Vacuum/volume 32/number 3fpages 127 to 135/l 982 Prlnted in Great Brltaln

0042-207X/82/0301 27-09$02.00/O Pergamon Press Ltd

Electrical breakdown between stainless-steel electrodes in vacuum A van Oostrom and L Augustus, Philips Research Laboratories, Eindhoven, The Netherlands

received 20 July 7 981

Electrical breakdown in vacuum is reviewed, particularly for the case of a small gap (~2 mm) between a pair of stainless-steel electrodes. Theoretical descriptions and experimental results are compared, and it is concluded that field-emitting sites on the cathode are responsible for the electron bombardment of the anode, the subsequent evaporation of anode material and the initiation of a breakdown.

I. Introduction

In many devices vacuum is used as the insulating medium. separating electrodes needed for the acceleration, focussing or deflection of charged particles. However. if a high voltage is applied between two electrodes in vacuum an electric current is often observed. This current will increase rapidly when the voltage across the clcctrodc gap is raised. An increase in voltage hcyond a critical value will cause an electrical breakdown, a sudden dissipation of energy which may produce considerable damage to the device or related circuitry. The insulating properties present initially at low voltages will fully disappear and an electric current up to thousands of amperes may flow between the electrodes. It is generally accepted that electrical breakdown is initiated by the presence ofeither cathode- or anode-vapour in the space between the electrodes. Ionization of the vapour by primary electrons and secondary processes at the cathode surface result in arc formation. The arc current is limited only by the resistance of the external circuit and the voltage supply. The gap between the electrodes has become an excellent conductor.

Over the past three decades the phenomenon of electrical breakdown has been the sub.iect of many experimental investi- gations and several theoretical models have been proposed’ ‘. Early investigations showed that pre-breakdown conduction could be described by field electron emission from the cathode. It was soon realized that electrode material and cleanliness played a ma.jor role in the breakdown mechanism. It is only in recent years that mass spectrometry, surface analytical techniques and other novel experimental methods have clarilied some of the complex mechanisms occurring during the initial stages of a breakdown. Experiments on well-defined electrode surfaces in a point-to- plane configuration under uhv conditions have provided a better insight into many aspects. For broad-area electrodes in a vacuum device the picture may well be less bright. Breakdown is a fast process which starts locally on an electrode surFace. These features often prevent thecorrelation ofa specific breakdown event with its origin or mechanism.

The initiation of breakdown and arc formation can be

descrihecl by one of three proposed models. all based on the assumption that metal vapour in the gap is an essential requirement.

(i) The first model is based on theexperimental observation that microparticles can be detached from one of the electrode surfaces by electrostatic forces in the vacuum gap and accelerated to the other electrodes’ “. Two cases can now be considered: When the velocity of the microparticle is above the threshold velocity of the target material for plastic deformation. impact craters and plasma will be produced. The threshold velocity is proportional to the yield strength of the electrode materials’. Since the yield strength of stainless steel is relatively high (3 x 10” N m-‘) plastic deformation is only expected for submicron particles, which have a sufficiently high velocity. In a recent investigation it was shown that clean polished stainless-steel electrodes produced micro- particles in the range ofdiameters from 3-12 Ltrn and a value ofthe particle density close to the one for irona. The measured velocity was about 75 m s-’ at a field of 1.9 x 10’ V m-l, well below the threshold velocity for plastic deformation of stainless steel of 530ms-‘. Impact craters with irregular ridges and lips which give rise to local field enhancement and lield-emitting sites are therefore unlikely to occur. In the second case the velocity of the microparticle is below the threshold velocity for plastic defor- mation. On impact the particle can partly evaporate due to conversion of its kinetic energy to heat or can be reflected”-‘“. For larger particles (> 10 pm diameter) trigger discharges have been observed”.‘5. These are explained by heating of the particle and evaporation in flight by electron bombardment from the cathode. The role of microparticles is considered to be of greater importance in large gaps (~5 mm) and high voltages (> 100 keV). For such gaps field emission phenomena and electron bombardment of the anode are less important. Thus, breakdown was observed with stainless-steel and titanium electrodes for a gap > lO’mm, while the voltage was > 300 kV and the field emission current only 10e9 A”.

