Surfaceand CoatingsTechnology, 68/69(1994)439—445 439

Ultrahighvacuumhigh resolution transmission electronmicroscopyofsputter-depositedMoS2 thin films

G. Jayarama,N. Doraiswamya, L.D. Marksa,M.R. Hilton1’

~DepartmentofMaterials Scienceand Engineering,NorthwesternUniversity, Evanston, IL60208,USAbMechanics and MaterialsTechnologyCenter,The AerospaceCorporation, El Segundo, CA90245, USA

Abstract

High resolution electronmicroscopyhasbeen usedto characterizethe structureof sputter-depositedMoS2 coatingsunder both

conventionalandultrahighvacuum(UHV) conditions.Asdeposited,thefilms have amixtureofshort-rangeorderedbasal-planeandedge-planeorientedgrainsnearthefilm substrateinterlace;structural changeswere characterizedin a UHY transmissionelectronmicroscopeas afunctionof two processingvariables:temperatureandAu deposition.Annealingin anoxygenenvironmentwasalsocarriedout to assesschemicalstability.During thermalannealingin UHV andin oxygen,substantial long-rangeorderingofthebasalislandsfollowed by grain growthwasobserved.Inhomogeneous oxidationresultingin the formationof MoO, in theinitial stagesfollowed by graingrowth, yieldingthefinal morphologyof a mixtureof MoO, crystallitesof 5—50nm size was seen onannealinginan oxidizing atmosphere.Au nucleationand growthon both thermally annealedandas-depositedfilms were seen to followtheVolmer—Webermode,i.e. three-dimensionalislands;theseislandswerealsoseen to be highlytextured.Also, in comparisonwithcarbonandSiO substrates,Au demonstratedhigherstability on MoS2 toelectronbeamfluxes,suggestinghigherbondingstrengthsto thesubstrate.Theseexperimentsdemonstratedtheparamountneed forUHY conditionsduringbothdepositionandcharacteriza-tion to avoiduncertaincontaminationartifacts.

1. Introduction incorporatingdopants(Au, Ni, SbO~,or polytetrafluo-roethylene)as co-sputteredspecies or asmultilayers

Theneedfor long-endurance thinsolid lubricantfilms [13—16]. Adjusting deposition conditions, i.e. highfor precisionmechanismsin spacecraftled to thedevel- powerdensities, lowpressures,and use ofion beamsoropmentof sputter-depositedMoS2 [1]. Thefirst genera- lasers, alsopromotesa densermicrostructure[17—20].tion of these films had a microstructure that resulted Detailedinvestigationsof the nanostructureof thesefrom competitivenucleation and growthbetweenbasal- materialsare incomplete.In particular, the morpho-plane (i.e. the (001) planes) and edge-planeoriented logical distribution of metal dopants in the newergrains (i.e. the (100) and the (110) planes) [2,3]. In co-sputteredand multilayer films has not been wellgeneral, puresputter-depositedMoS2 films have a characterizedand the relationshipof these nanostruc-porouscolumnar-plate morphologywith an edge-plane tures to tribological performanceand humidstoragepreferredorientation parallel to the substratesurface, stability is not well understood.There are also manyHowever,the columnarplatesare in the wrongorienta- issues concerningthe roleof contaminantsin both typestion for lubrication andtend to detachnear the film— of film; most sputter-depositedfilms preparedin highsubstrateinterfaceandreorient intolubricatingparticles vacuumsystems contain 5—15at.% oxygen (and/orearly in operation[1,4,5]. In addition, the edge-plane carbon). Some recent results[21,22] have highlightedorientationalso enhancesthe formationof MoO3 when this very dramatically; very pure thin films of MoS2MoS2is stored(or, worse,operated)in humidair [6—8]. grown underultrahigh vacuum(UHV) conditionshaveThis oxide has significantly lesserenduranceand a beenfound to yield friction coefficientsaboutan orderhigher friction coefficient than MoS2 [9—11]. ofmagnitudesmallerthan thoseobtainedwith conven-Experimentshave shown that basal-planeorientation tional high vacuumfilms.andlarge grainsize partially inhibit MoO3 formation in The availability of an UHY transmissionelectronhumidenvironments[6—11]. microscopewith a modularvacuumsystem designthat

