Nanomaterial Synthesis Method
Ri-ichi Murakami
Nanoscience and nanotechnology
Nanomaterial Synthesis Method
There's Plenty of Room at the Bottom
By Richard Feyman in 1959
Nanotechnology application in nowadays
Targeted drug delivery
Super nano-capacitors
CNT Transistor
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Outline
Emergence and Challenges in Nanotechnology
Bottom-Up and Top-Down Approaches
Introduction to synthesis of nanoparticles
Evaporation and Condensation growth
Lithography technology
Method to nano composite structure
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Emergence of Nano
• Moore’s Law
Original contact transistor
1947
~cm
Transistor
in Integrated circuit
Nowadays
~micrometer
CNT Transistor
Future
~nanometer
Moore’s Law plot of transistor size versus year
To meet the Moore’s Law, the size of transistor should be decreased
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Emergence of Nano
• In our life
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
LED for display
PV film
Self-cleaning window
Temperature control fabrics
Health Monitoring clothes
CNT chair
Biocompatible materials
Nano-particle paint
Smart window
Data memory
CNT fuel cells
Nano-engineered cochlear
The nanotechnology is changing our life, but not enough.
Energy crisis, environmental problem, health monitoring, Artifical joints
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Challenges in Nano
• Atomic scale imaging
TEM in biology
LaSrMnO and SrTiO superlattice
Understand and manipulate the target in nano scale
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Challenges in Nano
• Interdisciplinary Investigation
Protein TEM image
Biology
&
Medicine
Nano drug delivery
Nano
Mechanics
&
Electronics
Nano mechanics
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Physics
&
Chemistry
&
Materials
Approaches
Emergence and Challenges in Nanotechnology
Bottom-Up and Top-Down Approaches
Introduction to synthesis of nanoparticles
Evaporation and Condensation growth
Lithography technology
Method to nano composite structure
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Approaches
• Obviously there are two approaches to the
synthesis of nanomaterials and the
fabrication of nanostructures:
• Top-down
Lithography
• Bottom-up
Self-assembly
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Synthesis of Nanoparticles
Emergence and Challenges in Nanotechnology
Bottom-Up and Top-Down Approaches
Introduction to synthesis of nanoparticles
Evaporation and Condensation growth
Lithography technology
Method to nano composite structure
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Synthesis of Nanoparticles
• Homogeneous nucleation
A solution with solute exceeding the solubility or supersaturation possesses a high Gibbs free energy, the
overall energy of the system would be reduced by segregating solute from the solution.
G: Gibbs free energy
△G: Driving force for solidication
G
△G
GVS
GVL
△T
T*
Tm
At any temperature below Tm there is a driving force fro solidification.
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Synthesis of Nanoparticles
• Homogeneous nucleation
For nucleus with a radius r > r*, the Gibbs free
energy will decrease if the nucleus grow. r* is
the critical nucleus size, △G* is the nucleation
barrier.
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Synthesis of Nanoparticles
• Synthesis of metallic nanoparticles
Influences factors
A
Differenct reagents
A:sodium citrate
B: citric acid
B
A weak reduction reagent
induces a slow reaction rate
and favors relatively larger
particles.
Concentration
A: 0.25M AgNO3
B: 0.125M AgNO3
A large precursor
concentration induces a large
critical radius and favors
relarively larger particles.
A
Other factors: the surfactants, polymer stabilizer, temperature, ect
The details about the synthesis of nanoparticles via chemical
method would be introduced by other professors in this lecture.
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B
Evaporation and Condensation
Emergence and Challenges in Nanotechnology
Bottom-Up and Top-Down Approaches
Introduction to synthesis of nanoparticles
Evaporation and Condensation growth
Lithography technology
Method to nano composite structure
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Evaporation and Condensation
• The evaporation and condensation are the fundamental
phenomena in preparing thin films with nano meters
thickness.
Substrate
vapor
If a condensible vapor is produced by physical
means and subsequently deposited on a solid
substrate, it is called physical vapor deposition.
Condensation If a volatile compound of a material react, with or
without other gases, to produce a nonvolatile solid
film, it is called the chemical vapor deposition.
Although both are nonequilibrium processes, the
kinetics and transport phenomena are the
fundamental theory.
Evaporation
energy
Source
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Evaporation and Condensation
• The Kinetic theory Let’ s start with the equilibrium process.