(ii) The second model is based on the explosive evaporation of a cathode protrusion or whisker as a result of heating by its own

127

A van Oosfrom and L Augustus: Electrical breakdown between stainless-steel electrodes in vacuum

held emission current. It was shown for idcal tungsten held

emitters. as used in a field emission microscope. that thr local

electric fcld strength at breakdown is approximately constant“.

Alpert ct a/ using broad area tungsten clcctrodes wcrc able to

show that the breakdown field strength is independont ol’the gap

width’“. They assumed that protrusions on the cathode surface

enhanced the licld strength locally by two to three orders or

magnitude. This assumption also cxplaincd the held emission

nature of the pre-hrcnkdown current flowing hctwccn the

electrodes. Similar investigations on other electrode matcrisls

confirmed this behaviour. The independcnco of the hrcakdown

field strength on the gap width illustrates the high probability of

this mechanism for small gaps.

Although the held emission model has been found to give an

adequate description of breakdown induced by the explosive

evaporation of protrusions. it is often extremely dilhcult to (ind

protrusions of the right size on broad area clectrodes’g. Indeed.

there is considerable evidence that other emitting sites than

protrusions are responsible Ior breakdown. No protrusions or

whiskers were found on emitting electrode surfaces in a SEM’“.

on the other hand emitting sites sometimes show electro-

luminescence” and semiconductor properties”. Apparently,

emitting sites of different nature are feasible.

(iii) Electron bombardment. heating and evaporation of the

anode is the basis for the third model”’ 25. The electrons emitted

from a protrusion are accelerated and dissipate energy in a

localized region of the anode determined by the electron beam

size. The anode spot temperature can be calculated for a uniform

circular beam or a Gaussian intensity distribution”‘. The anode

temperature will also depend on the thermal conductivity. If the

temperature dependence of the thermal conductivity near the

melting point is known, excellent agreement between calculated

and measured temperatures is found”. Experiments on copper

electrodes showed that at breakdown the anode temperature at

the surface is almost equal to the melting point of copper”.

However, it has been argued that the vapour pressure at the

melting point is insufficient for avalanche ionization by electrons.

The temperature needed for breakdown should rather be closer to

the boiling point. In such a case the melted region will become

highly unstable and ejection of small particles from the melt

becomes probable, as in a vacuum arcZg.

So far. we have not made the distinction for the various models

between steady-state conditions and pulsed voltages. Usually.

however, a delay time r, is observed between application of the

voltage and the onset of breakdown. as characterized by a rapid

rise in current. This delay time depends on the over-voltage

applied, the heat capacity of the electrode material and most orall

on the mechanism involved. In the case of the microparticle-

initiated breakdown the transit time across the gap determines the

delay time. For the example given oriron particles detached from

a stainless-steel electrode the measured delay time is about 10e4 s.

In the case of a point-to-plane configuration breakdown occurred

for pulsed voltages in the range 5.10-g~4.10-” s. A current

density 5=4.10” A m-’ was observed for :I 5 ns pulse and the

product J’I,,, constant in the investigated range, indicating Joule

heating of the held-emitting point3”.3*. If the anode starts to

evaporate, as in the third model, breakdown occurs after a delay

time t do 10e6 s. So, we see that delay time can be a useful

parameter to distinguish between mechanisms.

In this paper we shall confine ourselves to the breakdown

phenomena between stainless-steel electrodes with an inter-

electrode distance smaller than 2 mm. For such small gaps field

128

cniission phcnomcna pIa! a dominant role. Firstly. wc ~liall

discuss to what cstcnt prc-hrcakdown currents can lx uwrtll in

predicting the breakdown voltapc for a pair of stainless-steel

elcclrodes. Scconclly. WC shall dcscribc expcrinicnlal results in

which the clcclrodc surhtcclr have hccn csan~inctl hcforc and d-k1

a conditioning process vvith the aid Or Scanning Aupcr

Microscopy.

