Enhanced performance and reliability of NILC-TFTs using FSG buffer layer

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MaterialsChemistryandPhysics132 (2012) 637–640

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Materials

Chemistry

and

Physics

jo u r n al h om ep a ge : w w w . e l s e v i e r . c o m / l o c a t e / m a t c h e m p h y s

Enhanced

performance

and

reliability

of

NILC-TFTs

using

FSG

buffer

layer

Chien-Chih

Chen, YewChung

Sermon

Wu

∗

, Chih-Pang

Chang

DepartmentofMaterialScienceandEngineering,NationalChiaoTungUniversity,1001UniversityRoad,Hsinchu300,Taiwan,ROC

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received16December2010 Receivedinrevisedform 30September2011 Accepted30November2011

Keywords:

Fluorinated-silicate-glass(FSG)

Polycrystallinesiliconthin-ﬁlmtransistors (poly-SiTFTs)

Ni-metal-inducedlateralcrystallization (NILC)

a

b

s

t

r

a

c

t

AnewmanufacturingmethodforNi-metal-inducedlateralcrystallizationthinfilmtransistors (NILC-TFTs)usingfluorine-silicate-glass(FSG)wasproposed.InFSG-TFTs,fluorineionwasimplantedintothe bufferoxidelayertoformFSGbeforeNILCprocesses.ItwasfoundFSG-TFTsexhibithighfield-effect mobility,lowthresholdvoltage,lowsubthresholdslope,highON/OFFcurrentratio,lowtrapstatedensity, lowinterfacetrapstatedensity,andgoodreliabilitycomparedwithtypicalNILC-TFTs.

1. Introduction

Polycrystallinesiliconthin-filmtransistors(poly-SiTFTs)had beenwidely usedin active-matrix organiclight emitting diode (AMOLED) because they exhibit good electrical properties and canbeintegratedinperipheralcircuitsoninexpensiveglass sub-strates[1].Sincepoly-Si TFTsrequireglasssubstrates,intensive studieshavebeencarriedouttolowerthecrystallization temper-atureofamorphoussilicon(a-Si)films.Ni-metal-inducedlateral crystallization(NILC)wasoneoftheeffectivemethodsthatcould crystallizeatatemperaturebelow600◦C[2,3].Unfortunately, poly-Si/oxideinterfacesandgrainboundariestrapNiandNiSi2,which increasestrapstatedensity,shiftsthresholdvoltage,andlowers field-effectmobility[4,5].Recently,fluorineionimplantationwas employedtoimprovetheelectricalperformanceofTFTs[6,7].Itwas foundthatfluorineatomseffectivelyminimizethetrapstate den-sityandimproveNILC-TFTs’electricalpropertiesandreliabilities [8].

Inearlyresearch,fluorineionwasalsousedtoimproveTFT per-formance[9–11].Toavoidimplantationdamage,fluorineionwas implantedthroughapadoxidebeforeintoactivelayer[9,10].If flu-orineionwasimplanteddirectlyintoactivelayer(withoutapad oxide),theimplantationmaydamagethechannel(poly-Sisurfaces) andlowerfluorinepassivationeffects[11].

Inthispaper,ﬂuorinated-silicate-glass(FSG)processwas devel-oped to reduce this implantation damage and improve the

∗ Correspondingauthor.Tel.:+88635131555;fax:+88635724727. E-mailaddress:[email protected](Y.S.Wu).

performanceofNILC-TFTs.TheTFTdeviceswithFSGbufferlayer notonlycanpreventtheimplantdamagealsosaveapadoxide fabrication.

2. Experiment

TheNILC-TFTswithFSGlayer(FSG-TFTs)fabricationprocess began withcapping Si wafers with500-nm-thick wet thermal oxide.TofabricateFSG-TFTs,thefluorineionwasimplantedinto thethermaloxidelayertoformfluorine-silicate-glass(FSG).The projectionrangeoffluorineionwassetatunderthetopsurfaceof theFSGlayerandtheacceleratingenergywas30keV.

ToinvestigatetheeffectoffluorinecontentinFSGlayeronthe performancesofTFTs,twotypesofFSG-TFTswerefabricated.They weredenotedasF2E12-TFT(2×1012_cm−2_dosages)_and_F2E15-TFT (2×1015_cm−2 _dosages). _Next, _a _100-nm-thick _undoped amor-phoussilicon(a-Si)layerwasdepositedbylowpressurechemical vapordeposition(LPCVD)at550◦C.The5-nm-thickline-patterned Niwasdepositedonthea-Si,subsequentlyannealedat530◦Cfor 48htoformtheNILCpoly-Sifilm.Afterthecrystallizationofthe a-Si,theactiveregionsweredefinedbyreactiveionetching(RIE). The100-nm-thickTEOSoxidewasdepositedbyplasma-enhanced CVD(PECVD)forgateoxideand 100-nm-thickpoly-Si filmwas depositedbyLPCVDforgateelectrodes.P+_-ion_was_implanted_at adoseof5×15cm2_to_form_the_source/drain_and_gate_after defin-ingthegatepattern.Theactivationofthesource/drainregionswas realizedbythethermalfurnaceunderN2ambientat600◦Cfor24h. Forthepurposeofcomparison,theNILC-TFTswerealso sub-jectedtothesameprocessesbutwithoutfluorineionimplantation.