Substrate
Adsorption
Supersaturation condition:
Condensation
ji, incident flux
Tsub, temperature of substrate
The impingement rate:
the number of collisions per unit area per
second that a gas makes with a surface,
such as a chamber wall or a substrate
P, the gas pressure;m, the particle mass; k, Boltzmann’s constant, 1.38×10-23 J/K; T, the temperature
The substrate should be placed at relactively low temperature to meet the supersaturation condition.
The impingement rate indicates the equilibrium process between evaporation and condensation.
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Evaporation and Condensation
• The vapor source
The vapor is usually produced from a effusion cell, rather than a open system, therefore, we can solve the flow
density from the implingement rate.
J  A z
On a certain angle
J: flow density
A: area of the leak
z: implingement rate
n cos  vav
J 
4
The angle distribution is
important for a co-sputtering
condition.
Source
substrate
Co-sputtering
Tsource Peq
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Evaporation and Condensation
• The vapor source
If we use a beer can as source material, what vapor will we obtain?
Consider the the implingement rate
Al 97.7%
Mg 1%
Mn 1.3%
Alloy source
Al: 0.0001%
Mn: 0.01%
Mg:99.99%
Al, Mg, Mn have different atomic mass.
Mn atom It is not practical to use a congruent
evaporation temperature to deposit a
compound (or alloy) film from a compound
beer can (or alloy ) film with a certain stoichiometric.
Al atom
Mg atom
This result is obtained under consideringt the adsorption and desorption effect.
Diffusion cell at 900 K
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Evaporation
• How to get the stoichiometric vapor
Flash Evaporation
Flash evaporation utilizes very
substrate
rapid vaporization, typically by
dropping powders or grains of
the source material onto a hot
surface. The vapor condenses
rapidly onto a relatively cold
substrate, usually with the
same gross composition as
that of the source material.
The substrate was placed at a temperature
that was a supersaturation temperature for
each component.
AC
Heater
Flash Evaporation
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Evaporation
• How to get the stoichiometric vapor
E-Gun
substrate
Molten End
e-
AC
E-Gun
Rod-Fed Source
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In a rod-fed source, typically an electron-beam-heated
evaporator, the source material evaporators from the
molten end of the rod. The rod advances as material is
lost from the molten end. In steady state, the
composition of the vapor stream must equal that of the
rod. This requires that the molten end be enriched in
the less volatile component. The adjustment is
automatic, since diffusion in the liquid state is rapid.
Evaporation
• How to get the stoichiometric vapor
Coevaporation
The covaporation with the three-temperature
method has been an effective technique for the
compositionally accurate deposition of compound
semiconductor films whose components’ vapor
pressure differ greatly. It was the forerunner of
molecular beam epitaxy (MBE).
substrate
T3
T1
A
Effusion
T2 Cells
B
Co-evaporation
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Evaporation
• How to get the stoichiometric vapor
Sputtering
Sputtering of certain materials, whose ejected particles are
molecules, was utilized to obtain a stoichiometric vapor.
•Direct current sputtering
•Direct current reactive sputtering
•Radio-frequency sputtering
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Evaporation
• The evaporation source
The simplest sources to produce vapors of materials may be thermal sources. These are sources
where thermal energy is utilized to produce the vapor of the evaporant material. Even when the
energy that is supplied to the evaporant may come from electrons or photons, the vaporizing
mechanism may still be thermal in nature.
quasiequilibrium
Evaporation
Sources
Effusion cell
Effusion cell
nonequilibrium
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Evaporation source
• Ideal Effusion Cell
aorifice
δA
L
How to design a effusion cell
Lbody
Gas, Peq
Liquid
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1. The liquid and vapor are in equilibrium within
the cell. Pliq=Pvap, Tliq=Tvap, Gliq=Gvap
2. The mean free path inside the cell is much
greater than the orifice diameter.λ>>aorifice
3. The orifice is flat.
4. The orifice diameter is much less than the
distance to the receiving surface.
5. The wall thickness is much less than the
orifice diameter. L<<aorifice
Evaporation source
• Near-ideal Effusion cell
It is impossible to design an ideal effusion cell
Direct
Re-emitted
With a thick orifice lid,
diffuse and specular
reflection off the
sidewalls are possible.
L
Lbody
Lbody
Gas, Peq
Liquid
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It is the restriction due to the long
cell body that cause a
nonequilibrium behavior of vapor.
