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Comprehensive Summaries of Uppsala Dissertationsfrom the Faculty of Science and Technology 665
_____________________________ _____________________________
CVD and ALD of Group IV- andV-Oxides for Dielectric Applications
BY
KATARINA FORSGREN
ACTA UNIVERSITATIS UPSALIENSISUPPSALA 2001
Dissertationfor the Degree of Doctor of Philosophyin Inorganic Chemistrypresentedat UppsalaUniversityin 2001
ABSTRACT
Forsgren,K. 2001. CVD andALD of Group IV- andV-Oxidesfor DielectricApplications.Acta Univ. Ups.,ComprehensiveSummariesof UppsalaDisserta-tions from the Faculty of Scienceand Technology 665. 56 pp. Uppsala. ISBN91-554-5143-8.
Due to the constantlydecreasingdimensionsof electronicdevices,the conven-tionaldielectricmaterialin transistorsandcapacitors,SiO2, hasto bereplacedbyamaterialwith higherdielectricconstant.Someof themostpromisingcandidatesaretantalumoxide,Ta2O5, zirconiumoxide,ZrO2 andhafniumoxide,HfO2.
Thisthesisdescribesnew chemicalvapourdeposition(CVD) andatomiclayerdeposition(ALD) processesfor depositionof Ta2O5, ZrO2 andHfO2 usingthemetaliodidesasstartingmaterials.The layer-by-layergrowth in ALD wasalsostudiedin real time with a quartzcrystalmicrobalance(QCM) to examinetheprocesscharacteristicsandto find suitableparametersfor film deposition.
All theprocessespresentedhereproducedhigh-purityfilms at low depositiontemperatures.It wasalsofound that films depositedon Pt substratesgenerallycrystalliseat lower temperature,or with lower thickness,than on silicon andsingle-crystallineoxidesubstrates.Filmsgrown onMgO(001)andα-Al2O3(001)substrateswere strongly texturedor epitaxial. For example,monoclinic HfO2
depositedonMgO(001)wereepitaxialfor depositiontemperaturesof 400-500C
in ALD and 500-600C in CVD. Electrical characterisationshowed that the
crystallinity of thefilms hada strongeffect on thedielectricconstant,exceptincasesof very thin films, wherethe dielectric constantwasmoredependentonlayerthickness.
Key words: CVD, ALD, Dielectricconstant,Tantalumoxide,Ta2O5, Zirconiumoxide,ZrO2, Hafniumoxide,HfO2, QCM.
Katarina Forsgren,Departmentof MaterialsChemistry, TheAngstrom Labora-tory, Box538,SE-75121Uppsala,Sweden
c
KatarinaForsgren2001
ISSN1104-232XISBN 91-554-5143-8
Printedin Swedenby EklundshofsGrafiskaAB, Uppsala2001
Till Mor ochFar
Contents
Preface VII
1 Intr oduction 1
2 Dielectric Oxides 32.1 Tantalumoxide . . . . . . . . . . . . . . . . . . . . . . . . . . 42.2 Zirconiumoxide. . . . . . . . . . . . . . . . . . . . . . . . . . 52.3 Hafniumoxide . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3 DepositionTechniques 73.1 CVD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73.2 ALD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83.3 Precursorsfor CVD andALD . . . . . . . . . . . . . . . . . . 10
3.3.1 Previouswork . . . . . . . . . . . . . . . . . . . . . . 10
4 Experimental 134.1 CVD experiments. . . . . . . . . . . . . . . . . . . . . . . . . 134.2 ALD experiments. . . . . . . . . . . . . . . . . . . . . . . . . 144.3 Film characterisation. . . . . . . . . . . . . . . . . . . . . . . 15
5 Depositionof Tantalum Oxide 175.1 CVD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175.2 In situmonitoring . . . . . . . . . . . . . . . . . . . . . . . . . 195.3 ALD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
6 Depositionof Zir conium Oxide 256.1 CVD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256.2 In situmonitoring . . . . . . . . . . . . . . . . . . . . . . . . . 276.3 ALD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
6.3.1 Temperatureseries . . . . . . . . . . . . . . . . . . . . 296.3.2 Thicknessseries . . . . . . . . . . . . . . . . . . . . . 31
VI Contents
7 Depositionof Hafnium Oxide 347.1 CVD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 347.2 In situmonitoring . . . . . . . . . . . . . . . . . . . . . . . . . 377.3 ALD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
8 Depositionof Mixed TiO2-Ta2O5 Films 428.1 CVD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
9 Concluding remarks 45
Acknowledgements 47
Bibliography 49
Preface
This thesiscomprisesthepresentsummaryandthefollowing papers,which arereferredto in thesummaryby theirRomannumerals.
I Halide chemicalvapour depositionof Ta2O5.KatarinaForsgrenandAndersHarstaThin SolidFilms, 343-344(1999)111.
II Atomic layer deposition of tantalum oxide thin films from iodideprecursor.KaupoKukli, JaanAarik, AleksAidla, KatarinaForsgren,JonasSund-qvist, AndersHarsta,TeetUustare,HugoMandar, andAlma-AstaKi-islerChem.Mater., 13 (2001)122.
III Characterisation of Ta2O5 films prepared by ALCVD .KatarinaForsgren,JonasSundqvist,AndersHarsta,KaupoKukli, JaanAarik, andAleks AidlaIn ChemicalVapor DepositionFifteenth International CVD Confer-ence. Editors M. D. AllendorfandM. L. Hitchman(TheElectrochem.Soc.PenningtonN. J., 2000)p. 645.
IV CVD of ZrO 2 using ZrI 4 asmetal precursor.KatarinaForsgrenandAndersHarstaJ. Phys.IV France, 9 (1999)Pr8-487.
V Atomic layer depositionof zirconium oxide from zirconium tetra-iodide, water and hydrogenperoxide.KaupoKukli, KatarinaForsgren,JaanAarik, TeetUustare,AleksAidla,Antti Niskanen,Mikko Ritala,MarkkuLeskela, andAndersHarstaJ. Cryst.Growth,231(2001)262.
VI Iodide-basedALD of ZrO 2: Aspectsof phasestability and dielec-tric properties.KatarinaForsgren,JorgenWestlinder, JunLu, JorgenOlsson,andAn-dersHarsta
VIII Preface
Submittedto Chem.Vap. Deposition.
VII Depositionof HfO2 thin films in HfI 4-basedprocesses.KatarinaForsgren,JaanAarik, Aleks Aidla, JorgenWestlinder, JorgenOlsson,andAndersHarstaSubmittedto J. Electrochem.Soc.
VIII In situ preparation of Ti-containing Ta2O5-films by halide CVD.KatarinaForsgrenandAndersHarstaIn ChemicalvapourDepositionFifteenthInternationalCVD Confer-ence. Editors M. D. AllendorfandM. L. Hitchman(TheElectrochem.Soc.PenningtonN. J., 2000)p. 652.
1 Intr oduction
Themicroelectronicsindustryis constantlystriving towardsincreasedfunction-ality andperformanceof integratedcircuits,thatis, to producefaster, smallerandcheaperelectronicdevices.This is achievedby increasingthedensityof cellsonachip throughreductionof theminimumfeaturesizeandtheproductionof morecomplex circuits. Whenthe featuresize is reduced,a lower operatingvoltagemustbe appliedto maintaina constantelectricalfield, andthis in turn requiresthecapacitanceto increasein orderfor thedevice to functionproperly. Sincethecapacitanceis inverselyproportionalto the thicknessof the isolatinglayer, thisthicknessis reducedaccordingly. Theconventionalcapacitormaterialin randomaccessmemoriesandfield effect transistorsis silicondioxide,SiO2, andthesuc-cessfulscalingof devicedimensionsover thepast30yearshasleadto thecurrentdielectric thicknessof about2 nm. However, leakagecurrentsarebecomingaseriousproblemat thispoint,andfurtherscalingwouldverysoonleadto adirecttunnelingcurrentthroughthe oxide. In addition,an oxide layer this thin is aninsufficient barrieragainstdopantdiffusion. By replacingtheSiO2 with a mate-rial of higherdielectricconstant,therequiredcapacitancecanbeachievedwith athicker layer, therebyreducingtheleakagecurrents.Thescalingtowardshighercell densitiescanthuscontinue.
Theamorphousthermallygrown SiO2 currentlyin use,offersa stable,high-quality interfacewith silicon, in combinationwith superiorelectricalisolationproperties,and will thus be very hard to replace. The main requirementsforfuturegatedielectricsare[1]:
i) technologycompatible,
ii) homogeneousstructureandefficientbarrieragainstpenetrationof con-taminantspecies,
iii) largebandgap,high dielectricstrengthandlow loss,
iv) largebreakdown strength,
v) negligible chargeleakageandlow interfacetrapdensities,
vi) suitablethresholdvoltage,and
2 Intr oduction
vii) stableelectricalcharacteristics.
It shouldbepointedout thattherearesomedistinctionsbetweentherequire-mentsfor memoryand transistorapplications. Memory capacitorsrequireex-tremelylow leakagecurrentsandvery high capacitancedensityfor charge stor-age,but thequality of the interfaceis not ascritical. Sincetheelectricalfield isnot requiredto penetratebelow thebottomelectrode,theelectrodecanconsistofmetalor nitrided poly-silicon. For a field effect transistor, on theotherhand,itis essentialthat theelectricalfield penetratesinto theSi channelunderneaththegateoxideto modulatethecarriertransport.It is thereforenecessarythatthesub-strateis Si andthattheinterfacebetweenthedielectricandthechannelis of veryhighquality. Transistorshave lowerdemandson leakagecurrentthancapacitors,althoughahigh capacitanceis still needed[2].
A numberof materialshave someof the qualitiesnecessaryfor replacingSiO2, but very few areconsideredpromisingin all areas.Thematerialsthathavereceived the mostattentionareTa2O5, SrTiO3, TiO2, Al2O3, ZrO2 andHfO2.Amongthese,Ta2O5, SrTiO3 andTiO2 have thehighestdielectricconstants,be-tween25 and80 in thin films, but arenotstablein contactwith silicon. Reactionat the interfacebetweenfilm andsubstrateduring the depositionexperimentorsubsequentheattreatment,mayleadto formationof siliconoxideorsilicides,thataredetrimentalto theelectricalproperties.Ta2O5 andSrTiO3 arestill consideredfor memoryapplications,but TiO2 is generallycharacterisedby a high leakagecurrent,andcannotbeusedin its pureform. Furthermore,theprocessintegrationof the ternarySrTiO3 presentsa greaterchallengethanthatof thebinaryTa2O5
andTiO2. Al2O3, ZrO2 andHfO2 arethermodynamicallystablein contactwithsilicon [3], andif aninterfacial reactiondoestake place,theresultis likely to besilicates,thatexhibit intermediatedielectricconstantsandmayevenbebeneficialfor theleakagecharacteristics.Thedielectricconstantof Al2O3 is only 8-10,andsubstitutingSiO2 with Al2O3 wouldthusonly bea temporarysolution.ZrO2 andHfO2, on theotherhand,have dielectricconstantsof around20 andoffer morelong-termsolutions.
The electricalpropertiesof a materialaredeterminedby a complicatedin-teractionof many factors,mostof which will not bediscussedhere.This thesisdescribesnew chemicalvapourdeposition(CVD) andatomic layer deposition(ALD) processesfor depositionof thehigh dielectricconstantmaterialsTa2O5,ZrO2, HfO2 andmixedTiO2-Ta2O5 oxide.
2 Dielectric Oxides
Therearedifferentcontributions to the permittivity of a materialdependingonthefrequency of theappliedelectricalfield asshown schematicallyin Fig.2.1[4].Thecurrentfrequency rangefor transistor(CMOS)operationis indicatedin thefigure,andit canbeseenthatthemaincontributionsin this region areelectronicandionic polarizations.
Theelectroniccontribution comesfrom interactionof the“electroncloud” ofanion with theexternalelectronicfield,andionswith alargeradiuscangenerallybepolarizedto a greaterextent.Theelectroniccontribution tendsto increasethepermittivity for oxidesof metalswith high atomicnumbers.Theionic contribu-tion is causedby thedisplacementof certainions in theunit cell in responsetotheappliedelectricalfield. Thepolarizabilitycanvary betweenthepolymorphicformsof a material,probablydueto differentdensityor displacementpossibili-ties. An amorphousbodydoesnot exhibit ashigh a permittivity asa crystallineonesincethedisplacementof theions is not uniform over any extendedvolumeof material.It hasalsobeenfoundthattheadditionof asecondmetalto acertainoxidecanenhancethedielectricconstantconsiderably[5, 6,7]. Theexactreasonfor this effect is not known, however. This is a simplified description,but it is
ε
ε ε
Interfacial and space charge
Orientational, Dipolar
IonicElectronic
Radio Infrared Ultraviolet light
Figure2.1: Thefrequency dependenceof thepermittivity [4].