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638 C.-C.Chenetal./MaterialsChemistryandPhysics132 (2012) 637–640

Fig.1. ID–VGtransfercharacteristicsofTFTs.

3. Resultsanddiscussion

Fig. 1 illustrates the ID–VG transfer characteristics of TFTs (W/L=10/10␮m) with and without FSG. The measured and extractedkeydeviceparametersweresummarizedinTable1.It wasfoundFSG-TFTs(F2E12-TFTandF2E15-TFT)exhibithigh ﬁeld-effectmobility(FE),lowthresholdvoltage(VTH),lowsubthreshold slope(S.S.)andhighON/OFFcurrentratio(ION/IOFF)comparedwith NILC-TFTs.Thisindicatesthetrapstatedensity(NT)waseffectively reducedusingFSG.

ThetrapstatedensityoftheTFTswasextractedusingLevinson andProano’smethod,whichcanestimatetheNTfromtheslope ofthelinearsegmentofln[IDS/(VGS−VFB)]vs.1/(VGS−VFB)2atlow VDSandhighVGS,whereVFBisdeﬁnedasthegatevoltagethatyields theminimumdraincurrentatVDS=0.1[12,13].AsshowninTable1, thetrapstatedensitiesofF2E12-TFTandF2E15-TFTwerelessthan thoseofNILC-TFTs.

InNILCpoly-Si,therearetwokindsofdefectsrelatedtotrap statedensity(NT):(1)grainboundarydefectsand(2)Ni-related defects.Thesedefectswoulddegradeelectricperformancebecause theyintroduceddanglingbondsandstrainbonds[4,5]. Secondary-ionmassspectroscopy(SIMS)wasusedtostudythedistribution of Ni and F. Fig. 2 shows the depth profile of the F2E12 and F2E15gate-oxide/poly-Si/buffer-oxidestructure.HighNiandhigh Fcontentsarebothpresentatthepoly-Si/buffer-oxideinterface. Thisobservationsuggestedthatfluorineatoms havediffusedto theinterface/boundariestoterminateNi-related trapstates and boundary defects. As a result, the trap state density (NT) was reduced,andelectricalcharacteristicswereimproved[6–8].ThisF diffusionbehaviormightbebecausemostoftheNi-relateddefects (residues)werelocatedattheinterfacebetweenpoly-Siandoxide. Asaresult,Fatomsdiffusedtointerfacetopassivatethesedefects, whichincludingdanglingbondsandstrainbondsbyformingstrong Si Fbonds[8].Besides,sincetheNILCpoly-Sifilmisonly100nm, Fatomshaveenoughtimetoreachtheinterfaceduringthe subse-quentlyprocedures(including600◦C,24h).

Fig.2.SIMSdepthproﬁleofﬂuorine,oxygen,siliconandnickeltheatomsof F2E12-TFTandF2E15-TFT.

However,theperformanceofF2E15-TFTwaspoorerthanthat ofF2E12-TFT,asshowninFig.1.ThisisbecauseFcontentinthe activelayerwashigherthanFsolubility([F]RT)intheSiatroom temperature.As mentioned earlier, F ionswere implantedinto thethermaloxidelayer.Withthesubsequentlyactivationprocess (600◦C,24h),Fatomshaveenoughtimetodiffusetointerfaceto passivatedefects.Atthesametime,Fcontentintheactivelayer reachedFsolubility[F]600◦C,whichismuchhigherthan[F]RT.Asa result,whendeviceswerecooleddowntotheroomtemperature, FcontentintheactivelayerwashigherthantheFsolubility([F]RT). ThisisconsistentwithourSIMSmeasurements.Asshownin Fig.2,theFcontentinside twoFSG-TFTsfilmswerealmostthe sameexceptatinterfaceareas.Thisisbecauseinbothcase,F con-tentsreached[F]600◦C.ExtraFatomsweretrappedatinterfaces, wheremostofNi-relateddefects(residues)were.Asaresult,the excessFatomsinF2E15-TFTsegregatedandformedfluorine clus-tersatinterfaces.Thesefluorineclusterswouldnotpassivatethe trapstatesbutincreasethedefectsandthendegradetheelectrical performances[8,14].AsshowninTable1,theNTofF2E15-TFTwas higherthanthatofF2E12-TFT.

