Improvement mechanism of resistance random access memory with supercritical CO2 fluid treatment

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ContentslistsavailableatScienceDirect

The

Journal

of

Supercritical

Fluids

j o ur na l h o me p a g e :w w w . e l s e v i e r . c o m / l o c a t e / s u p f l u

Improvement

mechanism

of

resistance

random

access

memory

with

supercritical

CO

2

ﬂuid

treatment

Kuan-Chang

Chang

a

_,

_Jung-Hui

_Chen

b

_,

_Tsung-Ming

_Tsai

a

_,

_Ting-Chang

_Chang

c,i,∗

_,

Syuan-Yong

Huang

a

_,

_Rui

_Zhang

d

_,

_Kai-Huang

_Chen

e

_,

_Yong-En

_Syu

c

_,

_Geng-Wei

_Chang

f

_,

Tian-Jian

Chu

a

_,

_Guan-Ru

_Liu

c

_,

_Yu-Ting

_Su

c

_,

_Min-Chen

_Chen

c

_,

_Jhih-Hong

_Pan

a

_,

Kuo-Hsiao

Liao

a

_,

_Ya-Hsiang

_Tai

f

_,

_Tai-Fa

_Young

g

_,

_Simon

_M.

_Sze

c,h

_,

_Chi-Fong

_Ai

j

_,

Min-Chuan

Wang

j

_,

_Jen-Wei

_Huang

k

a_Department_of_Materials_and_{Optoelectronic}_Science,_National_Sun_Yat-Sen_University,₇₀_Lien-hai_Road,_Kaohsiung_804,_Taiwan b_Department_of_Chemistry,_National_Kaohsiung_Normal_University,_Kaohsiung,_Taiwan

c_Department_of_Physics,_National_Sun_Yat-Sen_University,₇₀_Lien-hai_Road,_Kaohsiung_804,_Taiwan d_School_of_Software_and_{Microelectronics,}_Peking_University,_Beijing_100871,_People’s_Republic_of_China e_Department_of_Electronics_Engineering_and_Computer_Science,_Tung-Fang_Design_Institute,_Kaohsiung,_Taiwan

f_Department_of_Photonics_and_the_Institute_of_{Electro-Optical}_Engineering,_National_Chiao_Tung_University,_Hsinchu_300,_Taiwan g_Department_of_Mechanical_and_{Electro-Mechanical}_Engineering,_Kaohsiung_804,_Taiwan

h_Department_of_Electronics_Engineering_and_Institute_of_Electronics,_National_Chiao_Tung_University,_Hsinchu_300,_Taiwan i_Advanced_{Optoelectronics}_Technology_Center,_National_Cheng_Kung_University,_Taiwan

j_Physics_Division,_Institute_of_Nuclear_Energy_Research,_Taoyuan,_Taiwan k_Department_of_Physics,_ROC_Military_Academy,_Kaohsiung_83055,_Taiwan

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received22August2012

Receivedinrevisedform30August2013 Accepted4September2013 Keywords: Supercriticalﬂuid RRAM Hydration–dehydrationreaction Tindoping

a

b

s

t

r

a

c

t

WedemonstratedthatthesupercriticalCO2ﬂuidtreatmentwasanewconcepttoefﬁcientlyreducethe

operationcurrentofresistancerandomaccessmemory.Thedanglingbondsoftin-dopedsiliconoxide (Sn:SiOx)thinﬁlmwerepassivatedbythehydration–dehydrationreactionthroughsupercriticalCO2ﬂuid

treatment,whichwasveriﬁedbytheXPSandFTIRanalyses.Thecurrentconductionmechanismoflow

resistancestateinpost-treatedSn:SiOxthinﬁlmwastransferredtohoppingconductionfromOhmic

con-duction.Furthermore,thecurrentconductionmechanismofhighresistancestateinthememorydevice wastransferredtoSchottkyemissionfromFrenkel–Pooleconduction.Thephenomenawereattributedto thediscontinuousmetalﬁlamentformedbyhydration–dehydrationreactioninSn:SiOxthinﬁlmthrough

supercriticalfluidtreatment.Finally,areactionmodelwasproposedtoexplainthemechanismofcurrent reductioninSn:SiOxthinfilmwithsupercriticalCO2fluidtreatment.

