Surfaceand Coatings Technology,42 (1990) 91—100 91

Current industrial practices— Technicalnote: Ion behaviorin the hot hollow cathodecoating of silver

Y. S. KuoBoeing Electronics,Mail Sto’p 9E-88, P.O. Box24969, Seattle,WA 98124 (US.A.)

R. F. Bunshahand D. OkrentSchoolof Engineering and Applied Science,BoelterHat4 University of California, LosAngeles,CA 90024 (U.S.A.)

(ReceivedMarch 24, 1989)

Abstract

Experimentaldataare presentedon two aspectsof the copiousions producedinthevacuumcoatingchamberin ahot hollow cathode(HHC) device. First, thegeneralbehaviorof both the ions of thesilverevaporantandthoseions of theprocessgasargonis describedin termsof the currentcollectedby the biasedsubstrateasafunction ofvariousoperatingparameters.This substratecurrent is usedin conductingaspecific ionbombardmentstep,prior to thedepositionstep, that etchesandmodifies thesubstratesurfaceto ensureextremelygoodcoatingadhesion.Second,a thresholdarcor cathodecurrentwaspinpointedasa vital parameterfor thedesign scale-upof theHHC coatingdevice.The thresholdvalue wasdeterminedby estimatingan effective sputteryield andcomparingit with the respectivetrendsof thesilver depositionrateand the ion currentcollectedfrom the substrateholderasa function of the arcor cathodecurrent.Abovethe thresholdvalue, netdepositionof silverwasdemonstratedto occur aspredicted.Itis concludedthat necessaryprecautionsneedto be takenfor a scale-up,in addition tothesubstratecurrentdensity requirement.

1. Introduction

Coatingtechnologyusingthehot hollowcathode(HHC) hasbeenreviewedin generaland more specifically on its useto coat silver metal [1]. In thisapplication,the silver film coatedis usedas half of the interlayerbetweentwo coatedmechanicalcomponents,when they are pressedtogetherin asolidstatebondingprocess[2]. The adhesionof the silver film to thesubstrateis therefore crucial. It is achievedby a very successfulcleaningor etchingof the substratesurfaceby ion bombardmentprior to the actualdepositionof thesilver. Both steps, theion etchand the silverdeposition,areconductedin the samepumpdown process.

The ions are produced mainly by a low pressurearc in the form of aplasmabeambentinto an archshapeby a magneticpole piece, both shownin Fig. 1. An argon flow is fed into the chamberthrough the HHC, in theform of atantalumtubewhich is not shown.This initiatesthe arcor plasmabeamfrom the left which in turnstrikesthe silver evaporantin the crucible

ElsevierSequoia/PrintedIn The Netherlands

92

Subatrate Holder

Coating

Arc Material

Pole PiCce_~N L~\ _~~Hearth

!~

M.~rret

Holl~=;de~~

Vacuum

Fig. 1. Schematicof a vacuumcoating chamberusingan HHC device to coatsilver. Thesilverevaporantis placedin a tantalumcrucible which in turn is placed in the water-cooledcopperhearthshown.

onthe right.Theplasmabeam,whichservesastheheatsourcefor evaporation,is theprimaryplasmafrom which ionsdiffuse out to fill the chamber,formingthe (secondary)chamberplasma.

Two questionsrelatedto the ion etchstepareinvestigatedin this study.First, when the (negative)substratevoltage is appliedto attractthe ions inthe chamberplasma(generatedby the plasmabeam),whateffectsarecausedby the varying of those operatingparameters?Second,is therea scale-uplimitation in termsof the ion etchingeffectivenessas imposedby the possiblenet depositionof the evaporantsilver?

2. Experimental set-up

A diffusion pump systemcapableof operatingin the iO~ Torr rangeof backgroundpressurewasusedwith the chambershownin Fig. 1 (interiorvolume approximately3600 in.3). The details are given in ref. 2. The air

93

leak was implementedwith a Granville—Phillips variable leak valve (series203).

A Controlarc 400 welder power supply, model 51E34-2,was usedtosupply the arc currentfor the HHC. An Allen—JonesElectronicsCorporationhigh-voltagesupply, model AJR5S which was ratedat 5 kV and 500 mA,was usedto apply the substratevoltage.The accuracywas cited as ±3%of full scale.Thecurrentwastypically rectifiedbutnotfiltered.Forcomparisonpurposes,an R—Cifiter box (100 k.12 and 10 mF) was sometimesattachedbetweenthe Allen.-~Jonessupplyandthe substrateholderwhenthe substratecurrentwasapplied.Alsofor comparisonpurposes,aregulatedpowersupply,madeby OREM Electronics, Inc. and rated at 2 kV and 2 A, was madeavailablefor a few measurementruns.A KEPCO power supply, modelJQE75-15 and ratedat 75 d.c. and 15 A, was usedto measurethe substratecurrent producedunder low substratevoltagesbut for a wide range of arccurrent.

