Fu-Hsiang
Ko
a,∗aDepartmentofMaterialsScienceandEngineering,NationalChiaoTungUniversity,Hsinchu300,Taiwan
bDepartmentofElectronicEngineering,NationalChin-YiUniversityofTechnology,Taichung411,Taiwan
cDepartmentofAppliedChemistry,NationalChiaoTungUniversity,Hsinchu300,Taiwan
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Articlehistory:
Received3May2012
Receivedinrevisedform21June2012
Accepted21June2012
Available online 28 June 2012 Keywords:
Zirconiaultrathinfilm
Sol–gel
Highdielectricmaterial
Out-gassingcontamination
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Thisstudysuccessfullypreparedzirconiaultrathinfilmsfromthesol–gelsolutionwithdispersionof zirconiumhalidein1-octanolsolvent.Thefilmwassubjectedtoannealingtreatmentsaftersol–gel spin-coating,andthefilmsofinterestwereevaluated.Theamorphousmorphologyofthezirconiafilmwas identifiedusinghigh-resolutiontransmissionelectronmicroscopyandX-raydiffractometry.Theplotof thecurrentdensitywithrespecttotheelectricfielddemonstratesthattheas-depositedfilmat500◦C
annealingexhibitedaninferiorleakagecurrent,whereas600◦Cannealingstabilizedthefilmwitha satisfactoryleakagecurrentof10−8to10–9A/cm2.Theout-gassingbehaviorofthesol–gel-derivedthin
filmwasevaluatedusingathermaldesorptionsystem,thatis,atmosphericpressureionizationmass spectrometry.Thedielectricconstantofthefilmwasdependentontheretentioneffectofthepreparation solvents.Thelowresidualsolventforthepreparationofthethinfilmwith1-octanolsolventand600◦C
annealingcontributedtothesuperiorhigh-kproperty.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
Silicon-basedtechnologyhasbeenusedinmicroelectronicsand electronicssystems formorethan 30years. Severalresearchers focusonscalingdownelectronicdevicestooperateathigherspeeds withmultiplefunctions.ConventionalSiO2 gatedielectricshave reachedtheirphysicalthicknesslimit(1.5nm)andcannotbeused asCMOSdevices becauseoftheirhighdirecttunneling current andpoorreliability.Advancedmaterialsarecrucialforsmaller sys-temswithscaled-downthicknesses.Toaddresstheissueofleakage current,varioushighdielectricmaterialssuchaszirconiumoxide, hafniumoxideortantalumoxidearethemostpromising candi-datesforuseasanalternatingthinmaterialtoreplaceSiO2film
[1–8].Theseelectronicmaterialsexhibithightemperature stabil-ityandexcellentinsulatorpropertiesagainstheatandelectricity. Infact,dielectric filmshavinghigherpermittivityallowtheuse ofthickerfilmsofequivalentelectricalthicknessassilicon diox-ide;thissituationwillreducetheleakagecurrentandimprovethe fabricationreliabilityofthedielectricfilms. Recently,numerous technologieshavebeendevelopedforthepreparationofvarious high-kfilms[9–12].Atomiclayerdeposition(ALD),physicalvapor deposition(PVD), andchemical vapor deposition(CVD)arethe
∗ Correspondingauthor.
E-mailaddress:[email protected](F.-H.Ko).
mostusefultechnologiesamongthevariousmethodstoprepare metaloxidefilms.Itiscrucialtodeterminethemannerinwhich todepositandpreparethezirconiamaterialforhighdielectricthin films.IntheALDprocess,theprecursorofZrCl4andH2Oareused topreparethezirconiafilms. For thePVDprocess,a zirconium metaltargetisusedforsputteringunderambientoxygentodeposit thezirconiafilms.IntheCVDmethod,aZrCl4precursorisusedto depositzirconiafilms.However,thesemethodsrequireultrahigh vacuumconditionsandexpensivetools.
