Chemical Vapour Deposition is a Chemical Synthesis route of Nanomaterials. Specially thin films like Graphene and Carbon NanoTubes are grown by this method.

1. Malaviya National Institute Of Technology,
Jaipur
Presented by:
Monika Shrivastav
(2019RPY9541)
Submitted to:
Dr. Srinivasa Rao
Assistant Professor,MNITJ

2. I. Chemical Vapor Deposition
• CVD

3. 3

4. It has it's name because during this method the chemical reactions take place
between the substrate molecules and precursor molecules on the surface of the
substrate.
Substrate: One on which surface reaction takes place or on that surface change
occurs.Ex: For CNT Powder activated carbon (PAC)
Catalyst: To enhance the reaction rate. Wev have to impregnatehe catalyst molecules
into the substrate. Ex: Ni, Fe etc.
Precursor gas: The gas contains required element to synthesis of particular
substance.Ex: For CNT Acetylene, Methane, Ethanol, Ethylene.
Career gas or Forced Gas: Put a force on precursor gas to adsorbed on
substrate surface. secondlyprovide essential energy to react the precursor gas with
substrate molecules. it also removes the byproducts (usually the gases) from the
chamber.Ex: Ammonia, Hydrogen and Nitrogen.

5. Chemical vapor deposition (CVD) systems
Atmospheric cold-wall system
used for deposition of epitaxial
silicon.
(SiCl4 + 2H2  Si + 4HCl)
Low pressure hot-wall system
used for deposition of
polycrystalline and amorphous
films, such as poly-silicon and
silicon dioxide.
5

6. Types of CVD
APCVD (Atmospheric Pressure CVD), mass transport limited growth rate, leading to non-
uniform film thickness.
LPCVD (Low Pressure CVD)
• Low deposition rate limited by surface reaction, so uniform film thickness (many wafers
stacked vertically facing each other; in APCVD, wafers have to be laid horizontally side
by side.
• Gas pressures around 1-1000mTorr (lower P => higher diffusivity of gas to substrate)
• Better film uniformity & step coverage and fewer defects
• Process temperature 500°C
PECVD (Plasma Enhanced CVD)
• Plasma helps to break up gas molecules: high reactivity, able to process at lower
temperature and lower pressure (good for electronics on plastics).
• Pressure higher than in sputter deposition: more collision in gas phase, less ion
bombardment on substrate
• Can run in RF plasma mode: avoid charge buildup for insulators
• Film quality is poorer than LPCVD.
• Process temperature around 100 - 400°C.
MOCVD (Metal-organic CVD, also called OMVPE - organo metallic VPE), epitaxial growth
for many optoelectronic devices with III-V compounds for solar cells, lasers, LEDs, photo-
cathodes and quantum wells. 6

7. 7

8. CVD sources and substrates
• Types of sources
o Gasses (easiest)
o Volatile liquids
o Sublimable solids
o Combination
• Source materials should be
o Stable at room temperature
o Sufficiently volatile
o High enough partial pressure to get good growth rates
o Reaction temperature < melting point of substrate
o Produce desired element on substrate with easily
removable by-products
o Low toxicity
• Substrates
o Need to consider adsorption and surface reactions
o Cu substrate , Silicon waffer , Si/SiOx etc.
8

9. SOURCE: http://www.azonano.com/images/Article_Images/ImageForArticle_3423(1).jpg

10. Reaction Process in CVD
• Mass transport of the
reactant
• Gas-phase reactions
• Mass transport to the surface
• Adsorption on the surface
• Surface reactions
• Surface migration
• Incorporation of film
constituents, island
formation
• Desorption of by-products
• Mass transport of by-
products

11. Reaction Process in CVD

12. a) Epitaxial Growth
The term epitaxy describes an ordered crystalline growth on a monocrystalline
substrate. Because the substrate acts as a seed crystal, the deposited film
takes on a lattice structure and orientation identical to those of the substrate.
Homoepitaxy: a crystalline film is grown on a substrate or film of the same
material. This technique can grow more purified films than the substrate,
can fabricate layers with different doping levels and layers of different
isotopes.
Heteroepitaxy: a crystalline film is grown on a substrate or film, but the
materials are different from each other. This technique is used to grow e.g.
GaN on Sapphire or AlGaInP on GaAs

13. Homoepitaxy:
• When the thin crystal layer lattice is
the same as that of the substrate
(e.g. Si film on Si substrate).
Heteroepitaxy:
• When the thin crystal layer lattice is
different from that of the substrate
(e.g. GaAs film on Si).
Terminology (Epitaxial Growth)

14. Epitaxial Growth
Homoepitaxy of Si on a Si substrate
SiCl4(g)+2H2(g = Si(s)+4HCl(g) at approx. 1000-1200 °C

15. • Epitaxial films can be grown from
solid, liquid, or gas phases.
• It is easier to control the growth rate
in gas phase epitaxy by controlling
the flow of gases.
• In CVD, gases containing the
required chemical elements are
made to react in the vicinity of the
substrate inside the reactor.
Chemical Vapor Deposition (CVD)