The need to minimize early debris generation in allows interconnectionto depositionchambersandotherrolling-contact applicationsled to the developmentof a analyticalsystemsoffers exciting possibilitiesfor studiessecond generation of sputter-depositedMoS2 [12]. of film growth processesof many material systems.InThesefilms generally have densermorphologies and thispaperwereport thefirst UHY transmissionelectronmorebasal-planeorientationbecausethe growthof edge microscopy (TEM) study of sputter-depositedMoS2.facets is slowed orsuppressed;this is accomplishedby Theaim of the presentstudywas twofold: (1) to investi-

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440 G. Jayaramet al. / TEM of MoS,thin films

gatethe structural andchemical stabilityof MoS2during oxidizing atmospherewere executed in an attachedthermaltreatmentsin UHV andoxidizing environments chamber(sampletransferwas conductedunder UHYas a function of microstructure to gain insight into conditions). The static operatingpressureinside bothstorage and contaminationissues; (2) to assess the chamberswas approximately1 x 10—8 Pa (for a reviewdesirability of connectingan MoS2depositionsystemto of the instrument,see[23]).the UHY transmissionelectronmicroscope.

TheUHV transmissionelectronmicroscopewas usedto characterizestructuralchangesin MoS2 films (which 3. Resultsfor theseexperimentswerepreparedex situ) as afunctionof two depositionvariables:temperatureandAu depos- 3.1. As-depositedfilmsition (executedin situ), the latter providing a basis Prior to any heat treatment,a microstructural maptowards understandingthe structure of Au used in of theas-deposited thinfilm was obtainedusing UHVco-sputtering andmultilayer applications, high resolution electron microscopy (HREM). The

as-depositedfilm microstructurefor both AT and HTfilms, on bothcarbon andSiOsubstrates,wasdominated

2. Experimentalprocedures by basalislands.The hexagonalpatternof these basalislands can be clearly seenin Fig. 1 which shows a

Thin films (10—20nm) of MoS2 weredeposited,by r.f. typical HREM image of an AT film together withansputtering accordingto proceduresdescribedearlier [4] inset diffraction pattern taken from the same region.on substratesof amorphousevaporated carbon andSiO However, these domains were extremely short rangeon Cu grids; the latter were usedfor oxygen annealing ordered; in comparison,basalislands in HT films hadexperiments.Films were preparedat substratetemper- relativelylonger-range order,as shown by thearrowedaturesof 70°C(designatedAT for ambienttemperature) regionsin Fig. 2. The edgeisland morphologywas alsoand220°C(designatedHT for high temperature). seen in small areasof the film; their smallpercentage

Imaging was carried out in a Hitachi UHV-H9000 wascorroboratedby low intensityrings in thediffractionmicroscopeoperatedat 300 keY. Thermalannealingin patterns (DPs); thicker films had more edgeislands.UHV andsubsequentdepositionof Au on annealedAT Theseedge islandswere not straight, i.e. they had anand HTfilms werecarriedout insidethe UHY transmis- associatedcurvature.Diffraction andimagedatacollatedsion electron microscopecolumn itself. Au deposition from different samplesindicatedthat the planarspacingson as-depositedAT films and heattreatmentunderan deviatedfrom ideal single-crystalvalues incertaincases.

Fig. 1. The typical microstructureof an as-sputteredAT MoS2 thin film, i.e. the short-range-orderedhexagonalpatternof the basalislandsshownin an HREM image, togetherwith theinset diffraction patternshowing thecorresponding(100) and(110) planarspacingrings.

G. Jayaramet al. / TEM of MoS, thin films 441

Fig. 2. A typical high resolution electronmicrographof anas-sputteredHT MoS2 thin film; basalislands with relativelylonger-rangeorder(cf.

Fig. 1) arearrowed.