Thesol–gelspin-coatingmethodisanefficientapproachto pro-ducecrack-freeandsmoothceramicfilmswithexcellentsurface conformityanduniformityoverlargeareas[13–15].Inaddition, such filmscan be fabricated at room temperature and normal pressure,therebyobviatingtherequirementsforhigh-vacuum sys-tems[16–18].Thesol–gelmethodcanprovidecolloidalsolventsor precursorcompoundswhenmetalhalidesarehydrolyzedunder controlledconditions[18].Inthesol–gelprocess,hydrolysis, con-densation, and polymerizationstepsoccurtoformmetal oxide networks. These reactions play decisive roles in determination ofthefinalmaterial’sproperties.Themostinterestingfeatureof sol–gelprocessingisitsabilitytosynthesizenewtypesof materi-alsknownas“inorganic–organichybrids.”Filmsformationwith the sol–gelspin-coating method is simpler methodthan using ALD,PVDorCVDbecauseofitscost-effectiveprecursorandtools. In addition, thefilm can be fabricatedin the normal pressure systeminsteadofhighvacuumsystem.Typically,zirconiaisan
0169-4332/$–seefrontmatter © 2012 Elsevier B.V. All rights reserved.
Fig.1. Cross-sectionalTEMimagesofsol–gel-derivedZrO2/Sistructuresannealedat(a)200◦Cand(b)600◦Cunderanoxygenatmosphere.
excellentheat-resistantand chemicallydurable materialthat is used, for example, as a material for furnaces [19]. Although zirconia-based materialsgenerally provide good high-dielectric and thermal property, the electrical insulation issues such as leakagecurrentand dielectricconstantfor sol–gel-derivedZrO2 ultrathinfilmstillrequirefurtherimprovement.Inaddition,the techniqueforachievingreliablepropertyofultrathinfilmunder cost-effectivemanufacturingisalsorequiredtodevelop.
Thisstudyproposedtheuseofmaterialfromthedispersionof zirconiumhalideinorganicsolventtofabricatetheultrathinfilmby sol–gelspin-coating.Thethicknessandstructureofthefilmwere identified by high-resolution transmission electron microscopy (HRTEM)andX-raydiffractometry(XRD).Todeterminethe resid-ualorganiccontaminationinthefabricatedzirconiaultrathinfilm, theout-gassingresidualfromthepreparedthinfilmatahigh tem-peraturewasevaluatedbya thermaldesorptionsystem,thatis, atmosphericpressureionizationmassspectrometry(TDS-APIMS). Theelectricalpropertyofthethin filmfromsimpleandreliable sol–gelprocessingwasevaluatedforthecapacitance–voltageand currentdensity–electricfieldcharacteristics.
2. Experimental
2.1. Thinfilmmaterialandfabrication
Zirconiathinfilmswerepreparedbyusingaspin-coatingsol–gel method.ZrCl4 (>99.5%,Aldrich,USA)wasusedastheprecursor forthesynthesisofzirconia.Thethinfilmswerepreparedthrough polymerizationinanorganicsolution.Amothersolsolutionwas firstpreparedbydissolvingZrCl4in1-octanol(Fluka,>99.5%)under vigorousstirringinanicebath.Thesolsolutionwasobtainedby fullyhydrolyzingZrCl4withastoichiometricquantityofwaterin 1-octanoltoyieldaZr:octanolmolarratioof1:1000.Themetalhalide solutionwassubsequentlysubjectedtoultrasonicationat0◦Cfor 20mintoacceleratethegellingrate.
