Ceramic processing of targets for Physical Vapor Deposition Barbara Malič, Marija Kosec Jožef Stefan institute Jamova Ljubljana, Slovenia

Outline: Introduction Ceramic powder Basics of solid state chemistry Powder synthesis Powder characterization Bulk ceramics Shaping Sintering Characterization of ceramics

Introduction Ceramics Inorganic nonmetallic materials, produced by the action of heat and subsequent cooling. Chemical composition: oxides, carbides, nitrides Polycrystalline, but also amorphous (glass) Medallion Pithoi", or storage jars, at the Knossos palace. Named from the raised disks, they date to MM III/LM IA. Knossos The word ceramic comes from the Greek word "κεραμικός" (keramikos), "of pottery" or "for pottery“, from "κέραμος" (keramos), "potter's clay, tile.Greek

Division of ceramics Technical: Engineering Electronic Bio … Ceramic Si 3 N 4 bearing parts, Ceramic_materials Approx (AC), A Kamare style vase, BCE, MI_-_Kamaresvase_2.jpg Traditional

Bulk ceramics processing * powder synthesis * shaping * sintering Typical manufacturing of piezoceramics using the mixed oxide route.

1µm1µm Crystallite or grain pore Ceramic microstructure Ceramic powder

Powders Basics of solid state chemistry Surfaces, surface energy Phase equilibria Diffusion Reactions in solid state Powder systems Powder synthesis Solid state synthesis Mechanochemical synthesis Powder characterisation Chemical and phase composition Particle size and shape Density, specific surface area Thermal analysis

Surfaces, surface energy Cross-section of a condensed phase illustrating the difference in surroundings of a surface atom and an interior atom Surfaces in liquids and solids have special properties because they terminate the phase. Surface energy (surface tension): extra energy of surface atoms  [ N/m, J/m 2 ] Liquids: Isotropic surface properties. Crystalline solids: Different properties for different crystal planes. Interface: Surface layer between two phases

Surface tension causes a pressure difference across a curved surface  P = 2  / r  P =  g h h r r dr PP Bubble in a liquid: pressure is for  P higher than the pressure of the liquid Vapor P > P 0 P = P 0 P < P 0 Vapor pressure of a condensed phase (liquid) varies with the curvature.

Capillary action of water compared to mercury  SV =  SL  +  LV cos  LV SV  : wetting angle Water drop on glass Wetting,   Non-wetting,   (mercury)

Phase equilibria Gibbs phase rule P + F = C + 2 P: number of phases C: number of components F: degree of freedom Simple binary diagram (schematic)

Atom mobility (diffusion) 1 st Fick law: J = - D dC/ dx Quantity of diffusing material is proportional to its concentration gradient (stationary conditions). J: flux C: concentration X: distance D: diffusion coefficient 2 nd Fick law: dC / dt = D d 2 C / dx 2 Describes the time-dependence of concentration gradient. Temperature dependence of diffusion coefficient D = D 0 e – Q/RT Q: activation energy

AOB2O3B2O3 AB 2 O 4 AOB2O3B2O3 Solid-state reactions (heterogeneous) -diffusion controlled dy/dt  D K/ y or: y 2 = K D t (parabolic growth rate) y: reaction product thickness K: constant Oxidation of metals is a diffusion controlled process. Chemical potential gradients across an oxide layer on a metal.

Reactions in powder mixtures y: thickness of the reaction product y = r [ 1 - (1 – x) 1/3 ] x: fraction of the reacted material ( x = 1 – everything has reacted) and: y 2 = K D t [ 1 - (1 – x) 1/3 ] 2 = ( KD / r 2 ) t Reaction rate constant Relation assumes many simplifications (perfect contact, spherical particles, no volume changes, …) At constant T the reaction rate is proportional to diffusion coefficient and inversely proportional to the square of particle size. Reaction rate exponentially increases with T.

