Chemical Engineering Principles of CVD Processes

A Review of Basics: Part II

Flame-Assisted CVD (FACVD) • Involves combustion of liquid/ gaseous precursors

injected into flames • Decomposition/ vaporization/ combustion/ chemical

reaction occurs in flame • Flame source also heats substrate • Fuel can be H2 or a hydrocarbon

– Hydrocarbon produces soot • Flame temperature 1727 – 2727 C

– Causes homogeneous reaction, deposition of powders • Widely used commercially for production of powder • For film deposition, flame temp needs to be reduced • Additives can be introduced into flame • e.g., TiO2, SiO2 powders in large quantities

FACVD: Advantages • Allows use of volatile as well as less volatile precursors • Allows formation of product in a single-step w/o post-

processing • Rapid mixing of reactants on molecular scale

– Reduced processing time – Better control of stoichiometry

• High deposition rate • Relatively low cost (open atmosphere process) Main drawback: large temperature fluctuation of flame

source during deposition – not widely used for deposition of uniform thin films

• Combustion CVD (CCVD): FACVD using specially-designed atomizer (Nanomizer™) & flame synthesis; stablized flame temperature using feedback loop

Electrochemical Vapor Deposition (EVD)

• Used to deposit dense ion or electron-conducting oxide films onto porous electrodes – temperatures between 1000 to 1327 C, – Reduced pressures (< 1 kPa)

• Developed for fabrication of gas-tight components in Solid Oxide Fuel Cell (SOFC) – E.g., yttria-stabilized zirconia (YSZ)

• Advantages: – Thin electrolyte film deposition – Precise control over microstructure – Can produce dense films on curved surfaces – Refractory & oxide materials can be deposited at a fraction of

their mwelting temperatures (</= 1200 C) – PVD methods, such as RF-sputtering, are line-of-sight – Successfully used to manufacture SOFCs for multi-kilowatt

generators

EVD Process Principle • Stage I:

– Closure of pores in substrate by direct reaction of metal source reactants, e.g., MeCl2, with oxygen source reactant, e.g., water vapor (or NiO)

• Characterized using Thiele modulus – Reactants delivered to opposite sides of porous substrate,

diffuse into substrate pores – Open porosity acts as reaction site for oxide deposition

• Stage II: – Electrochemical growth mechanism – Oxide film growth occurs by solid-state diffusion of oxygen ions

due to large oxygen activity gradient across deposited film – Reduction of H2O at water-vapor side produces oxygen ions, – which diffuse thro’ oxide film to metal-chloride side, react with

metal chloride to form oxide on growing oxide layer • Parabolic rate constant

EVD Process

Chemical Vapor Infiltration (CVI) • CVI first developed in 1962 • Variant of CVD, used to manufacture matrix material of fiber-reinforced

ceramic matrix composites (CMCs) – E.g., SiCfiber/SiCmatrix, carbon/SiCmatrix, Nicalon/Si3N4matrix

• Provides high strength (400 MPa), fracture toughness (> 10 MPa m1/2), corrosion & erosion resistance

• Used as high-T structural material for reusable space vehicle, re-entry nose cones, heat exchangers, aircraft brake applications

• Used commercially for manufacture of about 50% of carbon-carbon composites

• Gaseous reactants – diffuse & infiltrate through porous structures, – undergo decomposition, chemical reactions, – deposit matrix material on surface of fibers in preform

• Byproducts & unreacted reactants have to diffuse out of fiber preform (unlike in CVD)

• CVI kinetics different from CVD, but same thermodynamics & chemistry • In CVI, deposition occurs in kinetically-limited low-T regime to maximize:

– infiltration, densification of composites. • Normally carried out at atmospheric or low pressure

– Very little thermal, chemical, or mechanical damages to fragile reinforcing fibers (compared to conventional densification & hot-pressing methods)

• Applications limited to high-value products in aerospace – As technology matures, cost will be lowered

CVI Systems: Classification • Isothermal

– Low deposition temp, reactant conc – Needs to be interrupted to remove outer layer of deposit by machining

to open diffusion passages – Commercially viable method to fabricate thin-wall composites – Simpler, more economical – Can handle large # of parts in a large furnace

• With temperature gradients – Deposition rate higher near mandrel (hottest region) – Outer surface of fiber preform (coolest region) receives little deposit – Reduced infiltration, improved processing efficiency – Normally, atm pr, mandrel heated to 1100 C

• With temperature & pressure gradients – Developed by Oak Ridge National Lab, in R & D stage – Steep temperature gradient across fiber preform – Reactant gases delivered under pressure – Deposition zone moves progressively from hottest to cooler regions

