Outline Introduction Basics of Chemical Solution Deposition (CSD) of functional–oxide thin films Solutions for CSD: overview of synthetic approaches Sol-gel (Alkoxide – based) MOD Inorganic routes Stages of CSD Ink-jet printing: parameters to control Solutions for ink-jet-printing

Introduction - History Optical coatings on glass TiO2/Pd, SiO2/TiO2 -Schott: Geffcken, Berger (1939) Dislich (1971) Electronic ceramic films PT/PZT/PLZT - S. R. Gurkovich, J. B. Blum, (1984). - K. D. Budd, S. K. Dey, D. A. Payne, (1985). 1985

Applications of Films and Coatings Optical (antireflective, absorbing, ...) SiO2, SiO2/TiO2, TiO2/Pd (IROX) Protective (corrosion or abbrasion resistance, adhesion passivation, planarization,..) SiO2, ... ⇨ ORMOSIL (Schmidt, 1986) Electronic Dielectric: (Ba,Sr)TiO3 - BST Ferroelectric: (Pb(Zr,Ti)O3-PZT, SrBi2Ta2O9 - SBT) Piezoelectric: ZnO, PZT Pyroelectric: PbTiO3, Ti-rich PZT Sensors: TiO2 HTSC: YBa2Cu3O7 - YBCO Conductive / semiconductive : LaNiO3, (La,Sr)CoO3, La-ruthentates, RuO2, ITO (90% In2O3, 10% SnO2 by weight), In2O3–ZnO, … ... Chemical homogeneity Crystallinity (if crystalline: phase and orientation Microstructure Functional response

Chemical Solution Deposition (CSD) routes Organic routes Sol-gel (pure) alkoxide-based alkoxide + salt/oxide Metalloorganic decomposition (MOD) Routes involving polymers: in-situ polymerisation (‘Pechini’), polymer precursor route Metal ions are homogeneously distributed in the organic network and coordinatively bonded by polymer’s functional groups. Inorganic routes (nitrates, citrates, peroxo- compounds) Stability of water based solutions of different metal ions. REACTIVE NON-REACTIVE

precursor solution (‘Sol’) CSD processing steps: Synthesis of precursor solution (‘Sol’) Coating ‘Gel’ film Drying, pyrolysis Amorphous oxide film Crystallization Crystalline film substrate substrate substrate

Alkoxide based sol-gel route Precursors: Metal alkoxides M(OR)n For M = transition metal High reactivity of TM(OR)n towards water Tendency to increase the coordination number ⇨ oligomerization Metal - acetates - nitrates - oxides (problem of solubility) Ti(OR)4, R: n-Pr, n-Bu Z = 4 N = 5 Babbonneau et al., 1988.

Reactive solvents: alkyl-alcohols (R-OH) ether-alcohols: R-O-(CH2)n-OH poly-alcohols (diols, triols): 1,3 propanediol methanol/acetic acid CH2-OH CH2 CH3-O-C2H4-OH 2-methoxyethanol Turova, Turevskaya, Kessler, Yanovskaya, 2002.

Building blocks of the inorganic network. Two major reactions (shown for one alkoxide group): Hydrolysis: M-OR + HOH M-OH + ROH Polycondensation: M-OR + M-OH  M-O-M + ROH M-OH + M-OH  M-O-M + HOH Oxo-bridge Building blocks of the inorganic network. Heterometallic systems: Reaction between: two alkoxides TM alkoxide and a metal salt (acetate, nitrate). Ester elimination: -[Pb-CH3COO] + [RO-TM]  -[Pb-O-TM] + R-OOC-CH3 oxo bridge PZT: S. R. Gurkovich, J. B. Blum, 1984. K. D. Budd, S. K. Dey, D. A. Payne, 1985.