16. b) Vapor-Liquid-Solid (VLS) growth
• Catalytic nanodots on substrate (e.g. UTAM technique)
• Equilibrium vapor pressure of the catalyst must be small so
that the droplet does not vaporize
• Catalyst must be inert

17. Nanostructures by CVD
Chang et al. Chem. Mater., Vol. 16, No. 24, 2004
1D Zinc oxide (ZnO) nanowires and
nanorods fabricated by CVD.
• diameters from 20 to 300 nm
• Length 20 µm

18. CVD advantages and disadvantages
(as compared to physical vapor deposition)
Advantages:
• High growth rates possible, good reproducibility.
• Can deposit materials which are hard to evaporate.
• Can grow epitaxial films. In this case also termed as “vapor phase epitaxy (VPE)”. For
instance, MOCVD (metal-organic CVD) is also called OMVPE (organo-metallic VPE).
• Generally better film quality, more conformal step coverage (see image below).
Disadvantages:
• High process temperatures.
• Complex processes, toxic and corrosive gasses.
• Film may not be pure (hydrogen incorporation…).
• All substrates can't used to make thin films.
18

19. Doping in CVD films
• Doping is usually done for epitaxial (thus single crystal) film during film growth.
• Dopant will be grown directly onto crystalline site (no need of dopant activation).
• Doping is realized by adding gas containing the dopant. Such as PH3, B2H6, AsH3
(all gas phase at room temperature); or PCl3, BCl3, AsCl3 (all liquid at RT).
• They will go through: dissociation, lattice site incorporation, and burying of
dopants by other atoms in the film.
• The dopant concentration C: (P is partial pressure of he dopant species, and v
growth rate)
• However, there is also unintentional doping process:
o Out-diffusion of dopant from heavily doped substrate into the epi-layer.
o Auto-doping – dopant from substrate diffuses into gas stream first, then back
into epi-layer.

C  Pi for low growth rates
C 
Pi
v
for high growth rates
19

20. 20

21. Types of CVD
For R&D, PECVD is most popular, followed by LPCVD.
(can be higher)
21
and epitaxy Si…
(can have
high
deposition
rate)

22. Further Examples of CVD
• Dielectrics: silicon dioxide, silicon nitride…
• Metal: tungsten, copper, titanium, aluminium …
• Semiconductors: epitaxial silicon, germanium …
• Nitrides: TiN, TaN
• Many other nanostructures ,such as nanobelts, nanotube,
SnO2 nanoboxes….
SnO2 nanoboxe Carbon Nanotube (SEM) Carbon Nanotube (TEM)

23. 23

24. Metal-Organic Chemical Vapor Deposition
(MOCVD)
Example Reaction:
Ga(CH3)3 + AsH3  3CH4 + GaAs
Trimethal
Gallium Gas
Arsene
Gas
Methane
Gas
on the
substrate
• The reaction occurs in a sealed container (reactor)
NOTE!! Arsene gas is highly toxic & highly
flammable! Trimethal gallium gas is highly toxic!!
Methane gas is highly explosive!
• If you are British, MOCVD  OMCVD!

25. MOCVD: Dopants can be introduced in precisely
controlled amounts!
Dopants are introduced in precisely
controlled amounts!

26. PECVD – plasma-enhanced CVD:
– glow-discharge plasmas (usually RF field: 100 kHz – 40 MHz),
or MW – 2.54 GHz plazma at reduced gas pressure between
50 mtorr and 5 torr) are sustained within chambers where
simultaneous vapor-phase chemical reactions and film
depositions occur
– plasma activation of reactions (average electron energies
range from 1 to 10
eV) ! chemical reactions occur at much lower temperatures than
in thermal
CVD
Main applications of PECVD:
- microelectronics (DRAM cells)
- plasma modification of metal surfaces (nitriding,
carburizing):
the atoms of nitrogen and carbon that deposit on metal
surfaces
modify them by diffusing into the underlying matrix
- diamond films
26

27. RF power input
Electrode
Electrode
Wafers
Plasma
Gas outlet, pump
Heater
Gas inlet
( SiH4, O2)
• Use RF-induced plasma to transfer energy into the reactant gases, forming radicals that is very reactive.
(RF: radio-frequency, typically 13.56MHz for PECVD)
• Low temperature process (<300oC), as thermal energy is less critical when RF energy exists.
• Used for depositing film on metals (Al…) and other materials that cannot sustain high temperatures.
(APCVD/LPCVD at such low temperatures gives increased porosity and poor step coverage)
• Surface reaction limited deposition, thus substrate temperature control is important to ensure uniformity.
• At low T, surface diffusion is slow, so one must supply kinetic energy for surface diffusion – plasma (ion
bombardment) provides that energy and enhances step coverage.
• Disadvantages: plasma damage, not pure film (often lots of H incorporated into film).
“Good” quality films (though
generally not as good as LP or
APCVD films deposited at much
higher T): energy supplied by
plasma (i.e. ion bombardment of
film) increases film density,
composition, and step coverage.
Cold-wall
27