4 Dielectric Oxides
clearthatboththenatureof theelementsandtheir arrangementin thesolid stateareof importancefor thepermittivity.
The dielectricconstantincreasesuponcrystallisation,but at the sametime,grain boundariesform. In thegrain boundaries,theconcentrationof impuritiesanddefectsis higher, andtheelectricalconductivity is higherthanin the“bulk”of thegrains.Thesmallerthegrains,thelargerthepartof thefilm thatconsistsofgrainboundaries.This is normallythereasontheleakagecurrentincreaseswhenamaterialgoesfrom theamorphousto thecrystallinestate.Thecloseconnectionto the electricalpropertiesis a strongreasonto maintaincarefulcontrol of thephasecontentof thefilms.
2.1 Tantalum oxide
In theTa-Ophasediagram,Ta2O5 appearsasa line phase,i.e. it hasno extendedstability region, and shouldhave a well-definedstoichiometry. Orthorhombic[8], monoclinic [9], hexagonal[10] and tetragonal[11] phasesof Ta2O5 havebeenidentified,but despitemucheffort, their respective stability areasarenotfully known. Thereseemsto be generalagreementthat the hexagonalandor-thorhombicphasesarestableat low temperaturesandthemonoclinicandtetrag-onalphasesat high temperatures.Theorthorhombicβ-Ta2O5 andthehexagonalδ-Ta2O5 structuresarecloselyrelated:
a (β-Ta2O5) = 3 a (δ-Ta2O5) andb β-Ta2O5) = a (δ-Ta2O5)
The δ-phaseis often claimedto be understoichiometric,indicating that Ta2O5
could possiblyhave a stability region. However, in thin film deposition,the β-phaseof Ta2O5 is themostcommonlyobserved(Fig. 2.2).
Ta2O5 is bestknown for its highdielectricconstant,but it alsohasotherinter-estingpropertieslikepiezoelectricity, protonicconductivity, highrefractive indexandcorrosionresistance.Thin films of Ta2O5 canbeusedin electroluminescentdevices[12], biologicalandchemicalsensors[13, 14], corrosionresistantcoat-ings [15], andanti-reflective coatings[16, 17]. Thin films of Ta2O5 have beenproducedby laserablation[18], sputtering[19], evaporation[20], sol-gel [21]andALD [22], but themostcommonlyusedtechniqueis CVD [23, 24, 25].
2.2Zir conium oxide 5
O
Ta
Figure 2.2: Thestructureof β Ta2O5.
2.2 Zir conium oxide
Zirconium oxide, ZrO2, is known to have four different crystal modifications:monoclinic,tetragonal,cubicandorthorhombic.Dueto hysteresisin thetransi-tionsanda stronginfluenceof thepreparationtechniqueaswell asthemeasure-mentconditions,the reportson their stability regionsdiffer widely. Therefore,the following dataare not to be taken as universal truths. Under atmosphericpressure,themonocliniclow-temperaturephasetransformsinto tetragonalZrO2
around1145C [26], and the tetragonalphaseis stableup to 2370
C whereit
transformsto cubicZrO2 [27]. Thehigh-temperaturephasescanalsobeobtainedat low temperatureswhenstabilisedby otheroxides,for instanceY2O3 or CeO2
[28, 29], or in materialswith smallgrainsizes,oxygendeficiency or impurities[30]. An orthorhombicphaseexists at elevatedpressure,anda triple point hasbeenreportedfor themonoclinic,tetragonalandorthorhombicphasesat 600
C
and23kbar[31]. The homogeneityrangefor ZrO2 extendsdown to 63 atomicpercentoxygen[32]. TheZrO2 phasesareunusualin that thestructurebecomesmoresymmetriconheating.Thetruecubicfluoritephase( seeFig. 2.3) is stableabove 2450
C, but the monoclinicandtetragonalpolymorphscanbe described
asdistortedfluorite structures,with the low temperaturephasebeing the mostdistorted[33].
ZrO2 is a materialwith well-known physicalpropertiesthat rendersindus-trial applicationsin many fields. For instance,zirconiumoxidehaslow thermalconductivity, high refractive index anda high dielectricconstantandis usedinthermalbarriercoatings,high temperatureopticalfilters [34] andoxygensensors[35]. Someof thetechniquesthathave beenusedto producethin films of ZrO2
6 Dielectric Oxides
O
Zr
Figure 2.3: Thecubic(fluorite) structureof ZrO2.
aresol-gel[36], evaporation[37], sputtering[38], ALD [39] andCVD [30].
2.3 Hafnium oxide
Hafniumoxide,HfO2, is very similar to ZrO2 in chemicalandphysicalproper-ties,andtheoxidesarecompletelysolublein all proportionsin thepseudo-binarysystem[40]. TheHfO2 andZrO2 phasesareisostructural,but therearesomedif-ferencesin atomic positionsand transitiontemperatures,and the hysteresisisconsiderablysmallerfor HfO2. For HfO2, themonoclinicto tetragonaltransfor-mationtakesplacebetween1620and1650
C [40], andthetetragonalto cubicat
2700C [41]. Thetriple-pointrelationis analogousto thatfoundin theZrO2 sys-
tem,but locatedat approximately1200C and15kbar[31]. HfO2 hasthesame
homogeneityrangeasZrO2, down to 63 atomicpercentoxygen,but thedensityis higherdueto theheavier Hf atom.
As canbeexpected,HfO2 hasthesameexcellentmaterialpropertiesasZrO2:extremechemicaland thermalstability, good electricalproperties,and a highrefractive index, andmoreor lessthesameapplicationsresult:protectivecoatings[42], opticalcoatings[43] andoxygensensors[44]. Thin films of HfO2 havebeenproducedby laserablation[45] ion beamsputtering[46], sol-gel[47], ALD [48]andCVD [49].
3 DepositionTechniques
For mostthin film applications,thedepositionmethodis a crucial factorin de-terminingthequality andpropertiesof thelayer. Theprocessmustalsobecom-patiblewith otherfabricationsteps,becost-efficient andsuitablefor large-scaleproduction.CVD methodshavebeenappliedin variousfabricationprocessesforquitesometimewith greatsuccess,andin recenttime,thepulsedCVD technique,ALD, hasenteredtheindustrialscene.UsingCVD techniques,a largevarietyofmaterialscanbe depositedover a wide pressureandtemperaturerange.Dense,well-adherentfilms canbeformedwith excellentuniformity over largeareasandon complex shapes.With ALD, thecompositionandthicknessof the layercanbecontrolleddown to theatomiclevel. Thedifferenteffectsthatcontrolthefilmgrowth in CVD andALD will bediscussedfurtherin thefollowing paragraphs.
3.1 CVD
ThenameChemicalVapourDepositionimpliestheformationof a solidmaterialfrom thegaseousstateby wayof achemicalreaction[50]. Thisreactionnormallytakesplaceon, or in the vicinity of, a surfaceandis activatedby somekind ofenergy. CVD processescanbeclassifiedaccordingto i) their activation energy,for instancethermallyactivated,plasma-enhanced,laser-inducedandelectron-beamassisted,ii) thenatureof thestartingmaterial:metalorganicor halide,iii)the processpressure:atmospheric,low-pressure,high-vacuumetc. In addition,conventional,thermal-activation CVD chambersareusuallydenotedhot-wall orcold-wall reactors,dependingon which part is heated.The depositionprocesscanbedescribedastakingplacethroughthefollowing steps(Fig. 3.1):
1. transportof thereactantsto thevicinity of thesubstratesurface
2. diffusionof thereactantsto thesubstratesurface
3. reactantadsorptionon thesubstratesurface
8 DepositionTechniques
3
2
18
7
64
5
Substrate
Figure 3.1: TheCVD processdividedinto steps.
4. surfacechemicalreaction
5. surfacemigrationandlatticeincorporation
6. reactionproductdesorption
7. diffusionof reactionproductsaway from thesubstratesurface
8. transportof reactionproductsoutsidethedepositionzone.
In general,thevariablesaffecting thedepositionrateandfilm propertiesarethe natureof the reactantsandtheir purity, the amountof energy supplied,thesubstratetemperature,theratio of reactants,thegasflow rates,thesystempres-sure,thegeometryof thedepositionchamberandthesubstratesurfaceprepara-tion. CVD processesgenerallyhave high depositionratesandaresuitableforlarge-scaleproductionat low cost.Thechoiceof startingmaterialsis notveryre-stricted,althoughsomemayrequirehighdepositiontemperatures.In unfortunatecases,wheretheexperimentalparametershave not beenoptimised,reactioncantake placein thegasmixtureabove thesubstrate,so-calledhomogeneousnucle-ation.This leadsto formationof powderthat,whenit fallsdown on thesubstrateor thegrowing film, cancausepooradhesionof thefilm.
3.2 ALD
Throughoutthis thesis,the nameAtomic Layer Deposition,ALD, will be usedto describethetechniquethatwasoriginally calledAtomic LayerEpitaxy, ALE
3.2ALD 9
Reactant A After first purge
Reactant B A
fter second purge
Figure3.2: Processingstepsin thegrowth of abinarycompound.
[51]. The term “epitaxy” wasabandonedsincethe depositedfilms mostoftenarenotepitaxial.Anotherdenomination,whichemphazisestherelationshipwithconventionalCVD, is Atomic LayerChemicalVapourDeposition,ALCVD. TheALD techniqueis basedon sequentialadmissionof reactantsinto the reactionchamber. Onereactantatatimeis allowedtoadsorbto thesurfaceof thesubstrateor thegrowing film. After eachreactantpulse,thereactoris purgedwith inertgasto remove superfluousmaterialandto make surethe reactantsareseparatedintimeandspace.Fig. 3.2 illustratesthegrowth of abinarycompoundfrom binarysourcematerials.Thecentralfeaturesfor processcontrolareto achieve surfacesaturationin eachreactantpulse,andthatno morethanonemonolayerremainsafterthepurge,sincethesequencingalonedoesnot resultin asurfacecontrolleddeposition. Saturationalsomakes the thicknessproportionalto the numberofgrowth cyclesinsteadof to thereactantflux.
Theminimisationof electroniccircuits requiresanatomiclevel accuracy inthin film depositionthat is inherentin theALD technique.Theseparatepulsingof thesourcematerialsandlayerby layergrowth alsomakesALD anexcellenttool for producingcomplex andlayeredcoatings.Otheradvantagesarethat theseparationof the reactantseliminatesthe risk for gas-phasereactions,andthatlower depositiontemperaturescanbe usedin ALD thanin CVD. However, thechoiceof precursorsfor ALD is generallymorelimited, thanin CVD, seesec-tion 3.3,theprocessesareinherentlyslow, andlarge-scaleproductionis not veryeasyto accomplish.On the otherhand,with atomiclevel control of thickness,uniformity andcomposition,theuseof ALD processesin industrialfabricationcanstill becost-efficient. Thereactionsin ALD havetraditionallybeenthermallyactivated,but otherenergy sourceslike plasmasandlasersarenow comingintouse.
10 DepositionTechniques
3.3 Precursorsfor CVD and ALD
Unlike physicalfilm depositiontechniqueslike sputteringandevaporation,thechemicalprocessesCVD andALD arevery dependenton the propertiesof thesourcematerials. Chemicalandphysicalpropertiesdeterminewhich materialswill be possibleto usefor deposition,aswell asthe propertiesof the resultingfilms. Sincethefilm growth in CVD andALD processesis basedon somewhatdifferentprinciples,the demandson suitablestartingmaterialsareslightly dif-ferent. Both techniquescanutilize materialsthat aregases,liquids or solidsatroom temperature,as long as they are stableenoughto be evaporatedor sub-limated,andhave high enoughvapourpressures.Gaseouscompoundsare themosteasyto handle,andcanalsobe suppliedat a controllablerateby simplyusingamassflow controller. Theevaporationrateof liquid andsolidsources,onthe otherhand,is determinedby temperature,surfaceareaandcarriergasflux.In all classesof materials,it is of coursepreferableto avoid theonesthatarepoi-sonous,explosive or flammablefor thesake of personalsafety, andalsoto avoidcorrosive or etchingmediafor thesake of theequipment.
In ALD, it is essentialthat theprecursorsadsorbto thesurface,sinceother-wisenogrowth canbeachieved.Uniform,self-limiting growth demandsuniformsaturation,for which it is importantthat theprecursorsarestableat theprocesstemperature.Decompositiononthesurfaceor incompleteexchangereactionscancauselossof saturationandincorporationof impuritiesin thefilm. Theneedforthermalstability usuallylimits theALD processesto a narrower working rangethanwith CVD. Furthermore,theactivationenergy for surfacereactionshouldbelow, andsincethereactantsareseparatedin timeandspace,therisk for gas-phasereactionsis eliminated,whichfacilitatestheuseof morereactivesourcematerialsthanin CVD. With morereactivesourcematerials,lowerdepositiontemperaturescanbeused.