Solid-state synthesis Takes place upon heating typically 700 – 900 o C) Reagents (metal oxides, carbonates, … Product Homogenization (Milling) Heating (calcination) Milling BaCO 3 + TiO 2  BaTiO 3 + CO 2 T Particle size: 0.1  m-  m-range Diffusion distances: 0.1  m-  m range Chemical non-homogeneity Synthesis–temperature: high

Particle size Purity Contamination during milling A sequence of s.s. reactions leading to the target phase requiring high temperature Reaction products with different stoichiometries  non-homogeneity. Coarsening Solid-state synthesis Templeton, Pask, 1959, Beauger, Mutin, Niepce, 1983, Mutin, Niepce, 1984, Phule, Risbud, Example: BaTiO 3 : solid state reaction between BaCO 3 and TiO 2 BaCO 3 + TiO 2  BaTiO 3 + CO 2 BaCO 3 + BaTiO 3  Ba 2 TiO 4 + CO 2 Ba 2 TiO 4 + TiO 2  2 BaTiO 3 Also possible reaction products: Ba 6 Ti 17 O 40, Ba 4 Ti 13 O 30

Mechanochemical synthesis (activation) or High-energy milling or Mechanical alloying Milling: particle size reduction (comminution) amorphisation shape change agglomeration changing properties (flowability, density) blending (mixing) Solid state synthesisMechanochemical synthesis Thermal energyMechanical energy Synthesis temperature Milling parameters Synthesis time

Mechanochemistry A branch of chemistry dedicated to the research of physical and chemical changes in the solids induced by mechanical energy. elastic deformation (reversible) plastic deformation (irreversible) fracture, amorphization, mechanical activation, chemical reactions Applicable for different materials: Metal compounds, alloys Metal oxides (ceramics) Biomaterials Molecular crystals (Pharmaceutics)  Solid-state processing technique, occurs at nominally room temperature.  Metalic or non-metalic powder mixture is actively deformed under a highly energetic ball charge to produce homogeneous and fine grained products.

Equipment: Shaker millPlanetary millAttrition mill Characteristics: loading, ball diameter, materials, rotation speed, time Problems: contamination

Powder characterisation Chemical and phase composition Particle size and shape Density, specific surface area Thermal analysis ….. Single particle Particle system Chemical compositionDistribution of chemical composition Impurities Distribution of impurities Phase compositionDistribution of phase composition Crystal defects, domainsDistribution of crystal defects Structure of phasesParticle structure distributionPorosity Size and shapeParticle size distribution Density Density (bulk)Specific surface area

Phase composition X-ray powder diffraction: size of crystallites Bragg’s law n = 2 d sin  d: distance between atomic layers  wavelength of the incident X-ray n: integer Information from XRD Peak positions - Crystal system Unit cell size Peak intensities Quantiative phase determination Atomic positions Peak shape Crystallite size Strain Defects 2-theta (deg.) Int. (cps)

Particle size and shape Microscpoy techniques, Image analysis

Particle diameters Volume diameter d V : diameter of the sphere with the same volume as the particle Surface diameter d S : diameter of the sphere with the same surface area as the particle Projected area diameter d A : diameter of the circle with the same surface area as the projected area of the particle Feret’s diameter d F : distance between two parallel tangents which touch the outline of the particle projection d A d F

Measurements of particle size and size distribution: -Sieving -Sedimentation -Laser diffraction -Microscopy

Density, specific surface area, porosity D u : ultimate (‘XRD’) density, density of the solid D a : apparent density, density of the solid+ closed pores D b : bulk density, desnity of solid and all porosity (open+closed)

Porosity Mercury porosimetry Cumulative pore size distribution of a nonporous (solid line) and porous alumina powder.

Specific surface area (SSA): BET -physical adsorption of a gas - monolayer coverage of the powder SSA is inversely proportional to the particle size.

Thermal analysis Thermal analysis (TA) is a group of techniques in which a physical property of a substance is measured as a function of temperature while the substance is subjected to a controlled temperature programme. Several methods are commonly used - these are distinguished from one another by the property, which is measured:  Differential scanning calorimetry (DSC): heat difference  Differential thermal analysis (DTA): temperature difference  Thermogravimetric analysis (TGA): mass  Evolved gas analysis (EGA) : gaseous decomposition products  …. McNaught, Wilkinson, IUPAC Compendium of chemical terminology, 2 nd ed. (Blackwell Scientific Publications, Oxford,1997) The TA curves demonstrate the thermal decomposition of calcium oxalate hydrate (CaC 2 O 4 ·H 2 O): dehydration and the two-step decomposition of the oxalate groups with evolution of carbon dioxide.