A contractionin the spacingof the basalislandfringes stability to such heat treatments.The HT films whenandan increasein the spacingsof the edgeislandfringes heatedto 550 °Cshowedonly the initial nucleationofwere observedin theseinstances;these deviationsare very small domains,arrowedin Fig. 4 (similar to theattributedto the formation of MoS2- ~ which arises behaviorof AT films on heatingto 400°C),suggestingfrom oxygen,during the sampledeposition,occupying that furtherheattreatmentwasrequiredbeforedomainssubstitutional sites in the MoS2 lattice [24] (these of the size seen intheAT films heatedto 550°Ccouldstructuralresultsare consistentwith earlierTEM studies be observed.The small size of the domainsin the HTof similar films by other investigators[1,25]; see also filmcould also beinferredfrom the DPswhereno new[3] for reference onearlierTEM studiesof MoS2. polycrystalline rings could be seen.

3.2. Thermal treatmentin an ultrahigh vacuum 3.3. Thermal treatmentin an oxygenatmosphereenvironment When the AT film wasfurther annealedat 550°Cfor

TheAT and HTfilms weresubjectedto heatingcycles 5 mm in a partial pressureof oxygen ofapproximatelyin the 100—550°Crange(andheld at eachtemperature 1 x iO~Pa,no change inmorphologywas observed,i.e.for about 10 mm) in incrementsof roughly 50°C.Up to particles showing(103) and (105) fringes still existed in400°C, there were no changesin film morphologyin the images. However, after anannealat the same temper-eitherthe images or theDPs.At temperaturesjust above ature for 10 mm in a partial pressureof oxygen of400°C,nucleationof small crystallites was seen in the 1 x 10 ‘~Pa,a few new particleswere evident coexistingHREM images ofAT films. At 550°C,new spottypoly- with MoS2. HREM images of theseparticles enabledcrystallinerings appearedin the DPs; the corresponding their positive identification as MoO3 inhomogeneousHREM images showedparticleswith (103)- and (105)- oxidation had occurredand the absenceof any ring intype fringesas seen in Fig.3. The (100) and(110) fringes theDPs correspondingto the oxide indicateda smallstill existed in the background(particles showingthese population.fringes were alsoseen)with a smaller fraction of edge The identification of MoO3 was further confirmedislands, suggestingthat the film morphologyconsisted of during a 2 h time sequenceunder the electron beama mixture of randompolycrystalsthat hadundergonea which showedthat theseparticlesunderwentradiationloss of texturedue to heating.The intensity of the rings damage (MoO3as themaximumvalent transitionmetalin the DPssuggestedthat particles showing(103) fringes oxideundergoesradiation damagewhile MoO2 doesfar exceededthoseshowingthe (105) type. not [26]). Fig. 5 shows theearly stages ofthis process

In contrastwith AT films, HT films showedgreater for suchan oxide particle;as theparticle amorphized,it

442 G. Jayaramet al. / TEM of MoS2 thinfilms

Fig. 3. Particleswhich show the(103) and (105) fringesandwhich dominatethemorphologyof anAT film annealedto 500—550°Cinan UHVenvironment.The relativeintensitiesof the correspondingrings in theinsetdiffraction patternareproportional to their populationdensity.

Fig. 4. The microstructureof an HT film annealedto 500—550°Cin anUHV environmentidentical with that in Fig. 2 with one very subtlechange,i.e. the arrowed darkercontrastregionswhich representnucleationof very small domains similarto thoseseen in AT films annealedto 400°C.

reducedto Mo metal whichlatercrystallized underthe occurred,resultingin particlesof sizes ranging from 5electronbeam.Electronbeamexposureduring this time to 50 nm as seen inFig. 6. While the imageprovidesnoperiod also causedthe edgeislands of MoS2 to amor- information on the particles’orientation or chemistry,phize;however, the basalislandsstill remainedcrystal- the insetDP revealedgrosschangesin the latter. Ringsline during the sameperiodof observation, correspondingto the basal islands had disappeared;

As the film was heatedto higher temperatures,i.e. instead,spotty rings correspondingto the (300), (310),between700 and800°Cunderanoxygenpartialpressure (410) and (420) spacingsof MoO, were seen,i.e. theof 1 x 10 “ Pa, significant coarsening of the film film had undergonecompleteoxidation.

