Chemical Engineering Principles of CVD Processes. A Review of Basics: Part I

Transcription

1 Chemical Engineering Principles of CVD Processes A Review of Basics: Part I

2 THIN FILM COATINGS THERMAL EVAPORATION (PVD) SPUTTERING PROCESS CHEMICAL DEPOSITION

3 CHEMICAL DEPOSITION METHODS Electrolytic deposition Electroless deposition Anodic oxidation & CHEMICAL VAPOUR DEPOSITION

4 Topics 1. Applications of the CVD technique 2. What is CVD Film? 3. Working principles of CVD 4. The CVD System 5. CVD Reactors 6. Thermodynamics of CVD 7. Adhesion 8. Deposition Mechanisms 9. Morphology and Microstructures

5 Applications of CVD Coatings Primarily for microelectronics and cutting-tool industries Transition metal nitrides, carbides and to some extent oxides, including alumina For wear/ corrosion/ erosion resistance For high temperature protection Fine-grained Impervious High purity Harder than similar material produced by conventional ceramic fabrication processes Chemically inert, high mp Only a few nm-µm thick Slow deposition rates (a few hundred nm/hour)

6 Applications of CVD Metallic/ Ceramic Compounds Elements e.g., Si, Ge Metals & alloys Intermetallics Carbides, nitrides, borides, oxides High-temperature composite materials Cer-met (C-C, C-SiC, SiC-SiC, etc.) e.g., SiC on carbon fibers by CVI Single crystals For semiconductor & related devices For integrated circuits, sensors, optoelectronics Densification of surfaces e.g., after plasma-spraying Depositable on mandrel or fiber

7 Applications of CVD Powders Novel powders, fibers Catalysts Nanomaterials e.g., single-walled carbon nanotubes; small-diameter, few-walled carbon nanotubes Optical fibers for telecommunication Coatings on Glass e.g., Reflectasol (Si-) coating from St Gobain Glass Production of gaseous products Typically, byproducts

8 APPLICATIONS

9 APPLICATIONS

10 APPLICATIONS

11 APPLICATIONS

12 APPLICATIONS

13 CVD Coating Processes: Advantages Environmentally friendly, waste products easily neutralizable Able to coat complex shapes, internally & externally Able to deposit elements selectively for oxidation, wear, corrosion protection Able to produce graded and/or multi-layered coatings for a variety of metals, alloys, compounds Able to control coating structure & grain size Wide range of throwing power, deposition rates Dense coatings Excellent purity control

14 CVD Coating Processes: Disadvantages Up-front capital costs high Complex handling, safety, automation systems Violates KISS rule of manufacturing: Keep It Simple, Stupid! High temperatures required, which limits choice of substrates Some substrates can be attacked by coating gases Gas-phase depletion problems with some system designs Use multiple gas inlets Possibility of poor adhesion, lack of metallurgical bondings, due to: Chemical attack on substrate surface Presence of oxide films, other contaminants on substrate Formation of brittle, porous zones between substrate & coating Formation of powdery, flaky deposits due to gas-phase nucleation Masking may be difficult

15 CVD Coatings: General Reaction Types Thermal decomposition (e.g.) Ni(CO) 4 (g) Ni (s) + 4 CO (g) Reduction (e.g.) TiCl 4 (g) + 2H 2 (g) Ti (s) + 4HCl (g) Displacement or Exchange reactions: CrCl 2 (g) + Fe (s) Cr-Fe alloy + FeCl 2 (g) Disproportionation (e.g.) Ti (s) + TiCl 4 (g) 2TiCl 2 (g)

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18 CVD Coating: Significant Process Parameters Reaction temperature Carrier flow rate, vapor pressure, temperature Metal halide (SiCl 4, TiCl 4,..) concentration Carbon input (CH 4, C 3 H 8, ) for carbide formation N 2 / NH 3 input for nitride formation System pressure Gas purity Metal halide generator temperature (AlCl 3, HfCl 4,.) HCl or Cl 2 input rate H 2 O/ CO 2 flow-rate for oxide formation System geometry

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22 Monitoring of CVD Systems for Coating Deposition

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28 Metallurgical bonding of coating to substrate At interface, abrupt change in properties such as: Hardness Thermal conductivity Coefficient of thermal expansion (CTE), etc. This causes stress build-up Reduction of stress increases adhesion

29 Strategies for Stress Reduction Match mechanical properties (e.g., CTE) of coating & surface Form intermediate layers to reduce large property gradients Interlayer coating Control structure of coating Reduce coating thickness Increase radius-of-curvature of coated surface

30 Requirements for coating material to improve adhesion Have good mechanical properties over wide temperature range Be resistant to thermal & mechanical cycling Be fully dense Be hard & resistant to wear/ erosion Be chemically inert, resistant to corrosion/ hightemperature oxidation Be resistant to atomic diffusion/ inter-diffusion at high temperature

