Nanotechnology (elective)
VII Sem
UNIT 2: PREPARATION OF NANOMATERIALS
Preparation of nanomaterials: Bottom-up and Top-down approaches with examples.
Physical methods: Ball milling, Inert gas condensation, Arc discharge, Ion sputtering, Laser ablation,
Spray pyrolysis, Flame pyrolysis, Thermal evaporation, Pulsed laser deposition, Molecular beam epitaxy.
Chemical methods: Metal nanocrystals by reduction, Solvothermal synthesis, Photochemical synthesis,
Electrochemical synthesis, Micelles and Microemulsions, Chemical vapour deposition (CVD), Sol -gel
process.
Lithographic techniques: Photolithography, Electron beam and Focused ion beam lithography.
[Brief explanation for each]
8 Hrs.
1. Bottom-up and Top-down approaches
1.1. Bottom-up approach
1.2. Top-down approach
1.3. Advantage and disadvantages of bottom-up and top-down approaches
2. Preparation of nanomaterials
2.1. Physical methods
2.1.1. Ball milling
2.1.2. Inert gas condensation
2.1.3. Arc discharge
2.1.4. Ion sputtering
2.1.5. Laser ablation
2.1.6. Spray pyrolysis
2.1.7. Flame pyrolysis
2.1.8. Thermal evaporation
2.1.9. Pulsed laser deposition
2.1.10 Molecular beam epitaxy
2.2. Chemical methods
2.2.1. Micelles and Microemulsions
2.2.2. Chemical vapour deposition (CVD)
2.2.3. Sol-gel process
2.3 Lithography
2.3.1. Photolithography
2.3.2. Particle beam lithography: Electron beam and Focused ion beam lithography
Model questions
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1. BOTTOM-UP AND TOP-DOWN APPROACHES
Using the nanotechnology, we can arrange atoms and molecules exactly as we want. In nano
technology, manufactured products are made from atoms. The properties of these products depend
on how the atoms are arranged. For example, if we rearrange the atoms in coal, we get diamond. If
we rearrange the atoms in sand (and add a pinch of impurities), we get computer chips.
Manipulation and observation of nanostructures is possible due to the combination of
analytical tools e.g. Atomic Force Microscope (AFM), and Scanning Tunnelling Microscope
(STM), and instruments for refined processes (e.g. Electron Beam Lithography and Molecular
Beam Epitaxy).
Two main approaches are used for the synthesis of nanomaterials and the fabrications of
nanostructures in nanotechnology. They are:
a) Bottom-up approach
b) Top-down approach
The difference between nanotechnology and conventional technology is that, the bottom-up
approach is preferred in nanotechnology, where as conventional technology usually use the
topdown approach. The difference between two approaches can be explained simply by using a
example of powder production, where the chemical synthesis represents the bottom-up approach
while the crushing and milling of chunks represents the equivalent top-down process. An
illustration of the top-down approach vs. the bottom-up approach is shown in Figure 1.
Fig. 1: The top-down approach vs. the bottom-up approach.
1.1. BOTTOM-UP APPROACH
Bottom-up approach is often emphasized in nanotechnology literature, though bottom-up is
nothing new in materials synthesis. Bottom-up approach refers to the construction of nanomaterial
from the bottom, i.e. atom-by-atom, molecule-by-molecule or cluster-by-cluster.
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In bottom-up approach, the atoms or molecules are used as the building blocks to produce
nanoparticles, nanotubes, nanorods, thin films or layered structures. According to their
dimensionality, these features are also referred to as zero-, one- or two-dimensional nanostructures.
Following figure also demonstrates the building of particles, layers, nanotubes or nanorods from
atoms (ions) or molecules.
Fig. 2: Bottom-up approach result in particle, nanotube, nanorods, thin films or layers structures.
Examples for Bottom-up approach:
1. Synthesis of large polymer molecule is a typical example for bottom-up approach, in which
individual building blocks (i.e. monomers) are assembled to a large molecule or polymerized
into bulk material.
2. Fabrication of integrated circuits (assembling different components, e.g. resistors, capacitors,
transistors etc) on a PCB.
3. Crystal growth, where growth species either atoms, or ions or molecules orderly assemble into
desired crystal structure on the growth surface.
4. Colloidal dispersion in the synthesis of nanoparticles.
5. Nanolithography and nano-manipulation.
6. Production of salt and nitrate in chemical industry, the growth of single crystals and deposition
of films in electronic industry.
Although such processes provide tremendous freedom among the resultant products, the
number of possible structures to be obtained is comparatively small. In order to obtain the ordered
structures, bottom-up processes must be supplemented by the self-organization of individual
particles, in which the atoms or molecules arrange themselves into a structure due to their natural
properties. Crystals grown for the semiconductor industry provide an example of self assembly, as
does chemical synthesis of large molecules.
1.2. TOP-DOWN APPROACH
The word "top-down" means starting from large pieces of material and producing the intended
structure by mechanical or chemical methods. This situation is shown schematically in figure. As
long as the structure are within a range of sizes that are accessible by either mechanical tools or
photolithography processes, then top-down processes have an unmatched flexibility in their
application. Top-down is in general an extension of lithography.
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The principle behind the top-down approach is to take a bulk piece of the material and then
modify it into the wanted nanostructure. Cutting, grinding and etching are typical fabrication
techniques, which have been developed to work on the nano scale. The sizes of the nanostructures,
which can be produced with top-down techniques, are between 10 to 100 nm.
Fig. 3: The intended product is obtained by the application of mechanical or chemical processes.
Examples: 1. Creating circuits on the surface of a silicon microchip by etching.
2. Attrition or milling in making nanoparticles,
NOTE: Lithography may be considered as a hybrid approach, since the growth of thin films is bottom-up
whereas etching is top-down.
1.3. ADVANTAGE AND DISADVANTAGES OF BOTTOM-UP & TOP-DOWN APPROACHES
Both approaches play very important roles in modern industry and most likely in
nanotechnology as well. There are advantages and disadvantages in both approaches.
An advantage of the bottom-up approach is the better possibilities to obtain nanostructures
with less defects and more homogeneous chemical compositions. This is due the mechanisms
utilized in the synthesis of nanostructures reducing the Gibbs free energy, so that the produced
nanostructures are in a state closer to a thermodynamic equilibrium.
In bottom-up approach, for most materials, there is no difference in physical properties of
materials regardless of the synthesis routes, provided that chemical composition, crystallinity, and
microstructure of the material in question are identical. Of course, different synthesis and
processing approaches often result in appreciable differences in chemical composition,
crystallinity, and microstructure of the material due to kinetic reasons. Consequently, the material
exhibits different physical properties. Bottom-up approach also promises a better chance to obtain
nanostructures with less defects, more homogeneous chemical composition, and better short and
long range ordering. This is because the bottom-up approach is driven mainly by the reduction of
Gibbs free energy, so that nanostructures and nanomaterials produced are in a state closer to a
thermodynamic equilibrium state. On the contrary, top-down approach most likely introduces
internal stress, in addition to surface defects and contaminations.