Evaporation source
• Open-Tube Effusion Cell
The relative beam intensity
of the open-tube effusion cell
calculated for various tube
length-to-tube radius ratios
(L/a)
a
L
A quasiequilibrium source
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An open-tube effusion cell
Figure 2.56
Evaporation source
• E-Gun
A target anode is bombarded
with an electron beam given
off by a charged tungsten
filament under high vacuum.
The electron beam causes
atoms from the target to
transform into the gaseous
phase. These atoms then
precipitate into solid form,
coating everything in the
vacuum chamber (within line
of sight) with a thin layer of
the anode material.
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Evaporation source
• Pulsed Laser Deposition
A high power pulsed laser beam is
focused inside a vacuum chamber
to strike a target of the material
that is to be deposited. This
material is vaporized from the
target (in a plasma plume) which
deposits it as a thin film on a
substrate.
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Evaporation source
• Sputtering
•
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In sputtering, energetic ions
from the plasma of a gaseous
discharge bombard a target that
is the cathode of the discharge.
Target atoms are ejected and
impinge on a substrate, forming
a coating.
Evaporation source
• Plasma-enhanced chemical vapor deposition
Plasma-enhanced chemical vapor depostion is a process used to deposit thin films from a gas state
(vapor) to a solid state on a substrate. Chemical reactions are involved in the process, which occur
after creation of a plasma of the reacting gases. The plasma is generally created by RF (AC)
frequency or DC discharge between twoelectrodes, the space between which is filled with the
reacting gases. A plasma is any gas in which a significant percentage of the atoms or molecules are
ionized. Fractional ionization in plasmas used for deposition and related materials processing varies
from about 10−4 in typical capacitive discharges to as high as 5–10% in high density inductive
plasmas.
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Condensation
•
Condendation is the change of the physical state of matter from gaseous phase into
liquid phase or solid phase, and the reverse is vaporization.
condensation
re-evaporation
film growth
film
adsorption
at special sit
surface
diffusion
nucleation
Inter diffusion
Adsorption of atoms from gaseous phase
Cluster formation
Critical size islands growth
Coalescence of neighboring islands
Percolation of islands network
Continuous film growth
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Condensation
• Adsorption
gas
It is defined as chemisorption
coefficient that he fraction of
adsorbated atoms transferred from
physisorption into chemisorption but
not re-evaporated.
physisorption
transition
Van der Waals force
chemical bond
chemisorption
re-evaporation
An critical condition is that the
adsorption is equall to the
reevaporation.
Only the atoms adsorpted on
the substrate and condensed,
grow bigger the critical radius,
then the film would be deposited.
substrate
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Condensation
• Condensation coefficient
incident flux
The fraction of the incident
flux that actually condenses
jc  ac ji
re-evaporation
condensation
substrate
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ji: the incident flux density
ac: the condensation coefficient
jc: the condensation flux
Condensation
• Deposition Rate
Growth speed
Si
The deposition rate, or the growth speed
jc
vn 
nf
jc, the condensation flux
nf, the particle density,
how many particles per volume
An example
8 atoms per conventional unit cell
The volume per unit cell, (5.430 A)3=160.10 A3
The particle density, 8/(160.10 A3)=0.05 A-3
The growth speed
a
5.430A
cubic lattice parameter, 5.430 A
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jc 0.703 A2 / s
vn 

 14.06  / s
3
nf
0.05 A
Condensation
• Growth mode
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Condensation
• Non-epitaxial growth
For most film-substrate material combinations, film grow in the VolmerWeber (VW) mode which leads to a polycrystalline microstructure.
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Condensation
• Epitaxial growth---molecular beam epitaxy
Molecular beam epitaxy is a technique for epitaxial growth via the interaction
of one or several molecular or atomic beams that occurs on a surface of a
heated crystalline substrate.
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Condensation
• Epitaxial growth-Atomic layer deposition
based on the sequential use of a gas phase chemical process.
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Condensation
• Monolayer monitoring---RHEED
Reflection high energy electron diffraction, an extremely popular technique
for monitoring the growth of thin films.
In RHEED, electrons beam strikes a single
crystal surface at a grazing incidence, forming
a diffraction pattern on a screen. Electrons
with tenth of KeV order energy are focused
and incident onto the surface. Then, electrons
are scattered by the periodic potential of the
crystal surface, which results in a
characteristic diffraction pattern on the screen.