G. Jayaramet a!. / TEM of MoS,thin films 443

Fig. 5. Inhornogeneous oxidationwhich resultsin MoO3 particlesand which occurson AT films annealedto 500—550°Cin 02 at 1 x l0~ Pa.

The amorphizedfringesat the edgesof theparticledemonstratethe earlystagesof radiationdamage.

— IFig. 6. Film coarseningwhich yields 5—50 nm particlesof MoO3 and which occurson annealingAT films to 700—800°Cin 02 at 1 x iO~~Pa.Spottypolycrystallinerings in the insetdiffraction pattern revealtheir orientation;the absenceof theMoS2 rings suggestscomplete oxidation.

3.4. Audepositionon MoS2films and (220) rings of Aubeganto appearon subsequentAu in the submonolayerto a few(4—5)-monolayer deposition;these were, however, very weak inintensity

(ML) regimewas depositedonto bothas-depositedAT in comparisonwith the (111) ring, suggestingthat Aufilms and post-annealedAT and HTfilms. Irrespective growth on MoS2 was highly textured. The HREMof the thicknessregime, Au growth was in theform of images alsodemonstratedthat there seemedto be nothree-dimensional (3D)islandson all types of film (at preferentialsite, i.e. basalor edge,for the nucleationandhigher thicknesses, Auislands simply grew in size). growth of Au on eitherthe AT or the HT films.Fig. 7 is an HREM image of the very early stagesof A very surprisingresult wasthat in comparison withgrowth of Au (~1 ML or less) on an as-depositedAT other substrates,e.g. carbon or SiO, Au had a muchfilm with the insetDP showingthe (111) ring of Au.On higher stability when depositedon MoS2. Even withfurther deposition(up to 2 ML), only anincreasein the high beam flux values, i.e. two ordersof magnitudeintensityof the (111) ring of Au wasobserved.The (200) higherthan thoseusedin earlier studiesof Au on carbon

444 G. Jayaramet al. / TEM of MoS2 thinfilms

Fig. 7. Early stageof Au nucleationand growthon an as-depositedAT film; the growth modeis Volmer—Weber.The (111)ring in the insetdiffraction patternrevealsthe highly texturednatureof this process.

or Au on SiO [27], Au particles on MoS2 did not rangeorder of the AT films. In the timeperiods studiedfluctuate under the beam, suggestinga very strong (less than 15 mm) at 550°C,an oxygen atmosphereatparticle—substrateinteraction. 1 x iO~Pa (10—6 Torr) was requiredto cause MoO3

to form. This agreeswell with other oxidationstudieswhich report slightly lower temperaturesalbeit with

4. Discussion higher oxygenpressures,e.g. in anin-situ TEM studyofMoS2 flakes, Baker et al. [29] have observedMoO,

From the characterizationdataof films as deposited formationat 527°Cin oxygenat 400 Pa while Kim andand after thermal annealingin UHY and in oxygen, a Lieber [30], using atomic force microscopy haveconsistentdescriptionof these films emerges.The sput- reported MoO, formation on single-crystalMoS2 afterter-depositedfilms are nanophasematerialswhich,near atmosphericoxygenexposureat 480°Cfor 10 mm.the substrate,consistprimarily of basalorientedregions The occurrenceof thermallyinducedrecrystallizationwith edge oriented islands dispersedthroughout. The complicates studiesof the kinetics of MoS2oxidationbasalorientedregionshavenanometer-scale grains,sub- that usehighertemperaturesto acceleratereactions(andgrains or domains. In a thermodynamicsense,these obtain activation energies). In pure oxygen, withoutfilms are more defective or are farther from a single- watervapor, in the conditionsstudied,recrystallizationcrystalreferencematerialthan MoS2crystalsor particles occurs before or with oxidation. Of greaterinterest isused in bondedlubricants.Henceonewould expect the thestudyof MoS2 oxidationin humid air environments,depositedfilms to be metastablerelative to molybdenite whichcanoccurat roomtemperature. These experimentscrystals. Such (albeit small)molybdenite crystals are shouldbeconductedwithin time—temperaturecombina-used in some bonded lubricant films. In applications tions that avoid recrystallization.Our experiencewithwherethe designerhasa choice ofusingeither sputter- the UHV transmissionelectron microscopeindicatesdepositedor bondedMoS2,the sputter-depositedfilms that the instrumentprovidesa usefulpathfindercapabil-will have less oxidation resistancein humid storage ity in kinetics experimentsto find such transformationowing to the nanocrystallinestructureof the MoS2and boundarieswith limited samplesquickly. Once known,the lack of protective encapsulationthat some binders theseboundarieshelp to definean experimental matrixprovide [28]. consistingof several samples atdifferentannealingcondi-