Top-down approach has proven unsuccessful for several purposes. One of the problems with
the top-down approach is the imperfection of the surface structure. Such defects in the surface
structure can have a significant impact on physical properties and surface chemistry of the
nanostructure, since the surface area to volume ratio in nanostructures and nanomaterials is very
large. The surface imperfection would result in a reduced conductivity due to inelastic surface
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scattering, which in turn would lead to the generation of excessive heat and thus impose extra
challenges to the device design and fabrication. Even though there are problems connected to using a
top-down approach, this is the method of choice when highly complex structures are made. This is
the case in the integrated circuit industry, where nano-sized structures are cut in plain silica
wafers using laser techniques.
2. PREPARION OF NANOMATERIALS
There are physical as well as chemical methods to prepare nanomaterials. However, the most
powerful methods are the chemical methods.
It can be seen from the following that, for each type, there is a large number of possibilities.
The technique to be used depends on material of interest, type of nanostructures [zero dimensional
(0-D), one dimensional (1-D) or two dimensional (2-D) material], size, quantity etc.
1. Physical methods: Ball milling, Inert gas condensation, Arc discharge, Ion sputtering, Laser
ablation, Spray pyrolysis, Flame pyrolysis, Thermal evaporation, Pulsed laser deposition,
Molecular beam epitaxy.
2. Chemical methods: Metal nanocrystals by reduction, Solvothermal synthesis, Photochemical
synthesis, Electrochemical synthesis, Micelles and Microemulsions, Chemical vapour
deposition (CVD), Sol-gel process.
3. Lithographic techniques: Photolithography, Electron beam and Focused ion beam lithography.
2.1. PHYSICAL METHODS
2.1.1. BALL MILLING
It is one of the simplest ways of making nanoparticles of some metals and alloys in the form of
powder. There are many types of ball mills viz. planetary, vibratory, rod, tumbler etc. A ball mill (a
type of grinder) is a cylindrical device used for grinding (or mixing) materials to as small as few
nanometers.
A ball mill consists of a cylindrical capped container that sits on two drive shafts (pulleys and
belts are used for rotary motion) or directly connected to motor for rotation. Size of container, of
course, depends upon the quantity of interest. The container is partially filled with the material to
be ground (powder or flakes) plus the grinding medium (hard spherical balls). Initial material can
be of arbitrary size and shape. Different materials are used as grinding media, including tungsten
carbide balls, ceramic balls, flint pebbles and stainless steel balls. Larger balls used for milling,
produce smaller grain size and larger defects in the particles. Usually 2:1 mass ratio of balls to
material is advisable. If the container is more than half filled, the efficiency of milling is reduced.
When the container is rotating about a horizontal axis, the material is forced to the walls and is
pressed against the walls. This internal cascading effect reduces the material to a fine powder. By
controlling the speed of rotation of the container as well as duration of milling, it is possible to
ground the material to fine powder (few nm to few tens of nm) whose size can be quite uniform.
This process may add some impurities from balls. The container may be filled with air or inert gas.
However, this can be an additional source of impurity, if proper precaution to use high purity gases
is not taken.
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Fig. 4: Common ball mill
Unit 2: Preparation of nanomaterials
Fig. 5: Planetary ball mill
Some of the materials like Co, Cr, W, Ni- Ti, Al-Fe, Ag-Fe etc are made nanocrystalline using
ball mill. Few milligrams to several kilograms of nanoparticles can be synthesized in a short time
of a few minutes to a few hours.
PLANETARY BALL MILL:
Apart from common ball mills (Fig. 4) there is a second type of ball mill called planetary ball
mill (Fig. 5). Planetary ball mills are mainly used in laboratories for grinding sample material down
to very small sizes. A planetary ball mill consists of at least one grinding jar which is arranged
eccentrically on a so-called sun wheel. The direction of movement of the sun wheel is opposite to
that of the grinding jars (ratio: 1:2 or 1:1 or else). The grinding balls in the grinding jars are
subjected to superimposed rotational movements, the so-called coriolis forces. The difference in
speeds between the balls and grinding jars produces an interaction between frictional and impact
forces, which releases high dynamic energies. The interplay between these forces produces the
high and very effective degree of size reduction of the planetary ball mill. Figure 5 shows
schematic of a ball mill in planetary motion. Sketch shows the material thrown against the wall
during the course of rotation of a single container. Dark regions are illustrating the powder
material, while the rest is empty.
NOTE: Key properties of grinding media are size, density, hardness, and composition.
Size: The larger the media particles, the smaller the particle size of the final product. At the same time, the
grinding media particles should be substantially larger than the largest pieces of material to be ground.
Density: The grinding media should be denser than the material to be ground. It becomes a problem if the
grinding media floats on top of the material to be ground.
Hardness: The grinding media needs to be durable enough to grind the material, but where possible should
not be so tough that it also wears down the tumbler at a fast pace.
Composition: Various grinding applications have special requirements. Some of these requirements are
based on the fact that some of the grinding media will be in the finished product. Others are based in how
the media will react with the material being ground.
2.1.2. INERT GAS CONDENSATION (IGC)
The inert gas condensation (IGC) process is one of the most known and simplest technique for
production of nanoparticles, in particular, metal nanopowders.
In its basic form, the method consists of evaporating a material (e.g.: a metal) in a chamber
using resistive heating. This chamber has been previously evacuated to about 10-7 torr and
backfilled with inert gas (e.g. helium gas) to a low-pressure.
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The metal vapor cools through collisions with the inert gas atoms. Thus, metal vapour losses
their kinetic energy, becomes supersaturated and then nucleates homogeneously. The particle size
is usually in the range 1-100 nm and can be controlled by varying the inert gas pressure.
Ultimately, the particles are collected and may be compacted to produce a dense nanomaterial.
Fig. 6: Schematic lay out of a typical inert gas condensation system.
2.1.3. ARC DISCHARGE
This is one of the simplest and useful method which leads to mass scale production of fullerens
and carbon nanotubes.
The arc discharge set up is as shown in figure. Basically, it requires water cooled vacuum
chamber and electrodes to strike an arc between them. The positive electrode itself acts as the
source of material. If some catalysts are to be used, there can be some additional thermal source of
evaporation. Inert gas or reactive gas introduction is necessary.
Fig. 7: Schematic lay out of a typical arc discharge system.
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Usually the gap between the electrodes is ~ 1 mm and high current ~ 50 to 100 amperes is
passed from a low voltage power supply (12-15 volts). Inert gas pressure is maintained in the
vacuum system. When an arc is set up, anode material evaporates. This is possible as long as the
discharge can be maintained. The adjustment of the electrode gap without breaking the vacuum
becomes essential, as one of the electrode burns and gap increases.
