Surfaceand CoatingsTechnology,59 (1993) 97—100 97

Determinationof massandenergydistributionof ions in glow discharges

S. Peter,R. Pintaske,G.Hechtand F. RichterTechnischeUniversitdt Chemnitz-Zwickau,FachbereichPhysik,OberfrohnaerStrafie33, 0-9033Chemnitz(Germany)

Abstract

Equipmentwasdesignedfor thestudyof energyandmassdistributionofparticleshitting thecathodeof a glow discharge.Energyand mass analysiswere performedusinga two-stage differentially pumpedplasmamonitor. Theplasmamonitor consistedof aquadrupolemassspectrometerand a modified cylindrical mirror analyserthat can measureparallel ion beams.D.c. and pulseddischargeswereinvestigatedunderconditionstypicalfor plasmaassistedchemicalvapourdepositionandplasmadiffusion treatment.Gasatmospherescontainingnitrogen,hydrogen,argon and tetramethylsilane(TeMS) were studiedatpressuresof somemillibars.Ionsprevailingat thecathodeareformedby electronimpactionizationofspecieshavinglow ionization potential.The appearanceofArH + andN

2H+ ions canbeexplainedby ion—moleculereactions.Theaddition of nitrogento a TeMS—H2—Ar dischargecausedahigherdegreeof fragmentationof theorganometallicprecursorand anincreasein ion energy.Evenin thecaseof d.c.excitationtheenergyof nearlyall theionsexceedsthesputteringthresholdof materials(10—30eV for metals).

1. Introduction roundedby quartzglass shieldings in order to obtaindefinedelectrodeareas.Threedifferent excitationmodes

Plasmaassistedchemicalvapourdeposition(PACVD) were used: d.c., 40 kHz pulsedd.c. and 20 kHz pulsedandplasmadiffusion treatmentare of growing interest a.c.Thepulse-powersupplydevelopedin ourlaboratoryfor improving themechanicalsurfaceproperties.Surface is characterizedby a fixed pulse repetition frequency.modificationby plasmadiffusion is usedon anindustrial The presetaveragedischargecurrent (10 mA—5 A) isscale.The processof layer formation has beeninvesti- kept constantby electronicalpulse-widthcontrol. Forgated by many authors [1—3]. The models developed the first time we used bipolar pulsedexcitation. Thisdiffer from each other with respectto the role of ions excitation modeis referredto as pulseda.c.(see upperand the surfaceprocesses;however,thereare still some right of Fig. 1). Pulseddischargesare characterizedby aseriousproblemswhich affect the industrialapplication stablebehaviourin the abnormalrangeof the current—of PACVD of hard coatings.If metal halidesare used voltagecharacteristic,which is the rangeof most techni-as precursors,poor adhesionand the incorporationof cal importance.By varying the pulse-width repetitionhalogenatomsin the coatingwereobservedat deposition ratio,theoperatingvoltage UB andtheaveragedischargetemperaturesbelow 500°C[4]. Very low deposition current I can be decoupledfrom eachother. Thus, attemperaturescan be achievedby using organometallic constantpressureandcurrent,the cathodepotentialcanprecursors,but the tribological propertiesdo not meet be varied independentlywithin the limits given by thethe requirementsto date. Finally, up-scalingfrom a dischargecharacteristic.laboratory device to an industrial reactoris difficult. The working gaseswere fed through the poweredTheseproblems arise from the complexity of plasma electrode and the gas flow was controlled by masschemistryandplasma—surfaceinteractions. flowmeters.The total pressureof 1—10 mbar, measured

In the presentwork we focussedour attentionon the with a capacitancemanometer,was controlled by aion component of particle flux at the cathode. Ions throttlevalve with an accuracyof about 1%. Insulatingcontributeto film growth andinitiate surfaceprocesses Si(C, N) films weredepositedfrom a tetramethylsilane—such as sputtering,desorption,etching adatommigra- N2—H2—Ar mixture at a substrate temperatureoftion andimplantation[5]. 400°Cby plasma CVD using pulsed a.c. excitation

[6]. The chosenfrequencyof 20 kHz is sufficiently highto deposit thick insulating films. The liquid organo-metallic precursortetrarnethylsilane(TeMS, Si(CH3)4)

2. Experimentaldetails was evaporatedat room temperature(boiling temper-ature26°C).Owing to its highvapourpressure,the flow

The glow dischargewas sustainedin a parallel-plate rateof the gaseousTeMS wasadjustedby meansof aarrangement.The stainlesssteel electrodeswere sur- massflow controller.

