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 • Sol-gel technique
• Chemical Vapour Deposition • Spray pyrolysis
• Atomic Layer Deposition
• Molecular Beam Epitaxy
Fabrication Techniques: Atomic Layer Deposition
Atomic layer deposition
✓ Special type of Chemical Vapor Deposition system only
✓ Thin film deposition technique where a material is deposited on a substrate one
atomic layer at a time by sequentially exposing the surface to different
chemical precursors
✓ The precursors react with the surface in a self-limiting manner, allowing for
precise thickness control at the atomic level and uniform coating on complex
structures
✓ Basically, it builds a film layer-by-layer through controlled chemical reactions
between gaseous precursors and the substrate surface.
Fabrication Techniques: Atomic Layer Deposition
Atomic layer deposition
✓ Highly conformal
coating, on
complex structures
Fabrication Techniques: Atomic Layer Deposition
Atomic layer deposition
Thermal atomic layer Plasma-Enhanced atomic
deposition layer deposition
Substrate is heated and Substrate is introduced to
the molecules attach to plasma of the gases for the
the hot substrate ingredients to stick on the
surface, heating is not
required
Fabrication Techniques: Atomic Layer Deposition
Atomic layer deposition
✓ System similar to
CVD reactor
✓ However, process of
deposition is
different
Fabrication Techniques: Atomic Layer Deposition
Atomic layer deposition
▪ Example: Growth process of TiO2,
Reaction: TiCl4 + 2H2O = TiO2 + 4HCl
▪ Precursors: TiCl4 gas and H2O
▪ Carrier gas: Nitrogen N2
▪ At the reaction site, substrate is placed
and heated (hot-wall process of cold-
H H H H H H
O O O O O O
wall process)
Substrate
▪ Substrate has hydroxide (OH) dangling
Susceptor
bonds on the surface
Fabrication Techniques: Atomic Layer Deposition
Atomic layer deposition
▪ Step1: TiCl4 is introduced in the
chamber
H H H H H H
O O O O O O
Substrate
Susceptor
Fabrication Techniques: Atomic Layer Deposition
Atomic layer deposition
▪ Step1: TiCl4 is introduced in the
chamber
▪ Molecules diffuse towards the hot
substrate and react with hydroxide
dangling bonds on the substrate and
by-product HCl is left out
O O O O O O
▪ Since all OH bonds are filled on the
Substrate
substrate, no more TiCl4 can deposit
Susceptor
on the substrate
Fabrication Techniques: Atomic Layer Deposition
Atomic layer deposition
▪ Step 2: Nitrogen carrier gas is now let
inside the system. This does not react
with the layer or the substrate but
removes the remaining TiCl4 molecules
and the by-products HCl molecules
from the chamber. This process is
O O O O O O
called purging process.
Substrate
▪ In this way, first single-atomic layer of
Susceptor
the material is grown.
Fabrication Techniques: Atomic Layer Deposition
Atomic layer deposition
▪ Step 3: H2O molecules are introduced
to the chamber which diffuses to the
heated substrate and react with the
deposited layer on the substrate.
O O O O O O
Substrate
Susceptor
Fabrication Techniques: Atomic Layer Deposition
Atomic layer deposition
▪ Step 3: H2O molecules are introduced
to the chamber which diffuses to the
heated substrate and react with the
deposited layer on the substrate.
H H H H H H
▪ The Chlorine atoms are replaced with O O O O O O
oxygen atoms and the required layer O Ti
O Ti
O Ti
O Ti
O Ti
O Ti
O
O O O O O O
of TiO2 is formed, leaving the
Substrate
byproducts of HCl in the system.
Susceptor
Fabrication Techniques: Atomic Layer Deposition
Atomic layer deposition
▪ Step 4: Nitrogen carrier gas is now let
inside the system. This does not react
with the layer or the substrate but
removes the remaining H2O molecules
H H H H H H
and the by-products HCl molecules O O O O O O
from the chamber by the purging O Ti
O Ti
O Ti
O Ti
O Ti
O Ti
O
O O O O O O
process.
Substrate
▪ In this way, the first single-atomic layer
Susceptor
of the TiO2 material is grown.
Fabrication Techniques: Atomic Layer Deposition
Atomic layer deposition
▪ All steps described above can be
repeated again and again to get a H H H H H H
step-by-step deposition of single O O O O O O
H H
O O O
H
O
H H
O
H
Ti
O Ti
O Ti Ti Ti Ti
molecular layers of TiO2 material to O O O O O O
H H H H H
O O O
H
Ti
O Ti
O Ti Ti Ti
O Ti
O
get the film of desired thickness on the O O O O O O
substrate. O O O O O O O
Ti Ti Ti Ti Ti Ti
O O O O O O
▪ It is a highly controlled process of
Substrate
fabrication of nanosheets or thin films
Susceptor
on substrates.
Fabrication Techniques: Atomic Layer Deposition
Plasma-Enhanced atomic layer deposition
▪ If material or substrate cannot
sustain high temperature, plasma
enhanced ALD is used, where the
precursor gases are pushed inside
the chamber in form of plasma,
which reacts with the substrate.
▪ The precursor > purging alternating
steps remains the same
Fabrication Techniques: Atomic Layer Deposition
Atomic layer deposition
Key points about ALD:
▪ Sequential exposure: Two different precursor chemicals are introduced to the substrate
one at a time, with a purging step between each exposure to remove excess gas
▪ Self-limiting reaction: Each precursor reacts only with the available reactive sites on the
surface, effectively "saturating" the surface and preventing further deposition beyond a
single atomic layer
▪ Precise thickness control: By repeating the cycle, the film thickness can be precisely
controlled down to a fraction of a nanometer.
▪ Conformal deposition: Due to the self-limiting nature, ALD can deposit uniform films on
complex surfaces with high aspect ratios.
Fabrication Techniques: Atomic Layer Deposition
Atomic layer deposition
Key parameters:
• Precursor selection: The choice of chemical precursors is crucial as they directly determine
the material being deposited and influence the reaction kinetics
• Pulse duration: The time for which a precursor is exposed to the substrate, impacting the
amount of material deposited per cycle
• Purge time: The time between precursor pulses, necessary to remove excess gas from the
chamber and prevent unwanted reactions
• Pressure: The pressure within the ALD chamber affects the transport of precursors and the
overall reaction kinetics
• Substrate temperature: The temperature of the substrate influences the reaction rate and
the quality of the film formed
Fabrication Techniques: Atomic Layer Deposition
Atomic layer deposition
Advantages of ALD: Disadvantages of ALD:
➢ Precise thickness control ➢ Low deposition rate
➢ Excellent conformality ➢ Complex equipment
➢ Wide material compatibility ➢ Precursor limitations
➢ Low deposition temperature ➢ Scalability concerns
➢ High purity films