2. Field-emission-initiated breakdown

Prc-breakdown currents l~ctwccn clcctroclcs may initially IX

unstahls and show large Ructuations. Howcvcr. a slight con-

clitioning ol’ the clcctrodcs by cithcr ion bomhardmcnt or

breakdown will rcmovc the instabilities. The mcasurecl current i in

amperes and lhc applied vollagc I’ in volls arc rclatsd to the

current density J and the electric held strength E as i=J:l and

E= /It.. where :I is lhr emitting arca in iii’ and /I ii gcomclrical

rxtor in m I. By plotting log(i,‘I”) vs I,‘l’ the well-known

Fowler Nordhcim plot is obtainccl with a slope m at any point or

this plot given by

m = - 2.97 x IO 9 v2 B S(J’). (1)

where (I) is lhc work function nf the nialcrial and s(r) ;I slowly

varying h’nction close to unity. The Fowlcr Nordhcim cqu-

ation.” can bc written in ii siniplifictl for111 wilh constants ;I and h

LtS

(‘1

where in a good approximation r(jS)=.s(~)-cCE/@‘. If’ we now

substitute r(r) in (2) introduce K=nt/V and take s(y)=O.95 we

obtain

10.42 J= 1.03 x 10’” $ exp cD1” r 1 exp[2.30 K]. (3)

Since the term log @,’ exp[ 10.42/Q)’ ‘1 is fairly constant [or all 0 an

almost linear relation exists between log J and K. Consequently.

from the slope of the Fowlcr Nordheim plot log J and also log .:I

can be derived. The accuracy in log J will bc about I “e,32.33.

It has been shown that this method olcalculating the emitting

area :I is correct for clean metal protrusions. but rails in the case 0r

adsorbed species. In such cases the calculated areas are usually

one or two orders of magnitude smaller than the real ones, since

the electron tunneling probability is reduced by the adsorbate.

Ifthe voltage between two electrodes is raised to a critical value.

the corresponding rapid rise in current will result in breakdown.

In general an electrical breakdown will drastically alter the prc-

breakdown currents between the electrodes and hence the

Fowler Nordheim plot too. The critical value of the breakdown

voltage will generally increase by successive breakdowns. the

insulating properties of the electrode gap will improve and the

electrodes will become ‘conditioned’. The breakdown voltage will

reach a limiting value. It was shown by Alpert ct trl that thecritical

breakdown field strength at the cathode surface E, is constant and

independent of gap width’“. For tungsten electrodes he observed

E,=7 x IO’ V m- and thus a field enhanccmcnt factor cx ofabout

100, where a = E,,/E, and E, = VJd is called the macroscopic field

strength. Figure I shows measurements ofthc breakdown voltage

V, as a function of gap width (1 for ‘conditioned’ stainless-steel

A van Oos~om and L Augustus: ElectrIcal breakdown between stalnless-steel electrodes In vacuum

I /

r large

Vb(kV)

100 r=.gmm

ra .3mm

clectrodcs. It is seen that I’,, also tlcpends on lhc radius of

curvature I’ of the clcctrodes. WC shall prcscnt a more dctailcd

analysis of these elcctrodcs in Sections 4 and 5.

Further examinations or lield-emission-initiated breakdown

showed lhal E,, is dcpcndcnl upon lhe emilling arca :I. This we

could show for tungsten lield-emitters in ;I point-to-plant

conliguration and uhv conditions”4. Figure 2 prcscnts results for

-12-

Log A

Oxygen

l/Eb[10 -‘O”-lm]

Figure 2. Dependence oflhc breakdown tield strength E, in V II- ’ on the emitting arc;, ..I in m’ for clean tungsten field emitters. lor tungsten with il monolayer of adsorbed oxygen and ahcr argon ion homhardmem.

clean tungsten emitters, field emitters covered with about a monolayer of oxygen and after bombardment with I keV argon

ions at a dose 0r IO”’ ions m - ‘. The emitting area .4 used for all

cases is the one obtained from the Fowler Nordheim plot prior to

adsorption or ion bombardment. i.e. for the clean tungsten

surface. The breakdown field strength E, is apparently dependent

on surface area and surface treatment. We also found that tield

emitters bombarded with argon ions and subsequent annealing at 1200 C behave as clean emitters. The experiments were repeated

with rhenium emitters, producing very similar results.