2.1.4. ION SPUTTERING
Sputtering is a process in which atoms are ejected from a solid target material due to
bombardment of the target by energetic particles. Sputtering process is commonly utilized for
thinfilm deposition. It is a very good technique to deposit multilayer films for mirrors or
magnetic films for spintronics.
In sputter deposition, some inert gas ions (e.g. argon) are incident on a target at a high
energy. Target material may be some alloy, ceramic or compound. Depending on energy of the
ions and ratio of ion mass to target atoms mass, the ion-target interaction can be a complex
phenomenon. The ions become neutral at the surface but due to their energy the incident ions may
get implanted, get bounced back, create collision cascades in target atoms, displace some of the
atoms in the target creating vacancies, interstitials and other defects, desorb some adsorbates,
create photons while losing energy to target atoms or even sputter out some target
atoms/molecules, clusters, ions and secondary electrons. Fig. 8 shows a schematic picture of
various possibilities.
Fig. 8: Interaction of an ion with the target
Sputter deposition can be carried out using Direct Current (DC) sputtering, Radio Frequency
(RF) sputtering or magnetron sputtering. In all these methods, one uses discharge or plasma of
some inert gas atoms or reactive gases. The deposition is carried out in a required gas pressurized
high vacuum or ultra high vacuum system equipped with two electrodes (one is a sputter target
and the other is a substrate) and gas introduction facility etc. Although the system during
deposition is at high gas pressure, low base pressure ensures that the adequate purity is obtained if
high purity gases are used.
DC SPUTTERING: Target and substrate serve as electrodes and face each other in a typical
sputtering chamber. Target is held at high negative voltage [i.e. acts as cathode: conductive
material, e.g. metal] and substrate may be at positive voltage or ground [i.e. acts as anode] (Fig. 9).
Substrates may be simultaneously heated or cooled depending upon the requirement. Once the
required base pressure is attained in the vacuum system, an inert gas (e.g. argon gas) is introduced
into the system at a pressure < 0.1 torr as the medium to initiate and maintain a discharge.
When an electric field of several kilovolts per centimeter is applied between the electrodes, a
glow discharge is set up with different regions such as cathode glow, Crooke's dark space, negative
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glow, Faraday dark space, positive column, anode dark space and anode glow. These regions are
the result of plasma, i.e. a mixture of free electrons, ions, neutrals and photons released in various
collisions. The density of various particles and the length over which they are spread and distributed
depends upon the gas pressure.
The free electrons are accelerated by the electric fiel, and gain sufficient energy to ionize argon
atoms. If the gas density or pressure is too low, then the electrons will simply strike the anode without
having gas phase collision with argon atoms. However, if the gas density or pressure is too high, then
the electrons will not gain sufficient energy when they strike gas atoms to cause ionization. The
positive ions, Ar+ ions, produced in the discharge strike the cathode (the source target) resulting in the
ejection of neutral target atoms through momentum transfer. These ejected atoms move towards the
opposite electrode (anode: substrate) and deposit there. Thus, sufficiently large number of Ar + ions are
generated that can be used to sputter the target.
Fig. 9: Schematic layout of a typical DC sputtering unit.
RF SPUTTERING: If the target to be sputtered is insulating, it is difficult to use DC sputtering. This
is because it would mean the use of exceptionally high voltage (> 10 6 V) to sustain a discharge
between the electrodes. Such high voltage will harm the target source and produced film.
In DC sputtering, 100 to 3000 volts is a usual voltage. However if some high frequency lower
voltage is applied, the cathode and anode alternatively keep on changing their polarity and oscillating
electrons cause sufficient ionization.
In principle, 5 to 30 MHz frequency can be used and the electrodes can be insulating. However,
13.56 MHz is a commonly used frequency for deposition, as it is reserved worldwide for this purpose
and others are available for communication. Target itself biases to negative potential becoming cathode
when the arrangement as depicted in Fig. 10 is used. The capacitor in the circuit will have low RF
impedance and will allow the formation of a DC bias on the electrodes
Fig. 10: Schematic layout of a typical AC sputtering unit.
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Unit 2: Preparation of nanomaterials
RF or DC sputtering efficiency can be further increased by using magnetic field. By adding a
magnetic field in the system, the electron will move in spiral path. This will hugely increase the
ionization density of argon gas and opportunity of collision. The density will be 1010 ion/cm3 and
increase to 1013 ion/cm3. Therefore, the deposition rate can be increased.
When a charged particle (Q) moves in a region where both electric and magnetic fields are present,
then the force acting on it is given by Lorentz force equation.
F =QE +Q (u×B)
Where, u is a velocity vector.
If E and B are parallel to each other as illustrated in Fig.
11 and an electron leaves the cathode at angle θ = 0°, then
u × B = 0 and only electric field vector acts on an electron.
However, an electron making an angle θ > 0°, would have
both electric and magnetic fields acting on it. The
magnitude of velocity component in the direction of electric
field would be "Q u cos θ ". The component due to the
magnetic field would be perpendicular to both u and B with
a magnitude "Q B u sin θ ".
Fig. 11: Effect of E and B on electron.
As soon as electron enters into the magnetic field, it starts moving in a circular path (after
experiencing magnetic force). Here, magnetic force (Fm) balances centripetal force [m(usinθ)2/r:
experienced by a charged particle] while circulating in an orbit of radius "r".
⇒ Fm =
m usinθ)
2
Where, "u" is the velocity of a charged particle moving in a circular path of radius "r".
r
2
⇒
2
2
BQusinθ = mu sin θ
⇒
r
r=
musinθ
BQ
[∵ Fm = B Q u sinθ ]
Electron moves in a helical path and is able to ionize more atoms in the gas. In practice, both
parallel and perpendicular magnetic fields to the direction of electric field are used to further increase
the ionization of the gas, increasing the efficiency of sputtering. By introducing gases like O2, N2, NH3,
CH4, H2S etc. while metal targets are sputtered, one can obtain metal oxides like A1 2O3, nitrides like
TiN, carbides like WC etc. This is known as "reactive sputtering".
NOTE: Creating plasma: The terms glow discharge and plasma are often used to mean the same thing.
Plasma is a mixture of free electrons, ions and photons. Plasma in overall is neutral but there can be regions,
which are predominantly f positive or negative charges. One can get plasma in different gases at different
pressures by using DC or AC. Different regions of plasma generated by applying a high DC voltage can be
divided into five regions as:
Fig. 12: (a) Generation of plasma by applying high voltage between two
electrodes in an evacuated glass tube. (b) Different regions of plasma.
(i) Cathode or Crook's dark space
(ii) Negative space glow
(iii) Faraday's dark space
(iv) Positive column
(v) Anode dark space
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The extent of these regions depends upon the pressure of the gas. The glow discharge in a glass tube with
two electrodes at the end filled with air has following characteristics at different pressures.
