Transcript Chemical Vapour Deposition (CVD)

CVD & PVD
Presented by:
-Priyasi Singh
-Pooja Singh
Chemical Vapour
Deposition
(CVD)
Definition:
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Chemical Vapor Deposition is the formation of a nonvolatile solid film on a substrate by the reaction of vapor
phase chemicals (reactants) that contain the required
constituents.
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The reactant gases are introduced into a reaction
chamber and are decomposed and reacted at a heated
surface to form the thin film.
Principle:
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Fundamental principle is that a chemical reaction takes
place between the source gases.
The product of that is a solid material that condenses on
all surfaces inside the reactor
Precursor gases (often diluted in carrier gases) are
delivered into the reaction chamber at approximately
ambient temperatures.
Sequential Steps:
1.
2.
3.
4.
5.
Transport of reacting to the substrate surface
Absorption of species on the substrate surface
Heterogeneous surface reaction catalyzed by the
substrate surface.
Desorption of gaseous reaction byproducts
Transportation of reaction by-products away from the
substrate
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These steps for the CVD process are sequential; so the
one that occurs at the slowest rate will determine the
deposition rate and is called the rate-limiting step
If the deposition process is dominated by step-2,3, or 4as
numbered above, it is a surface controlled process
If a deposition process is dominated by step-1, it is called a
mass-transport controlled process
Coating Characteristics
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CVD coatings are typically:
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Fine grained
Impervious
High purity
CVD coatings are usually only a few microns thick
and are generally deposited at fairly slow rates,
usually of the order of a few hundred microns per
hour.
CVD Apparatus
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A CVD apparatus will consist of several basic components:
1.
2.
3.
4.
5.
6.
7.
8.
Gas delivery system – For the supply of precursors to the reactor
chamber
Reactor chamber – Chamber within which deposition takes place
Substrate loading mechanism – A system for introducing and
removing substrates, mandrels etc
Energy source – Provide the energy/heat that is required to get the
precursors to react/decompose.
Vacuum system – A system for removal of all other gaseous species
other than those required for the reaction/deposition.
Exhaust system – System for removal of volatile by-products from the
reaction chamber.
Exhaust treatment systems – In some instances, exhaust gases may
not be suitable for release into the atmosphere and may require treatment
or conversion to safe/harmless compounds.
Process control equipment – Gauges, controls etc to monitor process
parameters such as pressure, temperature and time. Alarms and safety
devices would also be included in this category.
Energy Sources
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There are several suitable sources of heat for CVD
processes. These include:
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Resistive Heating e.g. tube furnaces
Radiant Heating e.g. halogen lamps
Radio Frequency Heating e.g. induction heating
Lasers
Other energy sources may include UV-visible light or
lasers as a source of photo energy.
Precursors
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Materials are deposited from the gaseous state during CVD.
Thus precursors for CVD processes must be volatile, but at
the same time stable enough to be able to be delivered to the
reactor.
Typical Precursor Materials: Halides - TiCl4, TaCl5, WF6, etc
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Hydrides - SiH4, GeH4, AlH3(NMe3)2, NH3, etc
Metal Organic Compounds –
Metal Alkyls - AlMe3, Ti(CH2tBu)4, etc
Metal Alkoxides - Ti(OiPr)4, etc
Metal Dialylamides - Ti(NMe2)4, etc
Metal Diketonates - Cu(acac)2, etc
Metal Carbonyls - Ni(CO)4, etc
Others – include a range of other metal organic compounds,
complexes and ligands
CVD Reactors
CVD
Low
pressure
atmospheric
Hot wall
tube
Cold wall
Tube
Continuous
motion
Hot wall
tube
Batch
Hot wall
batch
PECVD
Vertical flow
Cold wall
single wafer
Hot wall
Cold wall
Types of CVD process
1.
2.
3.
APCVD (atmospheric pressure CVD)
LPCVD (low Pressure CVD)
PECVD (plasma enhanced CVD)
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4.
5.
6.
7.
8.