1. Introduction

Fortheincreasingdemands forportable electronicproducts, nonvolatilememoryhasbeenwidelyappliedasinformation stor-agedeviceduetoitslowpowerconsumptionproperties.Modern semiconductor nonvolatile memories are scaled constantly to achievelargecapacitywhiledevicefeaturesapproachthe sub-100-nmregime.However,theincreasingdemandfordevicedensities byscalingdimensionisexpectedtobeamajorchallengedueto thetechnicalandphysicallimitation.Tosurmountthetechnical andphysicallimitationissuesofconventionalchargestorage-based ∗ Correspondingauthorat:DepartmentofPhysics,NationalSunYat-Sen Univer-sity,70Lien-haiRoad,Kaohsiung804,Taiwan.Tel.:+86675252000x3708; fax:+8867.

E-mailaddress:[email protected](T.-C.Chang).

memories[1–8],theresistancerandomaccessmemory(RRAM) constructedofaninsulatinglayersandwichedbytwoelectrodes iswidelyinvestigatedbyindustriesandacademics.TheRRAMisa greatpotentialcandidatefornext-generationnonvolatilememory due to their superior characteristics such as lesser cost, sim-plestructure,high-speedoperation,andnon-destructivereadout [9,10].Variousmaterialshavebeenreportedtopossessresistive switchingbehaviors,suchassolid-electrolyte-basedRRAM[11,12], transitionmetaloxides(MnOx,ZrOx,HfOx)[13–15,9,16,17],and

organicmaterial[18].Inaddition,manyswitchingmechanismof RRAM have been proposedto explain resistive switching phe-nomenon,suchasconductiveﬁlaments[19],valencechange[20], and Schottkybarrier [21]. However,theunderlyingmechanism ofresistiveswitchingbehaviorisstillnotyetunderstoodclearly. Silicon-basedoxideisapromisingmaterialforRRAMapplications becauseofitsgreatcompatibilityinintegratedcircuit(IC)process. 0896-8446/$–seefrontmatter.Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.

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184 K.-C.Changetal./J.ofSupercriticalFluids85 (2014) 183–189 Therefore,theresearchusingsilicon-basedoxideasthe

resistance-switchinglayerwasworthyofinvestigation.

In our preceding research, supercritical CO2 (SCCO2) ﬂuid

technology was used to improve the dielectric properties and performance of various thin film transistors (TFTs), such as hydrogenated amorphous-silicon TFTs and ZnO TFTs [22–30]. Supercriticalphaseispeculiarwithitscharacteristicsofhigh pen-etrationof gas and solubility of liquid. The supercritical water fluidhastremendousoxidationproperty[31].However,high crit-icaltemperatureandhighcriticalpressureareessentialcondition toachieve supercriticalwater fluid, which isdifficult torealize throughmodernfacilities.Byaddingalittlewaterinto supercriti-calCO2fluids,theliquidwatercanattaintothesupercriticalfluid

phaseduetothephaseclosetoideasolution.

Inthiswork,tinmetaldopedintosiliconoxidebyco-sputtering atroomtemperaturewastakenastheresistanceswitchinglayer of RRAM.To evaluatethe resistiveswitching properties of tin-dopedsiliconoxide(Sn:SiOx)layer,thePt/Sn:SiOx/TiNdevicewas

fabricatedatcleanroom.Moreover,thematerialandconduction mechanism analyseswereexecuted toexplain theinﬂuence of Snmetaldopedinsiliconoxideonresistiveswitchingbehaviors. Inaddition,thePt/SCCO2-treatedSn:SiOx/TiNsandwicheddevices

were fabricatedto investigate the effect of SCCO2 on resistive

switchingpropertiesofSn:SiOxthinﬁlm.TheeffectsofSCCO2

treat-mentonresistiveswitchingbehaviorsofSn:SiOxthinﬁlmwasalso

evaluatedbymaterialandcarrierconductionmechanism analy-ses.Furthermore,thereactionmechanism inRRAMwithSCCO2

ﬂuidwasalsodiscussedtoexplainthereasonofelectricalproperty improvementonSn:SiOxRRAM.

2. Experimental

Theexperimentalspecimenswerepreparedasfollows:Inthe ﬁrstgroup,theSn:SiOxthinﬁlm(about30nm)wasdepositedon

theTiN/Ti/SiO2/Sisubstratebyco-sputtering withthepureSiO2

andSntargets.ThesputteringpowerwasﬁxedatRFpower200W and3Wfor SiO2 and Sntargetsrespectively.Theco-sputtering

wasexecutedinargonambient(Ar=30sccm)withaworking pres-sureof 0.789Paat roomtemperature. Inthesecondgroup,the Sn:SiOxthinﬁlmswereputintothereactivechamberof

supercrit-icalﬂuidsystem,andthentheSCCO2ﬂuidmixedwith0.5mlwater

weresyringedintothereactivechambertotreatthespecimens. Thepassivationefﬁciencyincreaseswiththerisingoftemperature andCO2pressureaslongasexceedingthecriticalpoint,whichare

