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1、第七讲第七讲第七讲第七讲Physical Vapor Deposition物理气相沉积物理气相沉积纳米材料和纳米结构纳米材料和纳米结构纳米材料和纳米结构纳米材料和纳米结构Physical Vapor Deposition(PVD)wDefinition Film deposition by condensation from vapor phasewThree Steps of PVDGenerating a vapor phase by evaporation or sublimation Electron-beam evaporation Molecular-beam epitaxy Th
2、ermal evaporation Sputtering Cathodic arc plasma deposition Pulsed laser depositionTransporting the material from the source to the substrateFormation of film by nucleation and diffusionwApplicationCoatings of electronic materialsInsulatorSemiconductorConductorSuperconductorNanometer scale multilaye
3、r structuresAdvanced electronic devicesAbrasion resistant coatingswConcerned Problems and ChallengesContamination at the interfaces or intermixingMulti-material systems involvedCost of equipment and maintenance Complexion of operationwSystems Described in This SectionSputteringPulsed laser depositio
4、n1 Sputtering（溅射）（溅射）1-1 Principle of Sputtering1-2 Sputtering System1-3 Preparing Multilayer Structures by Sputtering1-4 Current Status of Sputtering1-1 Principle of SputteringwEjection of Atoms from the TargetEnergetic particles bombarding a target surface with sufficient energy(50 eV 1000 eV)wTar
5、getCathode,connected to a negative voltage supplyComposed of the materials to be depositedwSubstrateAnode May be grounded,floated,or biasedwGlow Discharge Medium in Sputtering ChamberA gas or a mixture of different gases,most commonly Ar or HeIn reactive sputtering:introduce reactive gases such as O
6、2 or N2 Pressure:a few mTorr to several hundreds mTorrwProcedureGeneration of positive ions:ionizing the sputtering gas by glow dischargeBombarding:accelerated positive ions to strike the target surface and remove mainly neutral atomsCondensation:neutral atoms leave the target and condense on the su
7、bstrate surface,and form into thin films wAn Important Concept:Sputtering YieldA measurement of the efficiency of sputteringRatio of the number of emitted particles to the number of bombarding ones 1-2 Sputtering SystemwTypical Types of Sputtering SystemsDirect current(dc)diode sputteringUsed for sp
8、uttering conducting materialsRadio frequency(rf)diode sputteringUsed for sputtering insulating materialsMagnetron diode sputtering Most commonly used todayPlasma be confined around the target surface by a magnet fieldAdvantages of using magnetron sputteringlFeasibility of large cathode sizelHigh spu
9、ttering yieldlLess bombardment to the substrate用于用于制备制备TiN/VN 多多层膜层膜的磁的磁控溅控溅射系射系统统氩气氩气流量表流量表流量控制阀流量控制阀压力传感器压力传感器低温泵低温泵低温泵低温泵靶靶1靶靶2旋转衬底支架旋转衬底支架衬底衬底流量表流量表阀门阀门流量表流量表流量控制阀流量控制阀1流量控制阀流量控制阀2主流量控制阀主流量控制阀质谱仪质谱仪阀门阀门锁定装置锁定装置wWays to reduce the damage and re-sputtering of growing filmDamage caused by negative
10、ion effect and radiation enhanced diffusionImprovement methodUse high gas pressure:to reduce the energy of the negative ionsUse off-axis sputtering:to avoid the substrate directly facing the cathode Disadvantage of off-axis sputtering:llow deposition ratelsmall deposition areawDeposition of magnetic
11、 materials:facing target sputtering systems偏轴溅射系统偏轴溅射系统示意图示意图Schematic of off-axis sputtering system可可360度旋转的衬底支架度旋转的衬底支架陶瓷加热器陶瓷加热器负离子撞击区负离子撞击区衬底衬底靶靶溅射枪溅射枪溅射源溅射源屏蔽闸屏蔽闸空间屏蔽区空间屏蔽区正面溅射系统正面溅射系统示意图示意图Schematic of the facing target sputtering system衬底衬底靶靶磁体磁体冷却水冷却水氩气氩气1-3 Preparing Multilayer Structures b
12、y SputteringwTypes and Properties of Multilayer StructuresTypes of architecturesMetal/metalCeramic/ceramicMetal/ceramicSemiconductor/semiconductorStructural and physical propertiesWith structurally modulated architecturesWith compositionally modulated architecturesHigh interface volume fractionLarge
13、 intrinsic stressWith structural and/or compositional gradientExhibiting unique and enhanced electric,dielectric,magnetic,and mechanic propertieswBaTiO3 Nanolayer Ferroelectric Thin Film CapacitorsAdvantage:higher relative dielectric constantDisadvantage:high leakage currentElectrical properties str
