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Transcript
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 postprocessing
• 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 speciallydesigned 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
Energy
Precursors
Reactor
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
Ion-beam induced CVD
-------------- EBCVD
----------------- 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
2AX (g) + H2 ---- A(s) +2HX (g)
The Principle of CVD
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