EachCVD precursordoesnot needto adsorbto thesurfacein orderfor filmgrowth to occur, andit is not necessarythat it is stableon thesurface.However,the precursorsshouldnot be too reactive, sincea certainreactionthresholdisneededto avoid reactionsin the gasphaseandfor the depositionto occuruni-formly over thesurface.
3.3.1 Previous work
The metalsourcesthat have beenusedin depositionof Ta2O5, ZrO2 andHfO2
by CVD andALD aremetalhalidesandmetalorganics,andin somecases,ni-trate compounds.Among these,both liquid and solid materialsare available.
3.3Precursorsfor CVD and ALD 11
Material Precursorsin CVD Precursorsin ALD
Ta2O5 TaCl5 [49, 53, 25] TaCl5 [22, 54, 55]Ta(OC2H5)5 [56, 23, 57] Ta(OC2H5)5 [58, 59]
Ta[N[CH3]2]5 [60]t(BuN)-Ta(NEt2)3 [61]
ZrO2 ZrCl4 [62, 63, 64] ZrCl4 [65, 54, 59]Zr(NO3)4 [66, 67] Zr[OC(CH3)3]4 [68]
Zr(acac)4 [56, 34, 30]Zr(NEt2)4 [69]
Zr[OC(CH3)3]4 [70]HfO2 HfCl4 [71] HfCl4 [72, 73, 54]
Hf(acac)4 [49]Hf(thd)4 [49]
Hf(NO3)4 [67]
Table3.1: Precursorsutilisedin previousworks.
Table3.1 is a compilationof precursorsthat have beenusedin CVD andALDof therespective oxides.It appearsthatfor Ta2O5, Ta(OC2H5)5 is themostcom-mon precursorin both CVD and ALD. For ZrO2 andHfO2, the chloridesarealmosttheonly sourcesusedin ALD, whereasa numberof metalorganicshavebeenusedin CVD. Thechloridesaretheoutstandinglymostpopularhalidepre-cursors,andthereareonly a few exampleswhereotherhalideshave beenused.With chloride sources,there is a risk for etchingreactionswith the substrate,thegrowing film or thereactor, andin CVD ratherhigh depositiontemperaturesareusuallyrequired.Themetalorganicprecursorsseldometchthesubstrate,andthegrowth temperaturesarelow. However, a commonproblemfor chlorideandmetalorganicstartingmaterialsis thatchlorideor carbonimpurities,respectively,often areincorporatedinto the films. For example,Ta2O5 films containing2%Cl [52], andHfO2 films containing5% Cl [39], have beengrown by ALD usingtherespective chloride,andZrO2 films containing15%C [34] have beengrownby CVD usingzirconiumacetylacetonate.Nitrateprecursorshave beenusedinCVD of ZrO2 andHfO2 at low temperatures,apparentlyleaving no impuritiesinthefilms, but they maynotbecompletelysafeto handle.
Metal iodideshave earlierproven to besuitableprecursorsfor depositionofthin films of for exampleBi2Sr2CaCu2O8 x [74], Bi4Ti3O12 [75] andTiO2 [76]by CVD andTiO2 by ALD [77, 78]. The CVD processesemployed O2 asthe
12 DepositionTechniques
only oxygensource,andin ALD, O2 [78] or H2O2 [77] wasused.ThegroupIV-andV-metaliodidesareall solidsat roomtemperature,but the thermalstabilityis goodandthe vapourpressurehigh enoughto give a reasonableevaporationrate[79, 80]. Accordingto literature,TaI5 [81] andZrI4 [49] reactwith oxygenalreadyat100
C to form their respectiveoxides.Informationaboutthereactivity
of HfI4 is scarce,but it is usually said to have similar propertiesto TiI4 andZrI4 [82]. Basedon the said reactivities and resultsfrom previous studies,itshouldbe possibleto depositof Ta2O5, ZrO2 andHfO2 usingiodidesourcesatlow temperatures.It alsopointsto the possibility of usingO2 assingleoxygensource,at leastin CVD, therebyminimisingthenumberof elementsinvolved. Inmostchlorideprocesses,H2O mustbe introducedto remove thechlorinein theform of HCl.
4 Experimental
4.1 CVD experiments
Thefilms weredepositedin ahorizontalhot-wall CVD reactorconsistingof twoconcentricquartztubes(Fig. 4.1). Thereactoris heatedby a four-zonefurnacethatallows carefuladjustmentof thetemperatureprofile. Thesolid metalsourceis evaporatedfrom anopenboatin the inner tubeanda flow of inert gasis usedfor transportingthevapourinto thedepositionzone.Theoxygensourceis sup-pliedby theoutertubedirectly to thedepositionzone,wheretheinnertubeends.Theflow of reactantscanbecontrolledby theevaporationtemperatureandcar-rier gasflow. All gasflows aremonitoredby massflow controllersanda throttlevalve is usedfor maintaininga constantpressureof 10 torr in thechamber. Ar-gon (99.9999%)wasusedascarriergaswith a flow of 150 sccm,andoxygen(99.998%)wassuppliedasthesingleoxygenprecursorwith a rateof 175sccmandwasdiluted with an additional100 sccmof argon. The linear gasflow ve-locity was105cm s 1 andthe air leak rateinto thesystem(1 10 6 Pa m3 s 1)correspondsto anair contaminationlevel of lessthan1 ppm.
O
Ar2
Ar
to pump
Figure4.1: Schematicillustrationof theCVD reactor.
14 Experimental
Metal source Tevap Treactor Substrates Paper(purity) (
C) (
C) no.
Si(100)TaI5 (99%) 275 300-800 Pt/Ti/SiO2/Si, I
MgO (001)Si(100)
ZrI4 (99.5%) 200 400-700Pt/Ti/SiO2/Si
IV
Poly-SiHfI4 (99%) 300 300-700
MgO (001)VII
Ti-Ta-OTaI5 (99%) 265-275TiI4 (99%) 135-145
500,600 Poly-Si VIII
Table4.1: Processparametersfor theCVD experiments.
4.2 ALD experiments
Theexperimentswereperformedin ahot-wall flow-typeALD reactorthatcanbefitted with aquartzcrystalmicrobalance(QCM) masssensorfor in situmonitor-ing. Thereactorconsistsof anouterstainlesssteeltube,linedwith aquartztube,andan inner quartztubethat connectsthe precursorevaporationzonewith thedepositionzone(Fig. 4.2). Thegasesarefedthroughvalvescontrolledby micro-processorsthatallow theflows to beswitchedon andoff in lessthan0.1s. Themetalsourcewasevaporatedfrom a silica crucibleandcarriedto thesubstratesby aflow of inertgas.Theevaporationrateof themetalsourcewascontrolledbythechoiceof evaporationtemperature.An aqueoussolutionof H2O2 (30%H2O2)wasusedasoxygenprecursorfor depositionof Ta2O5 andZrO2, anddeionisedwaterfor depositionof HfO2. The liquids werekept in anexternalcontainerat20
C, andthepartialpressureof H2O andH2O2 wasregulatedby aneedlevalve.
Nitrogen(99.999%)wasusedasbothcarrierandpurgegas,andwith a total re-actor pressureof 250 pascal,the linear flow ratewas 5 m/s. The film growthcharacteristicswerestudiedin real time by QCM monitoringwith thesusceptorplacedin thereactionzoneinsteadof thesubstrateholder. TheQCM consistsofa quartzcrystalconnectedto anelectricalcircuit anda read-outunit. Thecrystalhasa certainvibrationfrequency, andwhengasesadsorbto its surfaceandit be-comesheavier, thevibrationfrequency is reduced.Thereductionin frequency isrecordedasa gainin mass.Analogously, desorptionleadsto increasedvibration
4.3Film characterisation 15
frequency, which is recordedasa lossof mass.
N2+ Reactants
N2
N2
Exhaust
Mass sensorSubstrate
T
hermocoupleSubstrate holder
HeaterReactor tubes
Figure 4.2: Schematicillustrationof theALD reactor.
Metal source Tevap Treactor Substrates Paper(purity) (
C) (
C) no.
Si(100)TaI5 (99%) 245 250-400 Pt/Ti/SiO2/Si II,III
MgO (001)Si(100)
ZrI4 (99.5%) 240 250-500 Pt/Ti/SiO2/Si, Poly- V,VISi, α-Al2O3(001)Poly-Si,Si(100)
HfI4 (99%) 205 225-500MgO (001)
VII
Table4.2: Processparametersfor theALD experiments.
4.3 Film characterisation
Thephasecompositionof thefilms wasexaminedwith X-ray diffraction(XRD),anddependingon thecharacterof thefilm, differentanalysingmodeswereused.For thin films of randomorientation,usingalow incidenceangle,so-calledGraz-ing Incidence,isusuallysuitable.It makestheanalysismoresurfacesensitiveand
16 Experimental
helpsraisingthepeakintensitiesandavoiding overlapwith substratepeaks.Forthicker films, standardθ-2θ scansaremoreappropriate.Also, stronglytexturedor epitaxialfilms mustbeanalysedin θ-2θ mode,sinceatomicplanesthatarepar-allel to thesubstratecannotbeobservedin GrazingIncidence.In caseof stronglytexturedfilms, RockingCurve (ω-scan)analysiscanbeperformedto investigatethefilm quality. In RockingCurve, theBraggangleis keptconstantwhile vary-ing the incidenceangle,andthe full width at half maximum(FWHM) valueoftheresultingpeak,givesanindicationof thefilm quality. A smallFWHM valueindicatesthe possibility of epitaxy, which canbe confirmedby a ϕ-scan. A ϕ-scanis doneby recordingtheintensityfrom aplanenon-parallelwith thesurface,while rotatingthesamplearoundits normal,whereϕ denotesthe rotationalan-gle. Comparingϕ-scansperformedfor both the film and the substraterevealsthein-planerelationshipbetweenthem.In somecases,Ramanspectroscopy wasusedasa complimentto the phaseanalysisby XRD, sinceRamanhashighersensitivity for short-rangeorder.
Thepurity andcompositionof thefilms wereanalysedby X-ray photoelec-tron spectroscopy (XPS)andX-ray fluorescencespectrometry(XRFS).In XPS,the analysiswas performedafter removing the surfacecontaminationand thetopmostlayersof film by sputteringwith Ar ions, and in somecases,depthprofiling wasperformed.Surfacemorphologieswerestudiedby scanningelec-tron microscopy (SEM) andatomicforce microscopy (AFM). Film thicknessesweredeterminedin differentwaysdependingonthethicknessrangeof thefilms:Large film thicknessesweredeterminedby surfaceprofilometryover a stepinthe film or by measuringthe cross-sectionof the film in SEM. EllipsometryorX-ray reflection(XRR) wasusedfor thinnerfilms. For theelectricalcharacter-isation,capacitorswerefabricatedby depositingmetalliccontactson top of thefilms. Leakagecurrentsin thefilms weremeasuredasa functionof voltage.Ca-pacitancewasmeasuredasa functionof voltageor frequency, andthedielectricconstantsof thefilms werecalculatedfrom
εr = (ε0 A C) / d
where εr = relative permittivityε0 = permittivity of vacuumA = areaof contactC = capacitanced = film thickness
Somefilms werestudiedby transmissionelectronmicroscopy (TEM) to eval-uatethequality of theinterfacebetweenfilm andsubstrate.
5 Depositionof Tantalum Oxide
5.1 CVD
The orthorhombicβ-Ta2O5 [8] phasewasobtainedwithout post-depositionan-nealing in the temperaturerange300-800
C. The films were texture free on
Si(100) substrates,but showed preferentialgrowth in the c-axis direction onMgO(001) substrates.X-ray diffractogramsfor films depositedat 600
C can
beseenin Fig. 5.1.
20 30 40 50 60
MgO
(001)
(112)
(112)
(111)(110)
(310)
(002)
(001)
Inte
nsity
(a.
u.)
2
θ ( )o
Figure 5.1: X-ray diffractogramsfor Ta2O5 films depositedat 600C.
No iodine could be detectedin the films for any of the depositiontempera-turesby XRFS(thedetectionlimit for iodine is about0.01%).Furthermore,thefilms werecontinuousandhada very smoothsurfaceascanbeseenin theAFMmicrographin Fig. 5.2 for afilm grown on Si(100)at 600
C.