3. Results and discussion

3.1. Ion current collection by the substrateOneuniquefeatureof the HHC coatingprocessis that copiousamounts

of ions areproducedin the chamber.The majority of them arebelievedtobe generatedby the plasmabeambecausethe high densityelectroncurrentcan ionizeboth argon and silver by an electronbombardmentmechanism.Othermechanismslike Penningionization andionization by chargetransferarealso conceivablyinvolved. This is discussedin more detail in chapter4of ref. 2. A substantialfraction of theseionscanbe collectedby the substrateholderwhen the latter is properly biased.

The ion current registeredwasashigh asnearly 14 A for an arc currentoperatedat 200A, asshownin Fig. 2 with the substrateholderexceptionallylocatedin amostfavorableposition “RL” (seeref. 1 for the exactdefinitionof theselocations). For amore typical substrateholder position “0” (usedfor HHC silver coating),two sets of dataare shownin Fig. 3 for a modestbias of —40 V. The ion current at an arc current of 200A (typical valuefor silver deposition)is about5 A. Even this latter value,when divided bythe surfaceareaconcerned,would give avery highcurrentdensityof 10—20mA cm2, muchhigher than typical valuesof mostothercoatingprocesses.The curve representedby the closedcircles gives datacollectedwith a flatdisc-shapedshutterplacedabout3 in. abovethe silverpool, thuspreventingmost of the silver ions from being collected by the substrateholder. Thelargegapbetweenthesetwo curvessuggeststhatmostof the ions collectedby the biased substrateholder are that of silver, rather than argon. Thisreasoningis further supportedby the large gap betweentwo sets of opensymbolsof thedatashownin Fig. 4. Thetop curvethereingivesapproximatelythe ion current which may be used for the ion etch step of the coatingoperation.

94

140 ___________________________________________

AA A

A A

120 —

A -

AA AA

100 — —

AS HHC Arc CArrerrl. 200 Aocp

Scbrlr~reHoldcr Poiirro~.RL

c- 8.0— -z

A

Ar Flow

60 Syorbol SlPccfrrrirrl —0

A 105A

200U~ 40— -

WOO Arrfrerl Ic 195 A

20 6 —

1.0 — —

0 I10 20 30 40 50 60 70 80

INeg4rrceI SUBSTRATE BIAS VOLTAGE, ccFlr

Fig. 2. Thebias (ion) currentvs. the (externallyapplied)biasvoltagefor thesubstrateholderposition RL and two different argon flows, when the cathodearc was operatedat 200 A.

At least two variableshavebeenfound to affect the ion current consi-derably.Oneis the argonflow fed into the chamberthroughthe HHC whichcould vary from about 16 to 45 cm3 min~ at STP while still maintainingthe deviceoperation.A quite modestchange,seeFig. 2 for instance,givesrise to approximately15% changein the substrateion current. The ions,especiallythe silver ions producednear the crucible, are collectedby thesubstratethrough gas phase diffusion. The presenceof more argon atomsto scatter the silver ions makes it more difficult for them to diffuse to thesubstrate.Their lower arrival rate at the substratesurfacecausesa reducedsubstrateion current.

The other important variable is the magnetcurrent which determinesthe degreeof magneticbendingof the plasmabeam,whichin turndeterminesa lessobvious parameter.With an increasein magneticbending, the sizeof the impingementspotof the plasmabeamonto the silver pool surfacedecreases;andboth the electroncurrent densityand the silver evaporation“jet” increaseat the spot. The combinedresult is a considerablyenhancedproductionrateof silver ions there(mainlyowing to electronbombardment).The dependenceon the spot size was studied experimentallywith an airleak duringthe HHC operation,usingagraphiteblock in placeof thetantalumcrucible (seeAppendix E of ref. 2). Figure 5 shows the substantialimpact

95

10, I

AR Flow: 25 STPcC/minw ~ - Pressure: 7.1 x io’ Torr0 - Substrate Position: 0

- Substrate Bias: -40 Volts A

~ A::

3 0.1 — —0 -

w • SHUTTER CLOSED

• A SHUTTER OPEN

31.I3 -

0.0170 80 90 100 110 120 130 140 150 160 170 180 190 200 210220

ARC CURRENT (amperes)

Fig. 3. The biasessubstratecurrent as a function of the HHC current at a fixed bias and afixed argon flow (and therefore a fixed chamber pressure).

dueto the magneticcurrentvariation.The middle curvecorrespondsto thedata collectedwhenthe beamimpingementspot is locatedat roughly thecenterof the silver pooi surface.