Pressure (torr)
Appearance of discharge
~ 10
~ 1-0.5
~ 0.5
~ 0.3
~ 0.1
~ 0.05
~ 0.005
General glow discharge
Closely spaced striations in positive column
10 mm spacing of striations
Crook's dark space 5 mm long
Crook's dark space 10 mm long
Crook's dark space 20 mm long
Discharge disappearance
2.1.5. LASER ABLATION
Laser ablation is the process of removing material from a solid surface (or occasionally liquid)
by irradiating it with a laser beam.
The basic schematic layout of laser ablation system is as shown in the figure 13. Essentially it
consists of a high vacuum or ultra high vacuum (UHV) system filled with inert gas or reactive gas,
laser beam, solid target and cooled substrate. Usually laser giving UV wavelength such as eximer
laser (see Table 1) is necessary because lasers with other wavelengths like IR or visible often
reflected by some of the metal surfaces.
Table 1: Wavelengths of some commonly used excimer lasers
Fig. 13: Schematic layout of laser ablation system.
In a typical procedure, atoms from solid surface of the target are evaporated by making
powerful laser beam to fall on the surface of the target. These atoms collide with inert gas atoms
(or reactive gases) forming clusters. These clusters are condense on the cooled substrate. The
method is known as laser ablation.
Gas pressure in system is very critical in determining the particle size and distribution.
Simultaneous evaporation of another material and mixing the two evaporated materials in inert gas
leads to the formation of alloys or compounds. This method can produce some novel phases of the
materials, which are not normally formed by other techniques.
For example: Single wall carbon nanotubes (SWNT) are mostly synthesized by this method.
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2.1.6. SPRAY PYROLYSIS
The word pyrolysis has originated from the Greek words "pyro" (means fire) and "lysis"
(means separating). Pyrolysis is a thermochemical decomposition of organic material at elevated
temperatures without the participation of oxygen. It involves the simultaneous change of chemical
composition and physical phase of the starting material, and is irreversible.
Spray pyrolysis is basically a solution process and has been widely used in the preparation of
metal and metal oxide powder. The process can be simply described as converting microsized
liquid droplets of precursor (starting source material) or precursor mixture into solid particles
through heating. In practice, spray pyrolysis involves several steps:
1. Generating microsized droplets of liquid precursor or precursor solution
2. Evaporation of solvent
3. Condensation of solute
4. Decomposition and reaction of solute
5. Sintering (heating without melting) of solid particles
Schematic of spray pyrolysis system is shown in the figure 14. Spray pyrolysis is a process in
which a thin film is deposited by spraying a solution on a heated surface, where the constituent
reacts to form a chemical compound. The chemical reactants are selected such that products other
than the desired compound are volatile at the temperature of deposition. The process is particularly
useful for the deposition of oxides and has long been a production method for a long period for
applying a transparent electrical conductor of Tin oxide (SnO2) or Stannic oxide to glass.
Fig. 14: Schematic layout of spray pyrolysis system.
2.1.7. FLAME PYROLYSIS
The word pyrolysis has originated from the Greek words "pyro" (means fire) and "lysis"
(means separating). Pyrolysis is a thermochemical decomposition of organic material at elevated
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temperatures without the participation of oxygen. It involves the simultaneous change of chemical
composition and physical phase of the starting material, and is irreversible.
Flame pyrolysis or thermolysis is a chemical decomposition of chemical compound caused by
heat. Flame spray pyrolysis can be used to produce a wide array of high purity nanopowders
ranging from single metal oxides such as alumina to more complex mixed oxides, metals and
catalysts. The reaction is usually endothermic as heat is required to break chemical bonds in the
compound undergoing decomposition. If decomposition is sufficiently exothermic, a positive
feedback loop is created producing thermal runaway, which avoids a possible explosion.
Schematic of flame pyrolysis system is as shown in figure 15. Flame pyrolysis is a gas phase
synthesis method, where flame heat is used to vaporize precursor material and initiate the chemical
reactions to produce nano-particles. Flame spray pyrolysis is a one step process in which a liquid
feed (a metal precursor(s) dissolved in a solvent) is sprayed with an oxidising gas into a flame
zone. The spray is combusted and the precursor(s) are converted into nanosized metal or metal
oxide particles. The technique is flexible and allows the use of a wide range of precursors, solvents
and process conditions, thus providing control over particle size and composition.
Fig. 15: Schematic layout of flame pyrolysis.
2.1.8. THERMAL EVAPORATION [or RESISTIVE EVAPORATION]
Thermal/resistive evaporation is a proven and economical method of depositing thin film
coatings in vacuum. Resistive evaporation is used in various applications ranging from decorative
coating to semiconductor manufacturing processes..
Resistive thermal evaporation is one of the most commonly used metal deposition techniques.
Metals such as iron, nickel, aluminum, copper, tin, silver, gold and platinum are frequently
deposited using thermal evaporation methods. It consists of vaporizing a solid material (pure metal,
eutectic or compound) by heating it to sufficiently high temperatures and re-condensing it onto a
cooled substrate to form a thin film. As the name implies, the heating is carried out by passing a
large current through a filament container (usually in the shape of a basket, boat or crucible) which
has a finite electrical resistance. The choice of this filament material is dictated by the evaporation
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temperature and its inertness to alloying/chemical reaction with the evaporant. Traditionally the
filament material is made by tungsten [W: melting point=3380 °C] or thallium [Ta: M.P=2980°C)
or molybdenum [Mo: M.P =2630 °C] which is with very high melting temperature.
Typical layout of thermal evaporation system is as shown in the figure 16. In the thermal
evaporation process, the material to be deposited (source material: metal) is loaded into a container
called a crucible which is kept inside a vacuum chamber. The crucible is resistively heated by
passing a high current to a temperature equal to the melting point of the source material. A high
current flowing through the crucible heats it up, melts and causes evaporation of source material.
These vapours of atoms travel in straight lines and they strike the surface of the substrate where
they accumulate to form a film. This method is useful for depositing many layers of different
materials without chemical interaction between different layers.
Factors that influences the deposition thickness:
1. Pressure inside the vacuum chamber.
2. Temperature of the substrate.
3. Power flowing through the crucible.
4. Distance between source to substrate.
Vacuum pump
Fig. 16: Schematic layout of thermal evaporation system.
Advantages: 1. Instrumentation is relatively simple and cheap.
2. Less substrate surface damage.
3. Excellent purity of films.
4. Source material (material to be deposited) can be to different shape depending on the need.
Disadvantages: 1. Limited to low melting point metals, e.g.: Al, Au, Ni, Cr…
2. Fit only for metals, the dielectric material is difficult to melt and evaporate to form thin films.
3. The speed of deposition is very slow.
4. The hardness of the film is not good and the density is poor.
5. The materials that deposit on the wall of the chamber can contaminate later depositions.
6. Sometimes there are problems with film-thickness uniformity and uniform doping over largearea substrates.
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2.1.9. PULSED LASER DEPOSITION
Pulsed laser deposition (PLD) is an efficient method to produce thin films by utilizing a
technique called laser ablation. PLD can be used to deposit a wide range of materials from
polymers to metals. PLD is applicable to almost any material, in particular to compounds
(especially multi-element oxides) that are difficult or impossible to produce in thin-films by other
techniques. Examples of such materials include complex ceramic materials such as
hightemperature superconductors and certain magnetic materials [e.g. yttrium iron garnet (YIG)
and ferromagnetic shape-memory (FSM) alloy Ni-Mn-Ga]. PLD was the first technique used
to successfully deposit a superconducting YBa2Cu3O7 thin film.