Then-typewafers (10–15cm), whichwereservedas sub-stratesforthemetalinsulatorsemiconductor(MIS)structure,were RCAcleaned toremove thenativesilicon oxidefilm and parti-cle. Zirconiafilmswere deposited by spin-coating at 3000rpm for60satambienttemperature(25◦C)toobtaina thicknessof ca.9.5nmonthen-Si (100)substrate.The filmswereinitially annealedat200◦Cfor10min.Oneofthesampleswasmaintainedat 200◦Cfor1mindensification;theothersampleswererapid ther-malannealed(RTA)atdifferenttemperatures(500or600◦C)for 1minunderanoxygenatmosphere.Zirconia/n-Sistructureswere usedinourexperimentsforthefurthercharacterization.
TheAltopelectrodeswithasurfaceareaof3.14×10–4cm2were depositedontothetopsurfaceofthezirconiausingashadowmask andpureAltargetevaporation.Thethicknessofthetopelectrode was400nm.Abufferedoxideetch(BOE)solutionwasperformed toremovethesiliconbackoxideandthenthesamplewas thor-oughly rinsed withdeionized water. Finally, bottom electrodes wereattachedonthereversesideofthen-Si(100)substratesby vaporizingapureAltarget.Thethicknessofthebottomelectrode was100nm.Thedielectric propertiesweresubsequently deter-minedfortheMIS capacitorswithanAl/ZrO2/n-Si/Alstructure. Thedepositiontechniqueallowstheproductionofultrathingate dielectricfilmsunderexcellentthicknesscontrol.
2.2. Characterizations
Themorphology of thezirconiaultrathinfilm wasobserved byafield-emissionTEMtool(JEOLJEM-2100F).Thestructureof theobtainedultrathinfilmwascharacterizedwithanXRD instru-ment.Theorganiccontaminationofthesol–gel-derivedthinfilm wasanalyzedbytheTDS-APIMStoolfromHitachicompany.The thermaldesorptionspecieswerepurgedbynitrogengas,and var-iousspecies ofinterest werecontinuallymonitoredusingmass spectrometry.ThehighdielectricfilmswerepreparedintheMIS structuretocharacterize theleakagecurrentsand capacitances. Thecapacitance–voltagewasmeasuredusinganAgilent4284AC–V analyzer;thecurrentdensitywithrespecttoelectricfieldplotwas measuredusinganAgilent4156Canalyzer.
trapsandenhancethedevicereliability.Ensuringa smoothand crack-freeinterfacefortheMISdeviceiscriticaltoachievethehigh dielectricproperty.OurultrathinfilminFig.1suggeststhe pro-posedsol–gelmethodisanefficientmeansforhighdielectricdevice application.
ThecrystallinityoftheZrO2 film-based sampleswas investi-gatedusing1.5◦glancing-angleXRD.Thecrystallinityofthehigh-k dielectricfilmwasofconcernbecauseofthepossibledeterioration effectscausedbytheleakagepathsanddopant/impuritydiffusion alongthegrainboundaries,aswellasforthecontrolofdevice uni-formity.Hence,anamorphousstructureofhighdielectricfilmwas necessary.Liuetal.[3]usedthesol–gelmethodtofabricatethe 450-nmthicknessofZrO2film,andthecrystallinestructureswere
Fig.3.TDS-APIMSanalysesofthesol–gel-derivedZrO2filmspreparedfrom(a)
1-octanolsolventandRTA500◦C,(c)1-octanolsolventandRTA600◦C.
dispersedintheorganicsolvent,theresidualsolventafter anneal-ingmaydegradetheelectricalpropertiesforhighdielectricfilm. However,few studieshave addressedthis issue.We used TDS-APIMScharacterization tomeasure residualorganiccompounds presentinthefilmsafterannealingof500and600◦C.Weramped thedesorptiontemperatureoftheTDS-APIMSsystemfromroom temperatureto800◦C at20◦C/minunderanN2 atmosphere.In
Fig.3,wesimultaneouslyanalyzethesurfaceout-gassesat mass-to-chargeratio(m/z)of16,44,and82.Thesol–gelfilmcontainedZr, Cl,C,H,andOelements,whichwerederivedfromthepreparation solventsandtheprecursor.Theout-gassingspecieswith molec-ularweightsof16and44mayrepresentcarbonelementsinthe
Fig.4. High-frequency(at1MHz)C–Vcharacteristicsofthesol–gel-derivedZrO2
Fig.5.Gatecurrentdensityvselectricfield(J–E)characteristicsoftheultrathinfilm
aftervariousRTAtreatments.