14、ongly depending upon the processing condition,microcrystal structure,and choice of bottom electrodeAmorphous:low dielectric constant(16 at 105 V/cm),low leakage currentPolycrystalline:high dielectric constant(400 at 105 V/cm),high leakage currentAim of nanolayer structure BaTiO3 film capacitor:high
15、dielectric constant and low leakage currentRealization and effectsSubstrate:Ru/SiO2/SiTechnique:rf magnetron sputtering,sputtering interruption between layers to change the substrate temperatures(680 C C,60 C C)Layer structure:n-cycle alternate layers of amorphous and polycrystalline BaTiO3(microcry
16、stalline be obtained by annealing amorphous layer)Results obtainedlLeakage current density be considerably reduced,and the effect becoming better with increasing cycle numberlDielectric constant be two or three times higher than single amorphous layer but lower than a single polycrystalline layer具有纳
17、米多层结构的具有纳米多层结构的BaTiO3薄膜电容器横截面示意图薄膜电容器横截面示意图wNanolayer MoSi2/SiCSubstrate:single crystal silicon(sc-Si)Techniques:Magnetron sputtering for deposition of MoSi2rf sputtering for deposition of SiCMoSi2/SiC layered composites be prepared by cyclically passing the samples beneath the two targets with a sp
18、eed(thickness of 10 nm/3 nm)Heat treatment or annealing:inducing recrystallization in the MoSi2/SiC multilayered filmProperties after annealing:Superior oxidation resistanceSignificant hardness MoSi2/SiC多层膜的剖面透射电镜图片多层膜的剖面透射电镜图片Cross-sectional TEM image of MoSi2/SiC multilayered filmMoSi2/SiC多层膜退火前的电
19、子衍射花样多层膜退火前的电子衍射花样Electron Diffraction Pattern of MoSi2/SiC multilayered film before annealing经过经过800C，1h退火处理的退火处理的MoSi2/SiC多层膜多层膜的低放大倍数亮场电镜照片的低放大倍数亮场电镜照片wNanolayer Cu/NbSubstrate:(100)sc-SiTechniques:dc magnetron sputteringLayer thickness:(100 nm/100nm)Properties:wHigh strengthwSuperior thermal con
20、ductivitywSuperior electrical conductivity通过磁控溅射技术沉积的通过磁控溅射技术沉积的Cu/Nb多层膜的剖面透射电镜图像多层膜的剖面透射电镜图像Cu/Nb多层膜的电子衍射花样多层膜的电子衍射花样Electron diffraction pattern of Cu/Nb multilayer1-4 Current Status of Sputtering wAdvantagesMost widely used sputtering methodWell-established coating techniques for microelectronic
21、applicationsMany nanometer multilayer structures be prepared by sputteringwShortcomingsMaterials system limitation:mainly conductors or nitridesDifficulty in control stoichiometry,low deposition rate etc.Be questionable to be used as the main coating tool in microelectronics industry(although succes
22、sful for SrTiO3,BaTiO3,and Ba1-xSrxO3)2-1 Principles of PLD2-2 Deposition of Nano-Scale Metal Oxide Thin Films2-3 Multilayer Structures Prepared by PLD2-4 Current Status of PLD2Pulsed Laser Deposition(PLD，脉冲激光沉积，脉冲激光沉积)2-1 Principles of PLDwAdvantages and Properties of PLDSimplest deposition techniq
23、ue among all thin film growth techniquesStoichiometric removal of constituent species from targetVersatile deposition of a wide variety of materialsMetalsSemiconductorsNitridesDielectric materialsOxidesOrganic compounds/ploymersTernary compoundswTechnical Description of PLDBased on physical vapor de
24、position Impact of high-power short pulsed laser radiation on solid targetsRemoval of materials from impact zoneEquipment constituentHigh power laser:external energy source to vaporize target materialsVacuum chamber with a quartz windowTarget holder or multiple target holderSubstrate holder(with a h
25、eater)Integration with other type of evaporation sources脉冲激光沉积系统脉冲激光沉积系统（示意图）（示意图）Schematic diagramof a PLD system激光束激光束加热器加热器衬底衬底喷流喷流靶靶2-2 Deposition of Nanoscale Metal Oxide Thin FilmswImportanceMetal oxides could exhibit versatile properties High temperature superconductivity Ferroelectricity Col
26、ossal magnetoresistivity Non-linear optical propertiesMetal oxides be recognized as possible candidates for next generation electronic materials due to their diverse propertieswSubstrates for Metal Oxide FilmsImportance:proper choice of substrate be essential for accomplishing perfect 2-dimensional