Thedepositionratewasfound to bestronglydependenton the reactortem-perature.Theoxidethicknessincreasedwith 1 µm/h at 600
C, but loweringthe
18 Depositionof Tantalum Oxide
1µm
2µm
1µm
0µm2µm0µm
Figure 5.2: AFM micrographof a Ta2O5 film depositedon Si(100)at600
C.
reactortemperatureto 500C reducedthe growth rate to about0.5 µm/h. At
300C, thegrowth ratewasfoundto bevery low, about0.015µm/h,andthiswas
thereasonfor notattemptinganevenlower depositiontemperature.At theotherextreme,800
C, the depositionratewasagainsubstantiallyreduced,probably
dueto thermaldecompositionof TaI5 beforereachingthedepositionzone.FromtheSEM cross-sectionof a film depositedat 600
C shown in Fig. 5.3, it canbe
seenthattheoxidehasgrown with adense,columnarstructureandformsasharpinterfacewith theSi(100)surface.
Electricalcharacterisationof a film depositedat 600C indicatedthe value
of thedielectricconstantto be25.8with no measurabletemperaturedependencebetween24and90
C. However, a relatively highdcconductivity wassimultane-
ouslyobserved,which wasbelievedto becausedby hydrogenincorporatedintothefilm duringdepositionor from wateruptake uponstoragein air. A morede-tailedstudyof theelectricalbehaviour of afilm depositedat600
C film hasbeen
publishedelsewhere[83].In most other studies,the electricalcharacteristicsare given for annealed
films. However, Burte and Rausch[84] performedmeasurementsboth beforeandafterthermaltreatmentof theamorphousfilms.They foundthatthedielectricconstantincreasedfrom 24.5 to 32 while the leakagecurrentdecreasedconsid-
5.2 In situ monitoring 19
Figure 5.3: SEM cross-sectionof a Ta2O5 film depositedon Si(100)at600
C for 5h.
erably. Therearenumerousexamplesof investigationsreportingthatannealinghelpsto improvetheelectricalcharacteristics,but for mostindustrialapplications,high processtemperaturesshouldpreferentiallybeavoided.
It is interestingto notethatpure,crystallineβ-Ta2O5 films canbeproducedalreadyat 300
C, usingTaI5 asprecursorin CVD. In studiesemploying other
tantalumprecursors,the lowestdepositiontemperaturewherecrystallineTa2O5
hasbeenobtainedby thermallyactivatedCVD hasbeenreportedto be 625C
[85]. In most cases,however, somesort of thermaltreatmentabove 700C is
requiredfor crystallisation.
5.2 In situ monitoring
The depositionkinetics were investigatedby recordingthe masssignalduringrelatively long depositioncycles(Fig. 5.4). During the TaI5 pulse,t1, themassincreasedsharplyandstabilisedat a certainlevel, ∆m1. Thestabilisationof theQCM signalindicatesthat theprecursoradsorptionis saturative andtheprocesstherebyself-limiting. During the H2O-H2O2 pulse,a massdecrease,denoted∆m2, was recorded,andafter completionof the growth cycle, the massof thedepositedlayerwasobservedas∆m0. For temperaturesin therange240-325
C,
themasssensorsignalremainedstableaftersaturationhadbeenreached,but athighertemperatures,themassdecreasedcontinuouslyduringextendedTaI5 ex-posures(Fig. 5.4).Thelossof masswasenhancedby increasingthetemperature,andconcurrently, ∆m0 decreased.At sufficiently long pulsetimesandhigh sub-stratetemperatures,∆m0 becamenegative, indicatingthatsomeof thepreviously
20 Depositionof Tantalum Oxide
0 1 0 30 4
0 50 60 0 8 9 "! 0 #
TR = 363 oC TR = 295 oC
- m0
m0
m2
mp1
m1
H2O2 off
H2O2 on
TaI5 off
T
aI5 on
QC
M S
IGN
AL
(a.u
.)
TIME (s)
Figure 5.4: Themasssensorsignalasa functionof time duringa longTa2O5 depositioncycle.
depositedmaterialwasremovedby etchingreactionswith TaI5. Theetchingre-actionwas,however, notedto beslower thanin theTaCl5-H2O precursorsystem[86].
At sourcetemperatures(Ts) above 245C, theTaI5 adsorptionstabilisedand
remainedfairly insensitive to furtherchangesin Ts. During thesubsequentstud-ies,Ts waskeptatabout245
C. Theamountof tantalumoxidedepositedin each
cycle, ∆m0, increasedrapidly with the TaI5 pulselength,t1, between0.5 and1s (Fig. 5.5). Upon further increasesin t1, the ∆m0 valuecontinuedto increasebut at a considerablylower rate. This non-saturative increasein massmight beattributed to thermaldecompositionof the metal precursor. The samekind ofbehaviour hasbeenobserved, for instance,in the TiI4-H2O2 precursorsystemin the temperaturerange200-400
C [87], and in the Ta(OC2H5)5-H2O system
at temperaturesabove 300C [58]. During thefirst purge period,t2, a consider-
ablemassdecreasewasobserved (Fig. 5.4), the magnitudeof which increasedwith temperature.This effect wasattributedto thereleaseof I2, formedby pre-cursordecomposition,which wasalsoconfirmedby the fact that ∆m0 wasnotsignificantlyaffectedby variationsin t2. Therefore,the purging processcouldnot includedesorptionof Ta-containingsurfacespecies.
At thesametime, ∆m0 only hada weakcorrelationwith thegrowth temper-atureandwasactuallydecreasingwith increasingtemperature.This indicatedthat the contribution of the etchingeffects to the growth ratewasmoresignifi-
5.2 In situ monitoring 21
0 10
5
10
15
2$
0
25
30
35
t1-2-2-2 s 1-2-t3-2 s 1-2-0.5-t4 s
TR = 295 oC
TS = 243 oC
TIME (s)
MA
SS
INC
RE
ME
NT
(a.u
./cyc
le)
2 3 4
Figure 5.5: Dependenceof thegrowth rate,∆m0, on the lengthof theTaI5 pulse,t1, H2O2 exposuretime, t3, andsecondpurge time, t4. Thefirst purgeperiod,t2, waskeptconstantat 2 s.
cantthantheinfluenceof thermaldecompositionof theprecursorduringt1. Thegrowth ratewasnot noticeablyaffectedby the lengthof the H2O-H2O2 pulse,t3, but it wasaffectedby thevapourpressureof the liquid. For this reason,theH2O-H2O2 dosewaskeptrelatively high in thesubsequentexperimentsto ensurerapidcompletionof theexchangereactions.
Assumingthat theALD-grown oxidesurfaceis terminatedwith OH-groupsaftertheendof eachwaterpulse,thesubsequentTaI5 adsorbsto thehydroxylatedsurfacethroughanexchangereaction.Calculatingthemassexchangeratio fromQCM datarecordedatdifferenttemperaturesgives∆m0 ∆m1 valuesrangingfrom0.333to 0.345,without clearcorrelationwith growth temperature.Thesevaluescorrespondquitewell to asituationwhereTaI5 reactswith oneOH-groupduringt1, accordingto
%Ta-OH+ TaI5(g) & %
Ta-O-TaI4 + HI(g)
For thedepositionof samplefilms, theTaI5 exposuretime wassetto 2 s inorderto achieve a fairly high degreeof saturation,while minimizing theeffectsof etchingandprecursordecomposition.Although the growth ratewas ratherindependentof the purging time, both the first, t2, andsecondpurge, t4, werechosento be aslong as2 s to ensurereliableseparationof theprecursors.Theoxygenprecursorpulsewasalsosetto 2 s.
22 Depositionof Tantalum Oxide
250 300 350 400
0,5
1,0
1,5
2,0
2,5
3,0
3,5 Si(100) MgO(100) Pt/Ti/SiO ')(+*-, .0/213154
Gro
wth
Rat
e (Å
/ cy
cle)
Deposition Temperature ( C)o
Figure 5.6: Ta2O5 growth rateasa function of temperatureandsub-strate.
5.3 ALD
It wasconcludedthatthegrowth ratewasinfluencedbothby reactortemperatureand substrate(Fig. 5.6). For Si(100) and MgO(100) substratesthe depositionratedecreasedwith increasingtemperaturewhich canbeexplainedby a certaindegreeof etchingof the growing film by the oncomingTaI5 [88]. This trendagreeswith theresultsfrom theQCM studies.For thePt substrate,thethicknesswasthe sameat the lower temperatures,but startingat 350
C, the growth rate
increaseddrasticallywith the temperature.At 300C, the depositionrate was
approximately0.8A percycle,independentof thesubstrate,but for Ptit increasedto 3 A per cycle at 400
C. XPS analysisshowed that the depositedfilms were
iodine-freefor all reactortemperaturesandall substrates.Thesubstratealsohadastronginfluenceonthecrystallisationtemperatureof
thefilms. Thefilms grown on Si(100)substrateswerefoundto beamorphousinthewholetemperatureinterval 250-400
C.FilmsonMgO(100)substratesstarted
to crystalliseat 350C, andat 400
C strongly[001]-texturedTa2O5 hadformed
(Fig. 5.7). For Pt substrates,therewereno signsof crystallisationof the filmsat 325
C andbelow, but above this temperature,randomlyorientedTa2O5 had
formed. The strongestXRD peakscanbe assignedto either the hexagonalδ-phase[10] or the orthorhombicβ-Ta2O5 [8], but while thepeaksat d-valuesof
5.3ALD 23
400 o6C
350 o6C
MgO(001)
20 40
60 80 100
2θ ( )o
Inte
nsity
(a.
u.) (0
01)
(110
)
(002
)(0
20)
(310
) (003
)
(004
)
Figure 5.7: X-ray diffractogramsfor Ta2O5 depositedon MgO(001)substrates(pleasenotethelogarithmicintensityscale).
1.82A and1.80A bothcanbeindexedasorthorhombic,only the1.82A peakfitswith thehexagonalphase.Thus,theTa2O5 phaseis identifiedastheorthorhombicβ-phase.
Surfacestudiesby SEM andAFM revealedthat the amorphousfilms wereextremelysmooth. Rmsvaluesof 0.5 nm weremeasuredfor films on Si(100)andMgO(100)substrates,and1.7nmfor films onPt. Thecrystallinephasegrewaslargegrainsthatincreasedthesurfaceroughnessconsiderably, anddependingon the substrate,different shapeswere observed. For Si(100) at 400
C, large
outgrowths werefoundembeddedin theamorphousmatrix with randomdistri-bution over thesurface.Theshapeof theseoutgrowths indicatedthat they werecrystalline,but nocrystallinephasecouldbedetectedby XRD. The[001]-textureof theTa2O5 on MgO(100)at 400
C causedgrainsof hexagonalshapeto grow
out of thefilm (Fig. 5.8). The randomlyorientedTa2O5 depositedon Pt at 350and400
C hadadifferentappearancewhichcanbeseenin theSEMmicrograph
displayedin Fig. 5.8. Theincreasedsurfacetopographyof thefilms grown onPtleadsto increasedsurfaceareafor adsorption,which seemsto be thereasonfortheenhancedgrowth rateon Ptabove 350
C.
Electricalcharacterisationwasperformedfor filmsdepositedonPtsubstrates.Thecapacitancemeasurementfailedfor the250
C film, but it wasfoundthatthe
samplesof amorphousfilms depositedat300and325C haddielectricconstants
of approximately26,whereasthecrystallisedTa2O5 films at350and400C both
24 Depositionof Tantalum Oxide
Mag = 100kX1µm Mag = 100kX 1µm
Figure5.8: SEMmicrographsof Ta2O5 grown at400C onPtsubstrate
(left) andMgO(001)(right).
gave thevalue66. Theseareveryhighdielectricconstantsfor pure,as-depositedTa2O5, and other ALCVD studies[59] have reportedvaluesof 25 for weaklycrystallisedfilms. It is known that thecrystallisationenhancesthe permittivity,andin addition,with thePtsubstrate,no interfacialoxidecanform, loweringthetotalpermittivity. However, it cannotberuledout thatinterfacialchargetrappingaffectsthemeasurements[89].
6 Depositionof Zir conium Oxide
6.1 CVD
From the X-ray diffraction studies,it wasconcludedthat all films depositedonSi(100)werecrystallineas-depositedandconsistedof themonoclinicZrO2 phase[90]. Thefilms werestronglytexturedwith preferencefor the002,020and200reflections,but the 020 reflectionwas usually the strongest. Fig. 6.1 shows atypicalX-ray diffractogramwheretheverystrong020reflectionindicatesthatthefilm hasastrong[020] texture.However, theω-scanfull width athalf maximum(FWHM) valuefor the020reflectionwasashighas15
, ruling outthepossibility
of epitaxialgrowth.
001
031
-111
002
020
Inte
nsity
(a.
u.)
20 30 40 50 60
2θ ( )o
Figure 6.1: X-ray diffractogramfor a ZrO2 film depositedon Si(100)at 500
C.