When a floated dark spaceshield was usedwith the regulatedpowersupply, the substrateion currentwas considerablyreducedby the fact thatmuch of the substrateholder surfaceareawas coveredand isolatedby theshield from the plasma.The data shownin Fig. 6 were collectedat 1 kVas a function of the HHC arc current.The two curves again give a rangefor the substratecurrentdefinedby the extremevaluesof the magnetcurrent(numbersattachedto datapoints). The two datapoints on the left, whichconspicuouslyfall out of the curves,correspondto the arc current regimein which the evaporantsilver in the cruciblebecameonly partially melted,rather than fully melted. It is notedthat data in Fig. 5 were also takeninthe partially meltedregime.

Figure6 relatessomeof theion etchuncertaintiessince1 kV isfrequentlychosenfor the etch operationas the substratevoltage is limited by thearcingtendencybetweenthe holderandits shield.For instance,if themagnetcurrentis not specifiedduringthe ion etchstep,atagiven substratecurrent,

96

I 1

7~ - 0 SUBSTRATE VOLTAGE: -2.0 KV

0 Without Ag Evaporation

8~Z 0 With Silver Eviporaliona 0 • With Ag EvaporationBut Shutter ClOsed

~500-

0 0z 0

400-0

00 oW300- 0000

0

~200- 0 I

• 0• 0

100 I I I I I

40 60 80 100 120 140 160 180 200 220

HOT HOLLOW CATHODE ARC CURRENT (amperes)

Fig. 4. The biasedsubstratecurrent as a function of the HHC current at a fixed bias of 2kVr data collectedfor threedifferent availabilitiesof the silver ions.

I I I I

600 —

Test IE e27 (0)

Ar Flow 22 5 STP cc/mm

p 5x10’ Torr Magnet3 500 — arc = 80 Amp 6 Current =

£ 0.78 Ampsz a

a~400- & £ ~0UI- a

070Am~

~3O0- a •.

U)

. . .—200- I O6OArnp

I• I

0I • • • I •100 -

I

I I I I I I

6 tO 20 tOO 200 1000 2000

SUBSTRATE VOLTAGE APPLIED WITH A VOLTAGE-REGULATED POWER SUPPLY (Volts)

Fig. 5. The regulatedsubstratecurrent varying with the substratebias voltage at a given arccurrent, 80 A, for three different magnet currents.

say 500 mA, the actualHHC arc current could vary considerablyfrom 102A up to 130 A without any change in the argon flow at all. If the argonflow is allowedto vary, the uncertaintyrangein the HHC arc currentwouldbe evengreater.

97

I I

A Floated Dark-Space Shield was Used.

Substrate Voltage t.0 Kilovolts

o Approximate) Maximum Magnetic Bending0 Approximate) Minimun Magnetic Bending

2xt:~

O9~ -

3 0.630(7 50 -w

HHC Coating System #2Argon Flow = 22.5 SIP cc/win

Pressure = 5x10” Torr

I I I I I80 90 too tto 120 t30 t40

HOT HOLLOW CATHODE ARC CURRENT. AMPERE

Fig. 6. The regulatedsubstratecurrent, collectedwith the useof a dark-spaceshield,as afunction of the HHC arc currentat a givensubstratebias voltage, 1 kV. Two datasetsshowthe effect causedby theupperandthe lower extremesof the magneticbendingof the plasmabeam.

3.2. Net depositionas a scale-upCOncernOne unique featureof the HHC ion etch stepis thatevaporationoccurs

andthe evaporantprovidesaportion of the ions which performthe etchingaction.’Thismeansthation bombardmenthasto removeall thesilverdeposited(includingboth silver neutralsandions) beforeit canactually carryout theintendedcleaningor etching of the substratesurface.it is noted that thisevaporationfeature is not a simple constant,but varieswith anumberofoperatingparameterssuchas the argonflow andthe magnetcurrent.Figure7 gives the depositionrate at agiven argon flow (approximately22.5) cm3min STPas a functionof the arc current,showingagainthe rangecausedby the magnetcurrent extremes.

98

10_C I 67

— 100

a

z3.0— 2

,l 0 1.-HO ~

2,0—

0— 0E H.3 0 -1

~

o Substrate Holder Position 0 >:H

0.t = — 1.57 ~

I- -1.0— MagnetIc BendIng of ES

— ~~At Appenoiwneely Maxireun,1~At Approxlwetely Minir,rarnt U

0I-z

-g>30

sot I I I I I Igo ioo 120 140 160 180 200 220

HHC ARC CURRENT, AM#SRES

Fig. 7. The deposition, rate, obtained for the substrateposition 0 without any bias, as afunction of the HHC current. The extremeeffects of the magneticcurrent are also shown.