Upon heating in vacuum,recrystallizationoccurs as tions. After thermal processing,thesesamplescan beevidenced by the 3D particles with (103) and (105) characterizedby conventionalTEM.fringes.The datashow that the AT films are less stable UHY TEM canclearly characterizeMoS2 films afterthan the HT films; this is consistentwith the shorter- in-situ heatingor metal deposition,two known process-

G. Jayaramet al. / TEM of MoS2 thinfilms 445

ing variables.The apparentstability of Au evaporated Referencesonto MoS2 is very surprising. These resultswill haveto be compared with the structure of MoS2 films [1] T. Spalvins,ASLE Trans.,14(1971)267;17(1973)1;Thin Solid

co-sputtered withAu or having Au multilayers. One Films, 96 (1982) 17.

drawbackto our presentinvestigationis that the Au [2] MR. Hilton and PD. Fleischauer,New Material Approaches toTribology: Theoryand Applications, Materials Research Society

was depositedonto MoS2 films exposedto air; the Symp. Proc.,Vol. 140, Material, ResearchSociety, Pittsburgh,

natureandrole of adsorbedcontaminantsis unknown. PA, 1989, p. 227.Even in case of theas-depositedsamples, changesin [3] MR. Hilton andPD. Fleischauer,J. Mater. Res., 5(2)(1990)406.

morphologyare observedwithin the sametype of films [4] PD. Fleischauer andR. Bauer, Tribol. Trans., 31(2) (1988)

(AT or HT) and are not surprising since water vapor 239.[5] MR. Hilton, R. Bauer and PD. Fleischauer,Thin Solid Films,

which influences themorphologyis the major residual 188 (1990)216.

gas in the deposition system (base pressure,about [6] PD. Fleischauerand LU. Tolentino, Proc. 3rd Int. Conf. on

1 x iO~Pa); theseresultsdemonstratethat it is impera- Solid Lubrication, in Spec. Pub!. ASLE SP-14, 1984, p. 223,

tive to attach an UHV sputter depositionchamberto (Societyof TribologistsandLubrication Engineers).

the UHY transmissionelectronmicroscopeto character- [7] PD. Fleischauer,ASLETrans.,27(1) (1983)82.

ize the structureof very purefilms grown under well- [8] PD. Fleischauerand TB. Stewart,Effectsof crystallite orienta-tion on the oxidation of MoS, thin films, Tech. Rep.,controlledconditions. TR-0089A(5945-03)-02,9 September 1985 (The Aerospace

Corporation).

5. Conclusions [9] E.W. Roberts, Proc. Inst. Mech. Eng. Conf.in Tribology—Friction, Lubrication, and Wear, F(fty Years On, Vol. 1, London,Institution of MechanicalEngineers,1987, p. 503.

(1) Thesputter-depositedMoS2 filmsinvestigatedare [10] TB. StewartandPD. Fleischauer,Inorg. Chem.,21(1982)2426.

nanophasematerialswhich undergosignificantrecrystal- [11] L.E. PopeandJ.K.G.Panitz,Surf. Coat. Technol.,36(1988)341.

lization during thermal annealingin UHY at 550°C. [12] MR. Hilton and PD. Fleischauer,Surf. Coat. Technol.,54—55(1992)435.Films depositedat 70°Cwere less stablethermallythan [13] B.C. Stupp,Thin Solid Films,84(1981)257.

films depositedat 220°C. [14] B.C. Stupp, Proc. 3rd Int. Conf. on Solid Lubrication, in Spec.