According to XRFS analysis,the films were iodine-free,even for the low-est depositiontemperatures.The depositiontemperaturewas found to have a
26 Depositionof Zir conium Oxide
10µm
5µm
0µm5µm 10µm0µm
Figure6.2: AFM micrographof ZrO2 depositedon Si(100)at500C.
stronginfluenceon thegrowth ratewith a maximumof 0.3µm/h at 500C. The
reductionin growth rateat highertemperaturescannotbeexplainedby reactionkinetics. Onepossibleexplanationis that the depletionof the gasmixture in-creaseswith temperature,causinga steeperthicknessgradientanddisplacementof the actualdepositionzone. It is alsopossiblethat iodine-containingspeciesetchthegrowing oxide to someextent,aneffect that is likely to be larger at el-evatedtemperatures.Anotherpoint that hasto be consideredis that ZrI4 hasaboiling point of approximately600
C, andsomeauthorsclaim thatthemolecule
is dissociatedat650C [79]. If this is thecase,partof theZrI4 shoulddecompose
or reactbeforethe depositionzoneis reached,therebyreducingthe depositionratefor thehighesttemperatures.
SurfacemorphologystudiesusingAFM showedthatthedepositiontempera-turehadno significanteffect on thesurfaceroughnessor grainsizeof thefilms.Continuousfilms with smoothsurfacesweregrown atall temperatures,andfromthemicrographin Fig. 6.2 theaveragegrainsizecanbeestimatedto 0.3µm.
A dielectric constantof 18 was calculatedfrom the capacitancemeasuredat 1 Mhz for a film depositedon Pt(2000A)/Ti(200A)/SiO2(800A)/Si(100) sub-strateat 500
C. Measurementof the electricalresistanceof the samefilm gave
a conductancevalueof 145 µΩ 1. Thesevaluesarewithin the rangeof what
6.2 In situ monitoring 27
canbe expectedfor thin films of ZrO2 without any post-depositiontreatments.However, this processhasother featuresthat areworth specialnotice. First ofall, by usingZrI4 asmetalsource,films of pureZrO2 canbe depositedalreadyat 400
C, whereasthe typical depositiontemperaturefor thechlorideprocessis
around900C [62]. It is alsointerestingto notethatall reflectionsin XRD belong
to themonoclinicphase,independentlyof film thicknessanddepositiontemper-ature.In many studiesusingmetalorganicprecursors[69, 91], films depositedinthe range400-600
C containboth monoclinicandtetragonalZrO2. Generally,
thetetragonalphasehasbeenfoundto befavouredby thin films.
6.2 In situ monitoring
QCM studiesrevealedthatZrI4 hadtobeevaporatedattemperaturesabove235C
to achieve film depositionat an appreciablerate. The ZrI4 adsorptionwasde-tectedasanincreasein theQCM signal,denoted∆m1 (Fig. 6.3),duringtheZrI4
exposure.Thesignalcontinuedto increaseup to 40 s, which shows that thead-sorptionof ZrI4 wasnot completelyself-limiting in this temperaturerange.Thiskind of behaviour is analogousto thatobservedfor adsorptionof TiI4 on a TiO2
surfacetreatedin H2O-H2O2 flux [92]. In bothcases,theeffectmaybeexplainedby thedecompositionof metaliodideon theoxidesurface. With increasingex-posuretime,therateof themasschangedecreasedsignificantly:abouttwo timeshighermassincreasewasobtainedduringthefirst 5 s thanin thefollowing 35 s.Onecanthusrely on thesurfacecoveragewith iodidespeciesto becloseto themaximumvaluealreadyafterthefirst few seconds.
Using long purge timesbetweenthe ZrI4 andH2O-H2O2 exposurescausedthethefilm masstodecreaseby anamountdenoted∆mp1 in Fig.6.3.Thisisprob-ably dueto thedesorptionof iodinereleasedin theZrI4 decompositionprocess.Desorptionof ZrI4 itself canprobablyberuledout sinceno decreasein themassincrementper completeALD cycle wasobserved uponthe prolongationof thepurgetime from 2 to 10 s. During theH2O-H2O2 pulse,thefilm massdecreasedabruptlyby ∆m2 (Fig. 6.3). This decreasewasrelatedto an exchangereactionbetweenthe -ZrIx adsorbedon thesurfaceandtheoncomingoxygenprecursor,wheretheheavy iodineis replacedby oxygenor -OH groups.Whenthecycle iscompleted,anew layerwith themass∆m0 hasformed.A H2O-H2O2 pulseof 2 swaslong enoughto recover theadsorptioncapabilityof thesurfacetowardsZrI4
andachieve stablegrowth in a seriesof repeatedALD cycles(Fig. 6.4),andwasthereforeusedin furtherstudies.
As could be expected,the massincrement∆m0 asa function of ZrI4 pulse
28 Depositionof Zir conium Oxide
0 10 20 30 40 50 60
c7 ycle times: 40-10-58
s
Tgrowt9h = 300 oC, Tsource = 238 oC
m0
m2
mp1
m1
H2O2 off
onH2O2
Z:
rI4 off;
Z:
rI4 on
QC
M S
IGN
AL
(a.u
.)
TIME (s)
<>= ? =
Figure 6.3: QCM signal recordedduring an extra long ZrO2 growthcycle.
0 10 20
m0
m0
m2
m1
H2O2off;
off;
on
H2O2
Z:
rI4
ZrI4 on
m0
c7 ycle times: 2-2-2-2 s
Tgrowt9h = 300 oC, Tsource = 238 oC
QC
M S
IGN
AL
(a.u
.)
T@
IME (s)
30 40
Figure6.4: Consecutive ZrO2 growth cyclesusing2-2-2-2spulses.
6.3ALD 29
duration,did not saturate.After asteepincreasefor pulsetimesbetween0.4and1.0 s, an almostlinear increasein ∆m0 with a considerablylower but constantratewasobserved. Extendingthe ZrI4 pulsedurationfrom 2 to 10 s madethevalueof ∆m0 increaseby a factorof 1.8. To avoid thecontribution from theZrI4
decomposition,probablyresponsiblefor this increase,ZrI4 pulsesof 2 s wereusedfor thedepositionof thefilms for post-depositionstudies.A morecompletesaturationof themassincrementpercyclewasachievedby increasingtheoxygenprecursorpulseduration. The valueof ∆m0 increasedby a factorof 1.3 whenthe exposuretime of H2O-H2O2 increasedfrom 2 to 10 s. The QCM studiesalsoshowed that purge periodsof 2s weresufficiently long to remove gaseousproductsfrom thereactorandto avoid overlapbetweentheprecursorpulses.
6.3 ALD
6.3.1 Temperature series
TheXPSanalysisshowedthatsmallamountsof iodine,1.3-0.8atomic%, wereincorporatedinto films depositedat 250 to 350
C, but for highertemperatures,
no contaminationwasfound in the films. Films depositedon both Si(100)andPt substratesat temperaturesbetween250and500
C werecrystalline. Fig. 6.5
shows how the diffractogramsfor films depositedon Si(100)changewith tem-perature,but thesametrendsapplyfor films onPt. PeaksthatarecharacteristicofmonoclinicZrO2 [93] weredetectedin thewhole temperatureinterval (d-valuesof approximately3.14,2.84,2.62,2.32,2.19,1.84and1.65A). The remainingpeaksin thediffractogram,atd = 2.93, 2.58,2.54, 2.10,1.80, 1.69,1.55and1.53A, couldall beattributedto thetetragonalphase[94], andtheonesin bold printalsoto thecubic phase[95]. The intensityof thepeaksat 2.54and1.53A firstincreasesandthendecreasesabruptlywith increasingtemperature.At thesametime, the intensityof thepeaksat 2.58and1.55A that clearlydo not belongtocubicZrO2, increasesstronglywith depositiontemperature,startingfrom 300
C.
Thesetrendscould indicatea changein phasecontent,from cubicto tetrag-onal, with increasingtemperature,but it could alsoreflecta changeof orienta-tion of the tetragonalgrainswith depositiontemperature.Indeed,Ramanspec-troscopy studiesverifiedthattetragonalZrO2 hadformedevenat thelowesttem-peratures.A few peaksof both tetragonalandmonoclinicphase[96] appearedalreadyat 275
C, andthenumberaswell astheintensityof thepeaksincreased
with temperature.Thedepositionratereachesa sharpmaximumof 1.25A/cycle at 275
C, and
decreasesto approximately0.7 A/cycle for the highertemperatures.No differ-
30 Depositionof Zir conium Oxide
30 40 50 60
2.58
2.19
2.32 1.
531.
55
1.80
1.84
2.10
2.54
2.61
2.84
2.93
3.14
375oC
500oC
450oC400oC
350oC325oC300oC
250oC275oC
Inte
nsity
(a.
u.)
2θ ( )o
1.69
1.65
Figure 6.5: X-ray diffractogramsfor ZrO2 films depositedon Si(100)at differenttemperatures.
encecanbeseenin thedepositionrateonPtandSi(100)substrates,andtheSEMstudiesshowed that the surfacemicrostructureis practically identical for filmsdepositedon thetwo substrates.Thesurfacemorphologychangeswith tempera-tureascanbeseenfrom theSEMmicrographsof Fig. 6.6.
100nm Mag = 150kX 100nm Mag = 150kX
3A
00 Co 5B
00 Co
Figure 6.6: SEM micrographsof films grown on Si(100)substrateat300
C and500
C.
6.3ALD 31
25Å
Figure6.7: TEM micrographof afilm depositedonSi(100)at500C.
TEM studiesperformedon films depositedon Si(100) at 325 and 500C
showed thatan amorphouslayer of approximately2 nm hasformedat the sub-strate/filminterfaceof bothsamples,seeFig. 6.7 for the500
C film. EDSanal-
ysispointsto thecompositionof this layer to beSiO2. Thegrainsarerandomlyorientedfor both temperatures,and in low-magnificationcross-sectionsof thewhole films (not shown here),it wasobserved that large equi-axed grainshaveformedat 325
C whereasa columnarstructurehasdevelopedat500
C.
6.3.2 Thicknessseries
Films of thicknessesbetween3 and30 nm weredepositedat anaveragerateof0.09nm/cycle. No significantdifferencesin depositionratebasedon substrateor temperaturecould be seen.The growth rateat 275
C wasapparentlynot as
highaswasfoundpreviously (seeabove),andeventhoughasmalloffsettowardslower temperaturewould easilyreducethegrowth rate,this cannotfully explainthediscrepancy betweenthetwo results.
FromXRD studies,it couldbeconcludedthatnot only thetemperature,butalso the substrateinfluencesthe crystallisation. The ZrO2 films crystallisedatlower thicknesseson Pt substratesthanon poly-Si andα-Al2O3(001),an effectthat wasstrongerat the lower temperature.At 275
C, the sametetragonalre-
flectionsappearedasfor thethicker films depositedat thesametemperature,i.e.
32 Depositionof Zir conium Oxide
d = 2.93, 2.54, 1.80 and1.53 A. For depositionat 325C, the first peakswere
thesameasfor 275C, but with increasingthickness,thetetragonalreflectionsat
d = 2.58and1.55A, aswell asmonoclinicreflectionsappeared.For both tem-peratures,films depositedon α-Al2O3(001)substratesexhibitedonly onestrongreflectionatd=2.54A, whichwasassignedto the(110)planeof tetragonalZrO2.Rockingcurve (ω-scan)analysisof this peakgave full-width-at-half-maximum(FWHM) valuesof about4.5
for both depositiontemperatures.Consequently,
thefilms have astrongtexture,but areprobablynotepitaxial.The SEM studiesshowed that films on Pt andpoly-Si substratesdeveloped
verysimilarmicrostructureswith increasingthickness,andtheappearanceof the30 nm thick films werealreadyvery similar to thatof thethicker films. A minoreffectof thegrowth temperaturecouldbeseenin thatthefilms depositedat325
C
hadsomewhat largergrainsandhigherroughnessthanat 275C. Independentof
temperatureandthickness,thefilms depositedonα-Al2O3 (001)wereextremelysmoothandarebarelyvisible in theSEM.
Fig. 6.8 shows thedielectricconstantasa functionof film thickness,wherethevaluesvary between2.5and17 for thicknessesbetween3.5and20 nm. Thedielectric constantappearsto be higher for films grown at 325 thanat 275
C,
andalsosomewhatenhancedfor Ptsubstrates,but thedatapointsaretoo few forconclusionsto bedrawn. However, thefilm thicknesshasa stronginfluenceonthe permittivity, regardlessof temperatureandsubstrate.The leakagecurrentsin thefilms weredifficult to determinereproduciblyandno generaltrendscouldbeseen.Excellentreproducibilitywas,however, obtainedfor a 10 nm thick filmgrown on Pt at 275
C for which a leakagecurrentof 1 µA/cm2 at 1 MV was
measured.Thebreakdown voltagefor thisfilm was4MV/cm. Theinstabilityin theleak-
agecurrentscouldpossiblybe causedby oxygenvacanciesin theoxide,whichwouldalsohave astabilizingeffect on thetetragonalphase.