28. PECVD process parameter
Substrate temperature (100-300oC, up to 1000oC PECVD available)
• Control by external heater, very little heating from PECVD process
Gas flow (10s to 100s sccm – standard cubic centimeter per minute)
• Higher flow rates can increase deposition rate and uniformity
Pressure (P  50mTorr – 5Torr )
• Changes the energy of ions reaching electrodes
• Can change deposition rate
• Increases pressure may lead to chemical reactions in the gas
• Effects also depend on gas concentration
Power (10s to 100s watts)
• Affects the number of electrons available for activation and the energy of
those electrons
• Increased power may lead to chemical reactions in gas
• Increased power increases deposition rate
Frequency (mostly 13.56MHz, same for plasma etching and sputter deposition)
• Changes plasma characteristics
• Changes ion bombardment characteristics
28

29. • High density plasma CVD gives dense layers
(SiO2) at low T (150 °C) and low P (1- 10
mTorr); T increases to 400°C by bombardment.
• Separate RF (gives substrate biasing for
bombardment) from plasma generation
(electron cyclotron resonance ECR and
inductively coupled plasma ICP).
• Simultaneous deposition and sputtering/
bombardment. Improved planarization and filling
due to preferential sputtering of sloped surface.
Mostly used for SiO2 deposition in backend
processes.
29

30. Microwave plasma chemical vapor deposition system for high quality
poly-, mono- and nanocrystalline diamond films.
MW frequency – 2.54 GHz.
PECVD (MW) Plasma Enhanced CVD
30

31. Diamond films
31
Polycrystalline diamond films
Nanocrystalline diamond films

32. Application of diamond films
32
- Semiconductor Devices, RF MEMS,
- Creation of novel surface materials, i.e.
super‐hydrophobic surfaces,
super‐hydrophilic surfaces (biocompatible surfaces)
- Fabrication of 3‐D diamond probes and structures
for field emission
- High selectivity and high voltage range electrochemical
sensors and electrodes
- Defects in diamond (famous NV centers) for single photon sources,
quantum computers (qubit), quantum cryptography,…
- Sensors (quantum sensing in biology and medicine)
- Tribology.

33. • LPCVD reactors use: P = 0.25 – 2.0Torr, T = 500 – 900°C.
• Transport of reactants from gas phase to surface through boundary layer is still
not rate limiting (despite the high T), so wafers can be stacked vertically for
high throughput (100-200 wafers per run).
• Because LPCVD operates in reaction limited regime, it is VERY sensitive to
temperature and so temperature needs to be controlled closely (within +/- 1oC),
so use hot walled reactor for this precise control.
• Again, a 5-25oC temperature gradient is often created to offset source gas
depletion effects (or one can use distributed feeding).
• Requires no carrier gas, and low gas pressure reduces gas-phase reaction
which causes particle cluster that contaminants the wafer and system.
• Less auto-doping (at lower P), as out-diffused dopant gas pumped away
quickly.
33

34. Possible disadvantages:
• For too low temperature, deposition rates may be too low, film quality
decreases.
• Shadowing (less gas-phase collisions) due to directional diffusion to the
surface, so deterioration of the step coverage and filling.
Low Pressure Chemical Vapor Deposition (LPCVD)
Seems cold wall reactors also exist: cold wall
reduce deposition on walls, which leads to
depletion of deposition species and particle
formation that may flake off walls and fall on
wafers.
Besides poorer temperature control than hot
wall, gas convection is another problem.
Cold-wall
Hot-wall
34

35. Selective deposition:
• Especially important in microelectronics, surface
patterning and 3D-growth.
• Reaction rate of precursor is limited on a non-growth
surface. E.g. deposition of Cu from (hfac)Cu(PMe3) occur
on Cu, Pt… but not on SiO2.
• Growth surface acts as co-reactant, and is selectively
consumed. E.g. Si reacts with WF6 or MoF6, while
reaction at SiO2 or Si3N4 is slower.
• A chemical reaction of a gaseous co-reactant occur on
the growth surface. E.g. H2 dissociation on a metal
surface, but not on SiO2 or metal oxide surfaces.
Miscellaneous: selective deposition and laser CVD
Tungsten spring
grown by laser
CVD.
Laser CVD
(energy provided by laser)
35

36. CVD reactor types: quick summary
36
According to the LPCVD slides, APCVD growth rate should be lower, which is not true. Because: (?? I think)
• In APCVD reactive gas partial pressure could be set much higher than that in LPCVD.
• Its pressure could be much lower (by 10) than 1atm and is still called APCVD.
• Gas transport actually increases with T as T3/2 (APCVD is usually done at higher T than LPCVD).
• When putting wafer side-by-side facing the gas, more exposed to gas, thus faster transport.

37. 37
Atomic scale deposition. ALD is similar in chemistry to CVD, except that the ALD
reaction breaks the CVD reaction into two half- reactions, keeping the precursor
materials separate during the reaction.