Fabrication Techniques
Nanomaterial Fabrication Techniques
Top-Down approach Bottom-Up approach
breaking or etching bulk materials assembly of atoms or molecules
to create nanoscale structures to build nanostructures
Physical methods Chemical methods
• Physical Vapour Deposition • Pulse Laser Deposition • Self-assembly process
• Chemical Vapour Deposition • Sol-gel technique
• Molecular Beam Epitaxy
• Atomic Layer Deposition
Fabrication Techniques: Chemical Vapour Deposition
In Physical vapor deposition (PVD), vapors were generated physically, by
evaporation or sputtering process and transported and deposited onto the
material.
In Chemical vapor deposition (CVD), vapors are generated chemically, by
chemical reaction of gases inside a chamber, which are then transported and
deposited onto the material.
Fabrication Techniques: Chemical Vapour Deposition
Chemical vapor deposition
System consists of 3 parts:
➢ Precursor injection module,
consisting of gases
➢ Reaction site, chamber
where the chemical reaction
happens and material is
deposited on the substrate
and
➢ Gas ejection module, which
removes the byproducts
Fabrication Techniques: Chemical Vapour Deposition
Chemical vapor deposition
Precursor injection module :
➢ Three types of gases are used:
Source gas (key reactant gas
that directly contributes to the
formation of the film), carrier gas
(inert gas, used to transport the
source gas to the reaction site,
helps to evenly distribute the
source gas throughout the
chamber), dopant gas
Fabrication Techniques: Chemical Vapour Deposition
Chemical vapor deposition
Precursor injection module :
dopant gas (optional, dopant
gas introduces impurities into the
deposited material, changing its
electrical properties)
➢ Flow of gas is highly regulates
using mass flow controllers (mfc)
for each type of gas.
➢ For example, for making CNT,
methane (CH4) and hydrogen
are source gases, Ar is carrier gas
Fabrication Techniques: Chemical Vapour Deposition
Chemical vapor deposition
Reaction site :
➢ Consists of a quartz tube. In the
beginning of the procedure,
vacuum is generated inside the
tube, to remove contaminants.
Substrate onto which deposition is
required is placed inside the tube.
➢ The tube/substrate is heated
which (a) provides the necessary
energy to activate the chemical
reactions and (b) facilitates the adsorption and reaction of the precursor molecules onto substrate
Fabrication Techniques: Chemical Vapour Deposition
Chemical vapor deposition
Reaction site heating system:
➢ Hot walled CVD reactor: Both
substrate wafer and walls of reactor
are heated by using an external
furnace system, homogeneous
temperature is maintained inside
reaction chamber
➢ Cold-walled CVD reactor: Only the
substrate is heated by using
localized heaters, inhomogeneous
temperature inside the chamber (not preferred, difficult to get uniform layer)
Fabrication Techniques: Chemical Vapour Deposition
Chemical vapor deposition
Reaction site :
1. Transport of reactants by forced convection to
the deposition region
2. Transport of reactants by diffusion from main gas
stream to substrate wafer through the boundary
layer
3. Adsorption of reactants on the wafer surface
4. Surface processes: Chemical decomposition or reaction, surface migration to attachment sites,
site incorporation
5. Desorption of byproducts from the surface
6. Transport of byproducts by diffusion through boundary layer back to main gas stream
7. Transport of byproducts by forced convection away from the deposited region
Fabrication Techniques: Chemical Vapour Deposition
Chemical vapor deposition
Gas ejection module:
➢ By-products of the chemical reaction on
the substrate surface are transported
out of the reaction site via the gas outlet
using the vacuum pump.
➢ Special gas handling module
connected, which helps in neutralizing
the by-products. Corossive by-products
are neutralized using liquid nitrogen
trap. Inflammable by-products are
burned off. Sometimes, the by-products
are just collected and recycled.
Fabrication Techniques: Chemical Vapour Deposition
Chemical vapor deposition
Key parameters:
• Temperature: considered the most critical parameter, as it directly affects the
chemical reaction rate and the film growth process.
• Pressure: pressure within the CVD chamber influences the gas transport and the
collision rate between molecules, impacting film thickness and uniformity.
• Precursor gas composition and flow rates: type and concentration of precursor
gases used, along with their flow rates, determine the chemical composition of
the deposited film.
