第四章-1-铁电存储

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.铁电体与自发极化自发极化（Spontaneouselectricpolarization):在没有外电场的作用时，晶体内部某些区域的正、负电荷中心不重合而呈现电偶极矩，这种现象称为自发极化。铁电体：具有自发极化的电介质称为铁电体。铁电体的特征：具有高的介电常数，几百~几万；介电常数与电场强度大小有关；P~E的关系为电滞回线（Hysteresiscurve)oEP矫顽电场Ec自发极化Ps常见的铁电体有：酒石酸钾钠（NaKC4H4O6·4H2O）磷酸二氢钾（KH2PO4）钛酸钡（BaTiO3）二、铁电体的微观结构与介电性能按微观结构，铁电体可分为偶极矩有序型和离子位移型两类。偶极矩有序型：晶体内含有能够旋转或反转的固有偶极矩，在居里温度以下，由于强烈的内电场作用，这些偶极子形成长程有序，因而出现自发极化“电畴”。如KH2PO4。离子位移型：晶体内部的离子，在居里温度以下的温度内，由于强烈的离子位移引起晶体的对称性降低，而形成自发极化的电介质，如BaTiO3。一、铁电体的电畴结构和特性具有宏观偶极矩的区域称为“电畴”。注意薄膜的电畴在初始状态，就铁电体整体而言，对外界将不呈现电荷和极化状态（相当与回线的O点）。2.无应力的晶体S不变时S=？TCT铁电体介电常数与温度的关系铁电相顺电相Examples:BariumStrontiumTitanate(BST)B.位移型铁电体结构特点：钙钛矿结构通式：ABO3，其中A为一价或二价金属，B为四价或五价金属ABO3立方钙钛矿结构AOBBaO立方晶系四角晶系正交晶系三角晶系PsPsPsBaTiO3晶体的晶格参数与温度的关系四角相BaTiO3的自发极化强度与温度的关系BaTiO3单晶的介电常数与温度的关系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.反铁电体、压电体与热释电体1、反铁电体锆酸铅（PbZrO3）具有钙钛矿型结构。是最早发现的具有“反铁电性”的介质材料。反铁电体与铁电体的相同点：晶体结构与同型铁电体相近；介电常数和结构在居里温度附近出现异常，在居里温度以上，介电常数与温度遵从居里-外斯定律。TCT不同点：反铁电体随温度改变发生相变，但在高温下是顺电相，在居里温度以下为对称性较低的反铁电相，从宏观上看，反铁电体的自发极化强度为零。T（ºC）100200300100020003000如图为PbZrO3的~t曲线，t=230ºC时，出现一峰值。当t>230ºC，转变为立方钙钛矿结构，t<230ºC，并不能观察到电滞回线，这时P~E呈线性关系。EPEC当E>EC(E>20kV/cm)时，反铁电体转变为铁电体。2、压电体

当晶体上特定方向上施加压力或拉力，晶体的一些对应的表面上分别出现正、负束缚电荷，其电荷密度与外施力的大小成正比例。压电体的必要条件：晶体不具有对称中心。（在32个点群中，有20个具有压电性）+++++------+++++-----正压电效应：在外力的作用下，晶体表面出现束缚电荷。逆压电效应：在外电场的作用下，晶体发生形变。压电效应表示为：Pi=dijkTjkPi——极化强度或Pi=eijkSjkTjk——应力；Sjk——应变逆压电效应表示式为：Sjk=dijkEiTjk=eijkEidijk为压电应变常数，eijk为压电应力常数压电参数微区测量

PFM原理图

3、热释电体热释电晶体只要温度变化，由于其自发极化强度随温度变化的缘故，会在特定方向产生表面电荷，这就是最先由Brewster命名的热释电现象。当晶体中存在与其它极轴都不同的唯一极轴时，才有可能由于热膨胀引起电矩变化而导致热释电效应。有10种点群的晶体具有热释电效应，如钛酸钡，硫酸三甘酞，一水合硫酸锂，铌酸锂等。铁电材料的应用电子技术红外探测技术超声（和微波声学）技术固态记忆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=(½V)ΣiZi

Δi

IV.ApplicationsApplicationsfordielectricmaterialsDielectricmaterialstoinsulateelectricalconductorsDielectricmaterialsusedincapacitorsCommunications(radio,radarandmicrowave)MicroelectronicsApplicationsforFerroelectricMaterialsSwitchingapplicationNonswitchingapplicationApplicationsfordielectricmaterialsDielectricconstantDielectricconstant’sreactiontotemperaturelevelsDielectricstrengthWeightDimensionalstabilityMoistureresistanceCorrosionresistanceThermalconductivityThermalexpansionLowcurrentleakageProcessingmethodsMicroelectronicsBenefittomicroelectr