Typical layout of pulsed laser deposition (PLD) system is as shown in the figure 17. In
general, the idea of PLD is simple. A high power pulsed laser beam is focused onto the surface of a
solid target. The strong absorption of the electromagnetic radiation by the solid surface leads to
rapid evaporation of the target material. Finally, the evaporated material get deposited onto the
substrate. The evaporated materials consist of highly excited and ionized species.
Fig. 17: Schematic layout of pulsed laser deposition system.
Factors that influence deposition thickness:
1. Nature of target material.
2. Pulse energy of laser beam.
3. Distance between target to substrate.
4. Type of gas and pressure in chamber (e.g. oxygen, argon, etc.).
Advantages of Pulsed Laser Deposition:
1. Conceptually PLD is a simple process: a laser beam vaporizes a target surface, producing a
film with the same composition as the target.
2. Cost-effective: one laser can serve many vacuum systems.
3. Fast process: high quality thin films can be grown reliably in 10 or 15 minutes.
4. Films made by PLD can be extremely smooth and amorphous, or crystalline depending on
the chamber atmosphere.
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2.1.10. MOLECULAR BEAM EPITAXY (MBE)
Unit 2: Preparation of nanomaterials
EPITAXY: The term epitaxy is derived from the Greek words "epi" means "above" and "taxis"
means "in ordered manner". So, the word epitaxy means "to arrange upon".
Epitaxy is the method of depositing a thin film of single crystal material on a mono-crystalline
substrate. Epitaxial films can be grown from gaseous or liquid precursors, and the substrate acts as
a seed crystal. Also, orientation of the film deposited is identical to those of the substrate. The
other thin-film deposition methods grow polycrystalline or amorphous films even on single-crystal
substrates. If the film and substrate are same materials, then the process is called homoepitaxy or
simply isoepitaxy. If the film and substrate are different materials, then the process is called
heteroepitaxy.
The epitaxial growth techniques are widely used for the synthesis of nanostructured materials
and semiconductors. This is an affordable method of high quality crystal growth for semiconductor
materials like silicon (Si), germanium (Ge), gallium nitride (GaN), gallium arsenide (GaAs) and
indium phosphide (InP). There are different type of epitaxial growth techniques, such as vapour
phase epitaxy, molecular beam epitaxy, liquid phase epitaxy, solid phase epitaxy and atomic layer
epitaxy.
MOLECULAR BEAM EPITAXY (MBE): Molecular beam epitaxy is an effective epitaxial
growth technique to fabricate metal, insulator, semiconductor or superconductor epitaxial layers.
This technique of deposition can be used to deposit a precise nanometre length elemental or
compound quantum dots, quantum wells, quantum wires etc. in a very controlled manner.
Fig. 18: Schematic layout a typical of molecular beam epitaxy system.
Schematic representation of a typical MBE system is shown in figure 18. MBE essentially
consists of an ultra-high vacuum chamber [UHV: ~ 10 −11 mbar] into which a substrate is loaded
onto a heated sample holder. Precursors (i.e. source materials) of desired elements (e.g.: Ga, As,
Al, P, In, etc.) are then loaded into heated crucibles or furnaces called Knudsen cells or effusion
cells, outfitted with computer controlled shutters on their exits. The precursors are then heated such
that, when the shutters are opened, one obtains a beam of atoms directed towards the substrate.
Under such low pressures, the atomic species have very long mean free paths allowing them to
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reach the substrate without collisions with other gas phase species in the chamber (i.e. geometrical
size of the chamber is less than the mean free path of the particle). The substrate is sufficiently
heated and frequently rotated to improve the growth homogeneity. By controlling the temperature
as well as the sequence/timing of opening and closing the shutters, one can deposit uniform films
of semiconductor materials.
For example: In solid-source MBE, ultra-pure elements such as gallium and arsenic are heated in
separate effusion cells until they begin to slowly evaporate. The evaporated elements are then
condensed on the wafer, where they may react with each other to form thin film of
singlecrystalline gallium arsenide (GaAs).
Characteristics of MBE: Following are some of the important characteristics of MBE,
1. Low growth rate of ~ 1 monolayer (lattice plane) per sec.
2. Precise control of surface composition and morphology.
3. Grow films with good crystal structure.
4. A very smooth surface and interface is achievable through controlling the growth at the
monoatomic layer level.
5. Low growth temperature (~ 550°C for GaAs). A low growth temperature limits diffusion and
maintains hyper-abrupt interfaces, which are very important in fabricating two-dimensional
nanostructures or multilayer structures such as quantum wells.
2.2. CHEMICAL METHODS
2.2.1. MICELLES AND MICROEMULSIONS
Synthesis of nanoparticles in the cavities produced in microemulsion is a widely used method.
Advantage of this method is the biocompatibility and biodegradability of synthesi zed materials.
Biocompatibility is useful in novel applications like drug deliver y of nanomaterials and
biodegradability is sometimes environmentally useful.
Whenever two immiscible liquids are mechanically agitated or stirred together, they are
known to form what is called "emulsion". The tendency of the liquids is such that the liquid in
smaller quantity tries to form small droplets, coagulated droplets or layers so that they are all
separated from the rest of the liquid (for example droplets of fat in milk). The droplet sizes in
emulsions are usually larger than 100 nm upto even few millimetres. Emulsions are usually turbid
in appearance.
On the other hand, there is another class of immiscible liquids, known as microemulsions
which are transparent and the droplets are in the range of ~ 1 to 100 nm. This is the size needed for
the synthesis of nanornaterials.
Microemulsions are stabilized using surfactants (surface stabilized active agents). When an
organic liquid or oil (O), water (W) and surfactant (T) are mixed together, under some critical
concentration, "micelles" or "inverse micelles" are formed, depending upon the concentrations of
water and organic liquid. As shown in Fig. 19, micelles are formed with excess water and inverse
micelles are formed in excess of organic liquid or oil.
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Unit 2: Preparation of nanomaterials
Fig. 19: Formation of micelles and inverse micelles.
As shown in Fig. 19, micelles are formed with the head groups floating in water, whereas tails
and tail group filling the cavity along with organic liquid inside. Reverse is the case for inverse
micelles. They can form various shapes as illustrated in Fig. 20.
Fig. 20: Different types of micelles.
The ratio of water (W), oil (O) and surfactant (T) is important to decide which type of micelle
will be formed and can be represented in a ternary phase diagram using a triangle (Fig. 21).