• Substrate surface properties: surface characteristics of the substrate, such as its
cleanliness and crystal structure, influence the nucleation and growth of the
deposited film.
Fabrication Techniques: Chemical Vapour Deposition
Chemical vapor deposition depending on pressure inside the chamber:
APCVD: Atmospheric LPCVD: Low pressure UHVCVD: Ultrahigh
pressure CVD CVD vacuum CVD
✓ Works at atmospheric pressure, ~ 760 Torr
✓ Used to deposit thick layer (micron sized
thickness)
✓ Since vacuum generating assembly is not
present, APCVD system have low
operating cost
✓ Extremely susceptible to oxidation and
other types of contamination
Fabrication Techniques: Chemical Vapour Deposition
Chemical vapor deposition depending on pressure inside the chamber:
APCVD: Atmospheric LPCVD: Low pressure UHVCVD: Ultrahigh
pressure CVD CVD vacuum CVD
✓ Works at sub-atmospheric pressures, ~10
m Torr – 1 Torr
✓ Reduced pressure tends to reduce
unwanted gas-phase transitions and
improve uniformity of film across the wafer
✓ Low pressure increases precursor diffusion
through gas and hence the mass transfer
rate of reactants becomes higher
✓ Excellent technique for uniformity and
high purity films
Fabrication Techniques: Chemical Vapour Deposition
Chemical vapor deposition depending on pressure inside the chamber:
APCVD: Atmospheric LPCVD: Low pressure UHVCVD: Ultrahigh
pressure CVD CVD vacuum CVD
✓ CVD at very low pressure, typically below
~10−8 Torr
✓ Used to deposit extremely pure, high-quality
thin films with precise control over their
composition and structure
✓ Applications where minimal contamination
and atomic-level precision are crucial
(Semiconductor manufacturing, research on
novel materials)
Fabrication Techniques: Chemical Vapour Deposition
Chemical vapor deposition depending on pressure inside the chamber:
APCVD: Atmospheric LPCVD: Low pressure UHVCVD: Ultrahigh
pressure CVD CVD vacuum CVD
▪ Chemical Vapor Deposition (CVD) typically operates
within a temperature range of 600 to 1100°C.
▪ Temperature range – high for a few materials or
sometimes the substrate cannot withstand such high
temperature
Fabrication Techniques: Chemical Vapour Deposition
Chemical vapor deposition depending on pressure inside the chamber:
APCVD: Atmospheric LPCVD: Low pressure UHVCVD: Ultrahigh
pressure CVD CVD vacuum CVD
Chemical vapor deposition
PECVD: Plasma PICVD: Photo Initiated
Enhanced Chemical chemical vapor
Vapor Deposition deposition
Fabrication Techniques: Chemical Vapour Deposition
Plasma Enhanced Chemical Vapor Deposition
▪ PECVD utilizes plasma to provide energy for
the deposition reactions to occur rather
than having to heat the system to very high
temperatures.
▪ Substrate is placed in the deposition
chamber between two parallel electrodes
— a grounded electrode and an RF-
energized electrode
▪ The substrate is heated in the range of
250°- 350°C.
Fabrication Techniques: Chemical Vapour Deposition
Plasma Enhanced Chemical Vapor Deposition
▪ Precursor/Source gases (example silane
(SiH4) and ammonia (NH3)) are mixed
with inert carrier gases (example argon
(Ar) or nitrogen (N2)) to control
processes.
▪ Gases are introduced into the chamber
via a shower head fixture over the
substrate that helps to spread the gas
more evenly onto the substrate.
Fabrication Techniques: Chemical Vapour Deposition
Plasma Enhanced Chemical Vapor Deposition
▪ The plasma is ignited by electrical
discharge (100 – 300 eV) between the
electrodes.
▪ Plasma provides energy for chemical
reactions (In plasma, collisions between
electrons and gas molecules generate
highly reactive ions and radicals which
then readily participate in chemical
reactions on the substrate surface)
Fabrication Techniques: Chemical Vapour Deposition
Plasma Enhanced Chemical Vapor Deposition
▪ Plasma in dry etching process is used for ion bombardment and chemical
reaction for etching purposes (energy of plasma and gases are different)
▪ Plasma in sputtering physical vapor deposition is used for ion bombardment of
source material to form thin film on substrate by an atom-by-atom deposition
process (mostly inert gases used)
▪ Plasma in plasma enhanced chemical vapor deposition provides energy for
chemical reactions (ions and radicles are generated which enhance chemical
reaction)
Fabrication Techniques: Chemical Vapour Deposition
Plasma Enhanced Chemical Vapor Deposition
▪ The reaction products travel to the
substrate and are absorbed on the
substrate surface to grow films.