31◦Cwith7.3MPaCO2.Butowingtothelimitationofour

exper-imentequipment,thewater-mixedsupercriticalCO2ﬂuidswere

heatedandpressuredto120◦Cand20.4MPainthestainlesssteel chamberofsupercriticalﬂuidsystemfor1h.Finally,thePttop elec-trodeof200nmthicknesswasdepositedonSn:SiOxthinﬁlmto

formelectricaldeviceswithPt/Sn:SiOx/TiNsandwichstructuresby

DCmagnetronsputtering.Bycontrast,thePt/SiO2/TiNsandwich

structuresweremade bysame process procedurewitha sput-teredSiO2layerinsteadofaSn:SiOxlayerascontrolsamples.The

entireelectricalmeasurementsofdeviceswiththePtelectrodeof 250␮mdiameterwereperformedusingAgilentB1500 semicon-ductorparameteranalyzer.Besides,theFouriertransforminfrared spectroscopy(FTIR)measuredbyBrukerVERTEX70v spectrom-eterinfarinfraredregionandX-rayphotoelectronspectroscopy (XPS)wereusedtoanalyzethechemicalcompositionandbonding oftheseinsulatormaterials,respectively.

3. Resultsanddiscussion

The“formingprocess”isrequiredtoactivatealloftheSn:SiOx

RRAM devices, using dc voltage sweeping with a compliance

Fig.1. TheformingcurrentcurvesoftheSn:SiOxRRAMdevicesbeforeandafter SCCO2treatment.

currentof2mA.TheleakagecurrentoftheSn:SiOxRRAMdevices

after SCCO2 treatment was lower than that of pre-treatment

devices(Fig.1).Thisphenomenonisattributedtotheimprovement ondielectricpropertiesthroughSCCO2treatment,whichhasbeen

reportedbyourpreviousstudy[23].Thevoltagesweepbiaswas appliedonTiNelectrodewiththegroundedPtelectrodeasshown inthebottomleftinsetofFig.2.Aftertheformingprocess,a grad-ualdecreaseincurrentwasobservedwherethecellswitchesfrom lowresistancestate(LRS)tohighresistancestate(HRS),calledas “resetprocess”,bysweepingthevoltagefrom0to−1.8Vwithout currentcompliance.Oncontrast,asthevoltagewassweptfrom0to 1.2Vwitha5mAcurrentcompliance,theresistanceswitchedfrom HRStoLRS,calledas“setprocess”.InthePt/Sn:SiOx/TiNdevice,

theresistanceratioofHRSandLRSisabout102_times_at_a

read-ingvoltageof 0.1V. Theelectrical current-voltage propertiesof theSn:SiOxdeviceswerecomparedbeforeandafterSCCO2

treat-ment(Fig.2).ThecurrentofSn:SiOxdevicesisreduced at0.1V

readingvoltageafterSCCO2treatment.Toinvestigatethe

interest-ingphenomena,weanalyzedthecurrentconductionmechanism ofSn:SiOxthinﬁlmwithandwithoutSCCO2treatmentasshown

inFig.3.ThecarriertransportinLRSstateofSn:SiOxdevicewas

dominatedbyOhmicconductionintheSn:SiOxlayer.AfterSCCO2

treatment,thecurrentconductionmechanismwilltransferto hop-pingconductionbecauseofthechangeofmaterialproperties.In addition,wealsoanalyzedthecurrentconductionmechanismin HRSof Sn:SiOx withand withoutSCCO2 treatmentasshownin

Fig.4.Therelationshipinthecurveofln(I/V)versusthesquareroot

Fig.2.TheblackandredcurvesaretheresistiveswitchingcharacteristicsofSn:SiOx ﬁlmbeforeandafterSCCO2treatment,respectively.Thecurrentinhighresistance stateofpost-treatedSn:SiOxﬁlmisreducedabout15timesfrom9␮Ato0.6␮A.

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Fig.3. ThecurrentconductioncurvesintheLRSofSn:SiOxdevicesbeforeandafterSCCO2treatment.Thecurrentvs.voltagediagramsweremeasuredatdifferenttemperature environmentsintheSn:SiOxﬁlmbeforeandafterSCCO2treatment.

Fig.4.ThecurrentconductioncurvesintheHRSofSn:SiOxdevicesbeforeandafterSCCO2treatment.Theﬁttingofcurrentvs.voltagecurvesintheHRSofthedevicesbefore andafterSCCO2treatmentwasdrawninrightside.