+: starting compounds for a wide range of metals Alkoxide based sol-gel route +: starting compounds for a wide range of metals possibility to tailor the reactivity of the starting compounds effective solvents easy synthesis of different heterometallic solutions solutions are suitable for film deposition (viscosity, wettability) -: reactive starting materials inert atmosphere (dry-box, Schlenk glassware) toxic solvents syntheses require some knowledge of chemistry sometimes products with stoichiometries ≠ target material

MOD route Precursors: large metal carboxylates R-COO- or b-diketonates R-CO-CH2-CO-R’ Ba, Pb ethylhexanoate (CH3-(CH2)6-COO-) Zr neodecanoate (CH3-(CH2)8-COO-), Zr acetylacetonate (CH3COCHCOCH3-) Ti di-methoxydineodecanoate (Ti(CH3C2H4O)2(CH3-(CH2)6-COO)2 Zn naphtenate (= salt of cyclic carboxylic acid obtained from petroleum, MW ~200) Solvents: Non-polar: toluene (methyl-benzene), xylene (di-methyl-benzene),... Polar: alcohols, alcohol/carboxylic acid, ... Mixing at RT. No reaction between the starting chemicals. No reactive reagents (no need for protective atmosphere). Variables: chemicals, solution concentration. Neodecanoate = C10H19O2 R. W. Vest, J. Xu, 1988 Hoffman, Klee, Waser, 1995

Water – based solutions for CSD Heterometallic systems: problems with common solubility of different metal species. To overcome such problems, water-based heterometallic solutions often contain coordinating ligands, such as: -acetato -nitrato -carboxylato -peroxo.

[M-(OH2)] z+  [M-(OH)] (z-1)+ + H+  [M=O] (z-2)+ + 2H+ Transition metal ions (La3+, Ti4+, Zr4+, Nb5+, etc.) in water are solvated by water molecules: Aquo-ligand (H2O)n-1 - (H2O)n-1- Z: oxidation number CN: coordination number Depending on the pH of the solution the equilibrium between three types of solvated species is established (hydrolysis of metal ions – shown for one group): [M-(OH2)] z+  [M-(OH)] (z-1)+ + H+  [M=O] (z-2)+ + 2H+ Aquo- Hydroxo- Oxo- ligands The type of the complex depends on: Z CN pH

Thin film deposition methods: Dip coating Spin coating Spray coating Spin-up Spin-off Evaporation Four stages: Substrates: Single crystals: sapphire (A, C, R), SrTiO3 (100), ... Ceramics (Al2O3) Pt (/TiO2)/SiO2/Si - Pt typically strongly (111) oriented Metal foils (Cu) Glass Variables: Solution concentration Viscosity Volatility Surface tension Wetting

Processes/reactions taking place in the as-deposited ‘gel’ film Solvent evaporation ⇨ strong concentration of solute species ⇨ viscosity increases. ‘Physical’ aggregation: = concentrating of solute species. (MOD) ‘Chemical’ aggregation: Promoted polymerisation or polycondensation (= more cross-linking) leads to a gel. (Alkoxide sol-gel) Viscosity increases Thickness of a deposited film depends on solution properties and deposition conditions. Typically it is in the range a few 10 nm - 100 nm. Efficiency of ‘packing’ depends on branching of solute species (linear or branched).

Spin coating of a PZT sol ( solvent: 2-methoxyethanol): influence of the sol concentration, rotation of the spinner on evolution of cracks upon drying at different temperatures. C=1M, t=400 nm Temperature Speed rotation 60°C 100°C 200°C 350°C 500 rpm wet film cracks no film 1000 rpm 1500 rpm 2000 rpm 3000 rpm Temperature Speed rotation 60°C 100°C 200°C 350°C 500 rpm cracks no film 1000 rpm no cracks 1500 rpm 2000 rpm 3000 rpm C=0.5M, t=150 nm C=0.25M, t=50 nm Temperature Speed rotation 60°C 100°C 200°C 350°C 1000 rpm no cracks 1500 rpm 2000 rpm 3000 rpm

Processes taking place upon heating Drying Pyrolysis Crystallization 100- 200 oC 350 – 450 oC 400 – 800 oC Evaporation of the trapped solvents Dehydroxylation: removal of –OH groups from the network (continued polycondensation – alkoxide route) Thermolysis/pyrolysis: decomposition/oxidation of functional (organic/carbonate) groups Structural rearrangement: - film shrinkage change of coordination environment - long-range ordering Two-step: One-step:

Crystallization and evolution of microstructure: Nucleation and growth from the amorphous phase (Ba,Sr)TiO3 (BST): Pb(Zr,Ti)O3 (PZT) Homogeneous nucleation Heterogeneous nucleation - Granular micorstructure - Columnar microstructure

