Applications of Thin Films 1 • Protective coatings - Hard films • Optical coatings - Filters, mirrors, lenses • Microelectronic devices • Optoelectronic devices, Photonic devices • Electrode surfaces • Photoelectric devices, photovoltaics, solar cells • Xerography, Photography, Lithography • Catalyst surfaces - Heterogeneous catalysis • Information storage, magnetic, magneto-optical, optical memories 2 • Crystalline, amorphous, microcrystalline • Monolayer, multilayer, superlattice, junctions • Free-standing, supported • Epitaxial (commensurate), incommensurate Properties of Thin Films Superlattice 3 Properties of Thin Films • Thickness (1 Å – 1 m) • Surface-to-volume ratio • Structure - surface versus bulk, surface reconstruction • Surface morphology, roughness • Chemical composition (oxides, metals, nitrides, carbides,…) • Hydrophobicity, hydrophilicy (Si-OH vs. Si-H) • Texture: single crystal, microcrystalline, domains, orientation: Si (100) vs. (111) • Form - supported or unsupported, nature of substrate 4 Surface Energy Surface energy [J m2] depends on: • The distance of the face from the center of the crystal • Miller indices • Surface roughness • The radius of curvature Surface energy [J m2] a scalar Surface stress [J m2] a tensor Same for liquids, different for solids Surface tension [J m2] the work done in creating unit area of new surface (= Surface energy in one-component systems) 5 Surface Model The Terrace-Step-Kink (TSK) Model of a Surface (Kossel/Stranski) • Terrace • Step/Ledge • Kink • Vacancy • Adatom • Island 6 Surfaces The acetone molecule (in colored spheres) attached to (A) the edge of bilayer graphene (carbon atoms are represented as black balls), (B) the edge of four layer graphene, (C) on a step formed from bilayer graphene, and (D) on a step formed from trilayer graphene 7 Surfaces Screw dislocation on graphite Spiral growth 8 Thin-Film Growth Mechanisms The growth of epitaxial (homogenous or heterogeneous) thin films on a single crystal surface depends on the interaction strength between adatoms and the surface • Volmer–Weber (VW) growth - adatom-adatom interactions are stronger than those of the adatom with the surface, the formation of three-dimensional adatom clusters or islands, coarsening, rough multilayer films • Frank–van der Merwe (FM) growth - adatoms attach preferentially to surface sites, atomically smooth layers, layer-by-layer growth, epitaxy • Stranski–Krastanov growth - an intermediary process, both 2D layer and 3D island growth, transition from the layer-by-layer (wetting layer) to island-based growth occurs at a critical layer thickness, dependent on the chemical and physical properties, such as surface energies and lattice parameters, of the substrate and film 9 Thin-Film Growth Mechanisms Θ < 1 ML 1 < Θ < 2 Θ > 2 Surface coverage, Θ VW (island) FM (layer-by-layer) SK (layer-plus-island) 10 Symmetry at Surfaces AFM of C atoms within the hexagonal graphite unit cells Image size 2×2 nm2 11 Symmetry at Surfaces (111) (100) (110) 12 Higher temperature = Faster diffusion Surface Diffusion Surface diffusion coefficient D D = a2 ks a … effective hopping distance between sites ks … site-to-site hopping rate of an adatom ks = A exp(─Vs/kbT) Vs … energy barrier to hopping from site to site T … substrate temperature 13 Surface Diffusion The dissociative collision of a CH4 molecule with a nickel surface does not significantly perturb the nickel atom at the impact point 14 Si(111) Surface Silicon "diamond lattice" structure a = 5.463 Å Si(111) = a set of atomic planes One plane outlined with red Si (111) etches more slowly than (001) Si (111) oxidizes twice as rapidly as (001) A top view of the atomic arrangement for the (111) plane orange = the top layer green = deeper layers 15 Si(111) Surface 16 Reconstruction Relaxation = energy lowering, no change in symmetry Reconstruction = the surface atoms rearrange to a more energetically stable configuration Symmetry lowering 2D symmetry – 17 plane groups / 230 bulk space