第四章--1--铁电存储

1、Ferroelectric Memory Reliability and QualificationGary F. Derbenwick and Stephen C. Philpy Celis Semiconductor CorporationColorado Springs, Colorado 80918(719) 262-5144garycelis-2001 MRQWDec. 11-12, 2001Pasadena, CAFerroelectric Memory StorageDielectric and Ferroelectric Materials and ApplicationLou

2、isiana Tech UniversityOutline I. Introduction II. Theory for dielectric materials III. Theory for ferroelectric materials IV. Application V. Recently development VI. SummaryI. Introduction Ferroelectricity is a phenomena which was discovered in 1921. Ferroelectricity has also been called Seignette e

3、lectricity, as Seignette or Rochelle Salt (RS) was the first material found to show ferroelectric properties. A huge leap in the research on ferroelectric materials came in the 1950s, leading to the widespread use of barium titanate (BaTiO3) based ceramics in capacitor applications and piezoelectric

4、 transducer devices .Ferroelectricity Ferroelectricity derives its name from ferromagnetic. A magnetization can be observed that is reversible by applying a certain magnetic field. Ferroelectrics show a reversibility, but dealing with applied electric fields to reverse a materials polarization.: 在没有

5、外电场的作用时，晶体内部某些区域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，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。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。铁电体的特征：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；PE的关系为电滞回线（Hysteresis curve)oEP矫顽电场Ec自发极化Ps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。：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。：晶体内

6、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。具有宏观偶极矩的区域称为具有宏观偶极矩的区域称为“电畴电畴”。注意薄膜。注意薄膜的电畴的电畴在初始状态，就铁电体整体而言，对在初始状态，就铁电体整体而言，对外界将不呈现电荷和极化状态（相当外界将不呈现电荷和极化状态（相当与回线的与回线的O点）。点）。绝对温度熵内能TSUEPTSUG 最小稳定系统 G)0(EP)EPmax(TCTExamples: Barium Strontium Titanate (BST)AOBBaO立方晶系四角晶系正交晶系三角晶系PsPsPsBaTiO3晶体的

7、晶格参数与温度的关系III.Theory for FerroelectricFerroelectric materialCrystal SymmetrySpontaneous Polarization and Pyroelectric EffectFerroelectric Domains and Hysteresis LoopCurie Point and Phase TransitionsPerovskite StructuresDielectric HysteresisTheory for Ferroelectric(cont.) Magnitude of Spontaneous Pol

8、arization Atomic Arrangement and Ferroelectricity Absolute Sense of Spontaneous Polarization Dimensionality of Ferroelectric Crystals Changes in Properties Near the Curie Temperature Antiferroelectric Materials Representative Ferroelectric Materials Material Preparation and PolingFerroelectric mater

9、ial A ferroelectric material falls into a group of dielectrics that exhibit spontaneous polarization. Ferroelectrics have extremely high dielectric constants at low applied field frequencies. For example, the dielectric constant for barium titanate at room temperature can be as high as 5000.Barium T

10、itanate and its tetragonal crystal structureCrystal SymmetryCrystal StructurePoint GroupsCentro-SymmetricNon-centrosymmetricPiezoelectricPyroelectricTriclinic_1,1_111Monoclinic2,m,2/m2/m2,m2,mOrthorhombic222,mm2,mmmmmm222,mm2mm2,Tetragonal_4,4,4/m,422,4mm,42m,(4/m)mm4/m,(4/m)mm_4,4,422,4mm,42m4,4mmT

11、rigonal_3,3,32,3m,3m_3,3m3,32,3m3,3mHexagonal_6,6,6/m,622,6mm,6m2,(6/m)mm6/m,(6/m)mm_6,6,622,6mm,6m26,6mmCubic_23,m3,432,43m,m3mm3,m3m_23,43m-Spontaneous Polarization and Pyroelectric Effect The spontaneous polarization is given by the value of the dipole moment per unit volume or by the value of th

12、e charge per unit area on the surface perpendicular to the axis of spontaneous polarization. change in the spontaneous polarization vector Ps is given by:Ps = T The T dependence of Ps for BaTiO3Ferroelectric Domains and Hysteresis Loop Ferroelectric crystals possess regions with uniform polarization

13、 called ferroelectric domains. Polarization vs. Electric Field (P-E) hysteresis loop for a typical ferroelectric crystal is shown on the right.Curie Point and Phase Transitions the Curie-Weiss law: e = e 0 + C/(T-To)Variation of dielectric constants (a and c axis) with temperature for BaTiO3 is show

14、n rightPerovskite Structures Temperature dependence of Perovskite crystal structureAtomic Arrangement and Ferroelectricity The arrangement of the atoms in all ferroelectric crystals result in an equally stable state but with reoriented Ps. A simple example is BaTiO3 for which the prototype is cubic.