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186 K.-C.Changetal./J.ofSupercriticalFluids85 (2014) 183–189

Fig.5.ThecomparisonofFTIRspectraofSn:SiOxﬁlmbeforeandafterSCCO2 treat-ment.BothintensityofSn OandSi O SibondsareincreasedinSn:SiOxﬁlmafter SCCO2treatment.

oftheappliedvoltage(V1/2₎_is_linear._According_to_the

relation-shipofFrenkel–Pooleconduction,I∝Vexp[(q/kT)(2a√V−Bt)],

whereais

q/4εid,Btisthetrapbarrierheight,anddisthe

insu-latorthickness.TheFrenkel–Pooleconductionisduetoemissionof trappedelectronsintoconductionband. Thesupplyofelectrons fromthetrapsisthroughthermalexcitation.Thebarrier reduc-tionis largerthanin thecase ofSchottky emissionby a factor of2,which canbeobtainedascompared withtheslopeofthe plotofln(I)versus(V1/2₎_based_on_the_formula_of_Schottky

emis-sion,I∝I2_exp[(q/kT)(a√_V₋

B)],whereBistheSchottkybarrier

height.TheresultsrevealedthatthecarriertransportofSn:SiOxﬁlm

wasdominatedbyFrenkel–Pooleconductionduetothetrapinthe ﬁlm.AfterSCCO2 treatment,thecurrent conductionmechanism

willtransfertoSchottkyemissionbecauseoftheimprovementof dielectricproperties.Therefore,weutilized thematerial spectra analysestoﬁndoutthereasonofdifferentelectricaltransfer mech-anismsinconductioncurrentbeforeandafterSCCO2 treatment.

ComparedtheFTIRspectraofSn:SiOxﬁlmwithandwithoutSCCO2

treatment(Fig. 5), we foundthat the absorption peak of Sn-O bondat586cm−1wasincreasedafterSCCO2treatment.Theresult

impliesthatthedensityofSn ObondwasincreasedintheSn:SiOx

ﬁlmafterSCCO2treatment.Inaddition,theabsorptionofSi O Si

stretchbondat 450cm−1 wasalsoincreasedafterSCCO2

treat-ment,illustratingthecontentofsiliconoxidebondingintheﬁlm alsoincreased [32,33].To analyzethe chemical composition of Sn:SiOxﬁlminthisstudy,X-rayphotoelectronspectroscopy(XPS)

of Sn3d5/2,Si 2p and O1s peakswere performed.After

com-paredwiththepeakareaofSn,SiandOXPSspectra,themole fractionofSn: Si:Oin theco-sputteredSn:SiOx ﬁlmwas0.3%:

29.5%:70.2%.AccordingtoXPSspectraanalysesforSn3d5/2core

level(Fig.6),themolefractionofSn Obondwasobviouslyrisen butthatofSnelementwasdecreasedinSn:SiOxﬁlmafterSCCO2

treatment.Besides,themolefractionofSi Obondwas substan-tiallyincreasedincontrastwiththatofSi OHbondafterSCCO2

treatmentintermsoftheXPSspectraanalysesofSi2pcorelevel asshowninFig.6.Table1showsthecomparisonoftheareasof deconvolutionpeaksofSn3d5/2andSi2pcorelevels.Wefoundthe

molefraction(SnO2:Sn=50.6%:49.4%)inSn:SiOxﬁlmischanged

tothat(SnO2:Sn=70.6%:29.4%) afterSCCO2 treatment.Also,the

molefraction(SiOH:SiO2=18.9%:81.1%)inSn:SiOxﬁlmischanged

tothat(SiOH:SiO2=66.4%:33.6%)inthepost-treatedSn:SiOxﬁlm.

Therefore,weinferthatthelevelofoxidationwouldincreaseand

Fig.6. XPSspectraofSn3d5/2andSi2pcorelevelsinSn:SiOxﬁlmbeforeandafterSCCO2treatment.ThemolefractionofmetallictinandSi OHbondsinSn:SiOxﬁlmare reducedobviouslybutthatoftinoxideandsiliconoxidebondsareincreasedafterSCCO2treatment.

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Fig.7. Theschematicdiagramofhydration–dehydrationreactionmechanismonSn:SiOxﬁlmtoillustratedefectpassivationthroughSCCO2treatment.

Table1

ComparisonoftheareasofdeconvolutionpeaksofSn3d5/2andSi2pcorelevels beforeandafterSCCO2treatment.