groups 3D representation of the Si 7x7 STM image The image area is 18 x 8 nm2 the height of the "bumps" is about 0.04 nm 17 Si 7x7 Reconstruction When (111) surface of Si is heated to high temperatures under the Ultra-High Vacuum conditions the surface atoms rearrange to a more energetically stable configuration called 7x7 reconstruction STM image of Si(111) surface 18 Si(100) Surface ideal reconstructed STM images of the silicon-silicon dimers imaged with (a) Vsample = -2.0 V (b) Vsample = 2.3 V The filled and empty states of these highly ordered dimers can be probed by biasing the surface in the opposite directions The dimensions of the figure are 2.3 nm x 7.7nm Critical Parameters Synthesis of Thin Films 19 High-quality films for the electronics and optics industries Chemical composition control - variety of materials to be deposited Purity of precursors: usually less than 10-9 impurity levels Challenge for chemistry - purifying and analyzing at the ppb level Exceptionally clean growth systems Impurities destroy controlled doping of films for device applications Good film uniformity over large areas covered > 100 cm2 Precise reproducibility Precise control of film thickness = accurate control of deposition, film growth rate, 1 - 2000 nm layer thickness Crystal quality, epitaxy - high degree of film perfection Defects degrade device performance Alternating composition and graded composition films 30 - 40 sequential layers 0.5 - 50 nm thickness required with atomic level precision Interface widths - abrupt changes of composition and dopant concentration required, quantum confined structures 20 Synthesis of Thin Films MAIN METHODS OF SYNTHESIZING THIN FILMS: CHEMICAL, ELECTROCHEMICAL, PHYSICAL • Cathodic deposition, Anodic deposition, Electroless deposition • Thermal oxidation, nitridation • Chemical vapor deposition (CVD) • Metal organic chemical vapor deposition (MOCVD) • Cathode sputtering, vacuum evaporation • Molecular beam epitaxy, supersonic cluster beams, aerosol deposition • Photoepitaxy • Electrochemical deposition • Laser ablation • Plasma spraying • Self-assembly, surface anchoring, SAM • Dip coating • Evaporation-Induced Self-Assembly • Spin coating 21 Electrochemical Synthesis of Thin Films CATHODIC DEPOSITION Two electrodes, dipped into electrolyte solution External potential applied Metal deposition onto the cathode as thin film Anode metal slowly dissolves ELECTROLESS DEPOSITION Spontaneous, no applied potential, cementation Depends on electrochemical potential difference between electrode and solution redox active species to be deposited Both methods limited to metallic films on conducting substrates ANODIC DEPOSITION Formation of oxide films, such as alumina, titania Oxide films grow on a metallic electrode in aqueous salts or acids Deposition of conducting polymer films by oxidative polymerization of monomer, such as thiophene, pyrolle, aniline 22 Anodic oxidation of aluminum in oxalic or phosphoric acid Al | H3PO4, H2O | Pt electrolytic cell Al  Al3+ + 3 e- anode = oxidation 2 Al3+ + 3 O2-  -Al2O3 (annealing)  -Al2O3 PO4 3- + 2 e-  PO3 3- + O2- cathode = reduction Overall electrochemistry: 2 Al + 3 PO4 3-  Al2O3 + 3 PO3 3The applied potential controls the oxide thickness and the rate at which it forms, oxide anions from solution have to diffuse through an Al2O3 layer of growing thickness on the reacting Al substrate, to attain an equilibrium thickness of the alumina film Porous Alumina Films AFM Image of Porous Alumina Film 23 Porous Alumina Films Self-organizing process observed A regular array of size tunable hcp pores form and permeate orthogonally through the alumina film Exceptionally useful process for creating • Controlled porosity membranes • Photonic gap materials • Hard template for synthesizing semiconductor/metal nanostructures • Host for synthesizing and organizing aligned carbon nanotubes • Fuel cell electrode materials Electrochemical Deposition 24 HAp protective films on Ti implants Ca5(PO4)3(OH) Ionic species