6.3ALD 33
0 50 100 150 200 250 3000
2
4
6
8
10
12
14
16
18
20
Pt 325 C poly-Si 275 C poly-Si 325 C
Die
lect
ric c
onst
ant
TC
hickness (nm)
o
o
o
Figure6.8: Dielectricconstantof ZrO2 films asafunctionof thickness,temperatureandsubstrate.
7 Depositionof Hafnium Oxide
7.1 CVD
The film thicknessmeasurementsshowed that the growth rate increasedexpo-nentially with temperature,from 0.5 nm/min at 300
C to 90 nm/min at 700
C.
XPS analysisshowed that films depositedat 300C containedabout0.8 atomic
% iodine,andvery small amounts( D 0.1 atomic%) werealsodetectedin filmsdepositedat 400and500
C. Fromthediffractionstudies,it wasfoundthatmon-
oclinic HfO2 [97] hadgrown on poly-Si andMgO(001)substratesin thewholetemperaturerange300-700
C. Depositionon poly-Si resultedin poly-crystalline
films with a preferencefor the [001] orientationthat becamemorepronouncedwith temperature.FilmsonMgOshowedageneralpreferencefor the[001], [010]and[100] orientations,but thecontributionsfrom thedifferentorientationsvariedwith temperature(Fig. 7.1). For 300
C, thefilms wereonly partly crystallised,
with broadpeaksof very low intensity.The500and600
C films gaveverystrong0k0reflections,androckingcurve
analysisof the 020 peakgave FWHM valuesof approximately1.4 and1.8 de-grees,respectively. For 700
C, theoverlappingpeaksmadeRockingCurveanal-
ysis impossible,but a comparisonof the resultsfrom θ-2θ and GIXRD scansshowedthat the(0k0) planeswereexclusively orientedparallelwith thesurface,in contrastwith theotherplanes.ϕ-scanswereperformedfor the(024)planeofMgO andthe(041)planeof HfO2 for the500and600
C films, resultingin four
peaksfrom thefilm coincidingwith thepeaksfrom thesubstrate.ThisshowsthatanepitaxialrelationshipexistsbetweentheHfO2 andthesubstrate:thea-axisofHfO2 is parallelwith thea- or theb-axisof theMgO. Dueto low peakintensityin theθ-2θ scan,noattemptsweremadeto analyzetheorientationalrelationshipsfor thefilm depositedat 400
C.
Thedevelopmentof themicrostructureof thefilms asa functionof temper-atureandsubstratewasstudiedby SEM. Fig. 7.2 a) andb) show micrographsof films depositedat 400 and600
C on MgO substrates.At 400
C, spherical
crystalliteswith small diametersarevisible andthe surfaceis very smooth. In
7.1CVD 35
30 32 34 36 38 40 42 44 46 48 50
0 50 100 150 200 250 300 350
4E
00oC
500oC
600oC
700oC
In
tens
ity (
a.u.
)
2θ ( )oF
Inte
nsity
(a.
u.)
( o )ϕ
Figure 7.1: θ-2θ X-ray diffractogramsfor HfO2 films depositedonMgO substrates,at400-700
C with theϕ-scanfor 600
C in theinset.
contrast,the films depositedat 600C consistof oblongcrystallites,whoseor-
dering wasstrongly influencedby the substrate.On MgO, the crystallitesareorientedin oneof two possibledirections,90
rotatedfrom eachother, while on
poly-Si,they arerandomlyoriented.Thisstructurewasdevelopedgraduallywithincreasingtemperatureandis themostobviousin thethick films grown at700
C,
wherethecrystalliteshavegrown to about200nmlongneedles(notshown here).
Thedielectricconstantincreasedrathersharplyfrom 10.5for the300C film
to 17 for the400C film, but wasalmostconstantbetween400and700
C where
it reacheda value of 19 ( Fig. 7.3). This shows that the dielectric constantisdependenton the crystallinity of the film. For films that have almostthe samethickness,the 300
C film that is only partly crystallisedhasa muchlower per-
mittivity thanthecrystallinefilms. No obvioustrendscanbeseenfor theleakagecurrentsmeasuredfor thedifferentgrowth temperatures.However, asanexam-ple, a leakagecurrentdensityof 0.1µA/cm2 at 1 MV/cm anda breakdown fieldabove 1.5MV/cm wasmeasuredfor the300
C film.
36 Depositionof Hafnium Oxide
Mag = 200kX100nm Mag = 200kX100nm
a) b)
Figure 7.2: SEM micrographsof HfO2 films depositedon MgO(001)substratesata)400
C andb) 600
C.
300 400 500 600 70002468
1012141618202224
Die
lect
ric c
onst
ant
Temperature ( C)o
Figure7.3: Dielectricconstantof HfO2 asafunctionof depositiontem-perature.
7.2 In situ monitoring 37
7.2 In situ monitoring
TheQCM studiesrevealedthataHfI4 vapourpressuresufficiently highfor ALD-typegrowth at acceptableexposuretimes,wasobtainedat HfI4 sourcetempera-turesexceeding195-200
C. Fig. 7.4shows a typicalbehavior of theQCM signal
recordedatTG = 300C duringarelatively longALD cycle. Themostsignificant
amountof HfI4 is adsorbedin thebeginningof thepulse,but thefilm masscon-tinuesto increaseatexposuretimesreaching20s. Themassdecreasesduringthesubsequentpurge,indicatingdesorptionof somespeciesfrom thefilm surface.
-100
0
100
200
300
400
500
tG
4tG
1 tG
2tG
3
TH
G = 300oC
TH
S = 200oC
HfI4 on
H2O off
H2O onHfI4 off
HfI4on
QC
M s
igna
l, ar
bitr
ary
units
Time, s
0 20 40 60 80 100
Figure 7.4: QCM signalrecordedin an HfO2 ALD cycle during longexposureandpurgetimes.
However, as can be seenin Fig. 7.5, the amplitudeof this massdecrease,∆mp, doesnot influencethemassincrementpercompleteALD cycle,∆m0. Con-sequently, the amountof hafniumreleasedduring the purge cannot be consid-erableandthemassdecreaseis mostprobablydueto desorptionof iodine fromthesurfacespeciesformedduringtheHfI4 pulse.Exposureof thesurfaceto H2O(Fig. 7.4 and7.5) causesanabruptdecreaseof thefilm mass.Suchresponsetothe oxygenprecursoris typical for ALD processesin which a halideandwatervaporareusedasprecursors[73, 87] andis explainedby substitutionof theheavyiodineligandswith muchlighteroxygenatomsand/orhydroxylgroups.As a re-sultof theexchangereaction,anew layerof oxidewith themass∆m0 hasformedanda relatively stablesurfaceis obtained(Fig. 7.4).
38 Depositionof Hafnium Oxide
-100
0
100
200
300
400
500
mp
m0
m1
H2O offH2O off H2O on
H2O onHfI4 offHfI4 off
HfI4 onHfI4 on
TG = 300oCI
TS = 207oCI
QC
M s
igna
l (a.
u.)
TC
ime (s)0 10 20 30 40 50 60
Figure 7.5: Time dependenceof theQCM signalin ALD cycleswithdifferentpurgetimeaftertheHfI4 pulse.
The massincrement∆m0 dependson the HfI4 pulsedurationaswell asonthe oxygenprecursorpulseduration,but both dependenciessaturatewhensuf-ficiently long pulsesareused. The relatively high sourcetemperatureof HfI4
(205C) limits the rangeof substratetemperaturesthat canbe usedbecausethe
adsorptionof themetalprecursordoesnot saturatewhenthesubstratetempera-tureis lower thanthesourcetemperature.
By comparingcalculated∆m0/∆m1 ratioswith experimentalvalues,the av-erageI/Hf ratio in the adsorbatelayer formedduring the HfI4 pulsecanbe es-timated. The obtainedI/Hf ratiosare1.9, 2.0 and2.3 at 220, 260 and290
C,
respectively. This increasein the I/Hf ratio with temperaturecouldbe aneffectof decreasingconcentrationof surfacehydroxyl groupswith increasingtemper-ature. For comparison,the ligand/metalratios,which weredeterminedfor thesurfaceintermediatelayerformedduringtheHfCl4 pulsein theHfCl4-H2O pro-cess,were 2.4, 2.5 and 3.0 at substratetemperaturesof 225, 300 and 400
C,
respectively [73]. Thus, theseQCM resultsshow that HfI4 decomposesmoreeasilythanHfCl4.
7.3 ALD
Thegrowth ratedeterminedfrom film thicknessmeasurementswas0.075J 0.007nm per cycle, andwas independentof the depositiontemperaturein the range
7.3ALD 39
225-500C. Smallamountsof iodineweredetectedin thefilms depositedat 225
and300C, 0.85and0.35atomic% respectively, but thefilms depositedathigher
temperatureswerefreefrom contamination.Thefilms grown at 225
C weremainly amorphous,whereasthosegrown at
300-500C werecrystallineandconsistedmainly of monoclinicHfO2 [97]. The
diffractogramsfor films depositedon poly-Si substratesat 400 and500C also
containeda peakat about2θ K 30L 4 , that could be attributed to orthorhombic[17], tetragonal[98], or cubic[99] HfO2, but thepeakwasnotpossibleto identifyunambiguously.
Thesubstratehadaverystronginfluenceonthetextureof thegrowing HfO2.On poly-Si, thefilms werepoly-crystallinewith a slight preferencefor the[001]orientation,whereason MgO, only reflectionsoriginatingfrom the(0k0) planesweredetected(Fig. 7.6). Rockingcurve analysisof the020peakfor the400and500
C films on MgO gave FWMH valuesof approximately1 degree,indicating
the possibility of epitaxy. ϕ-scanswereperformedfor the (024) planeof MgOandthe(041)planeof HfO2, seetheinsetin Fig.7.6. for the500
C film. Boththe
400and500C films gavefourpeaksthatcoincidedwith thesubstratepeaks.This
showsthatthereis anepitaxialrelationshipsuchthatthea-axisof HfO2 is parallelwith eitherthea- or theb-axisof thesubstrate.Dueto low peakintensityin theθ-2θ scan,noattemptsweremadeatanalyzingtheorientationalrelationshipsforthefilm depositedat 300
C.
20 40 60 80
Inte
nsity
(a.
u.)
2θ ( )oM
Inte
nsity
(a.
u.)
( N )ϕ500 100 150 200 250 300 350
Figure 7.6: X-ray diffractogramfor a HfO2 film depositedon MgO at500
C. Theinsetshows theϕ-scanfor thesamefilm.
40 Depositionof Hafnium Oxide
IL
Si
23Å
132Å HfO2
O
5nm
Figure7.7: TEM micrographof afilm depositedonSi(100)at500C.
TheSEM-studiesof thesurfacemicrostructureshowedthatextremelysmoothfilms weredepositedat all temperatures.Thepoly-Si substratein itself is roughcomparedto theMgO surface,but thefilms depositedon MgO andpoly-Si hadthe samemicrostructuralfeatures.All the films hada small andvery uniformgrainsizeof approximately15 nm. TEM analysisof films depositedon Si(100)at 500
C showed that a 2-3 nm thick layer of amorphousSiO2 had formedat
the film-substrateinterface,Fig. 7.7. The films werecrystallinethroughoutthethicknessandconsistedof randomlyorientedmonoclinicHfO2.
Thedielectricconstantof thefilms wasfound to increasewith temperature,from 10 at 225
C to 18 at 500
C (Fig. 7.8). Sincethefilm thicknessesareequal,
but the crystallinity of the oxide increaseswith growth temperature,it can beconcludedthat the permittivity is dependenton the crystallinity. Typical DCcurrent-voltage(I-V) characteristicsareshown in Fig. 7.9,whereit canbeseenthattheleakagecurrentincreaseswith thedepositiontemperature,exceptfor the500
C film. Thebreakdown voltageincreasesstrictly with decreasingdeposition
temperature,andreachesveryhighvaluesfor thelow temperaturefilms. It is alsoworthnotingthattheleakagemechanismis fairly similar for all thefilms.
7.3ALD 41
200 250 300 350 400 450 50002468
1012141618202224
Die
lect
ric c
onst
ant
Temperature ( C)o
Figure7.8: Dielectricconstantof HfO2 asafunctionof growth temper-ature.
1E-8
1E-7
1E-6
1E-5
1E-4
1E-3
0,01
Electrical field (MV/cm)
Leak
age
curr
ent (
A/c
m )2
0 2 4 6
500 Co225 Co300 Co400 Co
Figure 7.9: Current-voltagecharacteristicsof HfO2 films depositedonpoly-Si at225-500
C.
