第四章-1-鐵電存儲

FerroelectricMemoryReliabilityandQualificationGaryF.DerbenwickandStephenC.PhilpyCelisSemiconductorCorporationColoradoSprings,Colorado80918(719)262-5144gary@2001MRQWDec.11-12,2001Pasadena,CAFerroelectricMemoryStorageDielectricandFerroelectricMaterialsandApplicationLouisianaTechUniversityOutlineI.IntroductionII.TheoryfordielectricmaterialsIII.TheoryforferroelectricmaterialsIV.ApplicationV.RecentlydevelopmentVI.SummaryI.IntroductionFerroelectricityisaphenomenawhichwasdiscoveredin1921.FerroelectricityhasalsobeencalledSeignetteelectricity,asSeignetteorRochelleSalt(RS)wasthefirstmaterialfoundtoshowferroelectricproperties.Ahugeleapintheresearchonferroelectricmaterialscameinthe1950's,leadingtothewidespreaduseofbariumtitanate(BaTiO3)basedceramicsincapacitorapplicationsandpiezoelectrictransducerdevices.FerroelectricityFerroelectricityderivesitsnamefromferromagnetic.Amagnetizationcanbeobservedthatisreversiblebyapplyingacertainmagneticfield.Ferroelectricsshowareversibility,butdealingwithappliedelectricfieldstoreverseamaterial’spolarization.0.鐵電體與自發(fā)極化自發(fā)極化（Spontaneouselectricpolarization):在沒有外電場的作用時，晶體內(nèi)部某些區(qū)域的正、負電荷中心不重合而呈現(xiàn)電偶極矩，這種現(xiàn)象稱為自發(fā)極化。鐵電體：具有自發(fā)極化的電介質(zhì)稱為鐵電體。鐵電體的特征：具有高的介電常數(shù)，幾百~幾萬；介電常數(shù)與電場強度大小有關(guān)；P~E的關(guān)系為電滯回線（Hysteresiscurve)oEP矯頑電場Ec自發(fā)極化Ps常見的鐵電體有：酒石酸鉀鈉（NaKC4H4O6·4H2O）磷酸二氫鉀（KH2PO4）鈦酸鋇（BaTiO3）二、鐵電體的微觀結(jié)構(gòu)與介電性能按微觀結(jié)構(gòu)，鐵電體可分為偶極矩有序型和離子位移型兩類。偶極矩有序型：晶體內(nèi)含有能夠旋轉(zhuǎn)或反轉(zhuǎn)的固有偶極矩，在居里溫度以下，由于強烈的內(nèi)電場作用，這些偶極子形成長程有序，因而出現(xiàn)自發(fā)極化“電疇”。如KH2PO4。離子位移型：晶體內(nèi)部的離子，在居里溫度以下的溫度內(nèi)，由于強烈的離子位移引起晶體的對稱性降低，而形成自發(fā)極化的電介質(zhì)，如BaTiO3。一、鐵電體的電疇結(jié)構(gòu)和特性具有宏觀偶極矩的區(qū)域稱為“電疇”。注意薄膜的電疇在初始狀態(tài)，就鐵電體整體而言，對外界將不呈現(xiàn)電荷和極化狀態(tài)（相當與回線的O點）。2.無應(yīng)力的晶體S不變時S=？TCT鐵電體介電常數(shù)與溫度的關(guān)系鐵電相順電相Examples:BariumStrontiumTitanate(BST)B.位移型鐵電體結(jié)構(gòu)特點：鈣鈦礦結(jié)構(gòu)通式：ABO3，其中A為一價或二價金屬，B為四價或五價金屬ABO3立方鈣鈦礦結(jié)構(gòu)AOBBaO立方晶系四角晶系正交晶系三角晶系PsPsPsBaTiO3晶體的晶格參數(shù)與溫度的關(guān)系四角相BaTiO3的自發(fā)極化強度與溫度的關(guān)系BaTiO3單晶的介電常數(shù)與溫度的關(guān)系III.TheoryforFerroelectricFerroelectricmaterialCrystalSymmetrySpontaneousPolarizationandPyroelectricEffectFerroelectricDomainsandHysteresisLoopCuriePointandPhaseTransitionsPerovskiteStructuresDielectricHysteresisTheoryforFerroelectric(cont.)MagnitudeofSpontaneousPolarization

AtomicArrangementandFerroelectricityAbsoluteSenseofSpontaneousPolarizationDimensionalityofFerroelectricCrystalsChangesinPropertiesNeartheCurieTemperatureAntiferroelectricMaterialsRepresentativeFerroelectricMaterialsMaterialPreparationandPolingFerroelectricmaterialAferroelectricmaterialfallsintoagroupofdielectricsthatexhibitspontaneouspolarization.Ferroelectricshaveextremelyhighdielectricconstantsatlowappliedfieldfrequencies.Forexample,thedielectricconstantforbariumtitanateatroomtemperaturecanbeashighas5000.BariumTitanateanditstetragonalcrystalstructureCrystalSymmetryCrystalStructurePointGroupsCentro-SymmetricNon-centrosymmetric