as precursors: Ca2+, PO4 3-, may form undesired Ca/PO4 phases Nanoparticulate HAp dispersed in aqueous solution using stabilizing agents (tri-sodium citrate and sodium polyacrylate) Oxidation of water, a reduction in the pH in vicinity of the implant surface, the protonation of the carboxylic residues of the dispersants, diminishes the repulsion interactions among the NPs Irreversible aggregation of NPs on the surface Resistivity Measurements in Thin Films 25 The sheet resistance = surface resistivity V = the measured voltage (V) I = the source current (A) k = a correction factor based on the ratio of the probe to wafer diameter and on the ratio of wafer thickness to probe separation Four-Point Collinear Probe Synthesis of Thin Films 26 THERMAL OXIDATION Oxides - metal exposed to a glow discharge in O2 Al + O2  (RT) Al2O3, thickness 3-4 nm Similar method applicable to other metals, Ti, V, W, Zr etc. Nitrides, exceptionally hard, high temperature protective coating Ti + NH3  TiN Al + NH3  AlN 27 Synthesis of Thin Films CHEMICAL VAPOR DEPOSITION Pyrolysis, photolysis, chemical reaction, discharges, RF, microwave Epitaxial films, correct matching to substrate lattice EXAMPLES OF CVD CH4 + H2 (RF, MW)  C, diamond Et4Si (thermal, air)  SiO2 SiCl4 or SiH4 (thermal, H2)  a-HSi SiH4 + PH3 (RF)  n-Si Si2H6 + B2H6 (RF)  p-Si SiH3SiH2SiH2PH2 (RF)  n-Si 28 Synthesis of Thin Films METAL ORGANIC CHEMICAL VAPOR DEPOSITION, MOCVD Invented by Mansevit in 1968 Recognized high volatility of metal organic compounds as sources for semiconductor thin film preparations MOCVD PRECURSORS, SINGLE SOURCE MATERIALS Me3Ga, Me3Al, Et3In NH3, PH3, AsH3 H2S, H2Se Me2Te, Me2Hg, Me2Zn, Me4Pb, Et2Cd All toxic materials – a problem of safe disposal of toxic waste Example - IR detectors: Me2Cd + Me2Hg + Me2Te (H2, 500 oC)  CdxHg1-xTe 29 Synthesis of Thin Films MOCVD reactors Controlled flow of precursors to single crystal heated substrate Most reactions occur in range 400 – 1300 oC Hot-wall or cold-wall reactors Photolytic processes (photoepitaxy) help to decrease the deposition temperatures REQUIREMENTS OF MOCVD PRECURSORS RT stable, no polymerization, decomposition Easy handling, simple storage Not too reactive Vaporization without decomposition at modest T < 100 oC Low rate of homogeneous pyrolysis (gas phase) wrt heterogeneous decomposition (surface) HOMO : HETERO rates ~ 1 : 1000 30 Synthesis of Thin Films Gaseous precursor flow Adsorption at the surface Heterogeneous reaction on substrate Surface diffusion, nucleation, film growth Desorption of byproducts Synthesis of Thin Films 31 CATHODE SPUTTERING Bell jar equipment 10-1 to 10-2 torr of Ar, Kr, Xe Glow discharge created by high voltage Positively charged rare gas ions Accelerated by the electric field to cathode target High energy ions collide with cathode Sputter material from cathode Deposits on substrate opposite cathode to form thin film Multi-target sputtering creates composite or multilayer films THERMAL VACUUM EVAPORATION High vacuum bell jar - 10-6 torr = the mean free path > 1 m Heating by e-beam, laser, joule heating of the resistive boat Evaporation - gaseous material deposited on a substrate Thin films nucleate and grow Containers must be chemically inert: W, Ta, Nb, Pt, BN, Al2O3, ZrO2, Graphite Substrates - insulators, metals, glass, alkali halides, silicon Sources - metals, alloys, semiconductors, insulators, inorganic salts 32 Epitaxy Epitaxial reactions = surface structure controlled reactions Crystallographic orientation of the film is controlled by the substrate Kinetic control – TD metastable phases YMnO3 • hexagonal in bulk • cubic perovskite film on NdGaO3 substrates Homoepitaxy – same compound/orientation in substrate and film Heteroepitaxy – different compounds in the substrate and film Strain engineering = the tuning of material properties via lattice distortions Physical properties can be changed through lattice distortions (strain) - the bond lengths, the electron density, the orbital overlap Strain