formsofCH4andCO2,respectively.Incontrast,thechlorine com-ponents(inprecursor)of thefilmcombinedwithcarbonatoms (insolvent)toformCCl2(m/z=82).Incomparisonwithour previ-ousstudy[1],wefindthatthedesorptiongasesorspeciesfromthe sol–gelfilmpreparedby1-octanolsolventwereconsiderablylower thanthepreparation solventof2-propanol/hexanol.This obser-vationsuggeststhe1-octanolsolventwasmoreeasilyremoved fromthehigh-kfilmbyusingtheannealingmethodthanthe 2-propanol/hexanolsolventbecauseoftheintermolecularhydrogen bondinginthehigh-kfilm.Theproportionofthehydroxylgroup inthe1-octanolmoleculewaslowerthanthatinthe2-propanol orhexanolmolecule,whichindicatedthatthe1-octanolmolecule hadalowhydrogenbondinginteractionwiththeZrO2 molecule. Therefore,thefilm preparedfromthe1-ocatanol solventhad a lowerdesorptionpeakthanthatfromthe2-propanol/hexanol sol-vent.Thesolventtypeplaysacriticalroleinthedielectricproperty ofthesol–gel-derivedthinfilm.Thisresidualspeciesinthehigh dielectricfilmmayaffectthedielectricconstantandleakage cur-rentmentionedlater.
3.3. Electricalpropertyofsol–gel-derivedultrathinfilm
We measuredthequantitative capacitanceand leakage cur-rentto evaluatethe dielectric performance of thehigh-k ZrO2 ultrathinfilmintheMISconfiguration.Fig.4illustratesthe high-frequency(1MHz)capacitanceversusgatevoltagecharacteristics (C–Vcurves)fortheMIScapacitorspreparedfrom1-octanol sol-ventunderthevariousannealingconditions.Thehysteresisfrom theC–Vcharacteristicswasusedtoevaluatethereliabilityofthe film.Bysweepingthevoltagefromaccumulationtoinversionand
Table1
Dielectricconstant(k)andequivalentoxidethickness(EOT)ofZrO2thinfilms
pre-paredusingtwodifferentsolventsandannealedat600◦C.
Solventtype Anneal(◦C) k EOT(nm) Reference
1-octanol 500 12.8 2.89 Thiswork
1-octanol 600 16.4 2.26 Thiswork
2-propanol/hexanol 600 13.9 3.04 [1]
2-propanol/hexanol 900 12.8 3.91 [1]
sweepingback(from+2.0to−2.0to+2.0V),allofthefilms exhib-itedashiftoflessthan30mVforthehysteresisphenomenon.The C–Vcurveforthesampleannealedat500◦Cwasinferior;several stepsoccurredinthecurve,whichindicatesthatthefilmswere unstableafterannealingat500◦C.Hence,thesampleannealedat 500◦Cdidnotexhibitanexcellentelectricalpropertybecauseofthe excessiveresidualorganicsinthefilm.Whentheannealing tem-peraturewasincreasedto600◦C,theZrO2filmsexhibitedexcellent andreversibleelectricalproperties.
Table1showsacomparisonofthedeviceswithvarioussolvents andannealedatdifferenttemperatures.Thevalueofequivalent oxidethickness(EOT)is3.04nmandthedielectricconstantis13.9 forthepreparationfrom2-propanol/hexanolsolventandannealed at 600◦C underan O2 atmosphere [1], while 2.26nm EOT and 16.4(dielectricconstant)from1-octanolat 600◦Cunder anO2 atmosphere.Ifannealedtemperatureisat500◦C,boththe dielec-tricconstantandEOTaredegradedduetocarbon-contamination issue.Thisobservationsuggeststhedispersionsolventfor sol–gel-deriveddevicesisfavorforlowportionofhydroxylgroupsolvent. Hence,the researchproposesthat 1-octanol solventis suitable fortheZrO2 sol–gelpreparationtofabricatethehigh-kultrathin film.
