint field emitters consisting of a cci~ducting substrate coated with a layer of insulating material of

controlled thickness. Several approaches were investigated, but the me that proved most successful

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987618

C6-110 JOURNAL DE PHYSIQUE

."\IS

tungsten micropoint emitter.

(4

Fig.2 Electron micrographs of (a) tungsten tip free from contamination (b) after been contami- nated with hydrocarbon. (Magnification=30240Xj

was based upon the type of tungsten micropoint emitters t h a t a r e conventionally used i n f i e l d emission microscopy. Such emitters typically have a t i p radius of 4 100 nm and a r e produced by the electrolytic etching t e d i q t l e used originally by Miiller (1937) [8]. I n this technique a 0.1 mm diameter wire i s etched a t a 2M solution of sodium hydroxide, and then cleaned ultrasonically in readiness f o r the coating procedures. Fig. 1 shows a scanning electron micrograph of such a tungsten tip.

2.2 liydnxarbon Coating

While viewing an uncoated tungsten t i p on one occasion in the TEM, it was noticed that on focusing the electron beam on t o the t i p a contaminant layer steadily grew on its surface,which was presumed to be formed by polymerized hydrocarbon molecules o r silicon base material from the pumping system i. e. the same type of insulating deposits that a r e well known to "soot up" lens apertures in e l e c t m microscopes e t c This e f f e c t can be seen i n Fig. 2, which shows micrographs of the t i p before and a f t e r contamination, whilst i n Fig. 3 the sequence (a) to (c) shows the growth of the contaminant film a t " 15 second intervals. The most l i k e l y origin of t h i s contamination is from the back-streaming organic molecules Le. e i t h e r hydrocarbon o r silicon arising respectively from the rotary o r diffusion pumps. To verify this, the emitter was subsequently mounted i n the SEM and amlysed using the X-ray energy dispersive analysis technique. Fig. 4 (a) is the spectrum obtained from the shank, well away from the contaminated region of the t i p , and shows tungsten peaks; whereas Fig. 4 (b) shows the spectrum obtained from the tip. Although both spectra w e r e obtained using the same sensitivity, the count r a t e f o r the peak a t 1.775 kev is considerably lower i n (b); this suggests that some contamination is present on the tip. However, as there are no new peaks _present there, the contamination must be due to elements with an atomic number l e s s than 9 a s t h i s represents the lower threshold of the a ~ l y s i s technique. By eliminating elements such a s nitrogen and oxygen, the contamination can be said t o consist predominantly of carbon in the form of hydrocarbon.

(4 (b)

Fig.3 A sequence hounng the butld tip. (Magnification=82080X).

Gaubi (1983) [41 and found to be "loA3 tom. Since this contamination provides the possibility of accurately controlled coating thickness, the process was potentially very interesting to the present investigation.

Ihe measurements presented here describe ^X- Ray energy ( keV) ( a )

300

"

W

200-

f

0 .L

i7 loo-

Z S

Fig.4 The X-ray energy spectra obtained from (a) 0

100

0

the emission characteristics of the composite

micropoint emitters consisting of tungsten core with a variable layer of hydrocarbon coating. These results include the current voltage (I-V) characteristics, emission images and electron spectra which were obtained using the experimental combination of a field emission microscope and a high resolution electron spectrometer (Mousa 1984 [ 7 ] and Latham and Mousa 1986 [6]). In additim, electron microscopy was used to investigate the details of emitter profile.

-

W(Mr*)

W

W peak with feww counts due t o Hydrocarbon contamination cn the

-

W ( M d

I I

Two general approaches have been used to present the I-V characteristics. TtE f i r s t i s a direct I-V plot showing how the total measured emission current, I depends on the voltage, V applied externally between the emitter and the anode. Such plots are very useful for identifying t f e important features of the emission bebviour. A schematic representation of the typical I-V ckaracteristics i s shown in Fig. 5. This seems to be initiated by a switch-on process in which there i s a threshold switching voltage Vsw a t which the emission dmges abruptly from an effective zewvalue to a saturated value ISAT

.