Scvcral dctailcd dcscriptinn5 Iixvc hwn proposed for the licld-

emission-initiated csplosivc cmis5ion nicchanism. One of the

propn5ctl modcl~. IIIC rcsihvc hting model. is lwscd on Juulc

hating and conduction cooling of ;I conical Ii&l cniiltcr’5. The

prcdictcd tcmpcraturc rise is ~issiinictl lo be indepcndcnl of lhc

height Or the prolruGon and is given l-f!

T= J’r,’ p 3 i tan fl,,

(4)

whcrc I’,. is the radius Or curvalurc Or tlic Ii&l eniillcr. It,, the cone

unglc. ~j the resistivity and i the thermal conductivity of the

emitter material. Substituting .I (2) we dcrivc

(5)

If hrcakdown occurs at the same critical tcmprrature. e.g. the

melting point. (I). p and i which are tempcrnturc dependent will he

constant. Since Azrj and E= E, at breakdown. li~rmuln (5)

describes the dependence olthc emitting area .‘I on the breakdown

licld strength E,. Since r(~)=.s(!.) -cE/@ and K =m I.it is easily

derived that

d( log .4 ) -=2E,(-K,f2) d[ b’E,1 where K,,= K at breakdown and is proportional to log J,. The

slope 0r the log .-I vs[ l/E,] plot is highly sensitive to the chosen

model. In the resistive heating model it is assumed that the

generated heat H=J”p and so surface heating is the dominant

process. If we slightly modify this assumption and assume volume

heating in the lield emitter tip with radius r,, we take H 2 i’R/rj.

In such a case the slope d(log .-I)/d[I/E,] is somewhat reduced.

Apart rrom the resistive heating model based on Joule heating

and conduction cooling. models based on the Nottingham effect

have been proposed”“.-“. At low temperatures most of the

electrons are emitted below the Fermi level and so contribute to

the heating of the emitter. At high temperatures the energy

distribution of the electrons is totally different and now. as in

thermionic emission. the electrons will cool the electron source,

since they are emitted above the Fermi level. For the models based

on Joule heating and Nottingham cooling and on Nottingham

heating and conduction cooling, a similar derivation can be made

as for the resistive heating model. In Table I. the average values of

the slope d(log A)/d[ l/E,,] are given for the three models in the

case of volume heating.

If we now compare the calculated values with the ex-

perimentally observed ones also given in Table I for tungsten field

emitters and derived from the results plotted in Figure 2, it is seen

that the case of clean tungsten field-emitters corresponds to Joule

heating and Nottingham cooling. On the other hand oxygen

adsorption on such emitters seems to point to a different

mechanism of Nottingham heating and conduction cooling. The

increase in work function on adsorption suppresses the current

density considerably and hence the Joule heating too. The

experimental results for emitters bombarded with argon ions do

not fit any of the mentioned models. Apparently, considerable

damage is produced in the tungsten lattice, reducing its stability

under the influence of the high electrostatic field stress.

For broad-area electrodes experimental results are more

difficult to interpret. Such electrodes will usually have a large

number oremitting sites all contributing to the electron emission,

129

A van Uusfrom and L Augustus: Electrical breakdown between stainless-steel electrodes rn vacuum

Table I. C;~lculatcd and measured valuc~ of the \lopc of the log .I \\ I E, plot for three theoretical models ant1 cspcrimcntal results glvcn in Figure 2.

Th 1

MECHANISM

Joule Heating

Conduction Cooling

d log A

d l/E,

5.6

Th 2 Nottingham Heating Conduction Cooling

9.8

Th 3 Joule

Heating Nottingham Cooling

4.3

Exp 1 W, clean 4.3

Exp 2 W, oxygen 8.7

Exp 3 Stainless Steel 3.4

while only one site will be responsible for the actual breakdown. The measured values of tn and .4 arc average values with a large mean deviation for fresh electrodes. Conditioned electrodes will have a much smaller mean deviation. Examinations of such electrodes in a SEM have clearly shown for various materials such as copper, titanium and stainless steel that the surface is full of craters3S.39. Lips and edges of these craters are potential candidates for field-emitting sites, however their field enhance- ment factors are considerably reduced.