PiezoelectricPyroelectric

Triclinic_1,1_1

1

1

Monoclinic

2,m,2/m

2/m

2,m

2,m

Orthorhombic

222,mm2,mmm

mmm

222,mm2

mm2,

Tetragonal__4,4,4/m,422,4mm,42m,(4/m)mm

4/m,(4/m)mm__4,4,422,4mm,42m

4,4mm

Trigonal__3,3,32,3m,3m__3,3m

3,32,3m

3,3m

Hexagonal__6,6,6/m,622,6mm,6m2,(6/m)mm

6/m,(6/m)mm__6,6,622,6mm,6m2

6,6mm

Cubic_23,m3,432,43m,m3m

m3,m3m_23,43m

------SpontaneousPolarizationandPyroelectricEffectThespontaneouspolarizationisgivenbythevalueofthedipolemomentperunitvolumeorbythevalueofthechargeperunitareaonthesurfaceperpendiculartotheaxisofspontaneouspolarization.changeinthespontaneouspolarizationvectorΔPsisgivenby: ΔPs=πΔTTheTdependenceofPsforBaTiO3FerroelectricDomainsandHysteresisLoopFerroelectriccrystalspossessregionswithuniformpolarizationcalledferroelectricdomains.Polarizationvs.ElectricField(P-E)hysteresisloopforatypicalferroelectriccrystalisshownontheright.CuriePointandPhaseTransitionstheCurie-Weisslaw:e=e0+C/(T-To)Variationofdielectricconstants(aandcaxis)withtemperatureforBaTiO3isshownrightPerovskiteStructuresTemperaturedependenceofPerovskitecrystalstructureAtomicArrangementandFerroelectricityThearrangementoftheatomsinallferroelectriccrystalsresultinanequallystablestatebutwithreorientedPs.AsimpleexampleisBaTiO3forwhichtheprototypeiscubic.TheparaelectrictoferroelectrictransformationatTcmaybeviewedintermsofalow-frequencytemperature-dependentmodeofthecrystallattice,observablebyopticalorneutronspectroscopy.AbsoluteSenseofSpontaneousPolarizationTherelativesenseofPsinacrystalisgivenbythechargedevelopedonthepolarfacesasasingledomaincrystaliscooledbelowTc.Thissensecanberelatedtotheatomicarrangementbymakinguseoftheanomalousscatteringinanx-raydiffractionexperiment.DimensionalityofFerroelectricCrystalsone-dimensional,BaTiO3:Ps>25×10-2Cm-2two-dimensional,BaCoF4:Psbetween10×10-2and3×10-2Cm-2threedimensions,Tb2(MoO4)3:Ps<5×10-2Cm-2CharacteristicsofsomeferroelectricmaterialsMaterial

Formula

Tc(K)

Ps(10-2Cm-2)a

Ammoniumdihydrogenphosphate(ADP)

NH4H2PO4

148

0bBariumcobaltfluoride

BaCoF4

c

8Bariumtitanate

BaTiO3

183,278,393

~20Boracite

Mg3B7O13Cl

538

0.05Guanidiniumaluminumsulfatehexahydrate(GASH)

C(NH2)3Al(SO4)2.6H20

d

3.5Leadtitanate

PbTiO3

763

~75Leadzirconate

PbZrO3

503

0bLithiumniobate

LiNbO3

1473

71Lithiumtantalate

LiTaO3

938

50Potassiumdihydrogenphosphate(KDP)

KH2PO4

123

5eRochellesalt

NaKC4H406.4H20

255,297

0.25fSodiumniobate

NaNbO3

73,627

0bTerbiummolybdate(TMO)

Tb2(MoO4)3

436

0.2Triglycinesulfate(TGS)

(NH2CH2COOH)3.H2SO4

322

2.8a)ValuesofPsareforsinglecrystalsatroomtemperatureunlessspecifiedotherwiseb)Antiferroelectricatroomtemperature

c)MeltsbelowTc

d)Decomposesatabout273K

e)At100K

f)At280K三.反鐵電體、壓電體與熱釋電體1、反鐵電體鋯酸鉛（PbZrO3）具有鈣鈦礦型結(jié)構(gòu)。是最早發(fā)現(xiàn)的具有“反鐵電性”的介質(zhì)材料。反鐵電體與鐵電體的相同點：晶體結(jié)構(gòu)與同型鐵電體相近；介電常數(shù)和結(jié)構(gòu)在居里溫度附近出現(xiàn)異常，在居里溫度以上，介電常數(shù)與溫度遵從居里-外斯定律。TCT不同點：反鐵電體隨溫度改變發(fā)生相變，但在高溫下是順電相，在居里溫度以下為對稱性較低的反鐵電相，從宏觀上看，反鐵電體的自發(fā)極化強度為零。T（oC）100200300100020003000如圖為PbZrO3的~t曲線，t=230oC時，出現(xiàn)一峰值。當t>230oC，轉(zhuǎn)變?yōu)榱⒎解}鈦礦結(jié)構(gòu)，t<230oC，并不能觀察到電滯回線，這時P~E呈線性關(guān)系。EPEC當E>EC(E>20kV/cm)時，反鐵電體轉(zhuǎn)變?yōu)殍F電體。2、壓電體