The diffracted intensity is displayed directly on
a screen, so the information is available
instantly, i.e, real-time analysis is possible.
Further, RHEED arrangement in UHV
chamber allows it to be used for in-situ
observation of MBE thin film growing process.
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Methods for deposition
Method
ALD
MBE
CVD
Sputtering
Evapor
PLD
Thickness Uniformity
good
fair
good
good
fair
fair
Film Density
good
good
good
good
fair
good
Step Coverage
good
poor
varies
poor
poor
poor
Interface Quality
good
good
varies
poor
good
varies
Low Temp. Depostion
good
good
varies
good
good
Good
Deposition Rate
fair
fair
good
good
good
Good
Industrial Application
varies
varies
good
good
good
poor
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Lithography
Emergence and Challenges in Nanotechnology
Bottom-Up and Top-Down Approaches
Introduction to synthesis of nanoparticles
Evaporation and Condensation growth
Lithography technology
Method to nano composite structure
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Lithography
•
We have discussed various routes for the synthesis and fabrication of
variety of nanomaterials; however, the synthesis routes applied have been
focused mainly on the chemical methods approaches, or the physical vapor
deposition. Now, we will discuss a different approach: top-down approach,
fabrication of nanoscale structures with various physical techniques--lithography.
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Lithography
Lithographic techniques
(a)Photolithography
(b)Phase shifting opitcal lithography
(c)Electron beam lithography
(e)Focused ion beam lithography
(f) Neutral atomic beam lithography
Nanomanipulation and nanolithography
(a)Scanning tunneling microscopy
(b)Atomic force microscopy
(c)Near-field scnning optical microscopy
(d)Nanomanipulation
(e)Nanolithography
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Photolithography
• Typical photolithographic process consists of producing a mask
carrying the requisite pattern information and subsequently
transferring that pattern, using some optical technique into a
photoactive polymer or photoresist.
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Photolithography
• Wafer preparation---cleaning
Typical contaminants that must be removed prior to photoresist coating:
•dust from scribing or cleaving (minimized by laser scribing)
•atmospheric dust (minimized by good clean room practice)
•abrasive particles (from lapping or CMP)
•lint from wipers (minimized by using lint-free wipers)
•photoresist residue from previous photolithography (minimized byperforming oxygen plasma ashing)
•bacteria (minimized by good DI water system)
•films from other sources:
–solvent residue
–H2O residue
–photoresist or developer residue
–oil
–silicone
Standard degrease:
– 2-5 min. soak in acetone with ultrasonic agitation
– 2-5 min. soak in methanol with ultrasonic agitation
– 2-5 min. soak in DI H2O with ultrasonic agitation
– 30 sec. rinse under free flowing DI H2O
– spin rinse dry for wafers; N2 blow off dry for tools and chucks
• For particularly troublesome grease, oil, or wax stains:
– Start with 2-5 min. soak in 1,1,1-trichloroethane (TCA) or trichloroethylene (TCE) with ultrasonic agitation prior to acetone
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Photolithography
• Wafer preparation---primers
 Adhesion promoters are used to assist resist coating.
 Resist adhesion factors:
•moisture content on surface
•wetting characteristics of resist
•type of primer
•delay in exposure and prebake
•resist chemistry
•surface smoothness
•stress from coating process
•surface contamination
Ideally want no H2O on wafer surface
– Wafers are given a “singe” step prior to priming and coating
•15 minutes in 80-90°C convection oven
Used for silicon:
– primers form bonds with surface and produce a polar (electrostatic) surface
– most are based upon siloxane linkages (Si-O-Si)
•1,1,1,3,3,3-hexamethyldisilazane (HMDS), (CH3)3SiNHSi(CH3)3
•trichlorophenylsilane (TCPS), C6H5SiCl3
•bistrimethylsilylacetamide (BSA), (CH3)3SiNCH3COSi(CH3)3
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Photolithography
• Photoresist Spin Coating
• Wafer is held on a spinner chuck by vacuum and resist is coated to uniform thickness by spin coating.
• Typically 3000-6000 rpm for 15-30 seconds.
• Resist thickness is set by:
– primarily resist viscosity
– secondarily spinner rotational speed
• Resist thickness is given by t = kp2/w1/2, where
– k = spinner constant, typically 80-100
– p = resist solids content in percent
– w = spinner rotational speed in rpm/1000
• Most resist thicknesses are 1-2 mm for commercial Si processes
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Photolithography
• Prebake
Used to evaporate the coating solvent and to densify the resist after spin coating.