Bulk ceramics Shaping (pressing) Sintering Solid state sintering Characterisation of ceramics: Density Chemical and phase composition Microstructure Sintering: -Temperature, time, heating and cooling rates, atmosphere, pressure.

Shaping: Powder compaction by uniaxial or isostatic pressing Schematic representation of stresses on a compact during uniaxial compaction. Stages of isostatic pressing: a) filling, b) loading, c) pressing, and d) decompression.

Pore size distribution in powder compacts Microstructure of PZT cermaics with a lens-shaped defect as a consequence of presence of agglomerates in the powder and consequently a non-uniform porosity distribtuion in the powder compact. Porosity distribution in a powder compact.

Various types of sintering: -Solid state -In the presence of a liquid phase -Reactive -… Sintering: Process which takes place upon firing: -Changes in grain size and shape -Changes in pore shape -Changes in pore size 1  m T, t, P, atm.,

Driving force for sintering: reduction of the total interfacial (surface) energy  A g: surface/inerface energy A: surface / interface area of the powder compact  (  A) =  A +  A  : consequence of densification (reduction of solid /vapor interfaces = surfaces by solid/solid interfaces)  A: consequence of coarsening = grain growth

Three stages of sintering (depending on the amount of densification achieved): Initial Relative density: 50-60% to about 70 % Porous skeleton of particles connected by necks Changes of particle and pore shapes Intermedaite Relative density: from about 70 % to about 92% Intensive densification Shrinkage of pores Final Densification is slower Grain growth Densification curve of KNN powder compact obtained by a heating stage microscope.A (H): mixture homogenized in acetone (n-hexane). The densitiies of the ceramics sintered at 1100 oC and at different times are inserted in a table. % TD Melting point of KNN: 1140 o C Density is most commonly expressed as relative density, that is, % of theoretical density (TD).

Factors affecting sintering: -Particle size: densification is enhanced if the powder is finer. -Pressure: applying pressure during sintering results in enhanced densification at a lower temperature. Finer Hot pressed

Temperature and time of sintering Temperature influences densification much more than time. (Sintering is a thermally activated process!) Shrinkage (%) t (min.) Shrinkage of CaF 2 depending on sintering conditions

Schematic presentation of sintering. Note the formation of necks between the particles, decrease of porosity and change of pore shape. Formation of a neck between the particles.

Characterisation of ceramics: Density: geometrical, Archimedes Chemical and phase composition: quantitative chemical analysis, XRD Microstructure Physical properties … Stereological analysis: measurement of grain size distribution No of grains: 238, D Feret = 0.32  m  = 0.22  m. Thermally etched microstructure of KNN with 1 mass% ZrO 2, sintered at 1125 o C, 2h. Density: 95.5 % TD.

Microstructure of In-Ga-Zn-O ceramic InGaZnO 4 In 2 O 3 50 mm ceramic target for sputtering ZnO-In 2 O 3 -Ga 2 O 3 phase diagram D. Kuščer et al., JSI, MULTIFLEXIOXIDE project

Acknowledgements Colleagues from Electronic Ceramics Department, Jožef Stefan Institute. Slovenian Research Agency (P2-0105). References (and source of the majority of figures): W. Kingery, H. Bowen and D. Uhlmann, Introduction to Ceramics, 2 nd ed., (Wiley, New York, 1976) D. Kolar, Tehnična keramika (ZRSŠŠ, Ljubljana, 1993) J.S. Reed, Principles of Ceramics Processing, 2 nd ed., (Wiley, New York, 1995) S. E. Dann, Reactions and Characterization of Solids, (RSC, Cambridge, 2000). Moulson, A.J. & Herbert, J.M. (2003): Electroceramics  Materials, Properties, Applications. 2 nd ed. (John Wiley & Sons) S.J. Kang, Sintering: Densification, Grain Growth and Microstructure, (Elsevier Butterworth- Heinemann, Oxford MA, 2005)