Fig. 21: Ternary phase diagram of water (W), oil (O) and surfactant (T) mixture.
Composition can be determined by drawing lines parallel to all three sides of the triangle as
shown in Fig. 21. Here point P denotes 50% water, 25% oil and 25% surfactant. A modified phase
diagram known as "Winsor Diagram" also can be constructed for finer details (not shown here).
The critical micelle concentration (CMC) depends upon all W, O and T concentrations as is
evident from above diagram. As shown in Fig. 22, the effect of T is to reduce the surface tension of
water dramatically below CMC and remain constant above it, as the organic solvent concentration
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Unit 2: Preparation of nanomaterials
is kept on increasing. Organic solutes also reduce the surface tension to some small extent. If there
are any electrolytes used, they slightly increase the surface tension.
Fig. 22: Surface tension vs temperature.
There are four types of surfactants in general:
1. Cationic − for example CTAB, C16H33N(CH3)3+Br−
2. Anionic − e.g. sulfonated compounds with general formula R-SO3−Na+ where R is CnH2n+1
3. Nonionic − for example R- (CH2-CH2-O)20-H
4. Amphoteric − some properties are similar to ionic and some to nonionic surfactants as in
betaines.
A large number of narioparticles of (metals, semiconductors and insulators) cobalt, copper,
CaCO3, BaSO4, CdS, ZnS etc. have been synthesized using microemulsions or inverse micelles. As
an example, consider the synthesis of cobalt nanoparticles. A reverse miceller solution of water
and oil can be stabilized us ing a monolayer of surfactant like sodium bis (2-ethylhexyl)
sulfosuccinate or Na(AOT). The droplet diametre is controlled simply by controlling the amount of
water. Two miceller solutions having same diameter of droplets can be formed. Thus one solution
should have Co(AOT) 2, i.e. cobalt bis (2-ethylhexyl) sulfosuccin ate and the other should have
sodium tetrahydroborate (NaBH4 i.e. sodium borohydride). When two solutions are mixed together
the solution appears clear but the colour changes from pink to black. One can find by electron
microscopy or some other analysis that cobalt nanoparticles are formed.
NOTE 1: AMPHIPHILIC MOLECULES IN LIQUIDS
If amphiphilic molecules are spread in an aqueous solution, they try to stay at air-solution interface with
hydrophobic groups in air and hydrophilic groups in the solution (see Fig. 23 (a)). Such molecules are known as
surfactants (surface active agents).
Fig. 23: Amphiphilic molecules in aqueous solutions
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Consider now a situation in which a hydrocarbon molecule solution is put in an aqueous medium. As shown
in Fig. 23 (b), the hydrocarbon solution itself would be separated from aqueous solution and float on it. When
surfactant molecules are mixed in large quantity in aqueous solution, then they would try to form what are
known as 'micelles' and 'inverse-micelles' when aqueous solution is mixed in oil. In micelles, the head groups
float in water and tails are inside, whereas tails point outwards in case of inverse micelles.'
NOTE 2: SURFACE TENSION OF LIQUIDS
Surface tension of a liquid can change if some electrolyte, organic or surfactant solutes are added. General
behaviour is shown in Fig. 24.
a)
b)
Fig. 24: For surfactant molecules γ decreases rapidly upto certain concentration known as critical miceller
concentration (c.m.c.).
2.2.2. CHEMICAL VAPOUR DEPOSITION (CVD)
Chemical vapour deposition (a hybrid method using chemicals in vapour phase) is
conventionally used to obtain coatings of a variety of materials viz. inorganic or organic materials.
It is widely used in industry because of its relatively simple instrumentation, ease of processing,
economical viability and possibility of depositing different types of materials. Under certain
deposition conditions nanocrystalline films or single crystalline films can also be grown.
Basically CVD process can be considered as a transport of reactant vapour or reactant gas
towards the substrate (Fig. 19) kept at some high temperature where the reactant cracks into
different products which diffuse on the surface (undergo some chemical reaction at appropriate
site), nucleate and grow to form the desired material film. The by-products created on the substrate
have to be transported back to the gaseous phase removing them from the substrate.
Fig. 19: Basic concepts of chemical vapour deposition (CVD) process and CVD chamber.
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Unit 2: Preparation of nanomaterials
Gas or Vapours of desired material may be often pumped into reaction chamber using some
carrier gas (Fig. 19). In some cases the reactions may occur through aerosol formation in gas
phase. There are various processes such as reduction of gas, chemical reaction between different
source gases, oxidation or some disproportionate reaction by which CVD can proceed. However, it
is preferable that the reaction occurs at the substrate rather than in the gas phase. Usually
temperature ~ 300 to l200°C is used at the substrate and gas pressures in the range of 0.1 torr to l.0
torr are used. Growth rate and film quality depend upon the gas pressure and the substrate
temperature. When the growth takes place at low temperature, it is limited by the kinetics of
surface reaction. At intermediate temperature, it is limited by mass transport, i.e. supply of reacting
gases to the substrate. Here the reaction is faster and supply of reactants is slower. At high
temperature, growth rate reduces due to desorption of precursors from the substrate.
GROWTH MECHANISMS:
When two types of atoms or molecules, say P and Q, are involved in the desired thin film
formation, then there are two mechanisms by which growth can take place.
A. Langmuir-Hinshelwood mechanism: Both P and Q type of atoms/molecules are adsorbed on
the substrate surface and interact there to produce PQ. When one species is adsorbed in excess
of the other, the growth depends on the availability of adsorption sites for both P and Q as
shown in a schematic diagram Fig. 20.
Fig. 20: Langmuir-Hinshelwood mechanism of growth.
B. Elay-Riedel mechanism: One species say Q adsorbs on the substrate and the species P from gas
phase interacts with Q. Thus, there is no sharing of sites. This type of mechanism is known as
Elay-Riedel mechanism and is as shown in Fig. 21.
Fig. 21: Elay-Riedel mechanism of Growth
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Unit 2: Preparation of nanomaterials
TYPES OF CVD: A variety of CVD methods and CVD reactors have been developed, depending
on the types of precursors used, the deposition conditions applied and the forms of energy
introduced to the system to activate the chemical reactions desired for the deposition of solid films
on substrates. For example, when metal organic compounds are used as precursors, the process is
generally referred to as metal organic CVD (MOCVD), and when plasma is used to promote
chemical reactions, this is a plasma enhanced CVD (PECVD). There are many other modified
CVD methods, such as low pressure CVD (LPCVD), laser enhanced or assisted CVD, and aerosolassisted CVD (AACVD).
2.2.3. SOL-GEL PROCESS
As the name suggests sol-gel involves two types of materials or components, 'sol' and 'gel'.
Sols are solid particles in a liquid (see Fig. 22). They are thus a subclass of colloids. Gels are
nothing but a continuous network of particles with pores filled with liquid (or polymers
containing liquid). A sol-gel process involves formation of 'sols' in a liquid and then connecting
the sol particles (or some subunits capable of forming a porous network) to form a network. By
drying the liquid, it is possible to obtain powders, thin films or even monolithic solid.