0257—8972/93/$6.00 © 1993 — Elsevier Sequoia.All rights reserved

98 S. Peter et al. / Determinationof massand energydistribution of ions in glow discharges

powersupp’y ~I I dc ion analysis,the ion transfer optics, both orifices andU Lii ~H H H Ii ~ pulseddc the ion sourceweregrounded.

gas ~~n ii ii _i-’ pulsedac As has been reported elsewhere[7], the deposition

inlet ~~ p [1 [1 [1 ~ processhas also beeninvestigatedby optical emissionanode ~ ~ ~ spectroscopy.(powered) _ -

4, ~ plasma 3 Resultsanddiscussion

~ ;:r~’~r chambercathode 3 1 Energydistribution measurements(grounded)1 I~ i Themeasuredenergydistributionsshow a maximum

orifice 2 L,/” ~ 2r~dstage and anextendedhigherenergy tail. Thesefeatureswere300 i.im 1111<10 bar pumping predicted by many works taking into account both

orifice 1 i~l ~ ~ chargeexchangereactionsandelasticcollision processes1 mm Ill i t ~ in the sheath[8, 9].

ion II p~J~ \_.........~ pumping We confirmedthe energycalibrationof the two-stagetransfer i i -~ plasmamonitor by meansof a thermally generatedandoptics liNt <10 mbar

B electron electrostatically acceleratedbeamof sodiumions. Until1 I~I impact now, the sensitivity of our instrumentin dependenceon

ionization the ion energy has not been measured.Therefore,thesource resultsobtainedwill bediscussedin a qualitativemanner.

V~a Energydistribution measurementswereperformedinII ‘ N2—H2—Ar plasmas,typical for surfacemodification by

~... modified CMA plasmanitriding. A comparisonof the threeexcitation

eneg~ga~ser modesused in our experimentsis given in Fig. 2. Thed d.c. energyspectrumis almostsymmetricalwith respect

_____ _____ to the maximumenergy.As hasbeenmentionedin theprevioussection,using pulsed excitation the operating

QMS sxp 300H voltage can be increasedwithout changing the meanpb 1 ...300 arnu discharge current. This is equivalentto a further trans-

ition into the abnormalrange of the glow discharge.Whereas the energy at maximum intensity remains

Fig. 1. Schematicof the experimentalarrangement. unaffected,the ion energy distributions in pulseddis-chargesare characterizedby a distinct high energy tail.Evenin the caseof d.c. excitation(seeFig. 3) the energy

Mass and energy analysis of particles striking the of nearlyall the ionsexceedsthe sputteringthresholdofcathodewas performedusinga two-stagedifferentially materials(10—30eV for metals).pumpedplasmamonitor(V.G. Quadrupoles).Theorificeof the secondstage(orifice 2) wasmountedin the centre 3.2. Typesofionsarriving at thecathodeof the cathode.Figure 1 showsthesystemschematically. In a self-sustainingglow dischargeions are mainlyThe main part of the measuringinstrument is the formed in the negativeglow by inelasticelectroncolli-modified cylindrical mirror analyser(CMA), in series sions. This primary ion massdistribution is modified bywith a quadrupolemassspectrometer(QMS). The ions ion—molecule reactions in the cathodesheath. It wasenter the CMA with an energyE.0~.The E10~is trans- shownthat NH~(x = 1—4) speciesare formedin N2—H2formed into the CMA passenergy(Epass= 10—25eV) by discharges[6, 10]. Owing to their low ionization poten-applying a retardingor acceleratingpotential VCMA. The tial, NH~+ ions are easily formed in the glow. For thepassenergydependson the potentialdifferencebetween same reasonwe assumethat a loss of these ions bythe two cylindrical electrodesof the CMA. The ion chargeexchangein the cathodesheathis unlikely. Otherenergyspectrumcanbe measuredby scanningthe VCMA. kinds of ions shown in the ion massspectrumof anAlter passing the CMA, the ions are retardedto the N2—H2—Ar discharge (Fig. 4) are createdby heavy-optimal energy of the QMS (3—4 eV) by applying the particle collisions. The ArH + ion is more stable thanpole bias voltage V1,1,. The quadrupolemassspectrome- the Ar + ion. The reactionbetweenAr + andH2 formingter, SXP 300H,has a high sensitivity acrossthe whole ArH + and H is characterizedby a large cross-section,mass rangeup to 300 amu becauseof its prefilter and reaching 10—14 cm