How to adapt the CSD-solution for ink-jet printing? DIMATIX ink-jet printer Requirements for the ink (given by the producer): Viscosity = 10 to 12 m Pas Surface tension = 28 to 32 mN/m http://www.dimatix.com

Parameters to control To realise the ink-jet printing and to obtain uniform and crack-free 1D/2D structure. Ink stability Viscosity Surface tension Concentration of the solution Wettability on the substrate Deposition parameters - Temperature of the cartridge - Temperature of the substrate - Drop spacing - Cleaning of the substrate Heating Design of the ink Printing procedure

In2O3/ZnO(IZO) CSD-solution Alkoxide and acetate based solution in 2-methoxyethanol 2MOE Dissolution In isopropoxide Zn acetate dissolution Clear and stable solution Polymer IZO, 4 deposited layers, 1h at 150°C in air after each deposition 0.5 mm Silicon C=0.25M J. Tellier et al., submitted.

Printing the CSD-solution Solution of IZO in 2-methoxyethanol, C=0.25M OM Ink-jet printing of IZO designed for spin coating 1 layer, annealing at 150oC 10 minutes Low viscosity Bad coverage Cracks (too thick) How to modify the solution ? Increase of the viscosity by adding a viscous solvent in the right proportion (1,3 propanediol) and also modify the surface tension Decrease the thickness by diluting Keep the stability of the original solution

Fluid parameters Weber number Adimentional number, related to surface tension Range 5-20 mPas and 35-40 mJ/N v: speed r: distance r: volume weight s: surface tension Reynolds number Adimentional number, related to viscosity Proper formation of drops v: speed r: distance r: volume weight n: cinematic viscosity h: dynamic viscosity r: nozzle diameter r: ink density h: dynamic viscosity s: surface tension R. Noguera et al, Journal of the European Ceramic Society (2005)

Increase of viscosity Properties of the solvent have a strong influence on the ink properties Different 2-metoxyethanol (2MOE) / 1,3-propanediol (13PD) volume ratios Stalagmometer Room temperature measurements

Design of IZO ink viscosity Solvent system: 2-methoxyethanol (2MOE) / 1,3-propanediol (13PD) Solvent Viscosity (mPas) 2MOE 3.4 13PD 41.5 Optimum : 2MOE / 13PD: 45 / 55

Printing the modified solution (ink) Solution of IZO in 2-methoxyethanol + 1,3-propanediol (45/55), C=0.25M 150°C Cracks (measured thickness 400 nm) Random nozzles clog Film not continuous Decrease concentration Possibility to form a single drop PZT thin films obtained by spin coating IZO thin films obtained by ink jet printing C=0.5M t=150 nm C=0.25M t=50 nm C=0.25M t=400 nm C=0.05M t=50 nm Decrease of concentration Decrease of concentration 500µm

Wettability of IZO ink Contact angle: Contact angle : high enough to ensure a good resolution Contact angle: Glass = 42.5° SiOx/Si = 27.6° PEN = 24.1° Solution of IZO in 2-methoxyethanol + 1,3-propanediol Viscosity h = 9.6 mPas Surface tension g = 34.5 mN/m ImageJ software: drop_analysis (Drop Snake)

Acknowledgements Colleagues from Electronic Ceramics Department, Jožef Stefan Institute. Slovenian Research Agency (P2-0105) EU 6FP project MULTIFLEXIOXIDES (NMP3-CT-2006-032231). References (and source of the majority of figures): : C. J. Brinker, G. W. Scherer, Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing, Academic Press, San Diego, 1990. M. Ohring, Materials Science of Thin Films, 2nd ed., Academic Press, San Diego, 2002. R. Waser (Ed.), Nanoelectronics and Information Technology, Wiley-VCH, Weinheim, 2003, P. Erhart, Film Deposition Methods, pp. 201 – 221. R.C. Buchanan (Ed.), Ceramic Materials for Electronics, 3rd ed. Marcel Dekker, New York, 2004, A. I. Kingon, P. Muralt, N. Setter and R. Waser, Electroceramic Thin Films for Microelectronics and Microsystems, pp. 465 – 526. G. Cao, Nanostructures and Nanomaterials, Imperial College Press, London, 2004. C. N. R. Rao, A. Mueller, A. K. Cheetham, The Chemistry of Nanomaterials (Vol. 1), Wiley, Weinheim, 2004.