第四章--1--鐵電存儲(chǔ)

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.: 在沒(méi)有

5、外電場(chǎng)的作用時(shí)，晶體內(nèi)部某些區(qū)域在沒(méi)有外電場(chǎng)的作用時(shí)，晶體內(nèi)部某些區(qū)域的正、負(fù)電荷中心不重合而呈現(xiàn)電偶極矩，的正、負(fù)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。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。鐵電體的特征：鐵電體的特征：具有高的介電常數(shù)，幾百幾萬(wàn)；介電常數(shù)與電場(chǎng)強(qiáng)度大小有關(guān)；PE的關(guān)系為電滯回線（Hysteresis curve)oEP矯頑電場(chǎng)Ec自發(fā)極化Ps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。：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(qiáng)烈的內(nèi)電場(chǎng)作用，這些偶極子形成長(zhǎng)程有序，因而出現(xiàn)自發(fā)極化“電疇”。如KH2PO4。：晶體內(nèi)

6、部的離子，在居里溫度以下的溫度內(nèi)，由于強(qiáng)烈的離子位移引起晶體的對(duì)稱性降低，而形成自發(fā)極化的電介質(zhì)，如BaTiO3。具有宏觀偶極矩的區(qū)域稱為具有宏觀偶極矩的區(qū)域稱為“電疇電疇”。注意薄膜。注意薄膜的電疇的電疇在初始狀態(tài)，就鐵電體整體而言，對(duì)在初始狀態(tài)，就鐵電體整體而言，對(duì)外界將不呈現(xiàn)電荷和極化狀態(tài)（相當(dāng)外界將不呈現(xiàn)電荷和極化狀態(tài)（相當(dāng)與回線的與回線的O點(diǎn)）。點(diǎn)）。絕對(duì)溫度熵內(nèi)能TSUEPTSUG 最小穩(wěn)定系統(tǒng) 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晶體的

7、晶格參數(shù)與溫度的關(guān)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，轉(zhuǎn)變?yōu)榱⒎解}鈦礦結(jié)構(gòu)，t230C，并不能觀察到電滯回線，這時(shí)PE呈線性關(guān)系。EPEC+-+-壓電效應(yīng)表示為：壓電效應(yīng)表示為：Pi=dijkTjk Pi極化強(qiáng)度極化強(qiáng)度或或 Pi=eijkSjk Tjk應(yīng)力；應(yīng)力；Sjk應(yīng)變應(yīng)變逆壓電效應(yīng)表示式為：逆壓電效應(yīng)表示式為：Sjk=dijkEiTjk=eijkEidijk為壓電應(yīng)變常數(shù)，為壓電應(yīng)變常數(shù)，eijk為壓電應(yīng)力常數(shù)為壓電應(yīng)力常數(shù)壓電參數(shù)微區(qū)測(cè)量壓電參數(shù)微區(qū)測(cè)量 333

18、30SZdEV當(dāng)晶體中存在與其它極軸都不同的唯一極軸時(shí)，才有當(dāng)晶體中存在與其它極軸都不同的唯一極軸時(shí)，才有可能由于熱膨脹引起電矩變化而導(dǎo)致熱釋電效應(yīng)。有可能由于熱膨脹引起電矩變化而導(dǎo)致熱釋電效應(yīng)。有10種點(diǎn)群的晶體具有熱釋電效應(yīng)，如鈦酸鋇，硫酸三種點(diǎn)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。甘酞，一水合硫酸鋰，鈮酸鋰等。壓電體鐵電體必是熱釋電體、電滯回線鐵電體鐵電體熱釋電體鐵電材料的應(yīng)用鐵電材料的應(yīng)用 電子技術(shù)電子技術(shù) 紅外探測(cè)技術(shù)紅外探測(cè)技術(shù) 超聲（和微波聲學(xué)）技術(shù)超聲（和微波聲學(xué)）技術(shù) 固態(tài)記憶固態(tài)記憶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