Chemical Vapor Deposition A process where gaseous precursors are introduced into a reactor and solid material is obtained by chemical reactions

Chemical Vapor Deposition

Precursors

Reactor

Energy

1. Solid products Thin films, powders 2. Gas phase products

Chemical Vapor Deposition Common Precursors

hydrides: MHx - SiH4, GeH4, AlH3(NMe3)2, NH3, PH3 ..

halides: MXy - TiCl4, TaCl5, MoF6, WF6, ...

metal-organics - metal alkyls : AlMe3, AliBu3, Ti(CH2tBu)4 .... metal alkoxides : Ti(OiPr)4, [Cu(OtBu)]4 .... metal dialkylamides : Ti(NMe2)4, Cr(NEt2)4 .... metal diketonates : Cu(acac)2, Pt(hfac)2 .... metal carbonyls : Fe(CO)5, Ni(CO)4 .... others: complexes with alkene, allyl, cyclopentadienyl, ..ligands many precursors have mixed ligands

Chemical Vapor Deposition Thermal Energy

resistive heating - tube furnace quartz tungsten halogen lamp (very good heat source) - radiant heating radio-frequency - inductive heating laser as thermal energy source

Photo Energy

UV-visible light laser as photo energy source

Glow Discharge-plasma

Chemical Vapor Deposition In CVD processes

- volatile byproducts are always formed

- these products are often neglected because

they are considered useless.

However, the analyses of these products can lead researchers to know more about CVD reaction mechanisms.

Consequently, to design better CVD systems

Frequently

The volatile products generated in CVD processes are

hazardous (poisonous, flammable, corrosive ...)

Careful treatments are needed before they are allowed to enter

the environment.

CLASSIFICATION OF CVD PROCESSES

Based on the activation energy Thermally-activated CVD ---------------------------TACVD Laser assisted CVD --------------------------- LACVD Plasma activated or plasma-enhanced CVD------------- PACVD/PECVD Electron beam induced CVD -------------- EBCVD Ion-beam induced CVD ----------------- IBCVD

CLASSIFICATION OF CVD PROCESSES

Based on the process pressure APCVD ------- Atmospheric pressure CVD LPCVD ------- Low pressure CVD UHVCVD -------- High vacuum range CVD Based on the precursors used MOCVD -------- metal-organic compound is precursor

PROCESS : BENEFIT

PRINCIPLES OF CVD A gas mixture is introduced into a reactor Near or on the usually heated substrate surface a chemical reaction: A solid material occurs

The Principle of CVD

2AX (g) + H2 ---- A(s) +2HX (g)

REACTION ZONES

Reaction Zones in CVD

Reaction Zones in CVD

Heterogeneous reactions in Zone 2 :

usually defines the microstructure - controls the properties of the deposited materials

Depending on the process :

the deposition temperature varies from RT to 2227 C

At high temp. : various solid –state reactions (Phase transformation, precipitation, recrystallzation, grain growth etc) may take place during CVD process Zones :3-5

Reaction Zones in CVD

Zone 4 : interdiffusion of the components in the coating and substrate can occur subsequent formation of intermediate phases Important Zone : adhesion of coating

THE CVD SYSTEM Usually constructed in three modules

(i) The reaction gas-dispensing system, (ii) The reactor (ii) The exhaust system, (including total pressure controller, vacuum pump and scrubber)

THE CVD SYSTEM Factors that influence the design of CVD system

The selection of the reactants Deposition temperature and pressure

determine the materials Process sensitivity: Contamination

-gases and from air leakage influence

the process economy

Gas-dispensing system

Source Material : Gas (at RT) includes pressure regulators, mass flow meters, shut-off & relief valves and particle filters for removal of particles in the gases

Source Material : Liquid/Solid (at RT) - Evaporator / sublimator can be used to heat the source material to a certain temperature A carrier gas is used to transport the evaporated material to the reactor - tubes to the reactor are heated, in order to avoid condensation of the reactants after evaporation or sublimation

Gas-dispensing system

Reactants can be generated in-situ in the gas-dispensing system - difficult to evaporate a solid materials ( at RT) or highly toxic materials

CVD REACTORS

A large variety of CVD reactor exists & reactor types can be classified / distinguished : 1.The cold wall reactor

2.The hot wall reactor

COLD-WALL CVD REACTOR The walls of the reactor are cold - usually no deposition occurs on them - with a low wall temp : the risk of contamination from vapor/wall reactions is reduced - Homogeneous reaction is suppressed (CH4 can not be used to reach acceptable deposition rates) Advantage: Flexibility, high cleanliness, high cooling rates & easy construction of automated substrate handling systems