當晶體上特定方向上施加壓力或拉力，晶體的一些對應(yīng)的表面上分別出現(xiàn)正、負束縛電荷，其電荷密度與外施力的大小成正比例。壓電體的必要條件：晶體不具有對稱中心。（在32個點群中，有20個具有壓電性）+++++------+++++-----正壓電效應(yīng)：在外力的作用下，晶體表面出現(xiàn)束縛電荷。逆壓電效應(yīng)：在外電場的作用下，晶體發(fā)生形變。壓電效應(yīng)表示為：Pi=dijkTjkPi——極化強度或Pi=eijkSjkTjk——應(yīng)力；Sjk——應(yīng)變逆壓電效應(yīng)表示式為：Sjk=dijkEiTjk=eijkEidijk為壓電應(yīng)變常數(shù)，eijk為壓電應(yīng)力常數(shù)壓電參數(shù)微區(qū)測量

PFM原理圖

3、熱釋電體熱釋電晶體只要溫度變化，由于其自發(fā)極化強度隨溫度變化的緣故，會在特定方向產(chǎn)生表面電荷，這就是最先由Brewster命名的熱釋電現(xiàn)象。當晶體中存在與其它極軸都不同的唯一極軸時，才有可能由于熱膨脹引起電矩變化而導致熱釋電效應(yīng)。有10種點群的晶體具有熱釋電效應(yīng)，如鈦酸鋇，硫酸三甘酞，一水合硫酸鋰，鈮酸鋰等。鐵電材料的應(yīng)用電子技術(shù)紅外探測技術(shù)超聲（和微波聲學）技術(shù)固態(tài)記憶II.TheoryforDielectricMaterials

BasicconceptionDielectricconstantDielectricmaterialBasicconceptionDielectric:materialthatiselectricallyinsulatingorcanbemadetoexhibitanelectricdipole.Ferroelectricity:propertyofamaterialtoexhibitpolarizationintheabsenceofanelectricfield.Dielectricconstant:εr=E/EoPermittivity:ratiooftheelectricdisplacementinamediumtotheintensityoftheelectricalfieldproducingit.Capacitance:basicbuildingblockofelectroniccircuits.Dielectricstrength:magnitudeoftheelectricfieldnecessarytoproducebreakdown.Dielectricloss:energylostinthereorientationofdipoles.Polarization---Qd=Eo(Er-1)(EF)=(E-Eo)(EF)

DielectricconstantCapacitanceisdefinedastheabilityoftwoconductorstostoreachargeQwhenapotentialVisappliedacrossthem.Co=Q/V=εoA/dεoisthepermittivityoffreespaceAistheareaoftheconductingplatesdisthedistancebetweenthetwoplatesDipolemomentsOrientationofdipolemomentsRelativePermittivityTheresultantcapacitancecanthenbemeasuredduetothedielectric:C=εrA/dthedielectricconstantεr=ε/εothedielectricconstant,orrelativepermittivity,istheratioofthepermittivityofthematerialtothepermittivityoffreespaceMeasuringDielectricConstantofthethinfilmthemostcommonmethodistheparallelplatemethodDielectricStrengthThedielectricstrengthisameasureofhowmuchvoltagecanbeappliedtoadielectricbeforeelectriccurrentbeginstoarcacrossthedielectricArcingacrossthedielectricisknownasdielectricbreakdown.DielectricstrengthhastheunitsofV/m.DielectricmaterialAdielectricmaterialisamaterialthatisnonmetallicandexhibitsormaybemadetoexhibitanelectricdipolestructure.Adielectricmaterialischaracterizedandselectedaccordingtoitsdielectricconstant,Σr,oftencalledtherelativepermittivity.Therearemanyceramicsandpolymersthatexhibitdielectricbehavior.DielectricHysteresisTheminimumdcfieldrequiredtomovedomainwallsisameasureofthecoercivefield.Atfullreversefield,thefinalPswillhavemagnitudeequaltotheoriginalfullPsbutofoppositesign.Thehysteresisobservedisafunctionoftheworkrequiredtodisplacethedomainwalls.MagnitudeofSpontaneousPolarizationThespontaneouspolarizationofsingle-domainmaterialsusuallylieswithintherange10-3to1Cm-2.ThemagnitudeofPsinasinglecrystalisdirectlyrelatedtotheatomicdisplacementsthatoccurinferroelectricreversalandmaybecalculatedfromtheatomicpositionswithintheunitcell.Ps=(?V)ΣiZi

Δi

IV.ApplicationsApplicationsfordielectricmaterialsDielectricmaterialstoinsulateelectricalconductorsDielectricmaterialsusedincapacitorsCommunications(radio,radarandmicrowave)MicroelectronicsApplicationsforFerroelectricMaterialsSwitchingapplicationNonswitchingapplicationApplicationsfordielectricmaterialsDielectricconstantDielectricconstant’sreactiontotemperaturelevelsDielectricstrengthWeightDimensionalstabilityMoistureresistanceCorrosionresistanceThermalconductivityThermalexpansionLowcurrentleakageProcessingmethodsMicroelectronicsBenefittomicroelectr