affects: the electronic bandgap, thermal conductivity, multiferroicity, catalytic properties, charge transport 33 Synthesis of Thin Films MOLECULAR BEAM EPITAXY 1968 Bell Laboratories Ultrahigh vacuum system >10-12 torr Elemental or compound sources in shutter controlled Knudsen effusion cells Ar+ ion gun for cleaning substrate surface or depth profiling sample using Auger analyzer Reflection high-energy electron diffraction (RHEED) for surface structure analysis Mass spectrometer for control and detection of vapor species Electron-gun for heating the substrate Fabrication of high quality artificial semiconductor quantum superlattices, ferroelectrics, superconductors Synthesis of Thin Films 34 PHOTOEPITAXY Making atomically perfect thin films under milder and more controlled conditions Mullin and Tunnicliffe 1984 • Photo chemical reaction • Pyrolytic reaction due to hot substrate Coherent laser source • Excimer lasers 351 nm (XeF), 308 nm (XeCl), 248 nm (KrF), 193 nm (ArF) and 157 nm (F2) • Ar+ ion laser • CO2 laser 9 -11 m Incoherent light source • High pressure Hg lamp • Xe, W, H2, D2 lamp Advantages of photoepitaxy • Lower temperature operation • Multilayer formation • Lower interlayer diffusion • Easy to fabricate abrupt boundaries • Less defects, strain, irregularities at interfaces Synthesis of Thin Films 35 LASER DIRECT WRITING Substrate GaAs Me3Al or Me2Zn adsorbed layer or in (g) UV laser beam focused on film Photodissociation of organometallic precursors: 2 Me3Al  2 Al + 3 C2H6 Me2Zn  Zn + C2H6 Creates sub-micron lines of Al or Zn ZnO epitaxial films on (0112) sapphire Temperature 350-500 °C, KrF excimer laser Me2Zn and NO2 Polymerization Quantum dots sintering Synthesis of Thin Films 36 LASER ETCHING GaAs substrate Gaseous or adsorbed layer of CH3Br Focused UV laser creates reactive Br atoms CH3Br (g) (h )  CH3 (g) + Br (g) Br (g) + GaAs (s)  GaAs…Brn(ad) GaAs…Brn(ad)  GaBrn (g) + AsBrn (g) Adsorbed reactive surface Br atoms erode surface regions irradiated with laser Vaporization of volatile gallium and arsenic bromides from surface creates sub-micron etched line (100) Si, excimer laser 308 nm or focused e-beam, Cl2 Surface chlorination due to photodissociation of Cl2 in the gas phase Induced etching of silicon to SiCl4 37 Pulsed Laser Ablation 38 Pulsed Laser Ablation (a) Initial absorption of laser radiation (indicated by long arrows), melting and vaporization begin (shaded area indicates melted material, short arrows indicate motion of solid–liquid interface) (b) Melt front propagates into the solid, vaporization continues and laser-plume interactions start to become important (c) Absorption of incident laser radiation by the plume, and plasma formation (d) Melt front recedes leading to eventual re-solidification Plasma Spraying 39 The plasma spray gun Cu anode, W cathode, water cooled Plasma gas (Ar, N2, H2, He) flows around the cathode and through the anode nozzle (50 l min−1) The plasma initiated by a high voltage discharge (70 V, 400 A) Localized ionisation and a conductive path for a DC arc to form between cathode and anode The resistance heating from the arc causes the gas to reach extreme temperatures (10 000 to 30 000 K), dissociate and ionise to form a neutral plasma flame (does not carry electric current) Powder/suspension is fed into the plasma flame (40 g min−1) Particles are melted and accelerated onto a prepared surface Upon impact, droplets cool down and solidify instantly by heat transfer to the underlying substrate and form a coating consisting of lamellae Plasma Spraying 40 Coatings applied with plasma spraying: • Pure metals (Cu, Al, Zn, Ni, Mo, W,…) • Alloys (NiCr, NiAl, NiMoAl, NiCrSiB, Tribaloy, Inconel, Stellite, …) • Carbides (WC/Co, CrC/NiCr, …) • Ceramics (Al2O3, TiO2, Cr2O3, ZrO2/Y2O3, …) • Abradables (Ni/Graphite, AlSi/Polyester, …) Vast variety of material combinations allow plasma spraying to be used in wide spectrum of industrial applications providing: • wear resistance • corrosion resistance • oxidation