Dependence of the breakdown field strength E, on the emitting area A has been observed for broad-area electrodes of molyh- denum, tungsten, titanium and stainless steel. Only in the case of tungsten has good agreement with the ideal point-to-plane configuration been obtained. The other materials have average values of the slope d(log A)/r(l/E,], ranging from 3.0 for molybdenum and titanium to 3.4 for stainless steelJ”.J’. This strong dependence of E, on A is either a result of ion bombardment ofthe emitting sites, as in Figure 2. or is caused by anode-initiated breakdown. Beukema has shown that E,, in the case of conditioned titanium and stainless steel electrodes depended on the anode material, while the slope of d(log A)/d[ l/E,] remained conslant4°.

It is now possible for conditioned electrodes to predict the breakdown voltage I’,, from the pre-breakdown currents. The conditioning process considerably improves the cleanliness of the electrode surfaces”. Although the anode may show segregation phenomena, the cathode is covered with evaporation products from the anode and may be treated in many cases as a clean metal surface. From the FowlerrNordheim plot n1 and A can then be found. Since the relation between E, and ,4 is known, we easily determine E,, Jbr K,. Such a prediction can he useful in many practical applications.

3. Experimental procedure

In actual devices electrical breakdown between a pair of stainless steel electrodes may he influenced by the operating conditions of

130

the dcvicc. Thcsc conditions may rliffcr consitlcrahl!~ from those prcscnt in a hakcd uhv sgstcni willi spcciallv prcparcrl clcctrodcs for breakdown csperinicnls. We have exaniincd stainless-slccl electrodes used for focusing in an clcctron gun. The clcctrode surfaces wcrc polished and ultrasonically clcancd bcforc IIWUI~~-

inp in the clcctron gun. .4fter evacuation the clectrotlcs wcrc conditioned by a numhcr ofsuccessivc breakdowns. resulting in a hrcakdown hehaviour as shown in Figure I. The clectrodcs wcrc subsequently demountcd and csaminccl in ;I scanning Auger microscope from Physical Electronics with a hcam size of three microns. Unless mentioned otherwise. the primary heam energy used was E,, = S keV and the primary beam current i, was ahout lo nA. Although this proccdurc has the disndvantagc of ;I brief esposurc to air during transfer of the etectrodcs from the tuhc to the microscope. it has the advantage of producing electrode surfaces representative of normal conditioning and operating procedure.

4. Cathode phenomena

Although conditioned electrode surfaces. as seen in a SEM. often show many sites that can produce lietd electron emission. fresh etectrodc surfaces seem to have a variety of possible causes of clcctrical breakdown. Such surfaces may have protrusions which arc stable or may show movement in the high clcctric f&i. However, loose particles or inclusions may also be responsible for breakdown and act as emitting sites.

In a vacuum device barium getters are often used to reduce the pressure level after the tube has been sealed off from the main pumping system. If flashing of the getter is not properly carried out, particles may he able to reach the electrode surfaces. Figure 3 shows the Auger spectrum of ;I micron sized BaO particle which served as an emitting site. The low work function of the particle contributed to the emission in this case. Loose particles on the cathode surface are also found prior to breakclown. These particles may have a poor electrical contact with the substrate and become charged by an electron beam. Figure 4 illustrates this point by showing the Auger images of Fe. Cr, 0 and C for a small section of60 x 60 Ltm’ of the electrode surface. The bright region in the carbon image corresponds to an insulating particle which causes the carbon peak in the Auger spectrum to shift by tens of volts. The shift of the carbon peak on the electron energy scale depends on the primary beam energy and current. We have not been able to observe such loose particles on conditioned electrode surfaces. Apparently breakdown removes such particles from the surface.

Fresh electrode surfaces may also contain insulating inclusions which may act as sites for electrical breakdown. Such inclusions may give rise to electroluminescence, as has been observed from copper, gold, molybdenum and stainless steel surfaces”. Farrall has shown that alumina inclusions in zone-refined copper electrodes caused electrical breakdown and crater formation3”. The craters appeared around the insulating inclusion. and sometimes partial melting of the inclusion occurred. Small inclusions were frequently removed by the vacuum breakdown.