Hot wall
Parallel type
Single wafer
MOCVD (metal organic CVD)
LCVD (laser CVD)
PCVD (photochemical CVD)
CVI (chemical vapor infiltration)
CBE (chemical beam epitaxy)
1. APCVD
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APCVD reactors operate in mass transport limited region
So they are designed such that equal flow of reactants is
delivered
This ensures uniform film deposition
This is done by placing the wafer horizontally and then
moving them under gas stream
They are used for depositing low temperature oxide films
Samples are carried through the reactor on a conveyer
belt
Reactant gases flowing through the centre of the reactor
are containing by gas curtains formed by fast flow of
nitrogen
Advantages:
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Disadvantages:
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Simple
High deposition rate
Low temp
Poor step coverage
Particle contamination
Require excess wafer handling
Application:
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Doped & undoped low temp oxides
2. LPCVD
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The reactor consists of a quartz tube heated by a three
zone furnace
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Gas introduced from one end & pumped out from the
other end
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Wafers stand vertically, perpendicular to the gas flow
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They are placed in a quartz holder
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It operates in a surface reaction rate limited mode
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Therefore supply of equal flux of reactants is not required
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Therefore geometry can be such that it can
accommodate a large no. of wafers approx 200 wafers at
a time
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Advantages:
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Excellent purity
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Comfortable step coverage
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Large wafer capacity
Disadvantage
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High temp
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Low deposition rate
Application
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Doped & undoped high temp oxides
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Silicon nitride
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polysilicon
3. PECVD
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PECVD system use an ‘RF induced’ glow discharge to
transfer energy into reactant gases
This procedure allows the substrate to remain at a low
temp than APCVD & LPCVD
Types:
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Parallel plate type
Hot wall type
Single wafer type
a) Parallel plate type
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Reaction chamber is cylinder & constructed of
Al- coated stainless steel
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There are Al plates on the top & bottom
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Samples lie on the grounded bottom electrode
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RF is applied to the top electrode which
creates a glow discharge between 2 plates
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Gases flow radially through the discharge
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Resistance heater heat the bottom, grounded
electrode to a temp b/w 100-400°C
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Gases are flowing from outer edges to the
center
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Advantages:
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Low temp deposition
Disadvantages
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Wafer must be loaded & unloaded indiviually
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Chance of contamination
Application:
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Silicon dioxide
b) Hot wall type
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The reaction takes place in a quartz tube
heated by a furnace
Samples are held parallel to the gas flow
The electrode assembly contains long
graphite or al slabs to support the wafers
Alternating slabs are connected to power
supply to generate discharge in the space
between the electrode (long slabs serve both
as electrode & holder)
Advantage:
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Uniformity
Large no. of wafer deposition
Disadvantage:
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Contamination while loading and unloading
c)Single wafer type
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The reactor is load locked
It offers cassette to cassette operation
It offers rapid radiant heating of each wafer
Wafer larger than 200mm can be loaded
4. MOCVD
MOCVD stands for MetalOrganic Chemical Vapour Deposition.This
is a technique for depositing thin layers of
atoms onto a semiconductor wafer. Using
MOCVD you can build up many layers,
each of a precisely controlled thickness, to
create a material which has specific optical
and electrical properties
Principle: Atoms that you would like to be in
your crystal are combined with complex organic gas
molecules and passed over a hot semiconductor wafer.
•The heat breaks up the molecules and deposits the desired
atoms on the surface,layer by layer.
•By varying the composition of the gas,you can change the
properties of the crystal at an almost atomic scale.
• It can grow high quality semiconductor layers
5. LCVD
•The
production of physical parts using LCVD involves generating solid deposits on the
surface of a substrate by inducing localized chemical reactions in a suitable vapor
reactant through the use of a laser beam.
•Materials
prepared by CVD, and presumably by LCVD, typically possess high purity,
low porosity, and a high degree of crystallinity. These attributes are the result of
deposition occurring one atom at a time, leading to materials having excellent
mechanical properties and thermal stability.
•The
deposition happens on a pyrolytic chemical reaction which occurs in the focus of
a laser beam.
6. PCVD
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Photo-Chemical CVD Reactor uses ultraviolet light as an
energy source for activating process gases for the
deposition of dielectric films at low temperatures
(<150ºC).
Films of silicon dioxide (SiO2), silicon nitride (Si3N4),
silicon oxy-nitride (SiON) and others can be deposited.
Minimal stress is observed in these films due to the low
deposition temperature.
Since the UV photon energy used does not ionize the
process gases, no radiation damage from charged particles
has been observed
7. CVI
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Chemical Vapor Infiltration method of Ceramic Matrix
Composites fabrication is a process, in which reactant
gases diffuse into an isothermal porous preform made of long
continuous fibers and form a deposition. Deposited material is a
result of chemical reaction occurring on the fibers surface.
The infiltration of the gaseous precursor into the reinforcing
ceramic continuous fiber structure (preform) is driven by either
diffusion process or an imposed external pressure.
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The deposition fills the space between the fibers, forming
composite material in which matrix is the deposited material
and dispersed phase is the fibers of the preform.
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Commonly the vapor reagent is supplied to the preform in a
stream of a carrier gas (H2, Ar, He). Silicon carbide (SiC) matrix is
formed from a mixture of methyltrichlorosilane (MTS) as the
precursor and Hydrogen as the carrier gas. Methyltrichlorosilane
is decomposed according to the reaction:
CH3Cl3Si → SiC + 3HCl
The gaseous hydrogen chloride (HCl) is removed from the
preform by the diffusion or forced out by the carrier stream.
Carbon matrix is formed from a methane precursor (CH4)
8. CBE
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Chemical beam epitaxy (CBE) forms an
important class of deposition techniques
for semiconductor layer systems, especially III-V
semiconductor systems. This form of epitaxial
growth is performed in an ultrahigh vacuum system.
The reactants are in the form of molecular beams of
reactive gases, typically as the hydride or a metal
organic.
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CBE combines the advantages of both metalorganic
chemical vapor deposition (MOCVD) and molecular
beam epitaxy (MBE) systems, which have been
widely used for the preparation of nitride-based
compounds.