Theelectric field(E) intheJ–Ecurveof Fig.5 wasobtained usingtheequationE=V/tox,whereVistheappliedvoltageand tox is theEOTdeterminedthroughC–Vmeasurement. The sam-plesannealedat 200and 500◦Ccontaineda coupleoforganics intheZrO2film.Hence,theirbreakdownelectricfieldswere rela-tivelylowat2.6and4.2MV/cm.Theunsatisfactorybreakdownfield wasrelatedtotheeffectofresidualorganiccontaminationas men-tionedearly.Thebreakdownelectricfieldincreasedto4.5MV/cm whentheannealingtemperaturewasincreasedto600◦C. Simi-larly,the600◦CRTAdemonstrated theleakage currentof 10−8 to10−9A/cm2.Ahighandsuitableannealingtemperatureledto theremovaloftheunstablecarbon-containingcompoundsfrom ZrO2 film and stabilized the thin film. Table 2 compares vari-ousmethods[1,2,4–7]forthefabricationofthinceramicfilm.On theissueof dielectric constant and leakagecurrent, ourdevice hasbetterperformancethan methods ofphoto-assisted sol–gel spin-coating,oxygenplasma-enhancedsol–gelspin-coating and chemicalvapordeposition[2,4,6,7].Incomparisonwithexpensive physical vapor deposition [5], the electrical property on leak-agecurrentfromoursimpleandcheapersol–gel-derivedmethod
Table2
Comparisonofelectricalpropertiesofvarioushigh-kdielectricmaterials.
Preparationmethod/solution Film/substratematerial Annealingtemperature(◦C) Dielectricconstant Lowestleakagecurrentdensity(A/cm2) Reference
SGSC/octanol 9.5-nmZrO2/Si RTA/600 16.4 1×10–9 Thiswork
SGSC/*1 9.3-nmZrO2/Si RTA/600 13.9 1×10−9 [1]
LA-SGSC/ethanol 50-nmZrO2/Si CA/400 – 8×10−8 [2]
PE-SGSC/ethanol 9.5-nmZrO2/PI CA/250 14.8 9×10−9 [4]
PVD/invacuum 5.0-nmZrO2/Si CA/550 18.4 2×10–1◦ [5]
PE-SGSC/ethanol 10-nmHfO2/PI CA/250 11.7 4×10−9 [6]
CVD/invacuum 7.0-nmTa2O5/Si RTA/800 7.91 4×10–8 [7]
*1:mixingsolventof2-propanolandhexanol.PI:polyimidesubstrate,SGSC:sol–gelspin-coating,LA-SGSC:photo-assistedsol–gelspin-coating,PE-SGSC:oxygen
plasma-enhancedsol–gelspin-coating,RTA:rapidthermalannealing(∼1min),CA:conventionalovenorfurnaceannealing,PVD:physicalvapordeposition,CVD:chemicalvapor
remainedinanamorphousphase.Theoutgassingstudyofthese ultrathinfilmsindicatedthe600◦Cannealingwasthemostsuitable conditionto minimize theresidual organics in the ZrO2 ultra-thinfilms.TheMIScapacitorwiththeproposedprocedureshad lowerfabricationcost,superiordielectricconstant,higher break-down field, and lower leakage current than those using other techniques.
Acknowledgments
TheauthorswouldliketothanktheNationalScienceCouncil oftheRepublicofChina,Taiwan,forfinanciallysupportingthis researchundercontractof99-2120-M-009-008andNSC 98-2113-M-009-017,respectively.
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