The stainless steel electrodes we examined showed insulating inclusions prior to breakdown, hut also after the conditioning process. We shalt discuss here a particular case in somewhat more detail. Scanning the electron beam across the electrode surface indicated the presence of a charged inclusion. The Auger peaks from the inclusion would shift along the energy scale. The energy shift was found to depend on the primary beam energy, current and area imaged. This latter parameter controls the number of

A van Ooslron~ and L Auyuslus. ElectrIcal breakdown between stalnless-steel elecrrodes In vacuum

dN YE

ELECTRON ENERGY, eb’

Fe

0 C

AUGER-IMAGES OF CATHODE Figured. Auger-images ofa small section of’thc cathode surface da pair of stainless-steel electrodes. Imaged area 60 x 60 [tm’. The Fe. Cr, 0 and C images are shown.

electrons per unit time striking the inclusion. The dimensions of

the inclusion were found to be less than the diameter of the beam

size. By comparing the Auger spectra of regions with and without

inclusion, it was easily derived that this particular inclusion had a

diameter of about 1.5 {tm.

The secondary emission coefficient (5 varies as a function of the

primary beam energy E, and its value determines the measured

shift in the secondary electron peak of the inclusion as a function

of E,. For E, = 2.4 keV no shift was observed, indicating 6 = 1. The

inclusion could also be charged to any selected value by adjusting

the primary beam energy and current. Consequently. continuous

charging to a constant value of -300 V required 42 nA at

E,=3.0 keV and only 2.6 nA at E,=5.0 keV.

The composition of the inclusion can be determined by

comparing the Auger spectra of the stainless-steel substrate and

the substrate with inclusion, using the minimum beam size of 3

microns. The Auger spectra were measured with a cylindrical

mirror analyzer (CMA) in the derivative mode d:C’!dE vs E.

Atomic concentrations wcrc calculated h! using the scnsitivit!

factors for the elcmcnts ;IS given in the I-ltrrldhool; (t/’ -I ~r!/c’r E/cc~,ojr S/IL”.II.~).S’.,‘/‘~~~‘, The peak \hape and position of Glicon and

aluminium peaks corrcbpand to the osidc phase of both clemcnt~

Correction\ wcrc ma& for the change in 5cnsilivily factor< for

thcbe clrment~. The rc\ults ofthi comparison arc summaripcd in

Table 2. whcrc the atomic concentrations arc given for the two

AT.CONC. %

I RON

CHROMIUM

NICKEL

CARBON

OXYGEN

MAGNESIUM

ALUMINIUM

SILICON

A : SUBSTRATE

A

44.6

14.2

69

10.5

168

13

1.9

38

1-

6 c

29.5 3

7.9 -

4.4 -

84 3

320 61

1.7 -

5.4 11

10.7 22

B: SUBSTRATE + PARTICLE

c: PARTICLE

regions A and B. From these results the composition of the

inclusion is found and given in column C. It is seen that the

inclusion consists mainly of silica and alumina in the ratio 2: I.

As we have seen, the insulating particle can easily be charged up

to a potential orseveral hundreds of volts. However. if we increase

the charging voltage above 300 V. the primary beam current i, for

a constant primary beam energy E,=J.O keV rises rapidly.

Apparently, charge is lost either by conduction or by emission. It is

found that the current increases exponentially with voltage and

behaves as a field emission current. Figure 5 shows a

Fowler Nordheim plot of the electron emission from the inclu-

sion to the matrix. From the slope we derive J=Z.lO .4 m-‘.

.-l~lO-‘imzand Ez5x IO9 Vm-‘. However.thesevaluesare

only an indication that we are dealing with a tunnel current either

through the insulator or across the vacuum gap to the matrix of

stainless steel. Although there is experimental evidence that

emitting sites may show semiconductor-type electron energy

distributions”, it is also experimentally observed that mostly

matrix material is evaporated during a breakdown38. The

electrical field at the stainless-steel surface is higher at the vacuum

interface than at the insulator interface. since the dielectric

131

A van Oosfrom and L Augusfus: Electrical breakdown between stainless-steel electrodes in vacuum

-11.

Log +*

-12.

-1%

--14- 20

-

.

30

Figure 5. Fowlcr-Nordheim plot of ~hc slcctron emission hctwccn an

insulaling inclusion in II stain&-slccl malrix and lhc mntris itself.

constant of 50, is about 3.9. We think that a direct breakdown

between the stainless-steel electrode surface and the insulator

surface is the more likely of the two possible processes. Although

we have shown that lield emission can occur and the potential on

the insulating inclusion can be sufficiently high, direct evidence is

at present not available.