8 Depositionof Mixed TiO2-Ta2O5
Films
Thin films of mixed tantalumandtitanium oxideshave beenproducedby sput-tering[100, 7], sol-gel[5], but noneof thesehave reachedashighpermittivity asthebulk material[6]. Oneof the techniquesthathave not previously beenusedfor depositionof mixedTi-Taoxidesis conventionalCVD.
8.1 CVD
Accordingto XRFSanalysis,thefilms depositedat 500C hadcompositionsin
the range5-100%titanium, andat 600C in the range40-65%titanium. XPS
depthprofilesof thefilms show acertaintitaniumenrichmenttowardsthesurfacewhichcouldbedueto a lowersurfaceenergy for TiO2. No iodineresiduescouldbedetectedat thesurfaceor aftersputtering.
Both theTi/Taratioandthedepositiontemperaturehadastronginfluenceonthecrystallinityof thefilms. Filmsdepositedat500
C wereamorphousfor com-
positionsup to 50%titanium,but highertitaniumcontentresultedin crystallineTiO2 (anatase).However, theanataseXRD peaksweredisplacedtowardshigherd-values(lower angle)thanin the literature,indicatingthat the cell parameterswereenlargeddueto solid solutionwith the larger Ta atom. It shouldbenotedthat pureTa2O5 grown from TaI5 hasbeenfound to be crystallineat this tem-perature[101]. The X-ray diffractogramsfor pureTiO2 andthe solid solution(approximately80%Ti) areshown in Fig. 8.1a) andb) respectively. Films con-taining approximately50% titanium depositedat 600
C containeda mixture of
crystallineβ-Ta2O5 andanatase,seeFig. 8.1 c). The samedisplacementof theanatasepeaksis observed as for the 500
C films, whereasthe positionsof the
β-Ta2O5 peaksremainat literaturevalues[102].SEM studiesof the surfacemicrostructuresrevealeda dependenceon tita-
nium concentrationthatcorrelateswith thephasecontent.Theamorphousfilmsare relatively smoothwith no specialfeatures,but upon crystallisationof the
8.1CVD 43
films, differentshapesareobserved(Fig. 8.2).
20 30 40 50 60
c)
b)
a)
= β-Ta2O5(0
01)
(112
)(2
11)
(105
)
Si
(002
)
(112
)(0
04)
(103
)
Si
(101)
Si
2
θ ( )o
Inte
nsity
(a.
u.)
Figure 8.1: Diffractogramsfor a) pureTiO2 grown at 500C, b) TiO2
with Tain solidsolutiongrown at500C,andmixedfilm, 50%Ti grown
at 600C.
10µm Mag = 10kX 1µm Mag = 100kX
a) b)
Figure 8.2: SEM micrographsof films containinga) 80%Ti depositedat 500
C, b) pureTiO2 depositedat500
C.
It wasfound that thedielectricconstantwasconnectedto thephasecontentandthe film thickness.For films depositedat 500
C, amorphousfilms (5-50%
Ti) haddielectricconstantsof approximately35, irrespective of thecomposition.For films of crystallisedanatasewith tantalumin solid solution the dielectricconstantwasapproximately60,againwithoutcompositiondependency. For pureanatase,the valueswere around70. For comparison,600
C films containing
44 Depositionof Mixed TiO2-Ta2O5 Films
about50%titaniumin amix of crystallisedβ-Ta2O5 andanatasehadadielectricconstantof about20. Theincreasein dielectricconstantwith increasingtitaniumconcentrationappearsto be relateddirectly to the phasecontent,which in turnis controlledby thecomposition.Within thesephasestability regions,the layerthicknesshada stronginfluenceon thedielectricconstant.
The dielectricconstantscalculatedfor thesefilms correlatewell with thoseof Umezawa et al. [7], who foundthatthedielectricconstantof thin films of Ti-Taoxideincreasedfrom 30 to 70 whenthetitaniumconcentrationwasincreasedfrom 10 to 90%. Ganet al. [100] studiedthin films of (TiO2)x-(Ta2O5)1 x pre-paredby sputteringandfoundthat8% TiO2 givesa maximaldielectricconstantof 30 for as-depositedfilms. High-temperatureannealinducedcrystallisationofthefilms andenhancedthepermittivity but themaximum,now 55,wasstill foundat 8%Ti.
9 Concluding remarks
Thisthesisdescribesthebasicfeaturesof iodide-basedCVD- andALD-processesfor depositionof Ta2O5, ZrO2 andHfO2. Theseprocesseshave beenstudiedforthefirst time, andtheprocessparametershave thereforenot yet beenoptimised.Already at this stage,however, it is clear that the iodide precursorsoffer thepossiblityto depositpure,crystallinefilms at lower temperaturesthanmostotherprocesses.In theCVD processes,thisisdoneusingO2 astheonly oxygensource,whereasH2O is usuallyneededin chloride-processesto remove Cl in the formof HCl. It is preferableto avoid introducinghydrogeninto theprocesssinceHIthat is formedmight etch the film, andhydrogencanbe incorporatedinto thefilms which affects the electricalproperties.Recently, it wasshown that TiO2
canbedepositedby ALD usingTiI4 andO2 [87]. Work is currentlyin progressto prove that O2 canbe usedin ALD of Ta2O5 aswell [103]. A reasonfor thelower contaminationlevels in iodide processescomparedwith thevery popularchlorideprocessescanbe found in the formationenthalpies.Themetaliodideshave lower formationenthalpies,andtherebylower thermalstabilitiesthat leadto easierremoval of theligandsin chemicalreactions.Thedifferencein thermalstability also meansthat the energy gain on going from a metal iodide to theoxide is larger thangoing from thechlorideto thesameoxide. Theenergy thatis releasedcanbe usedfor crystallisationof the oxide, which would thereforerequirea lower temperaturewhenusingan iodide precursorthanwhenusingachloride[104].
The metal iodidesusedin this work arevery similar both in their chemicalreactionsand thermalstabilities,but therearesomedifferencesin the processcharacteristics.For TaI5 and ZrI4, the growth rate in CVD increasesup to acertainpoint, and then decreaseswith increasingtemperature.In ALD usingTaI5, the growth ratedecreasesslightly with temperature,andfor ZrI4 thereisa maximumandthendecreasinggrowth ratewith increasingtemperature.HfI4,on theotherhand,givesagrowth ratein CVD thatincreasesmonotonicallywithtemperatureandin ALD that is constantwith temperature.It thusappearsthatHfI4 is morestableandcanmoreeasilygive self-limiting growth in ALD.
46 Concluding remarks
AlthoughZrO2 andHfO2 arepredictedto bestablein contactwith silicon, itwasshown thataninterfacial layerof SiO2 hadformedin theALD experiments.A SiO2 layerhasmostlikely formedin theTa2O5 samplesaswell, but no TEManalysishasbeenperformedto confirm this. The SiO2 can form either in thebeginningof thedepositionexperiment,whenthesubstratesurfacehasnot beencompletelycoveredandis exposedto oxygen-containingspecies,or later in theexperimentsinceoxygendiffusesrathereasilyin bothZrO2 andHfO2. It is alsopossiblethattheinterfaciallayerformedin asolidstatereactionbetweenthefilmand the substrate,but in that case,the ZrOx formed in the reactionmusthavereoxidisedto ZrO2. The seriesconnectionwith the SiO2 layer lowersthe totaldielectricconstantandmustsomehow beavoidedfor futureapplications.
Acknowledgements
Anders(Silverback)Harsta,you’ve beena greatsupervisor!Thankyou for al-waystakingthetime for all kindsof discussionsandfor generouslysharingyourknowledge.
ProfessorJan-OttoCarlssonis gratefullyacknowledgedfor giving methisoppor-tunity andfor puttingtheexcellentfacilitiesat TheAngstrom Laboratoryat mydisposal.
I amalsogratefulto Jan-OttoandEva Fredrikssonfor giving mea goodstartasaPhDstudent.
I would like to expressmy deepestgratitudeto JaanAarik andKarpo Kukli atThe University of Tartu for their enthusiasmandfriendlinessin a very fruitfulcooperation.
Thanksarealsodueto Prof.MarkkuLeskela andAss.Prof.Mikko RitalaatTheUniversityof Helsinki,whereI have hadthepleaseto beaguest.
David GilmeratMotorolais acknowledgedfor providing mewith valuablechar-acterisationandfor beingsuchaniceguy.
Thereis abunchof peopleatTheDep.of SolidStateElectronicsthathavehelpedme out in timesof needandI would especiallylike to thankJorgenWestlinderfor workingsohardon theelectricalcharacterisation.
It cannot be said too many times that this kind of work would be impossiblewithout the skillful assistanceof the technicalandadministrative staff. Thankyou Nisse,Torvald, Anders,Janne,Gunilla, Ulrika, andKatarinafor alwaysbe-ing helpfulandfriendly.
48 Acknowledgements
Specialthanksto:
Kajsa,whohasalwaysbeentherefor me.It wouldn’t havebeenthesamewithoutyou.
Sofia,Ulrika, Micke, Marie, Asa, Vickan, Nina, andothers..., for beinggoodfriends.
All thepeopleof TheDep.of Mater, Chem.,who aremorelike friendsthancol-leagues.
Mina underbaraforaldrar, som alltid har stott mina mer eller mindre konstigaprojektochsomaldrig ifr agasattnyttanav ennio ar langutbildning.
And of courseUli, thecoolestguyI know, for having thenerveto takecareof me.
Uppsala,September2001
Eject!
Bibliography
[1] C. Chaneliere, J. L. Autran, R. A. B. Devine, and B. Balland,Mater. Sci.Eng., vol. R22,pp.269–322,1998.
[2] G. D. Wilk, R. M. Wallace,andJ.M. Anthony, J. Appl.Phys., vol. 89,no.10,pp.5243–5274,May 2001.
[3] K. J.HubbardandD. G. Schlom,J. Mater. Res., vol. 11,no.11,pp.2757–2776,Nov. 1996.
[4] S.O.Kasap,Principalsof ElectricalEngineering, 2nded., (McGraw-Hill,New York, 2000).
[5] A. Cappellani,J.L. Keddie,N. P. Barradas,andS.M. Jackson,Solid-StateElectronics, vol. 43,no.84,pp.1095–1099,1999,
[6] R. F. Cava,W. F. Peck,andJ.J.Krajewski, Nature, vol. 377,no.84,1995,
[7] T. Umezawa,S.Yajima,andK. Matsumoto,ThinSolidFilms, vol. 52,pp.69–75,1978.
[8] L. D. Calvert andP. H. G. Draper, CanadianJournal of Chemistry, vol.40,pp.1943–1950,1962.
[9] N. C. StephensonandR. S. Roth, ActaCryst., vol. B27, pp.1037–1044,1971.
[10] N. Terao,Jpn.J. Appl.Phys., vol. 6, no.1, pp.21–34,1967.
[11] J.L. WaringandR. S.Roth,J. Res.Nat.Bur. Stand.Sect.A, vol. 72A, no.2, pp.175–186,1968.
[12] H. FujikawaandY. Taga,J. Appl.Phys., vol. 75,pp.2538,1994.
[13] A. Gebbert,M. Alvarez-Icaza,W. Stocklein, and R. D. Schmid, Anal.Chem., vol. 64,pp.997,1992.
50 Bibliography
[14] U. Teravaninthorn,Y. Miyahara,andT. Moriizumi, Jpn.J. Appl. Phys.,vol. 26,pp.2116,1087.
[15] P. M. Natishan,E. McCafferty, P. R. Puckett,andS.Michel, Corros.Sci.,vol. 38,pp.1043,1996.
[16] T. J.Rehg,J.A. Ochoa-Tapia,A. Knoesen,andB. G. Higgins,Appl.Opt.,vol. 28,pp.5215,1989.
[17] F. Rubio, J. Denis,J. M. Albella, andJ. M. Martinez-Duart, Thin SolidFilms, vol. 90,pp.405,1982.
[18] Y. Nishimura,H. Ujita, andM. Tsuji, Appl.Surf. Sci., vol. 89,no.84,pp.393–399,1995.
[19] C.-C.Lee,J.-C.Hsu,D. T.Wei, andJ.-H.Lin, Thin SolidFilms, vol. 308-309,no.84,pp.74–78,1997.
[20] F. Gitmans,Z. Sitar, andP. Gunter,Vacuum, vol. 46,pp.939,1995.
[21] N. Ozer and C. M. Lampert, J. Sol-GelSci. Tech., vol. 8, no. 84, pp.703–709,1997.
[22] K. Kukli, J. Aarik, A. Aidla, O. Kohan,T. Uustare,andV. Sammelselg,ThinSolidFilms, vol. 260,no.84,pp.135–142,1995.
[23] B. C.-M. Lai andJ.Y.-M. Lee,J. Electrochem.Soc., vol. 146,no.184,pp.266–269,1999.