Fig. 22: (a) sol, (b) gel and monolithic solid (aerogel).
Sol-gel method is particularly useful to synthesize glass materials, ceramics or metal oxides
although sulphides, borides and nitrides also are possible. Nowadays the sol-gel process is quite
often used for the fabrication of a variety of nanomaterials. Usually metal alkoxides and metal
chlorides are used as precursors. Figure 23 shows the typical sol-gel process.
Fig. 23: Schematic diagram of sol-gel process.
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Unit 2: Preparation of nanomaterials
POLYMERISATION OF METAL ALKOXIDE:
Metal alkoxides (R-O-M) are organic compounds with one or more metal atoms in the
molecule. They are like alcohols (R-OH) with a metal atom M replacing the hydrogen H in the
hydroxyl group. Metal alkoxides are widely used in sol-gel synthesis as ideal chemical precursors.
Silicon tetraethoxide or tetraethyl orthosilicate (TEOS) is an example for metal alkoxide. Its
chemical formula is Si(OC2H5)4 or Si(OR)4 where the alkyl group R = C2H5. Hydrolysis and
condensation are the two essential processes which takes place in sol-gel process.
Hydrolysis: Metal alkoxides (R-O-M) react readily with water, and a hydroxyl ion becomes
attached to the metal atom during the reaction. This reaction is called hydrolysis.
M-O-R + H2O → M-OH + R-OH
e.g.: Si(OR)4 + 4 H2O → Si(OH)4 + 4 R-OH
During the completion of hydrolysis process, all of the OR groups are replaced by OH groups,
depending on the amount of water and catalyst present. Any intermediate species [(OR)2-Si-(OH)2
or [(OR)3-Si(OH)] will be considered the result of partial hydrolysis.
Condensation: Two partially hydrolyzed molecules can link together in a condensation reaction to
form a siloxane [Si-O-Si] bond:
Water condensation: M-OH + HO-M → M-O-M + H2O
e.g.: (OR)3-Si-OH + HO-Si-(OR)3 → [(OR)3Si-O-Si(OR)3] + H-O-H
Alcohol condensation: M-O-R + HO-M → M-O-M + R-OH
e.g.: (OR)3-Si-OR + HO-Si-(OR)3 → [(OR)3Si-O-Si(OR)3] + R-OH
Thus, polymerization of metal alkoxides is associated with the formation of a 1, 2, or 3dimensional network of siloxane. [Si-O-Si] bonds along with the production of H-O-H and R-OH
species. This will give the required sol. Then, the sol is aged for a certain time to get long chains of
siloxane bonds, removed by hydroxyl groups, resulting in thickening of the sol to form the gel.
Finally, this gel is heated up in atmospheric air to get SiO 2. The condensation liberates a small
molecule, such as water or alcohol. This type of reaction can continue to build larger and larger
silicon-containing molecules by the process of polymerization.
2.3. LITHOGRAPHY
Mainly, there are two approaches for the fabrication of nanomaterials, bottom-up and
topdown approaches. In the earlier sections of this chapter, we have seen few methods to make
nanoparticles and thin films (or multilayers) [e.g. sputtering, CVD, thermal evaporation, pulsed
laser deposition, molecular beam epitaxy etc]. These methods are popularly known as bottom-up
approach. In bottom-up approach, atoms and molecules are assembled so as to form nanomaterials
of required size and shape by controlled deposition or reaction parameters. In top-down approach,
usually a bulk material is taken and machined to modify into the desired size, shape and product.
Ball milling is an example of top-down approach.
Lithography may be considered as a hybrid approach because growth is bottom-up while
etching is top-down. But nanolithography is commonly a bottom-up approach. Both approaches
play a very important role in nanotechnology. There are advantages and disadvantages in both
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approaches. The imperfection of the surface structure is the drawback of top-down approach. For
instance, in lithographic techniques the processed patterns contain crystallographic damages in
addition to the defects which occur during the etching process. Such imperfections would have a
significant impact on physical and chemical properties of nanomaterials, since their surface to
volume ratio is very large.
The word lithography has originated from Greek words "litho" (which means stone) and
"gramma" (which means writing). Therefore, lithography literally means carving a stone or writing
on a stone. Now, it is used to mean a process in which a sample is patterned by removing some
part of it (i.e. etching) or sometimes even organizing some material on a suitable substrate.
Lithography is very extensively used in electronics industry to obtain integrated circuits (IC) or
very large scale integration (VLSI) on small piece of semiconductor substrate often called a "chip".
Over the last 3-4 decades, different lithography techniques like photolithography, X-ray
lithography, electron beam lithography and some others have been developed. They depend upon
using photons or particle radiations for carving the materials. The lithography technique involves
transfer of some pre-designed geometrical pattern (called master or mask) on a semiconductor (like
silicon) or directly patterning (often known as writing) using suitable radiation. Mask is usually
prepared by creating radiation opaque and transparent regions on glass or some other material.
Predesigned patterns can be transferred on a substrate much faster as compared to direct writing.
Direct writing being a slower process is overall expensive.
PRINCIPLE AND PRODUCE OF LITHOGRAPHY:
Common principle in most of the lithography techniques is to expose a material sensitive to
either electromagnetic radiation or to particles at some regions. Such a radiation sensitive material
is known as resist. The selection of area is made using a mask which is transparent in some regions
and opaque in the other regions. This causes selective exposure of the resist, making it weaker or
stronger compared to unexposed material depending upon the type of the resist being used. By
removing the exposed or unexposed material in suitable chemicals or plasma, desired pattern is
obtained. This may be done in a number of steps depending upon the pattern and materials
involved.
Fig. 24 depicts schematically various steps involved in photolithography to transfer a pattern
on the surface of some semiconductor. A thin film coating of a metal (e.g. chromium) is deposited
on a suitable substrate (e.g. glass or silicon). A positive or a negative photo-resist (usually some
polymer) is coated on metal thin film. Positive photo-resist material has the property that, when
exposed to the appropriate radiation it degrades or some chemical bonds are broken. Negative
resist on the other hand is a material, which hardens (cross-links) on exposure to a radiation. A
mask is placed between the resist coated substrate and the source of light. By using a suitable
chemical (developer), the weakened portion is removed (or image is developed). Remaining
unexposed part also can be removed by appropriate chemical treatment. The remaining material
can be dissolved in one step and the hardened material in another step.
Depending on the radiation used like visible light, X-rays, electrons, ions etc. the lithography
name is tagged with it.
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Unit 2: Preparation of nanomaterials
Fig. 24: Photolithography process steps: (1) surface is coated with metal, (2) coating of photoresist on the substrate, (3) mask placed over upper layer, (4) exposed to UV radiation, (5)
resist development and striping and (6) etching to get final pattern.