• Typical thermal cycles:
– 90-100°C for 20 min. in a convection oven
– 75-85°C for 45 sec. on a hot plate
• Commercially, microwave heating or IR lamps are also used in production lines.
• Hot plating the resist is usually faster, more controllable, and does not trap solvent like convection oven baking.
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Photolithography
• Align/Expose/Develop
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Photolithography
• Etching/remove photoresist
photoresist has same polarity as final film;
photoresist never touches the substrate wafer.
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Photolithography
• Etching/remove photoresist
photoresist has opposite polarity as final film; excess
deposited film never touches the substrate wafer.
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Phase-shifting Photolithography
• Photolithography has a resolution limit. In order to
improve the resolution in photolithography, the phaseshifting method was developed.
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E-beam lithography
• The theoretical resolution of photolithography is
2bmin
d
 3  (s  )
2

The wavelength of the exposing radiation
s
The gap width maintained between the masi and the photoresist surface
d
The photoresist thickness
The wavelenght of electron beam is much smaller than UV light,
electron beams can be focused to a few nanometers in diameter and
can be deflected accurately and precisely over a surface.
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E-beam lithography
• Resist film
 Negative resist: After development, the
exposed structure is higher than the surrounding due
to crosslinking of polymer chains.
 Positive resist: After development, the exposed
structure is deeper than the surrounding due to
chopping of polymer chains.
 PMMA (Poly-methyle-metacrylate)
-one of the first e-beam resists (1968)
-standard positive resist
-resolution<10 nm
-medium sensitivity (150-300μC/cm2 )
-available with high (950K) and low (50k) molecular weight
-contrast: high for 950k-resist, low for 50k-resist
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E-beam lithography
• Challenge
 Charging effect: Complicate exact focusing
ofelectron-beam, displacement and distortion of
exposed structures.
 Proximity effect: Scattering of electrons in
resist film and substrate, unwanted additional
exposure.
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Focused ion beam lithography
• Advantages
-Ions have heavy mass than electrons.
-Less proximity effect than E-beam
-Less scattering effect
-High resolution patterning than UV, E-beam lithography
-Even smaller wavelength than E-beam
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Neutral atomic beam lithography
•
In neutral atomic beams, no space charge effects make the beam divergent;
therefore, high kinetic particle energies are not required. Diffraction is no severe limit
for the resolution because the de Broglie wavelength of thermal atoms is less than 1
angstrom.
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Nanomanipulation and nanolithography
(a)Scanning tunneling microscopy
(b)Atomic force microscopy
(c)Near-field scnning optical microscopy
(d)Nanomanipulation
(e)Nanolithography
Nanomanipulation and
nanolithography are
based on scanning probe
microscopy.
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Scanning tunneling microscopy
•
STM relies on electron tunneling, which is a phenomenon based on
quantum mechanics.
Principle
A famous sample
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Atomic force microscopy
•
In spite of atomic resolution and other advantages, STM is limited to an
electrically conductive surface since it is dependent on monitoring the
tunneling current between the sample surface and the tip. AFM was
developed as a modification of STM for dielectric materials.
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Atomic force microscopy
• Local oxidation nanolithography
Schematic diagram for the AFM based local oxidation lithography on both
silicon and Ag monolayer.
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Atomic force microscopy
• Effects of tip bias potentials on the lithography patterns.
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Atomic force microscopy
• AFM and KPFM(Kelvin probe force microscopy) images of the
patterned silver nanoparticle monolayer. Shaped patterns were
written on to the monolayer.
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Nanomanipulation and nanolithography
• Some examples
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Quiz
• How to get the stoichiometric vapor ?
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Quiz
•
How to get the stoichiometric vapor ?
1.
2.
3.
4.
Flash Evaporation
E-Gun
Covaporation
Sputtering
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Quiz
• Can we get the vapor with the same
stoichimometric as the source materials?
Why?
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Quiz
• Can we get the vapor with the same
stoichimometric as the source materials?
Why?
No
Because of the different impingement rate
for each element at the same vacuum
condition
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Quiz
• Describe a typical photolithographic process
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Quiz
• Describe a typical photolithographic
process
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Lecture by Ri-ichi Murakami
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