(2) Oxidation occurredduring annealingat 550°Cin Pub!. ASLE SP-14, 1984, p. 217 (Society of Tribologists and

an oxygenatmosphereat 1 x iO~Pa.Futurestudiesof Lubrication Engineers).

oxidation, particularly in humid air environments,that [15] MR. Hilton, R. Bauer, S.V. Didziulis, MT. Dugger,J. KeemandJ. Scholhamer,Surf. Coat. Technol.,53(1992)13.attempt to use elevated temperaturesto accelerate [16] P. Niederhauser,HE. Hintermann and M. Maillat, Thin Solid

reactions, will have to stay below recrystallization Films, 108 (1983)209.

temperatures. [17] C. Muller, C. Menoud,M. Maillat and HE. Hintermann, Surf.

(3) UHY TEM canalso beusedto studyfilm depos- Coat. Technol.,36 (1988) 351.

ition issues.Au was successfullydepositedonto MoS2 [18] A. Aubert, J.Ph.Nabot,J. Etnoult andPh. Renoux,Surf. Coat.Technol.,41(1990)127.

in situ. The Au particleshadunusuallystrong stability [19] RN. Bolster, IL. Singer, J.C. Wegand,S. Fayeulle and C.R.on basalorientedMoS2 relativeto Au on SiO or Au on Gossett,Surf. Coat. Technol.,46 (1991) 207.

carbon. Nucleationwas seen onboth basaland edge- [20] S.D. Walck, MS. Donley, J.S. Zabinski and V.J. Dyhouse,

orientedMoS2. J. Mater.Res.,9(1)(1994)236,andearlierreferencescitedtherein.

(4) Finally, it would be desirableto characterizeboth [21] Th. Le Mogne, Y. Chevallier, Ch. Donnet andJ.M. Martin,Proc. 5th Eur. SpaceMechanismsand Tribology Conf., in Publ.

the heatingandthe depositioncharacteristicsof sputter- ESA SP-334, 1992, p. 317 (European Space Agency)ESTEC,

deposited MoS2films preparedunder stringent UHV Noordwijk.

conditionsto eliminateuncertain contaminationartifacts. [22] C. Donnet.Tb. Le Mogne andJ.M. Martin, Surf. Coat. Technol.,

62(1993)406.[23] J.E. Bonevichand L.D. Marks, Microsc: Key Res. Tool,22(1)

Acknowledgments (1992)95.[24] JR. Lince, MR. Hilton and AS. Bommannavar,Surf. Coat.

ork was sponsored by the Air Force Material Technol.,43—44 (1990)640.Command, Spaceand Missile Systems Center, under [25] J. MoserandF. Levy, J. Mater. Res.,8(1) (1993)206, andearlier

referencescited therein.ContractFO4701-93-C-0094as part of The Aerospace [26] S.R. Singhand L.D. Marks, unpublished,1990.

Corporation Mission—Oriented Investigation and [27] N. Doraiswamyand L.D. Marks, Z. Phys. D,26 (1993)S70.

ExperimentationProgramand the US Air Force Office [28] EL. McMurtrey, Lubrication Handbookfor the Space Industry,

of Scientific Research on Grant F49620-93-1-0208 Part A: Solid Lubricants, In Tech. Memo.NASA TM-86556,

AFOSR. The authorswish to thank Reinhold Bauer 1985 (Marshall SpaceFlight Center,National AeronauticsandSpaceAdministration).(AerospaceCorporation)for preparingthe MoS2 films [29] R.T.K. Baker, J.J.Chludzinski,Jr.andRD. Sherwood,J. Mater.

and R. Plassand R. Passeri(NorthwesternUniversity) Sci., 22 (1987)3831.

fordesigningtheheaterstageandpreparingtheSiO grids. [30] Y. Kim and CM. Lieber,Science,257 (1992)297.