5. Anode phenomena

Important requirements for electrode materials are high yield

strength and oxidation resistance. These requirements limit the

choice ofsuitable metals or alloys considerably. It is not surprising

that Fen Cr alloys are often selected, since Cr,O, can be used as a

protective layer which acts as a barrier to reactive species. It is

rapidly formed in the surface region ofsuch alloys by oxidation at

IOOO-13Oo”C, producing a stoichiometric oxide with a low defect

concentration. The solubility for oxygen is only O.OOS”,, at 1200-C

and Cr,O, has a melting point of about 2400’P4.

We have investigated various stainless steels which were used as

electrode materials. Although the bulk composition of these

materials varied, it was observed that the composition of the

surface region was mainly determined by the heat treatment.

Fresh electrodes were hardly oxidized, but electrodes taken from

baked systems and conditioned by electrical breakdown always

showed a larger degree of oxidation. In all cases of conditioned

stainless-steel electrodes the surface composition of the anode

proved enriched in chromium oxide in contrast to the cathode. On

the other hand the thickness of the oxidized surface region was

larger for the cathode than for the anode. This was established

with combined scanning Auger microscopy and in-depth profiling

by sputter removal of the electrode material, as illustrated in

Figure 6 for a pair of conditioned electrodes. The peak-to-peak

heights of the oxygen KLL (510 eV), chromium LMM (529 eV)

and iron LMM (703 eV) transitions in the Auger spectrum are

given as a function of the sputter time with 1 keV argon ions. This

clearly indicates that the temperature of the anode was con-

siderably higher than that of the cathode. The anode also only

shows chromium enrichment in the region facing the cathode. in

other surface regions of the anode the initial composition prior to

conditioning is maintained.

CATHODE

dN m rl 0

ANODE pz

I

0 50 100 150

Sputter-time(min)

Figure 6. Auger in-depth prolilcs of the cn~hotlc and anodc of a pair of ‘conditioned‘ stxinlcss-slecl clcclrodcb. illuslmting lhc clTccl of con- ditioning on ~hc composition or 111s slcclrodcs in ~hc surface qion.

Oxidized stainless-steel surfaces consist of bulk material

covered with a protective scale of Cr?O,. a spincl structure

FeCr,O, and the iron oxides FeO. Fe,O, and Fc,O,.

Apparently. the anode surface has been depleted in iron oxides by

evaporation. The silica insulators close to the vacuum gap

between the electrodes are found to be covered with a submono-

layer of the evaporated species. Figure 7 shows the Auger

dN dE

r-4+-- “-+-

Cr Fe si C Si

0

I

0 400 800 1400 1000

ELECTRON ENERGY, eV

Figure 7. Auger spectrum of a silica insulator supporting rhc clecirodes. indicating deposition ol’ evaporating products mainly from the anode surlace.

spectrum of such an insulator surface after conditioning of the

electrodes. Both iron and chromium peaks are observed. The

thickness of the Cr,O, region on the anode surface is also a

function of the conditioning process. The thickness is increased, if

the amount of power dissipated in the discharge is limited. The

evaporation of material is also reduced in that case. The conclusion

drawn is that the surface composition clearly depends on the

conditioning process and on the geometry of the electrode

surfaces.

A closer examination of the anode surFaces of pairs of fully-

conditioned electrodes also indicates the enrichment of impurities

at the surface. Figure X shows the Auger spectrum in the derivative

mode dN/dE vs E of an anode surface. Cathode surfaces do not

show such segregation phenomena. The most striking feature of

this spectrum is the presence of large amounts of potassium. but

also of magnesium, aluminium, silicon, sulphur and carbon. As

132

A van Oosrrom and L Aclgusrus ElectrIcal breakdown between stamless-steel electrodes in vacuum

dE I I

indicated in Fifurc 9. the potassium is concentratccl in small

regions WI the surhce. The location of thcsc regions is not limited

to those whrrc chromium enrichment occurs. Surface scgrcgation

d potassium occur5 at a lower tempcruturc than chromium

enrichment and iron evaporation. If wc now sclcct ;I potaGum-

rich arca on the 5urfxc and remove material by argon ion bombardment. we c;ln compare the composltlon. prior to

sputtcrinp. in the oxide region and after removal of the oxide

phase. The results arc summarized for thc~ three stages in Table

3. It is seen that potassium is true surfacc scgrcgation. while silicon

and magnesium are also present enriched in the oxide phase.