[24] K. D. PollardandR. J. Puddephatt,Chem.Mater., vol. 11, no. 84, pp.1069–1074,1999.
[25] S.Tanimoto,N. Shibata,K. Kuroiwa, andY. Tarui, J. Electrochem.Soc.,vol. 141,no.584,pp.1339–1346,1994.
[26] A. Ono, Mineral J., vol. 6, no.6, 1972.
[27] R. C. Garie,High Temperature Oxides, (AcademicPress,1970).
[28] J.Cai,C. Raptis,Y. S.Raptis,andE. Anastassakis,Phys.Rev. B, vol. 51,no.1, pp.201–209,Jan.1995.
[29] M. Ozawa,S.Suzuki,C.-K. Loong,andJ.C. Nipko, Appl.Surf. Sci., vol.121/122,pp.133–137,1997.
51
[30] J.S. Kim, H. A. Marzouk,andP. J. Reucroft, Thin SolidFilms, vol. 254,pp.33–38,1995.
[31] G. Bocquillon,C. Susse,andB. Vodar, Rev. Int. HautesTemp.Refract.,vol. 5, no.4, pp.247,1968.
[32] L. G. Liu, J. Phys.Chem.Solids, vol. 41,pp.331–334,1980.
[33] C. J. Howard, R. J. Hill, andB. E. Reichert, Acta Cryst., vol. B44, pp.116–120,1988.
[34] S. B. Desu,T. Shi and,C. K. Kwok, Mater. Res.Soc.Symp.Proc., vol.168,pp.349–356,1990.
[35] M. SayerandK. Sreenivas,Science, vol. 247,pp.1056,1990.
[36] C. Mary, R. Guinebretiere,G. Trolliard, B. Soulestin,P. Villechaize,andA. Dauger,ThinSolidFilms, vol. 336,pp.156–159,1998.
[37] S. B. Qadri, E. F. Skelton, M. Z. Harford, C. Kim, and P. Lubitz,Surf. Coat.Tech., vol. 63,pp.155–157,1994.
[38] T. Ngai,W. J.Qi, R.Sharma,J.Fretwell,X. Chen,J.C. Lee,andS.Baner-jee, Appl.Phys.Lett., vol. 76,no.4, pp.502–504,2000.
[39] J. Aarik, A. Aidla, H. Mandar, V. Sammelselg,ang T. Uustare,J. Cryst.Growth, vol. 220,pp.105–113,2000.
[40] E. Ruh, H. J. Garrett,R. F. Domagala,andN. M. Tallan, J. Amer. Ce-ram.Soc., vol. 51,no.4, pp.27,1968.
[41] A. G. Boganov, V. S. Rudenko, and L. P. Makorov,Dokl. Akad.Naut.SSSR, vol. 160,no.5, pp.1065,1965.
[42] J.Wang,H. P. Li, andR. Stevens,J. Mater. Sci., vol. 27,pp.5397,1992.
[43] A. J. Waldorf, J. A. Dobrowolski, B. T. Sullivan, and L. M. Plante,Appl.Opt., vol. 32,no.28,pp.5583–5593,1993.
[44] V. V. Kharton,A. A. Yaremchenko, E. N. Naumovich, andF. M. B. Mar-ques,J. SolidStateElectrochem., vol. 4, pp.243–266,2000.
[45] G. Reiße,B. Keiper, S.Weißmantel,H. Johansen,R. Scholz,andT. Mar-tini, Thin SolidFilms, vol. 241,pp.119–125,1994.
52 Bibliography
[46] S. Capone,G. Leo, R. Rella, P. Siciliano, L. Vasanelli, M. Alvisi,L. Mirenghi, andA. Rizzo, J. Vac.Sci.Tech. A, vol. 16, no.6, pp.3564–3568,1998.
[47] T. Nishide,S. Honda,M. Matsuura,andM. Ide, ThinsSolid Films, vol.371,pp.61–65,2000.
[48] J. Aarik, A. Aidla, H. Mandar, T. Uustare,K. Kukli, and M. Schuisky,Appl.Surf. Sci., vol. 173,pp.15–21,2001.
[49] M. Balog,M. Schieber, M. Michman,andS.Patai, Thin SolidFilms, vol.41,pp.247–259,1977.
[50] J.-O.CarlssonandU. Jansson,Prog. SolidSt.Chem., vol. 22,pp.237–292,1993.
[51] T. SuntolaandJ.Antsom,USPatent4,058,430, 1977.
[52] H. Kattelus,M. Ylilammi, A. Saarilahti,J.Antson,andS.Lindfors, ThinSolidFilms, vol. 225,no.84,pp.296–298,1993.
[53] S. Tanimoto,N. Shibata,K. Kuroiwa, andY. Tarui, Appl. Surf. Sci., vol.79/80,no.84,pp.220–226,1994.
[54] H. ZhangandR. Solanki, J. Electrochem.Soc., vol. 148,no. 4, pp. F63–F66,2001.
[55] J.Aarik, K. Kukli, A. Aidla, andL. Pung,Appl.Surf. Sci., vol. 103,no.4,pp.331–341,1996.
[56] M. Balog,M. Schieber, S.Patai,andM. Michman, J. Cryst.Growth, vol.17,pp.298–301,1972.
[57] H. Ono,Y. Hosokawa, T. Ikarashi,K. Shinoda,N. Ikarashi,K. Koyanagi,andH. Yamaguchi,J. Appl.Phys., vol. 89,no.2, pp.995–1002,2001.
[58] K. Kukli, J. Aarik, A. Aidla, H. Siimon, M. Ritala, and M. Leskela,Appl.Surf. Sci., vol. 112,no.84,pp.236–242,1997.
[59] K. Kukli, J. Ihanus,M. Ritala,andM. Leskela, J. Electrochem.Soc., vol.144,no.184,pp.300–306,1997.
[60] H. Koyama,S. Tanimoto,K. Kuroiwa, andY. Tarui, Jpn.J. Appl. Phys.,vol. 33,no.11,part184,pp.6291–6298,1994.
53
[61] H.-T. Chiu, C.-N. Wang,andS.-H.Chuang,Chem.Vap. Deposition, vol.6, no.5, pp.223–225,2000.
[62] J.Minet, F. Langlais,R. Nasrin,andC. Bernard,J. Less-Comm.Met., vol.119,pp.219–235,1986.
[63] H. YamaneandT. Hirai, J. Cryst.Growth, vol. 94,pp.880–884,1989.
[64] R. N. Tauber, A. C. Dumbri, andR. E. Caffrey, J. Electrochem.Soc, vol.118,no.5, pp.747–754,1971.
[65] M. RitalaandM. Leskel, Appl. Surf. Sci., vol. 75,pp.333–340,1994.
[66] R. C. Smith,N. Hoilien, C. J.Taylor, T. Ma, S.A. Cambell,J.T. Roberts,M. Copel,D. A. Buchanan,M. Gribelyuk,andW. L. Gladfelter, J. Elec-trochem.Soc., vol. 147,no.9, pp.3472–3476,2000.
[67] R. C. Smith, T. Ma, N. Hoilien, L. Y. Tsung,M. J. Bevan, L. Colombo,J. Roberts,S. A. Campbell,andW. L. Gladfelter, Adv. Mater. Opt. Elec-tron., vol. 10,pp.105–114,2000.
[68] K. Kukli, M. Ritala,andM. Leskela, Chem.Vap.Deposition, vol. 6, no.6,pp.297–302,2000.
[69] A. Bastianini,G. A. Battiston,R. Gerbasi,M. Porchia,and S. Daolio,J. Phys.IV France, vol. 5, pp.C5525–531,1995.
[70] M. A. CameronandS.M. George, Thin SolidFilms, vol. 348,pp.90–98,1999.
[71] C.F. Powell, ChemicallyDepositedNonmetals, (JohnWiley & Sons,NewYork, 1966).
[72] K. Kukli, J. Ihanus,M. Ritala,andM. Leskela, Appl.Phys.Lett., vol. 68,no.2684,pp.3737–3739,1996.
[73] J. Aarik, A. Aidla, A.-A. Kiisler, T. Uustare,andV. Sammelselg,ThinSolidFilms, vol. 340,pp.110–116,1999.
[74] P. MartenssonandA. Harsta,J. Cryst.Growth, vol. 156,pp.67,1995.
[75] M. Schuisky andA. Harsta,Chem.Vap.Deposition, vol. 4, pp.213,1998.
[76] M. Schuisky andA. Harsta,J. Phys.IV France, vol. 9, pp.Pr8–831,1999.
54 Bibliography
[77] M. Schuisky, A. Harsta,A. Aidla, K. Kukli, A.-A. Kiisler, andJ. Aarik,J. Electrochem.Soc., vol. 147,pp.3319,2000.
[78] M. Schuisky, J.Aarik, K. Kukli, A. Aidla, andA. Harsta,,Langmuir, vol.17,pp.5508,2001.
[79] Gmelin Handbuch der anorganischen Chemie, Number 42. (VerlagChemie,Weinheim,1958).
[80] J.Gerlach,J.-P. Krumme,andF. Pawlek, J. Less-Comm.Met., vol. 15,pp.303–310,1968.
[81] P. J. DurrantandB. Durrant, Introductionto AdvancedInorganic Chem-istry, (Longmans,London,1962).
[82] E. deBarry BarnettandC. L. Wilson, Inorganic Chemistry, (LongmansGreen& Co,London,1953).
[83] M. StrømmeMattsson,G. A. Niklasson, K. Forsgren,and A. Harsta,J. Appl.Phys., vol. 85,no.4, pp.2185–2191,1999.
[84] E. P. Burte and N. Rausch, J. Non-Cryst.Solids, vol. 187, no. 84, pp.425–429,1995.
[85] K. Tominaga, R. Muhammet, I. Kobayashi, and M. Okada,Jpn.J. Appl.Phys., vol. 31,no.5A, part284,pp.L585–587,1992.
[86] J.Aarik, A. Aidla, K. Kukli, andT. Uustare,J Cryst.Growth, vol. 144,no.84,pp.116–119,1994.
[87] K. Kukli, A. Aidla, J. Aarik, M. Schuisky, A. Harsta,M. Ritala, andM. Leskela, Langmuir, vol. 16,pp.8122,2000.
[88] K. Kukli, J. Aarik, A. Aidla, K. Forsgren, J. Sundqvist, A. Harsta,T. Uustare,H. Mandar, and A. -A. Kiisler, Chem.Mater., vol. 13, no.1, pp.122–128,2001.
[89] K. Chen, M. Nielsen, E. J. Rymaszewski, and T. M. Lu,Mater. Chem.Phys., vol. 49,no.84,pp.42–45,1997.
[90] J.D. McCulloughandK. N. Trueblood,ActaCryst., vol. 12,pp.507–511,1959.
55
[91] C. S. HwangandH. J. Kim, J. Mater Res., vol. 8, no.6, pp.1361–1367,1993.
[92] J.Aarik, A. Aidla, T. Uustare,andV. Sammelselg,J. Cryst.Growth, vol.148,pp.268,1995.
[93] C. J. Howard, E. H. Kisi, R. B. Roberts,and R. J. Hill, J. Amer. Ce-ram.Soc., vol. 73,no.10,pp.2828–2833,1990.
[94] B. Bondars,G. Heidemane,J. Grabis,K. Laschke, H. Boysen,J. Schnei-der, andF. Frey, J. Mater. Sci, vol. 30,pp.1621–1625,1994.
[95] P. Duwez,andF. Odell, J. Amer. Ceram.Soc., vol. 33,pp.274–283,1950.
[96] B.-K. Kim, J.-W. Hahn,andK. R. Han, J. Mater. Sci.Lett., vol. 16, pp.669–671,1997.
[97] R. E. Hann,P. R. Suitch,andJ.L. Pentecost,Comm.Amer. Ceram.Soc.,vol. 68,no.10,pp.C–286–C–286,1985.
[98] C. E. Curtis,L. M. Doney, andJ. R. Johnson,J. Amer. Ceram.Soc., vol.37,no.10,pp.458–465,1954.
[99] I. A. El-Shanshoury, V. A. Rudenko, and I. A. Ibrahim, J. Amer. Ce-ram.Soc., vol. 53,no.5, pp.264–268,1970.
[100] J.-Y. Gan,Y. C. Chang,andT. B. Wu, Appl.Phys.Lett., vol. 72,no.384,pp.332–334,1998,
[101] K. ForsgrenandA. Harsta,Thin SolidFilms, vol. 343-344,pp.111–114,1999.
[102] K. Lehovec, J. Less-Comm.Met., vol. 7, pp.397–410,1964.
[103] J.SundqvistandA. Harsta,to bepublished.
[104] A. Harsta,Chem.Vap.Deposition, vol. 5, no.4, pp.191–193,1999.