27、epitaxy of metal oxide heterostructuresRequirements for a good substrateGood in plane lattice matchThermal expansion coefficient close to that of filmAtomically smooth surfaceGood chemical compatibility with the film Commonly used single crystal substratesYttria-stabilized zirconia(YSZ)MgOLaAlO3SrTi
28、O3NdGaO3(LaAlO3)0.3 (Sr2AlTaO6)0.7Sapphire Surface treatment of substratesIon milling+in situ annealingIon milling+pre-depositionChemical etching+annealingSurface terminatingwInitial Growth of Metal Oxide Films Observation and monitor techniquesIn situ:reflected high energy electron diffraction(RHEE
29、D),laser light scattering,real-time optical diagnosisEx situ:scanning tunneling microscope(STM),atomic force microscopy(AFM),cross-sectional transmission electron microscopy(TEM),X-ray diffractionGrowth ModeHighly depending upon the quality of substrateStranski-Krastanov mode(layer plus island growt
30、h)to Volmer-Weber mode(island growth)at a critical thicknessScrew-growth in thicker filmsLayer plus island growth(Thickness Critical Thickness)YBCO薄膜的早期生长模式及其转变薄膜的早期生长模式及其转变D.-W.Kim et al.,Physica C 313(1999)246wCharacterizing multilayer thickness by X-ray diffractionYBCO/PrBCO superlattice:a new ma
31、n-made periodicityThe modulation thickness in superlattices is measured by the position of satellites peaks,given by D=(/2)/(sin n+1-sin n)D is the modulation thickness with D=dYBCO+dPrBCO;is the wavelength of X-ray source;n+1 and n are positions of the nth and the(n+1)th satellite peaksThe satellit
32、e peaks up to the fourth order indicate atomically sharp and flat interfaces Nominal Thickness2.4 nm/12 nmCalculated Thickness14.6 nmC.Kwon et al.,Appl.Phys.Lett.62(2004)1289X-ray X-ray -2-2 scan around(001)and(002)peaks of a YBCO/scan around(001)and(002)peaks of a YBCO/PrBCOPrBCO superlatticesuperl
33、atticewCharactering film thickness by low angle X-ray diffractionwCharactering film thickness by STM,AFM,SEM,and TEMC.Kwon et al.,Appl.Phys.Lett.62(2004)1289A low angle X-ray reflection of a nominally 27.6 nm thick YBCO on NdGaO3wSuperconductivity in a Unit-cell Thick YBCOAimed Questions:What is the
34、 minimum unit needed for the occurrence of superconductivity?How essential is the interlayer coupling between Cu-O planes in determining the transition temperature?Way to the question:using superlattice structures as model system(possible coupling or other parameters can be changed artificially by i
35、nterposing other materials in between the Cu-O planes or unit-cells)System Composing:Ultrathin YBCO layers+nonsuperconducting PrBCO layers2-3 Examples of Multilayer Structures Prepared by PLDC.Kwon et al.,Superlattices and Micro structures 29(2005)169Resistive transition of 1,2 and 4 unit-cell thick
36、 YBCO layers sandwichedbetween(Y1-xPrx)Ba2Cu3O7 adjacent layers with x=1 and 0.6C.Kwon et al.,Superlattices and Micro structures 29(2005)169Dependence of transition temperature on the x value of(Y1-xPrx)Ba2Cu3O7 adjancentC.Kwon et al.,Appl.Phys.Lett.62(2004)1289The dependence of critical current den
37、sity on the magnetic field for1 unit-cell(1.2 nm)and 200 nm thick YBa2Cu3O7 filmswSuperlattice SrTiO3/BaTiO3SrTiO3/BaTiO3 can form solid solution with each other at all compositions due to their similar structures and the comparable ionic radii of Ba2+and Sr2+Strained SrTiO3/BaTiO3 superlattice Elec
38、tric/dielectric behaviourIntroducing lattice stress at the interfaceEnhanced dielectric constant H.Tabata et al.,Appl.Phys.Lett.65(2005)19702-4 Current Status of Pulsed Laser DepositionwMajor AdvantagesCongruent evaporationCrystallinityFast response timeEspecially suited for synthesizing high qualified metal oxide thin filmswMajor technological obstaclesParticulate emissionLarge area coatingHard to control down to the atomic scaleFurther optimizing of PLD processing condition and deposition techniques are needed Thank you for your attention