AT. CONC. % A B c

IRON 1 2.1 13.7 61.5

1 CHROMIUM / 16.9 ) 14.8 ) 17.8

1 OXYGEN 1 338 1 34.9 1 1.6

AUGER-IMAGES OF ANODE

Several diffrrent models have been proposed to dcscrihc surface

se_pregation. Rcccntly. Miedema4’ dcrivccl ;I formula for the

increased surface concentration d an clement A in ;I matris 0r material of element B.

G+ - = expCfAff&, + q(;$ - ;?,) I’!, “/3R T] C;

where the surface energy of A can be approsimated b!

(7)

Here AHto,. and AH: ,,p., are the heats of solution and evapor-

ation of element A. 1. , is the atomic volume and ,/‘and g are

constants. Table 4 gives numerical values for some of these

quantities for a number of elements in an iron matrix. The last

column gives the ratio C, Cl, of surface and bulk concentrations.

It is clear from the table that a strong enrichment of potassium is

predicted by this model. In a complex alloy system like srainless

steel the simple binary elrmen~s approach needs correction since more elements are present and some are in the oxidized state.

Nevertheless. the predicted magnitude of the effect is such that

surface segregation phenomena are to be expected.

Previous work has shown that electron-stimulated desorption

from 304 stainless steel will produce mainly F’ and H + ions”‘. In

the present experiments we have not been able to detect fluorine

on the surface (hydrogen is undetectable in AES). This could be

due to the near-coincidence of the Fe 651 eV and the F 647 eV

Figure 9. Distribution of iron. chromium and potassium at the surface of a transitions in the Auger spectrum or the large cross-section for

‘conditioned‘ anode, showing locali;red enrichment of chromium und desorption of the fluorine. Potassium was found on the surface in

polassium. the present experiments and the previous work.

133

A van Oostrom and L Augustus: Electrical breakdown between stainless-steel electrodes in vacuum

Table 4. Numerical values of the surrace energy 7. heat or evaporation

A%,, heat of solution AH,,, and ratio of surface and bulk concentrations C,/C, ror a number or elements in an iron matrix.

Al 1.20

tt Si I29

395 1 -61 0.771 I I I I 1

428 1 -6 1 0.5 1

283 ( ’ I 38 I

145 ) 78 ( 19 )

6. Conclusions

Electrical breakdown between a pair of stainless-steel electrodes depends on field emitting sites on the cathode surface. The nature of these sites can be either insulating inclusions or small protrusions on the edges of craters formed by previous break- downs. Loose particles and impurities do not seem to play an important role on the cathode side. The composition of the surface region of the cathode is not greatly affected by the conditioning process.

Major changes occur in the composition of the anode surface. Evaporation and surface segregation cause the region facing the cathode to become quite different from other anode regions. In the ‘conditioned’ state a stable chromium oxide layer protects the anode and reduces the vapour pressure on electron bombard- ment. However, impurity segregation has been observed, par- ticularly of potassium, which evaporates at relatively low temperatures. The amount of potassium seems too small to produce sufficient vapour in the electrode gap for breakdown. However, potassium or perhaps fluorine could play a role in the charging of the insulating inclusions in the cathode surface. This point deserves further investigation.

Dependence of the electrical breakdown field strength on the emitting area of the protrusion or emitting site has been shown to be a good criterion for distinguishing between various mech- anisms. For protrusions in ideal point-to-plane configurations, models based on Joule heating and the Nottingham effect seem to

work satisfactorily. However, broad-area stain&-steel elec-

trodes show that the breakdown field strength is dependent on the

emitting area. which seems to point either to cathode breakdown

influenced by ion bombardment or to anode breakdown. In view

of the difference in composition in the surface region of cathode

and anode. we conclude that anode breakdown is the major

mechanism in small gap stainless-steel electrode systems.

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