HOT-WALL CVD REACTOR The reactor is surrounded by furnace

-The substrate and reactor walls are at the same temperature

-The deposition is not only on the substrate but also on the inside of the reactor walls

Contamination: a risk that particles will become detached (spall) from the walls & fall down during the deposition resulting in PIN—HOLES in the coating

HOT-WALL CVD REACTOR Contamination: due to reactions between the hot reactor walls and the process gases

different techniques are used to improve the yield of coating: (i) Introduction of fresh reaction gas at different positions in the reactor

(ii) Arrangement of a longitudinal temperature gradient in the reactor to increase the yield of the process

HOT-WALL CVD REACTOR

(iii) Use of geometrical restrictions in the reactor to increase the linear gas flow velocity &

(iv) The addition of species to the reaction gas to reduce the deposition rate, particularly at the entrance of the reactor Hot-wall reactor – must necessarily be employed in processes where homogeneous reactions are important

Advantage - thousands of substrates can be deposited simultaneously

EXHAUST SYSTEM The exhaust system usually includes

- a vacuum pump - a total pressure control system & - a scrubber

The selection of vacuum pump depends

- The process e.g. pumping capacity required - pressure range to be used & - gases to be pumped

EXHAUST SYSTEM

SCRUBBERS: - an essential part of the system

- toxic , explosive and corrosive gases are formed in CVD process

-To remove these gases before exhaust to the atmosphere

- For some processes a combination of different scrubbers has to be used to reduce the concentration of the hazardous gases to an acceptable level

THERMODYNAMICS OF CVD

Thermodynamic analysis : better understanding of CVD chemical processes

Under given experimental conditions:

- Starting concentrations of reactants

- System temperature and pressure

- it is possible to predict theoretically both the feasibility of

the process and the nature (as well as the amount) of the

solid and gaseous species

THERMODYNAMICS OF CVD

Selection of Precursors : is a key parameter in a CVD Process

- Determines to a large extent the nucleation conditions & adhesion -the deposition temperature, the depositing rate Hence the microstructure and properties of the coating/substrate composite

Selection of Precursors Different halides Metal-organic compounds & in rare cases carbonyls are used as precursors for metals For oxide deposition : the O2 source is normally selected from 02, H20, CO2 or N20 For instance TiO2 can be deposited from TiCl4 and O2 at 1500 K (well crystallized structure is obtained) TiCl4 and H2O at RT ( amorphous structure) CVD processes at high temperature are usually needed to produce the desired microstructure and adhesion

Selection of Precursors

For nitrides : N2 or NH3

Carbides : CH4, C2H2 or C6H6 Precursors selected determine - the threshold temperature for the deposition and the deposition rate In CVD of TiN for instance, The use of NH3 instead of N2 lowers the deposition temperatures from 1300 K to about 950 K The deposition rate is also higher when NH3 is employed

Transport Phenomena

• Convection • Fick diffusion • Thermal (Soret) diffusion • Multicomponent

ADHESION

In CVD processes, a number of factors which can reduce coating-substrate adhesion:

a. High Processing Temperature employed during CVD

Mismatch in the thermal expansion coefficients between the substrate and the coating may cause poor adhesion

- due to the generation of thermal stress

Usually the residual stress pattern can be favorably changed by changing the deposition conditions

b. Formation of brittle intermetallic compounds and pores in the substrate-coating interface reduces adhesion - fracture are easily initiated in brittle materials and at voids By again pre-depositing an intermediate layer ( in this case acting as a diffusion barrier) adhesion can be improved

ADHESION c. Homogeneous nucleation in the vapor results in

formation of solid particles and a powdery deposit By reducing the driving force of the process the

homogeneous nucleation in the vapor can be eliminated

d. The substrate surface is usually contaminated by different impurities and oxides –which can cause poor adhesion.

By utilizing proper substrate cleaning procedures outside the reactor and

in-situ (e.g. hydrogen reduction of surface oxides)

coatings of acceptable adhesion for the application can be produced

ADHESION

e. Reaction between halide-containing vapor and a substrate can form various substrate halides.

The formation of substrate halides – cause poor adhesion

Thermodynamic calculations – used to predict product species

LIMITATIONS

• Some chemical precursors are hazardous or extremely toxic which necessitates a closed system

• Chemical reaction : generate byproducts The byproducts can be toxic and corrosive -disposal procedures – incurs additional costs

• Energy requirement is high , especially when

high deposition temperature are required

• Efficiency of the process is sometimes low, resulting in high costs