2 at thermal energies [Fl].its large9 in rod system. In order to perform external Furthermore,as has alreadybeen observedby Szabo

S. Peteret al. / Determinationof massand energydistribution of ions in glow discharges 99

xia2 ni2

H~ 1.2 Iii

4.0

~3.0 ~L8

1.0 (a) E — •‘. 1L. operaringvd~ge

I.0 100.0 2~.a ~.r 400.0 s~.a a. i~.e 2~.0 ~.i 400.0 508.0E)(RC? eV EWRCY eV

sputtering thresholdof materials

H~ Fig. 3. High energytail of the distributionofAr~ions atd.c. plasmac2~5 excitation.Conditions:200 seemN

2 20 seemAr; 200°C;500 Pa;d.c.

(410 V, 0.5A); Ar~(40 amu).xio2 H~

0.5 “.. -~,... •1-~~ . (la) S2.5 H~ N~

2 +

EPt~CVeV 81.5 H3

~ia2 ~i.a

2.1 0.5 N~ AIH~

0.0 2 I I01.5 H~ 0 5 18 15 28 25 38 35 48 45

005$

01.0 Fig. 4. Spectrum of ions extracted from an N2—H2—Ar plasma.I. Conditions: 100 seemN2 100 seemH2 20 seemAr; 200°C;200 Pa;

20 kHz pulseda.c. (550V. 100 mA); ion energy100 eV.(c)

0.0 - I impact ionization. This ion is also presentin the ion0.0 108.0 288.0 ~.0 400.0 500.0 . . .

spectrumof Fig. 5(a). From the relative ion intensitieswe concludedthat the dominantlight fragments(C ~,

Fig. 2: Energydistribution of H~ions at different excitation modes. Si4 + C

2H4~,SiCH3’) are formed by a sequenceofConditions:(a) 100 seemH2 100 seemN2 20 seemAr; 200 Pa;200°C;d.c. (257V 100 mA), (b) 40 kHz pulsedd.c. (500 V, 100 mA) and (c) dissociationprocesses.20kHz pulseda.c. (500V, 100 mA). If nitrogenwasaddedto the dischargethe ion energy

at maximumintensity increasedto 95 eV andthe inten-sity of ions containing Si—C bonds decreased(see

and Wilhelmi [10] at the cathodeof N2—H2 plasmas Fig. 5(b)). Only a weak SiCH3+ (43 amu) signal couldthe N2H+ ion appearsat the expense of the N2 + be detected. Theresultsobtainedindicateahigherdegreeion peak. of fragmentation of the TeMS molecule in the presence

The plasmadeposition processwas investigated in the of nitrogen.case of Si(C, N) deposition from tetramethylsilane A nitrogenadditionto thedischargecausedthedepos-(TeMS, 88 amu). In TeMS—H2—Ar dischargesheavy ition rate to increaseby about three times. The filmfragmentsof the TeMS molecule were detected (see density grew simultaneouslyfrom 1.9—2.0 g cm~ toFig.5(a)). The SiCH3 + ion (43 amu) was found to be 2.2—2.3 g cm~ The analysisof theseinsulating,amor-one of the strongest peaksin the ion spectrum. At phous films by meansof electron or ion assistedtech-maximum intensity the ion energywas about 65 eV for niques(electronbeammicroanalysisandglow dischargeall of the ions. The measuredenergy distributions of optical emissionspectroscopy)wasnot satisfactory.Fordifferent kinds of ions were similar at energiesup to this reason, no results concerning the chemical film200 eV. The Si(CH3)3+ ion (73 amu) is the dominant compositionare availableat present.peakin the crackingpatternobtainedby 70 eV electron Up to now depositionparameters(flow ratesof van-

100 S. Peteret al. / Determinationof massand energydistribution of ions in glow discharges

xii3 Si~+C H~ without changingthe meandischargecurrentby using

245.0 pulsedplasmaexcitation.This furthertransitioninto theSicH; abnormalrangeof the glow dischargecausedanincrease

~4.0 c~ in ion energy. Even in the case of d.c. dischargestheo majority of ions were found to be capableof sputtering~3i depositedfilms.