15、 The paraelectric to ferroelectric transformation at Tc may be viewed in terms of a low-frequency temperature-dependent mode of the crystal lattice, observable by optical or neutron spectroscopy.Absolute Sense of Spontaneous Polarization The relative sense of Ps in a crystal is given by the charge d

16、eveloped on the polar faces as a single domain crystal is cooled below Tc. This sense can be related to the atomic arrangement by making use of the anomalous scattering in an x-ray diffraction experiment.Dimensionality of Ferroelectric Crystals one-dimensional, BaTiO3 : Ps 25 10-2 C m-2 two-dimensio

17、nal, BaCoF4 : Ps between 10 10-2 and 3 10-2 C m-2 three dimensions, Tb2(MoO4)3 : Ps 230C，转变为立方钙钛矿结构，t230C，并不能观察到电滞回线，这时PE呈线性关系。EPEC+-+-压电效应表示为：压电效应表示为：Pi=dijkTjk Pi极化强度极化强度或或 Pi=eijkSjk Tjk应力；应力；Sjk应变应变逆压电效应表示式为：逆压电效应表示式为：Sjk=dijkEiTjk=eijkEidijk为压电应变常数，为压电应变常数，eijk为压电应力常数为压电应力常数压电参数微区测量压电参数微区测量 333

18、30SZdEV当晶体中存在与其它极轴都不同的唯一极轴时，才有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。甘酞，一水合硫酸锂，铌酸锂等。压电体铁电体必是热释电体、电滞回线铁电体铁电体热释电体铁电材料的应用铁电材料的应用 电子技术电子技术 红外探测技术红外探测技术 超声（和微波声学）技术超声（和微波声学）技术 固态记忆固态记忆II.Theory for Dielectric Mater

19、ials Basic conception Dielectric constant Dielectric materialBasic conception Dielectric:material that is electrically insulating or can be made to exhibit an electric dipole. Ferroelectricity:property of a material to exhibit polarization in the absence of an electric field. Dielectric constant:r =

20、 E/Eo Permittivity:ratio of the electric displacement in a medium to the intensity of the electrical field producing it. Capacitance:basic building block of electronic circuits. Dielectric strength:magnitude of the electric field necessary to produce breakdown . Dielectric loss: energy lost in the r

21、eorientation of dipoles. Polarization-Qd = Eo(Er - 1)(EF) = (E - Eo)(EF) Dielectric constant Capacitance is defined as the ability of two conductors to store a charge Q when a potential V is applied across them. Co = Q/V = oA/d o is the permittivity of free space A is the area of the conducting plat

22、es d is the distance between the two platesDipole moments Orientation of dipole moments Relative Permittivity The resultant capacitance can then be measured due to the dielectric: C = rA/d the dielectric constant r= /o the dielectric constant, or relative permittivity, is the ratio of the permittivi

23、ty of the material to the permittivity of free space Measuring Dielectric Constant of the thin film the most common method is the parallel plate methodDielectric Strength The dielectric strength is a measure of how much voltage can be applied to a dielectric before electric current begins to arc acr

24、oss the dielectric Arcing across the dielectric is known as dielectric breakdown. Dielectric strength has the units of V/m.Dielectric material A dielectric material is a material that is nonmetallic and exhibits or may be made to exhibit an electric dipole structure. A dielectric material is charact

25、erized and selected according to its dielectric constant, r, often called the relative permittivity. There are many ceramics and polymers that exhibit dielectric behavior.Dielectric Hysteresis The minimum dc field required to move domain walls is a measure of the coercive field. At full reverse fiel

26、d, the final Ps will have magnitude equal to the original full Ps but of opposite sign. The hysteresis observed is a function of the work required to displace the domain walls.Magnitude of Spontaneous Polarization The spontaneous polarization of single-domain materials usually lies within the range

27、10-3 to 1 C m-2. The magnitude of Ps in a single crystal is directly related to the atomic displacements that occur in ferroelectric reversal and may be calculated from the atomic positions within the unit cell.Ps = (V) i Zi i IV. ApplicationsApplications for dielectric materialsDielectric materials

28、 to insulate electrical conductorsDielectric materials used in capacitorsCommunications (radio, radar and microwave) MicroelectronicsApplications for Ferroelectric Materials Switching applicationNonswitching applicationApplications for dielectric materialsDielectric constantDielectric constants reac

29、tion to temperature levelsDielectric strengthWeightDimensional stabilityMoisture resistanceCorrosion resistanceThermal conductivityThermal expansionLow current leakageProcessing methodsMicroelectronics Benefit to microelectronic devices: Higher speed Smaller size Higher frequencies Lower operational power requirementSwitching application matrix addressed memories shift registers transchargers or transpolarizers oscillators and filtersNonswitching application Thin Film Capacitors BaTiO3 and PMN are the two important materials being