It was concludedin ref. 3 that a thresholdsubstratecurrent densityofabout 1.3 mA cm2 was necessaryin HHC silver coating to produce asufficiently clean substratesurfacefor an ion etch stepof 10 min or longer.If this kind of thresholdvalue is usedas a sufficient condition or criterion,evenwhencombinedwith asafetyfactor, for the designof a scale-upHHCcoatingequipment,the consequencecould be disastrous.For instance,witha safetyfactor of approximatelytwo to designfor asubstrateholder surfaceof 200 cm2,a substratecurrentof 500 mA is obtained(for a currentdensityof 2.5 mA cm2). As was pointed out, the arc current rangegoesfrom 102A to 130 A to generatesuch a substratecurrent.From Fig. 7 it is easytofind the correspondingdepositionratesat thesetwo arc currentvalues.Tomake the comparisonreadily meaningful,the depositionrate is convertedinto an equivalention current density, assumingevery depositedatom issingly ionized, on theverticalaxis at the right-handside of Fig. 8. One canseequickly that if the high extremearc current is usedduring the ion etchstep, 5 or 6 mA cm2 equivalentof silver would deposit; in other words,

99

~ 1.6- — — — — — — —

aCa

~14——----- —7—0 /C —o

Cfa

a0

80 100120 140 ibO 180200 220

HotHollow Cathode Arc Current, Amp

Fig. 8. The ratio of the silver depositionrate to the biasedsubstratecurrent, as a tendencyparameterof thenet depositionof silver duringthe ion etch, varieswith theHHC arc current.

every 2.5 ions bombardingthe substratesurfaceneedto removeor sputteroff about five or six equivalentions of depositedsilver atoms.The averageor effectivesputteringyield during the ion etchwould needto be significantlyhigher than two in orderto fulfil an effective ion etch. If it is lessthan two,therewould be a net depositionof silver during the ion etch, any originalcontaminationwould not be removedby the ion bombardment,and poorcoating adhesionwould thus ensue.

The kinetic energycarriedby eachbombardingparticle during the ionetch varies greatly from particle to particle, making the estimationof thesputteringyield very difficult. A crude estimateof the averagebombardingenergyduring the HHC ion etchis about 100V per particle [2], includingions andenergeticneutrals,basedon a simplified chargeexchangemodel.Suchan energyshould give a sputteryield of aboutoneor slightly greater(seepublishedsputteryield data in ref. 4). If so, the abovehypothetical,simplistic designapproach,requiring asputteryield of two or greater,hasalreadyproducedan unacceptableprocess.

In practice,the plasmabeamis usually aimedat nearly the centerofthe crucible.It is thuspreferredto take somemiddle valuesfor the magnetcurrent,which are not shown but canbe approximatelylocatedin Figs. 6and 7. By aiso taking note of the upper curve in Fig. 3 but selectingaproperscalefactor for 1 kV insteadof 40 V, a new plot can be madeonthe(required)sputteryield toavoidnetdeposition,whichcanbeapproximatedby the ratio of the depositionrate to the substrateion current density,asa function of the HHC arc current, as is given in Fig. 8. For an effectivesputteryield of 1.0, the thresholdHHC arc currentis approximately120 A.Interestingly,in a laboratoryexperimentit wasconfirmedat this arc current

100

valuethat netsilver depositionstartedto happenandpoor adhesionoccurred,sometimes,evenafter aprolongedion etch stepof as long as 1 or even2h.

4. Conclusion

The HHC coatingdeviceproducesavery largequantity of ions, mainlyfrom the evaporant.This providesthe processwith the capability of averyeffectiveion etchstep.However,the potentialdangerof silver net depositioncould totally eliminatethis advantageand causepoor coatingadhesion.Thisweaknesshas beenexperimentallydemonstratedand canbecomea seriousproblem in the scale-upfor a large substrateholder. It can be avoidedbythe judicious useof a relatively low HHC arc current and a relatively lowmagnetcurrent (to cut down the evaporation)at the requiredsubstrateioncurrent.

Acknowledgments

Theexperimentalwork wasconductedat RockwellInternational,Golden,CO. YSK wishes to thank all his previouscolleaguesat Goldenwho havebeen of assistance.

References

1 Y. S. Kuo, R. F. Bunshahand D. Okrent, J. Vac. Sci. Technol.,4A (3) (1986) 397.2 Y. S. Kuo, Ph.D. Dissertation,University of California, Los Angeles, (1985).3 D. Larson and L. Draper, Thin Solid Films, 107 (1983).4 R. Behrish(ed.), Sputteringby Particle Bombardment,Vols. 47 and52, Topics in Applied

Physics, Springer, Berlin, 1983.