CH~ The depositionprocessof Si(C, N) from a TeMS—22 HSiCH~

I + \ /2 ~ N2—H2—Ar plasma was investigated.The addition of

E HH ~ ~ L Si(CH~ S1(CH~ (a) dissociationof the TeMS molecule and increasedionnitrogen to the dischargecaused a higher degreeof

~I0.0U__T0 10 20 30 40 I energies,accompaniedby improved mechanical film

MASS properties.In order to understandthe variety of pro-

xie2 CH~+N~ cessesinvolved in PACVD and to optimize the depos-2.5 + ition process with respect to the mechanical film

properties,furtherin-situdiagnostics (massspectroscopy,~ 1+ N~4~+C2H5 optical emission spectroscopy,ion energy and mass

IH ~ analysis) are now in progress.

IIH~

I I ~ Acknowledgment~TI SiCH~ (b)3 This work was supportedby the Bundesministerium0.0 I I I I I I für Forschungund Technologie der Bundesrepublik0 10 20 30 40 58 60 70 80 98 DeutschlandunderContractNo. 13 N 5889.

MASSFig. 5. Spectraof ions extractedfroma depositionplasma.Conditions:(a) 200 seemH2 20 seemAr; 1 seemTeMS; 400°C;200 Pa; 20 kHzpulsedac. (550 V, 50 mA); ion energy 100eV and (b) 100 seemN2 References100 seem H2 20 seem Ar; I seem TeMS; 400°C;200 Pa; 20kHzpulsedac. (550V. 50 mA); ion energy100 eV. 1 Th. Lampe,St. EisenbergandG. Laudien,Hdrtereitechn.Mitteil.,

46 (1991) 5.

ous gases,substratetemperature,pressure,deposition 2 M. Hudis, 3. App!. Phys.,44 (1973) 1489.3 G.G.Tibbets, J. App!. Phys.,45 (1974) 5072.time, operatingvoltage and mean dischargecurrent) 4 F.H.M. Sanders,in E. Broszeit,W.D. Münz, H. Oeehsner,K.-T.

havebeenvaried in rather wide steps.Films deposited Rie and G.K. Wolf (eds.),Proc. 1st mt. Conf. on Plasma Surfaceon hardenedsteel substratesfrom nitrogen containing Engineering, DGM Informationsgesellsehaft-Verlag,Oberursel,

plasmasrevealedimproved wear resistanceand adhe- 1989, p. 163.sion, as wasdeterminedby scratchtests.Critical loads 5 J.M.E. Harper,J.J. Cuomo,R.J. GambinoandHR. Kaufmann,in

0. Auciello and R. Kelly (eds.),Ion BombardmentModification ofof up to 55 N were measured. Surfaces,Elsevier,Amsterdam,1984, p. 127.

6 F. Richter,G. Hecht,H. Kupfer,S. Peter,R. Pintaske,N.Schwarzer,F. Elstner,H. Giegengack,H.-J. Erler, B. Rau,H-i. Spie/l and K.

4. Conclusions Hock, Poster presentedat BMFT-Statusseminar“Dünnschicht-technologien‘92”, Fulda, 1992.

7 R. Pintaske,S. Peter,M. Beier, F. Richter andG. Hecht,Proc. 9thEnergyand massdistribution of ions arriving at the Symp. on ElementaryProcessesand Chemical Reactionsin Low

cathodeof a glow dischargewere studiedusing a two- Temperature Plasma, ~astd, 1992; Acta Physica Universitates

stagedifferentially pumpedplasmamonitor system.The Comenianae,acceptedfor publication.main featuresof measuredion energydistributionscould 8 J.Lju, G.L. HuppertandH.H.Sawin,3. App!. Phys.,68(1990)3916.

be explainedby theoreticallytaking into accountboth 9 P.W. May, D.Field and D.F. Klemperer, J. AppI. Phys., 71(1992) 3721.chargeexchangereactionsand elastic processesin the 10 A. Szaboand H. Wilhelmi, Hdrtereitechn.Mitteil., 39 (1984) 148.

cathodesheath.Three different excitation modeswere 11 RD. Levine and RD. Bernstein,Molekulare Reaktionsdynamik,investigated. The operatingvoltage can be increased Teubner,Stuttgart,1991, p. 72.