2.3.1. PHOTOLITHOGRAPHY
It is possible to use visible, ultraviolet, extreme ultraviolet (EUV) or X-rays to perform
lithography and wherever possible lasers are also used. Highest resolution of the generated features
ultimately depends upon the wavelength of radiation used and interaction of radiation with matter
as well as mask and optical elements used. Smaller the wavelength used smaller can be the feature
size which is limited by diffraction limit, ~ λ/2. Depth of focus depends upon the penetration of
incident radiation. For the lithography using electromagnetic radiation, optical elements and masks
have to be used for various purposes. In the visible range (~700 nm to 400 nm) glass lenses and
masks can be used. In the UV range fused silica or calcium fluoride lenses are used.
There are three methods (see Fig. 25) viz. "proximity", "contact" and "projection" which can
be used to pattern a substrate.
Fig. 25: (1) mask is close to the photo-resist, (2) mask is in contact with the resist and (3) focused
beam is scanned through the mask.
As the name suggests, in 'proximity' method, mask is held close to the photo-resist coated
metalized substrate, whereas in 'contact' method the mask is in contact with photo-resist. In both
proximity and contact methods a parallel beam of light fal1s on the mask, which transmits the
radiation through some windows but blocks through opaque parts. Although better resolution is
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Unit 2: Preparation of nanomaterials
achieved with contact method as compared to proximity method, in contact method the mask gets
damaged faster. In case of 'projection' method a focused beam is scanned through the mask, which
allows good resolution to be achieved along with the reduced damage of the mask. However,
scanning is a slow process and also requires scanning mechanism adding to the cost.
LITHOGRAPHY USING UV LIGHT AND LASER BEAMS
Using monochromatic light in the visible to UV range features as small as 1 to 1.5 µm size can
be routinely obtained. Often g-line [i.e. green colour] (436 nm) from mercury lamp is used. Laser
beam of KrF (248 nm) or ArF (193 nm) also are employed reaching ~150 nm as the smallest
feature size. However to obtain feature size below ~ 100 nm using photons is a difficult task.
LITHOGRAPHY USING X-RAY
Smaller features are possible to obtain by employing X-rays also. However, it is difficult to
make suitable masks for X-ray lithography. X-rays in the 0.1 to 5 nm range are used with
appropriate metal masks in proximate geometry. Absorption of X-rays in materials not only
depends upon the thickness of the material but is also complicated by the presence of absorption
edges. Depending upon the wavelength of X-rays used, metals of suitable elements are chosen.
Metal masks are fabricated in such a way that through thin portions rays are transmitted and
absorbed in thicker regions. Gold masks are often used. The masks themselves are made using
electron beam lithography discussed in the next section.
2.3.2. PARTICLE BEAM LITHOGRAPHY
We know that, all the moving particles have associated wavelength λ known as de Broglie
wavelength given by,
h
λ=
mv
Where h is Planck's constant, m is the mass and v is the velocity of the particle. All kinds of
particles can in principle be used. But to achieve high resolution λ should be as small as possible.
Thus, large mass and large velocity of particle makes it possible to get adequate resolution. In fact
it is possible using neutral atoms, ions or electrons to bring down the particle associated
wavelength to any desired value, even as small as 0.1 nm. However, ultimate resolution depends
upon the interaction of incident particles with resist material. Under certain conditions, features as
small as 2 nm have been patterned. Due to various reasons like electrons can be easily generated,
accelerated and focused, electrons are preferred for lithography purpose and often used.
ELECTRON BEAM LITHOGRAPHY
Figure 26 shows schematically electron beam lithography set up. It is very similar to a
scanning electron microscope (SEM) and requires vacuum (~ 10 −5 torr). Sometimes SEM is
modified in order to use it as a lithography set up. Electron beam lithography is a direct writing
method, i.e. no mask is required to generate a pattern. Rather, patterns or masters required for other
lithography processes (like optical lithography and soft lithography) can be generated using
electron beam lithography.
Electrons with high energy (usually larger than ~ 5 keV) are incident on the photo-resist. Here
also positive or negative photo-resists can be used. Common positive resists are poly-methyl26
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Unit 2: Preparation of nanomaterials
methacrylate (PMMA) and polybutane-l-sulphone (PBS). Negative resist often used in electron
beam lithography is poly-glycidyl-methacrylate co-ethyl-acrylate (COP). Developers used are
methyl-isobutyl-ketone (MIBK) and isopropylalcohol (IPA) in 1:1 ratio.
Fig. 26: Electron beam lithography set up.
A focused electron beam in electron beam lithography is used in two modes, viz. 'vector scan'
or 'raster scan'. In vector scan, the electron beam 'writes' on some specified region. After one
region is completed the X-Y scanning stage on which the substrate to be patterned is mounted
moves. During its movement electron beam is put off. Then a new region is selected and ' written'
with the beam. This is continued until whole pattern is generated. In 'raster scan' the beam is
rastered or moved continuously over a small area, line by line. The X-Y stage of the sample moves
at right angles to the beam. The beam is turned off or turned on depending upon the pattern.
Although very high resolution (~50 nm) is routinely possible using this lithography, due to
scanning mode it is rather slow. For example, if the optical lithography can generate 40 patterns
with 1µm resolution in one hour, only five similar patterns would be generated with electron beam.
However, larger layer depth is an added attraction of electron beam lithography as compared to
optical beam lithography.
ION BEAM LITHOGRAPHY
Very small features (~5-10 nm) can be written using high-energy ion beams. Major advantages
of using ion beams is that resists are more sensitive to ions as compared to electrons and have low
scattering in the resist as well as from the substrate. Commonly used ions are He+, Ga+ etc. with
energy in the 100-300 keV range.
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MODEL QUESTIONS
1. Explain bottom-up and top down approaches for fabricating of nanostructures with examples.
What are the advantages and disadvantages of both methods?
2. What is ball mill? Explain common ball miller and planetary ball miller with neat figures.
3. Explain with neat schematic lay out: a) inert gas condensation, b) arc discharge method and c)
laser ablation.
4. What is sputtering? Explain DC, RF and magnetron sputtering with neat diagrams.
5. What is pyrolysis? Explain the spray pyrolysis and flame pyrolysis with the neat diagrams.
6. Explain thermal/resistive evaporation with its schematic lay out? List the advantages and
disadvantages thermal evaporation.
7. Explain pulsed laser deposition with its schematic layout? List its advantages.
8. What is meant by epitaxy? Explain molecular beam epitaxy with its schematic layout. List its
characteristics.
9. What are Microemulsions? Explain the formation and types of micelles along with suitable
diagrams.
10. Explain chemical vapour deposition process and growth mechanism with neat diagrams.
11. Explain sol-gel process with schematic diagram and also polymerization of metal alkoxides.
12. What is lithography? Explain the basic principle and procedure involved in lithography with
neat diagram.
13. Explain photo-lithography with neat figure. Mention the types of photolithography.
14. Explain electron beam lithography with neat figure.
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