31 Aerospace Diffusion Coatings Earliest commercial development of CVD coatings ( 50s) Improved material performance for jet engines Diffusion process: intermingling of atoms of same/ different materials by pack cementation Pack components in a powder mixture of pure metal or alloy coating source elements, an activator (e.g., halide salt and an inert filler material like alumina) Place powder mixture in a retort or container with parts to be coated Heat to high-temperature, C Reaction occurs when activator decomposes, and liberated halogen reacts with coating element to form volatile metal halide, which then substrate to form coating Differs from conventional CVD in generation process for gaseous reactants within powder packing Aluminizing, boronizing, chromating, siliciding for o/ c/ e protection

32 Tool Coatings Second commercial application 1968: TiC coatings on WC substrate, 4 µm thick For wear/ chipping resistance of surface To reduce friction between machining chip & tool surface To prevent galling e.g., Al 2 O 3, HfN, TiN, carbides; composite layers of TiN & TiCN on TiC; Al 2 O 3 on TiC; TiC on WC

33 Thermal/ Diffusion Barrier Coatings, Erosion Resistant Coatings TiCN on SS turbine blades TiC, TiN on C/ SiC substrates On fibers from which composite material is constructed

34 Corrosion Resistant Metallic Coatings Corrosion cost in U.S.: > $ 300 billions per year (40% of GNP) Organic inhibitors/ coatings are soluble in concentrated salt solutions, and thermally unstable at high temperatures Ceramic/ inorganic coatings are not environmentally-friendly, costeffective Hence, use of surface-modification of low-cost alloys with corrosionresistant metal diffusion coatings Reactive metal diffuses up to 100 µm Diffusion coatings serve as surface alloys with gradient composition Mimic expensive superalloy composition on surface (bulk can be cheap steel) Deposited by CVD or MOCVD Pack cementation has low melting-point, boundary-layer problems FBR-CVD widely used Aluminide & silicide coatings on C for tubes & heaters in power-stations

35 FBR - CVD Reactive metal fluidized, deposited on substrate Lower temperature, shorter time, uniform Can coat fibers, particles, powders, fabricated parts Inert gas (e.g., Ar) used to fluidize C temperature Bed consists of reactive metal e.g., Cr, Si, Ti Mixture of H 2 / HCl gases introduced with argon stream Reactive metal chlorides formed in-situ, decomposed on substrate surface Process requires 10 minutes to several hours Coated samples kept in fluidized bed at preset temperature for predetermined period to anneal (interdiffused coatings) T & t based on thermochemical calculations for depositing metal

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37 Mullite Coatings Ceramic coatings for diesel/ engine/ turbine components Deposited on SiC, Si 3 N 4 -based substrates (susceptible in high-temperature corrosive environment) Structure: 3Al 2 O 3.2SiO 2 ; mole % Al 2 O 3 CVD gives dense, adherent coatings able to control microstructure & morphological properties Vertical, hot-wall CVD reactor 950 C, 75 torr Equilibrium thermodynamic analysis: AlCl 3 SiCl 4 CO 2 H 2 system CVD phase diagram

38 Polymer Coating with Ti-Based Layers Temperature sensitive, high T not feasible 60 C PET, PES, PVC, PTFE, PE, PP can be coated with very smooth (R a = 3 nm) thin (5 100 nm) layers with good adherence (> 10 N/mm 2 ) Tube & textile geometry can be coated e.g., medical devices to improve biocompatibility Precursor: Ti[N(C 2 H 5 ) 2 ] 4 Carrier gas: H 2, N 2 Polymer pretreatment with plasma improves adhesion

39 Aerosol Assisted CVD (AACVD) Variant based on use of aerosol precursors Chemical precursor prepared by dissolving S/L starting chemicals into a solvent Aerosol generated by atomizing precursors into sub-µm liquid droplets (aerosols) Droplets distributed throughout gas medium using Ultrasonic aerosol generator, Electrostatic aerosol generator, or Electrospraying Generated aerosols delivered into heated zone where solvent is rapidly evaporated Intimately-mixed precursors undergo decomposition and/ or reaction near/ on heated substrate to deposit film e.g., Pyrosol process for In 2 O 3, SnO 2, Li 2 B 4 O 7 ; CdS, Pt, Pd, Ru (CO gas sensor applications); TiO 2

40 AACVD: Advantages Simplified generation & delivery compared to conventional bubbler/ vaporizer method Hence, lower cost Uses single-source precursors Hence, well-controlled stoichiometry Rapid deposition at low temperatures Can be performed in open atmosphere for oxide deposition Hence, no need for sophisticated reactor/ vacuum system

41 AACVD: Aerosol Generation Methods Ultrasonic: Piezoelectric transducer placed below liquid precursor Aerosol properties depend on liquid properties (density, viscosity, surface tension), ultrasonic beam properties (frequency, amplitude) Droplet diameter is a function of wavelength (d = k λ) Narrow droplet size distribution achievable Hence, superior aerosol uniformity, coating quality Electrostatic: Aerosol generated ultrasonically, charged electrostatically Electrospray: Electric potential applied to cylindrical spray nozzle Causes atomization of liquid into fine charged spray droplets, formation of stable spray cone ( Taylor cone ) Droplet dia depends on liquid flow rate, relative permittivity of liquid, and conductivity

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43 AACVD: Four possible deposition mechanisms