,

-

conventional practice used in field emission studies. A schematic representation of the corresponding F-N plots for the uncoated and coated tips are shown, respectivdy, as A and B in Fig. 6. Occasionally few of the emitters studied exhibited atypical CU-t voltage characteristics. This was characterized by a series of current steps as the voltage

the ghank of the tungsten emitter. ( b ) the con- ^{0 1 2 3 4 5 6 7 8 9 1 0} X-Ray mergy (keV)

taminated tip. ( b ) r I

0 1 2 3 4 5 6 7 8 9 1 0

C6-112 JOURNAL DE PHYSIQUE

was slowly cycled. Subsequent recycling of the voltage led to exhibiting the typical characteristic hving a w e l l defined ,,V

.

3 4 I

A "3 I

I I

\ I I I I I

I I I I I I I

"L VSAT Vsw VM V(V0lt~)

Fig.5 The generalised form o f the current-voltage (I-V) characteristics o f a hydrocarbon-coated tungsten micropoint emitter.

Fig.6 Fowler-Nordheim plots o f the uncoated (curve A ) and hydrocarbon-coated (curve B) emit- ter whose I- V characteristics are presented i n Fig.5.

Fig.7 (a) Emission image from hydrocarbon-coated tungsten emitter i n its typical form. (6) Im- age showing electron emission originating from more than one sub-emission centre.

The c m e n t i d field emission microscope has been used to obtain the emission images of the composite emitters. This allows one to obtain information about the physical processes involved with these types of emitters and also to provide means of measuring such practical electron sources parameters such as beam brightness, current stability and lifetime A typical emission image of the hydrocarbon coated tungsten emitter i s s kin Fig. 7 (a) where it is seen to consist of a single bright spot whose diameter is field-dependent Fig. 7 (b) shows a multi-spot image which has been o c c a s i d l y observed due to secondary switctron processes during the i n i t i a l application of the applied field. This i n i t i a l image i s found to consist of two or more well-defined sub-emission centres. The image s h in Fig. 7 (b) is possible to be made to consist of a single spot by subjecting the emitter to certain conditioning treatments such as baking to a temperature ranging between 140' C and 200' C

The instrumental facility used to measure the electron energy spectra of the tungsten microemitters coated with hydrocarbon was h s i c a l l y a Van Oostrom retarding potential analyser with fully automated electmnic systems modified to give a differential output. The vital importance of these measurements l i e s in the information they contaip about the physical nature of mechmism operating with these composite emitters. The facility output gives the electron energy spectrum from which it i s possible to obtain the spectral half-width (F.WltM.), and the peak shift from the substrate Fermi level. The spectrum of the coated emitter i s shown in Fig. 8 and found to be displaced towards lower energies by -0.5 eV with respect to Fermi level of the tungsten substrate and to have larger F.W.H.M. Hence, it was difficult to obtain this spectrum due to the relative fluctuation of the emission. Thus, a sequence of spectra to demonstrate t3-e spectral shifts and hlf-widths as functions of the emission current and field were not m r d e d .

6on layer.

O"6eV'av

Fermi level

Energy decreasing

-

3. Discussion

The thickness of the hydrocarbon insulating film was measured by using a specially d e s m specimen holder (Mousa 1984) [7] in an 80 KV TEM. I t should be noted that in more sophisticated TEMs which involve the use of liquid nitrogen cold finger trap, better pressures can be obtained thereby eliminating the possibility of such contaminant layer formation.

Because of the ease with which controlled layers of this material could be deposited in-situ in a TEM, it was possible to make an additions1 investigation concerning the influence of the coating thickness on the emission characteristics. However, unlike the previous study on resin-coated emitters (Latham and Mousa 1986) [6], these emitters were baked up to a temperature of 140" C with the conseqwnt improvement in stability and reproducibility of the emission current 'Ihe increased brfghtness of this type of source following baking was particularly marked. In fact the phosphorus screen was "burnt ",presumably due to ion bombardment resulting frwn the evaporation of hydnmrbon from the emitter. However, whilst baking to 2 0 ° C produced a somewhat improved stability, a further bake-out a t ^" 350° C resulted in the destruction of the switching characteristics of the emitter. In the particular case of these hydrocarbon emitters, the baking procedure also led to an increase in the emission current which was observed before switch-on occurred. Initially, this p r e switch on current h d measured values as low as \< lo-" A prior to baking a t temperatures ), 14O0 C, but after baking currents as high as lo-' A were easily detected.

As a result of baking "tip explosions" were significantly reduced. This was an effect which occurred widely with UIIW emitters. These explosions were observed as a visual "flash" t h t caused the destruction of the tip. In less drastic form, "tip explosions" could result in either the electrons being emitted a t wide angles from sites on the side of the cone or causing "tip deformtion

".