SnO2 Sn SiOH SiO2

BeforeSCCO2 50.6% 49.4% 66.4% 33.6%

AfterSCCO2 70.6%↑ 29.4%↓ 18.9%↓ 81.1%↑

accompanywithdehydrationinthepost-treatedﬁlm.Theseresults

wereconsistentwiththeabove-mentionedFTIRanalyses.

Basedontheelectricalandmaterialanalyses, weproposeda

reactionmodetoexplainreactionmechanismofSn:SiOxﬁlmwith

SCCO2 treatmentasshowninFig.7.Asthesamplewasputinto

thewater-mixedSCCO2ﬂuidenvironment,theH2Omoleculewas

carriedinto thegrainboundaryof Sn:SiOx ﬁlm bySCCO2 ﬂuid,

whichisattributedtothehighpenetrationabilityofSCCO2ﬂuid.

AstheH2Omoleculeapproachedtograinboundaryoftheﬁlm,the

hydrationreactionoccurredintheSn:SiOxﬁlm.Then,

monomolec-ularCO2insupercriticalﬂuidsinducesthedehydrationofneighbor

hydroxylgroupssoastoformSi O SiandSn O Sinetwork-like bondinginthefilm.Themechanismiscallhydration–dehydration reactionofSCCO2fluidsinSn:SiOxfilm.AsfortheLRSofSn:SiOx

ﬁlm,theconductiveﬁlamentwillbeformedinpre-treatedSn:SiOx

filmaftertheformingprocess.Theconductivefilamentwillbe con-nectedwithdanglingbondintheswitchingregionofthefilm.The carriersweretransportedthroughthesedanglingbonds,leading tothecurrentconductiondominatedbyOhmicconduction.Ifthe Sn:SiOxfilmwasputintotheSCCO2fluidenvironment,theH2O

moleculewascarriedintothegrainboundaryoftheﬁlmbySCCO2

ﬂuid,whichisattributedtothehighpenetrationabilityofSCCO2

ﬂuid.Hence,thetinmetalinSn:SiOxthinﬁlmwillbeisolateddue

tohydration–dehydrationreaction bySCCO2 treatment.Onlyif

theconductive ﬁlamentformed in theSn:SiOxﬁlm, thecarrier

willhopthroughtheisolated tinmetalintheswitchingregion ofthepost-treatedﬁlm.Thisphenomenawillmaketheelectrical

currentconductioninLRSofSn:SiOxﬁlmtransferredfromOhmic

conductiontohoppingconductionasshownin Fig.8.Owingto thetrapofSn:SiOx ﬁlmcanbepassivated bySCCO2 treatment,

theelectrical currentconduction inHRSofSn:SiOxﬁlm willbe

transferredtoSchottkyemissionfromFrenkel–Pooleconduction asshowninFig.9.Thisphenomenonwillcausetheimprovementof dielectricpropertiesofthinﬁlm,leadingtodecreasetheoperation currentandthepowerconsumptionofRRAM.

Fig.8. TheschematicdiagramofcarrierhoppingmodelinSn:SiOxﬁlmafterSCCO2 treatment.

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188 K.-C.Changetal./J.ofSupercriticalFluids85 (2014) 183–189

Fig.9.TheschematicdiagramofthetransferoncarrierconductionmechanisminSn:SiOxﬁlmafterSCCO2treatment.

4. Conclusion

Insummary,theoperationcurrentofSn-dopedsiliconoxide RRAMdevicewasreducedbysupercriticalfluidtreatmentinthis study.Thewatermolecularcanbebroughtintothefilmto pas-sivatethedanglingbondofgrainboundaryinresistiveswitching layerbysupercriticalCO2 fluid.Invirtueofthephenomena,the

discontinuousconductiveﬁlamentinSn:SiOxﬁlmwasformedby

hydration–dehydrationreactionthroughSCCO2ﬂuids.The

opera-tioncurrentofRRAMcanbereducedduetothedecreaseofdefect inthelayer,which resultsin lowpowerconsumption. Besides, theeffectofjouleheatingcanalsobeimprovedforthedevice. Therefore,supercriticalﬂuidtreatmentcanenhancethe proper-tiesofresistiveswitchinglayerofRRAMdevice.Webelievethat thetechnologyisbeneﬁcialtothedevelopmentofRRAMforthe nextgenerationnonvolatileapplications.

Acknowledgments

ThisworkwasperformedattheNationalScienceCouncilCore Facilities Laboratory for Nano-Science and Nano-Technology in theKaohsiung-PingtungareaandwassupportedbytheNational ScienceCounciloftheRepublicofChinaunderContractNos. NSC-102-2120-M-110-001,andNSC101-2221-E-110-044-MY3.

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