▪ The chemical bi-products are then
desorbed and pumped away,
completing the deposition process.
▪ Disadvantages of this approach are that
the substrate can be damaged due to
direct bombardment by ions
Fabrication Techniques: Chemical Vapour Deposition
Plasma Enhanced Chemical Vapor Deposition
▪ Technological advancement: Inductively
coupled plasma source used to
generate the plasma on the side of the
deposition chamber, also called remote
PECVD reactors.
▪ Electrodes outside the chamber induce
plasma inside the chamber, which is
better as compared to CCP plasma.
Fabrication Techniques: Chemical Vapour Deposition
Plasma Enhanced Chemical Vapor Deposition
Key parameters:
▪ RF Power: Controls the plasma density and excitation level, directly affecting the
deposition rate and film characteristics.
▪ Gas Flow Rates: Determines the concentration of reactant gases in the chamber,
impacting the film composition and growth rate.
▪ Chamber Pressure: Influences the mean free path of gas molecules, affecting the
plasma uniformity and deposition rate.
▪ Substrate Temperature: Plays a crucial role in surface reactions and film morphology,
often impacting the film stress .
Fabrication Techniques: Chemical Vapour Deposition
Photo Initiated Chemical Vapor Deposition
▪ Uses ultraviolet (UV) light to trigger
chemical reactions within a vaporized
precursor mixture, causing the molecules to
decompose and deposit a thin film on a
substrate surface
▪ Initiating the deposition process through
light energy instead of heat as in traditional
CVD methods
▪ Allows for precise control over the
deposition location and can be done at lower temperatures
Fabrication Techniques: Chemical Vapour Deposition
Photo Initiated Chemical Vapor Deposition
▪ Precursor selection: suitable precursor gas
mixture is chosen, often containing a
photo-initiator molecule that readily
absorbs UV light.
▪ Vaporization: Precursor mixture is vaporized
and introduced into the reaction chamber
▪ UV irradiation: Vapor is exposed to UV light,
which excites the photo-initiator molecules,
causing them to break down into reactive
radicals.
Fabrication Techniques: Chemical Vapour Deposition
Photo Initiated Chemical Vapor Deposition
▪ Surface adsorption and reaction:
Activated radicals adsorb onto the
substrate surface and undergo chemical
reactions, forming the desired film
▪ Film growth: As the reaction continues, the
film gradually builds up on the substrate
Fabrication Techniques: Chemical Vapour Deposition
Photo Initiated Chemical Vapor Deposition
Key parameters:
▪ Photo initiator selection: Choice of photo initiator is crucial as it determines the efficiency of radical
generation upon UV exposure, impacting the polymerization rate and film quality.
▪ UV light source: Wavelength and intensity of the UV light directly affect the rate of photo-initiator
activation and polymerization
▪ Chamber Pressure: Influences the mean free path of gas molecules, affecting the plasma
uniformity and deposition rate
▪ Substrate Temperature: Plays a crucial role in surface reactions and film morphology, often
impacting the film stress
▪ Gas flow rates: Precise control of the gas flow rates of the monomer and photo-initiator vapors
ensures a consistent deposition process
Fabrication Techniques: Chemical Vapour Deposition
Chemical vapor deposition
Some materials that can be grown using CVD
▪ Polysilicon: Polycrystalline silicon is deposited from trichlorosilane (SiHCl3) or silane (SiH4)
▪ Silicon dioxide: Common source gases include silane and oxygen, dichlorosilane (SiCl2H2) and nitrous
oxide (N2O)
▪ Silicon Nitride: Common source gases include silane and ammonia gas
Fabrication Techniques: Chemical Vapour Deposition
Chemical vapor deposition
Some materials that can be grown using CVD
▪ Metal: achieved from tungsten hexafluoride (WF6)
▪ Graphene, carbon nanotubes can be grown using methane (CH4) and hydrogen gas