resistance • thermal and electrical insulation • electrical conductivity 41 Porous Si SEM of a porous silicon Electrochemical etching HF:EtOH = 1 : 2.5 j = 10 mA/cm2 t = 30 min IR spectrum of porous Si 42 Porous Si 500 600 700 800 900 0 1 2 3 4 2.4 2.2 2 1.8 1.6 1.4 PLIntensity[lin.u.] Wavelength[nm] Energy[eV] T = 295 K exc = 457.9 nm Luminiscence of p-Si 43 Chemistry on Si Surface 44 Hydrosilylation 45 Chemistry on Si Surface NBS = N-Bromo Succinimide Halogenation 46 Carbaanion LiR, RMgX 47 2+2 Cycloaddition 48 Secondary Chemistry 49 Secondary Chemistry Self-Assembled Monolayers 50 Self-assembly: spontaneous organization of molecules into stable, structurally well-defined aggregates Self-assembled monolayers (SAM): two-dimensional ordered assemblies of long hydrocarbon chains anchored through chemical bonds to surfaces of solid inorganic substrates Alkanethiolates on gold and alkylsiloxanes on silicon dioxide belong the most notoriously studied SAM systems Self-Assembled Monolayers 51 R S R S R S R S R S R S R S R S R S R S R S R S Metal Surface Metal surfaces Au, Ag, Cu, Pt, Hg, Fe,… react with Thiols M + RSH  M-S-R + 1/2 H2 Disulfides 2 M + RSSR  2 M-S-R Sulfides M + RSR  M-S-R Same products formed in all three reactions: thiolates RSH are more soluble and react 103 faster with Au than RSSR Substrates: gold polycrystalline films on Si(SiO2), glass, mica Thickness 5-300 nm, sputtering, evaporation Anealed to atomically flat surface Self-Assembled Monolayers 52 Tilt Twist Precession Au surface = ccp Self-Assembled Monolayers 53 Thermodynamics Au does not form surface oxide layer Reaction driving force:  Au-S bond energy 160-185 kJ mol-1  van der Waals attraction between alkyl chains 6-8 kJ mol-1 per CH2 In tBuSH and n-C18SH competition reaction, the linear alkyl thiol binds 300 – 700 times better Surface coverage 1014 molecules per cm2 C16 chain length 2.2 nm, 32-40 tilted, all-trans Chemical stability: Cu/C18SH sustains HNO3 Thermal stability: Au/RSH loses sulfur at 170-230 C Self-Assembled Monolayers 54 Binding modes on Au(111)  On-top sites  Hollow sites – threefold, more stable by 25 kJ mol-1  Bridging sites – the most stable!! (QM calculations) Au–S–C = 180, sp Au–S–C = 104, sp3, more stable by 1.7 kJ mol-1 barrier to interconversion 10.5 kJ mol-1 Au(111) Hexagonal array of S, S….S distance 4.97 Å, interchain distance in crystalline paraffins 4.65 Å, tilt angles 25 - 30 to reestablish alkyl chain contacts, hollow site binding, 21.4 Å2 per molecule Ag(111) Hexagonal array of S, S….S distance 4.41 Å, on-top site binding, more tightly packed alkyl chains, no tilt Self-Assembled Monolayers 55 Mechanisms: alkyl chain flipping, RS- lateral diffusion, equilibrium with dissolved RSH, Au atom diffusion, Au in solution Better crystallinity of films in polar solvents: MeOH, EtOH,… Kinetics Au(111) + RSH reactions proceed in two steps: 1. First step, fast (minutes), diffusion controlled Langmuir adsorption, concentration dependent (1 mM  1 min, 1 M  100 min) 2. Second step, slow (hours), disordered film orders to a 2D crystal, surface crystallization, defect healing, trapped solvent expulsion Self-Assembled Monolayers 56 Surface chemical derivatization HS–(CH2)n–X X = CH3, CF3, OH, NH2, SH, COOH, COOR, CN, CH=CH2, CCH, Cl, Br, OCH3, SO3H, SiMe3, ferrocenyl, …. Microfabrication  Self-assembly, at thermodynamic minima, rejects defects, high degree of perfection  Dimension in the range 1 nm to 1000 m, too large for chemical synthesis, too small for microlithography  High efficiency, spontaneous 57 SiO2 Surfaces- Native oxide on Si - Silicagel isolated vicinal geminal Chemical derivatization methods are based on the reactivity of the surface hydroxyl groups with various reagents 58 SiO2 Surfaces [O3Si]OH stands for the siliceous surface 1. Grafting Reactions with trifunctional reagents, such as alkyltrichlorosilanes and trialkoxyalkylsilanes, lead to the three-fold attachment of the Si-R groups 3 [O3Si]OH + Cl3Si-R  {[O3Si]O}3Si-R + 3 HCl 3 [O3Si]OH + (MeO)3Si-R  {[O3Si]O}3Si-R + 3 MeOH 59 SiO2 Surfaces 2. Chlorination/Displacement Method The first step is the replacement of the Si-OH groups by more reactive Si-Cl bonds by chlorination [O3Si]OH + SOCl2  [O3Si]Cl + HCl + SO2 [O3Si]OH + CCl4  [O3Si]Cl + COCl2 + HCl In the subsequent step, the surface is treated with a Grignard or organolithium reagent with the formation of strong Si-C bonds [O3Si]Cl + RMgCl  [O3Si]R + MgCl2 [O3Si]Cl + RLi  [O3Si]R + LiCl 60 SiO2 Surfaces 3. Post Modification Method The organic groups (Si-R) covalently anchored to the siliceous surface by the two previous methods can be subsequently chemically modified APTES (3-aminopropyl(triethoxy)silane) Large number of chemical transformations of the amino moiety to other functional groups are known 4. Hybrid sol-gel method (co-condensation) A thin layer of a hybrid (organically modified) silica gel can be deposited on the silica surface from a solution of TEOS and (MeO)3Si-R by controlled hydrolysis and condensation (MeO)3Si-R + (MeO)4Si + 7 H2O  [O3Si]R + 7 MeOH 61 SiO2 Surfaces 5. Organometallic modification method Organometallic reagents, such as metal alkyls, halides, amides, and alkoxides can be used to deposit a monolayer of metal complexes on the surface (MLn stands for an organometallic group, M for a metal, L for a ligand, R” for a short alkyl chain, X for halogen) [O3Si]OH + R”MLn  R”H + [O3Si]OMLn [O3Si]OH + XMLn  HX + [O3Si]OMLn [O3Si]OH + Me2NMLn  Me2NH + [O3Si]OMLn [O3Si]OH + R”OMLn  R”OH + [O3Si]OMLn These organometallic moieties can serve as attachment points for further modification with long chain alcohols, thiols, carboxylic acids, phosphates, and diketonates [O3Si]OMLn + HOR  [O3Si]OMLn-1OR + HL [O3Si]OMLn + HOOCR  [O3Si]OMLn-1OOCR + HL 62 Manipulations with SAM STM or AFM probe tips – mechanical or electrochemical 63 Manipulations with SAM Dip-pen nanolithography Dip Coating 64 Schott 1939 The substrates are dipped into a precursor solution - a sol or a slurry Withdrawal from the sol - a wet film is formed Solution viscosity, gravity force, surface tension gradient, particle size Drying atmosphere - the produced film is transformed into a xerogel Heat treatment - crystallization Control of the thickness: • Number of depositions • Loading of the slurry • Concentration of the sol • Withdrawal speed Dip Coating 65 The substrate pretreatment process Immersion - at a constant speed, the substrate is dipped into the coating solution Startup - the substrate remains in the solution for a designated time, and then it starts to be pulled out Deposition - while the substrate is being pulled out, the thin film coating starts to be deposited on it, the thickness of the coating is directly dependent on the speed by which the substrate is being pulled out Drainage - excess liquid is drained from the substrate surface Evaporation - solvent starts to evaporate from the surface of the substrate to form a thin film Coating Thickness 66 The drainage regime Low withdrawal speeds The Landau-Levich equation h0 = wet coating thickness, η = viscosity, γLV = liquid-vapor surface tension, ρ = density, g = gravity, U0 = withdrawal velocity, c = the curvature of the dynamic meniscus The viscous flow regime High velocities and viscous solutions The capillarity regime Very low withdrawal speeds The solvent evaporation faster than the movement of the drying line E = evaporation rate, L = the width of the film, hf = the final dry film thickness, ci = concentration of the solute, Mi = the molar weight of the solute, αi = porosity of the final film Evaporation-Induced Self-Assembly (EISA) 67 Spin-Coating 68 Four stages: • Deposition • Spin-up the liquid flows radially outward, driven by centrifugal force • Spin-off the excess of liquid is ejected as drops on the perimeter • Evaporation Surface tension coating (2 - 50 µm) Doctor-blade coating - solution deposition onto the surface of the substrate, moving the coating tool to spread the solution, drying of the deposited coating (below 5 µm)