Applications
CVD has applications across a wide range of industries such as:
• Coatings – Coatings for a variety of applications such as wear resistance, corrosion
resistance, high temperature protection, erosion protection and combinations thereof.
• Semiconductors and related devices – Integrated circuits, sensors and optoelectronic
devices
• Dense structural parts – CVD can be used to produce components that are difficult or
uneconomical to produce using conventional fabrication techniques. Dense parts produced via
CVD are generally thin walled and maybe deposited onto a mandrel or former.
• Optical Fibres – For telecommunications.
• Composites – Preforms can be infiltrated using CVD techniques to produce ceramic matrix
composites such as carbon-carbon, carbon-silicon carbide and silicon carbide-silicon carbide
composites. This process is sometimes called chemical vapour infiltration or CVI.
• Powder production – Production of novel powders and fibres
• Catalysts
• Nanomachines
PHYSICAL VAPOR
DEPOSITION
PVD is a process by which a thin film of material is
deposited on a substrate acc to following steps:
1) The material to be deposited is converted into
vapour by physical means
2) The vapor is transported across a region of low
pressure from its source to the substrate
3) Vapor undergoes condensation on the substrate to
form the thin film
PVD
Evaporation
thermal
E-beam
Sputtering
DC
RF
Magnetron
EVAPORATION
I)
Thermal
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A metal is evaporated by passing a
high current through a highly
refractory material contaminant
structure
Once the metal is evaporated, its
vapour undergoes collisions with the
surrounding gas molecules inside the
evaporation chamber.
As a result a fraction is scattered
within a given distance during their
transfer through the ambient gas.
Therefore pressure lower than 10¯⁵
is necessary to maintain for a straight
line path for evaporated molecules.
II) E-beam Evaporation
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In this mode of operation high
intensity electron beam gun is
focused on the target material i.e.
placed in a water cooled copper.
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The process begin under a vacuum.
A tungsten filament is heated so
that it will give e¯ which forms a
beam i.e. deflected & focus on the
material to be evaporated by the
magnetic field.
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When E-beam strikes the target
material, the kinetic energy of the
motion is transferred into thermal
energy.
SPUTTERING
Sputtering is a deposition tech consist of basic 4
steps;
1. Ions are generated & directed at the target material
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2.
The ion sputter atoms from the target material
3.
The sputter atom get transported to the substrate
through a region of reduced pressure
4.
This sputter atom condense on the substrate
forming a thin film
I)RF SPUTERRING
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RF sputtering will allow the
sputtering of target that are
electrical insulator.
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The target attracts Ar ions
during one half of the cycle.
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The electrons are more
mobile & build up a negative
charge called as self bias
which helps in attracting Ar
ions which does the
sputtering.
II) DC SPUTTERING
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Sputtering can be achieved by
applying a large DC voltage
(approx 2000v).
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A plasma discharge is established
& the argon ion will be attracted
to an impact sputtering of the
target atoms.
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In DC sputtering the target
must be electrically conducted
otherwise the target surface
will charge with the collection of
ion & repels other Ar ion.
III)MAGNETRON SPUTTERING
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In this technique, a magnetic field,
mainly parallel to the target surface,
is superimposed to the applied
electric field so that the secondary
electrons (emitted by the target
during its bombardment) are
trapped near the target surface.
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Thus, one single electron can induce
several argon ionizations before
being lost by recombination on the
chamber walls.
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This results in a large increase of the
plasma ionization rate at the target
surface and then in a significant
increase of the deposition rate .
Advantages
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Materials can be deposited with improved properties
compared to the substrate material
Almost any type of inorganic material can be used as well
as some kinds of organic materials.
Great variety of coating.
High wear resistance.
Low frictional co-efficient
NO toxic reaction product
Excellent adherence
Uniform coating thickness
Disadvantages
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It is a line of sight technique meaning that it is extremely
difficult to coat undercuts and similar surface features
High capital cost
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Some processes operate at high vacuums and
temperatures requiring skilled operators
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Processes requiring large amounts of heat require
appropriate cooling systems
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The rate of coating deposition is usually quite slow
APPLICATIONS
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PVD coatings are generally used to improve hardness,
wear resistance and oxidation resistance. Thus, such
coatings use in a wide range of applications such as:
Aerospace
Automotive
Surgical/Medical
Dies and moulds for all manner of material processing
Cutting tools
Fire arms
PVD
CVD
1.
Deposition occur by
condensation.
1.
Deposition occur by chemical
reaction.
2.
The material that is introduced
onto the substrate is introduced
in solid form
2.
Introduced in a gaseous form
3.
Atoms are moving and
depositing on the substrate
3.
The gaseous molecules will
react with the substrate.
4.
PVD coating is deposited at a
relatively low temperature
(around 250°C~450°C)
4.
CVD uses high temperatures in
the range of 450° C to 1050° C.
Cont….
PVD
5. PVD is suitable for
coating tools that are used
in applications that
demand a tough cutting
edge.
CVD
5. CVD is mainly used for
depositing compound
protective coatings.
6. Low capital cost
6.
High capital cost
7. Coating thickness up to
20micrometer
7. Coating thickness 3-5
micrometer