Asmentionedearlier,mostNiresidues(Ni-relateddefects)were locatedattheSi/oxide interface(Fig.2)and formedtrapstates. Theinterfacetrapstatedensity(Nit)nearthepoly-Si/gateoxide interfacecouldbecalculatedas:

Nit=

_S_.S. ln 10

_q kT

−1

_C ox q

whereCox wasgate oxidecapacitance [15].Asshown inFig.3, NitofFSG-TFTswaslessthanthoseofNILC-TFTs.Besides,Nit of TFTsincreasedwithtemperature.Thisincreaseimpliesthat leak-agecurrentincreaseswiththedevicesworkingtemperaturedueto thethermalexcitedtrappedcarriers[16].Comparedwiththoseof NILC-TFTs,theNitdegradation(thermalstability)ofFSG-TFTswas improvedbyFSGprocesses.Thiswasbecauseofthepassivationof danglingbondsandstrainbondsatthepoly-Si/oxideinterfaceby ﬂuorineatoms.

Table1

DevicecharacteristicsofNILC-TFT,F2E12-TFTandF2E15-TFT.

FE(cm2/Vs) VTH(V) S.S.(V/dec) ION/IOFF(106) NT(1012cm−2)

NILC-TFT 55.1 6.12 2.13 0.56 4.61

F2E12-TFT 97.5 2.48 1.14 2.36 2.25

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Fig.3.Theinterfacetrapstatedensityandtheirvariationversustemperaturefor NILC-TFT,F2E12-TFTandF2E15-TFT.

Theotherimportant issueof poly-SiTFTsis theirreliability,

whichwasexaminedunderhot-carrierstress.TheDCstress

condi-tionwassetgatevoltageVG,stressatVG−VTH=10Vanddrainvoltage

VD,stressatID=10−4A.Thestresstimewas10,000s.Asshownin

Figs.4and5,theVTHandtheIONofTFTsweredegraded.Thiswas becausewhenTFTswereundertheDCstressoperation,hot-carrier wouldimpactintodrainsideandcreatebrokenbondsduetothe weakSi-SibondsandSi Hbonds.Thosebrokenbondsordangling bondswouldincreasethetrapstatedensity,leadingtothe degra-dationofelectricalcharacteristics[17,18].Comparedwiththoseof NILC-TFTs,theVTHvariationandIONdegradationsofFSG-TFTswere greatlyimproved.ThiswasbecausetheweakerSi Sibondsand Si HbondsinFSG-TFTswerereplacedbythestrongerSi Fbonds whichappearedexcellentelectricalenduranceagainsthot-carrier impact.

Althoughtheelectricalreliabilitieswereenhancedby ﬂuorine-ion incorporating in poly-Si, it was also found the electrical degradationofF2E12-TFTwasworsethanthatofF2E15-TFT,as showninFigs.4and5.OnaccountoftheF2E12-TFThasthe high-estFE,thehot-carrierimpacteffectswouldbemoreserious.When TFTdeviceswereunderworking,theelectronwillbeaccelerated bytheapplieddrainelectricﬁeld.ThehigherFEimpliesthatthe lesselectronscatteringwhileelectrontransferringfromsourceto

Fig.4. ThresholdvoltagevariationversusstresstimeforNILC-TFT,F2E12-TFTand F2E15-TFT.

Fig.5. ON-currentdegradationversusstresstimeforNILC-TFT,F2E12-TFTand F2E15-TFT.

drain,sothedegradationofelectricalperformanceswasgreater duetotheelectronwithhigherenergy[19].

4. Conclusion

AninvestigationoftheeffectsofFSGbufferlayerontheelectrical characteristicsandreliabilityofNILC-TFTshasledtothe develop-mentofa simple,effectiveprocessforimprovingtheNILC-TFTs electricalproperties.ThiswasbecauseFatomscouldpassivate dan-glingbondsandstrainbonds.ItwasalsofoundthatFSGprocess couldgreatlyalleviatethethresholdvoltageandtheON-current degradationsunderhot-carrierstress.Thiswasduetotheweaker Si HandSi SibondswerereplacedbystrongerSi Fbonds,thus leadingtoimprovedelectricalreliability.However,theexcess ﬂu-orineatomswouldnotpassivatethetrapstatesbutincreasethe defectsandthendegradetheelectricalperformances.

Acknowledgments

This project was funded by Sino American Silicon Products IncorporationandtheNSCoftheROCunderGrantNo. 98-2221-E-009-041-MY3.TechnicalsupportsfromtheNationalNanoDevice Laboratory,CenterforNanoScienceandTechnologyandtheNano Facility Center of the National Chiao Tung University are also acknowledged.

AppendixA. Supplementarydata

Supplementarydataassociatedwiththisarticlecanbefound,in theonlineversion,atdoi:10.1016/j.matchemphys.2011.11.080.

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