There are general agreements and marked differences in the behaviour of the hydrocarbon coated microemitters and the resin-coated tungsten microemitters,described earlier [6] in their techological and theoretical implications. The similarities include a) current voltage

characteristics with both approaches discussed in the previous section, even with higher switching currents (" 12pA), b) the resulting improvements in the emission characteristics whenever the baking procedure was applied to the coated emitters. The distinctive phenomenon produced by baking i s the marked increase in the brightness of the emission image of this type of source which makes it potentially interesting to be used as an electron source, and c) the shift of the energy spectrum of the emitted electrons from the substrate Fermi level towards lower energies and hving larger F.W.H.M. The differences include a) stability of this source (hydrocarbon coated tungsten microemitters) i s less, b) the emission image i s less concentrated Le. less bright, and c) it i s believed that the resin coating i s more resistant to the back bombardment of electrons t h the hydrocarbon.

Because of the ease of depositing the insulating layer of hydrocarbon, the effect of baking on its chracteristics and the tendency of its emission spectra to shift towards lower energies, it is believed that this type of coating deserves further investigation as a potential electron source

C6-114 JOURNAL DE PHYSIQUE

It is clear t h t the coating of a metallic microemitter with a sub-micron thick insulating layer promotes the field-induced electron e m i s s i a From the similarity in the characteristcs of t h i s composite microemitter and the other metal-insulator structures (AttWal C S. and Latham R V. 1981

[ I ] , Bayliss K. H. and Latl-m R V. 1985 [2] & Lathm and Mousa 1986 [6]), the emission of electrons from this type of composite emitter is believed t o originate from the hot-electron emission mechanism. According t o this model (Latl-m R. V. 1982 [5], Latham and Mousa 1986 [6] &

Bayliss a d I a h 1986 131) the emission switch-on occurs when a n a m chumel (or filament) i n the insulating layer switches into a conducting state. The chamel formation facilitates the tunneling of electrons fmm the substrate metal (bingsten) into the insulatofs conduction band where they a r e heated by the penetrating e l e c t r i c f i e l d t o a certain energy. A population of hot electrons are gathered i n a high f i e l d une close t o the dielectric-vacuum interface. Some of these electrons are emitted over the surface potential barrier i n a quasi-themionic manner. The version of the model given by Bayliss and Latham (1986) 131 & Xu N. S. and Latham R V. (1986) [11]

explained the saturation of the emission current a s a manifestation of bulk-limited conduction.

Experimental evidence i n the form of electron emission c h r a c t e r i s t i c s of composite micropoint emitters consisting of tungsten core with a sub-micron layer of hydrocarbon coating, have been shown t o provide a potential electron sou- This evidence has a l s o shown tht the composite metal- insulator microstructures is i n accordance with a field-induced hot-electron emission m e d z m i s m .

TABLE 1 Collated emission data obtained from composite micropoint emitters consisting of tlngsten substrate coated with hydrccarba

References

'me of insulator

Hydrocarbon

1. Athwal C. S. and Lattam R V., Physica, 104C, 189, 1981.

2. Bayliss K. H. and L a t h m R V., Vacuum, 36, 211, 1985.

3. Bayliss K. H. and Latham R. V., Proc. R. Soc. Lond., A 403, 285, 1986.

4. Gaubi H., 'The Institution of Elec. Ehg., Science, Education and T e h o l o g y Div." Conference, 1983.

VSW Volt

3000 2600 2500 650 5000 4000 Insulator

t h i c h e s s P In

0.0231 0.0280 0.0305 0.0595 0.158

5. L a t h m R V., Vacuum, 32, 137, 1982.

6. Latham R V. and Mousa M. S., J. Phys D. Appl. Phys., 19, 699, 1986.

7. Mousa M. S., PHI thesis, University of Aston, Birmingham, UK, 1984.

8. Muller E. W., Z. Phys., 106, 541, 1937.

9. Mulvey T., Electron Microscopy, 1, 46, 1980.

10. Swanson L. W. and Bell A. E, Adv. Electron, 32, 193, 1973.

11. Xu N. S. ad Lathm R V., Joumal De Physique, C2, 47, 67, 1986.

ISAT P A

3.2 12 8 2 1 .O 12.0

12

V s ~ ~

Volt

2600 2500

2300 600

-

3800

v~

Volt A P P ~

1600 1900 1400 400

-

2900

Volt

800 1200 300 100

-

1900