ect 292 nanoelectronics

Module 2
1
EC 402 Nano Electronics
APJ Abdul Kalam Technological University
Department of Electronics and Communication Engineering
Musaliar College of Engineering and Technology, Pathanamthitta.

Production of Nanolayers
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Physical Vapor Deposition (PVD)
• Physical Vapor Deposition (PVD) is a process by
which a thin film of material is deposited on a
substrate.
• (i) evaporation
• (ii) sputtering
• (iii) ion plating
• (iv) laser ablation.

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Physical Vapor Deposition (PVD)
by Evaporation
1) the material to be deposited is converted into vapor
by physical
2) the vapor is transported across a region of low
pressure from its source to the substrate
3) the vapor undergoes condensation on the substrate
to form the thin film.

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• A crucible is heated up by a resistance or an electron gun
until a sufficient vapor pressure develops.
• As a result, material is deposited on the substrate.
• There are several ways by which heating is achieved in PVD
by evaporation.
– Technically, the resistance is wrapped around the crucible,
or a metal wire is heated up by a current and vaporized.
– Or an electron beam is directed at the material intended
for the deposition on the substrate
– In electron beam evaporation, a high kinetic energy beam
of electrons is directed at the material for evaporation.
– Upon impact, the high kinetic energy is converted into
thermal energy, heating up and evaporating the target
material,

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Advantages
1) high film deposition rates
2) less substrate surface damage unlike sputtering that
induces more damage because it involves high-energy
particles;
3) excellent purity of the film because of the high vacuum
condition used by evaporation;
4) less tendency for unintentional substrate heating.

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disadvantages
Difficult control of film composition

Sputtering
• Sputter deposition is a physical vapor deposition (PVD)
method of thin film deposition.
• An atom or a molecule, usually in its ionized form, hits a
solid state(target) and knocks out surface atoms. This
will be deposited on a solid state substrate.
• Sputtering involves ejecting material from a "target"
that is a source onto a "substrate" such as a silicon
wafer.
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• Mainly two types
– Glow discharge [ DC sputtering] .
– High frequency discharge [ RF sputtering].

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Glow discharge
• It is created by applying a voltage between two electrodes in a
glass tube containing a low-pressure gas.
• When the voltage exceeds a value called the striking voltage,
the gas ionization becomes self-sustaining, and the tube
glows with a colored light.
• The color depends on the gas used.
• Glow discharges are used as a source of light in devices
such as neon lights, fluorescent lamps, and plasma-screen
televisions.
• They are also used in the surface treatment technique
called sputtering.

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Glow discharge Sputtering
• Sputtering is achieved by glow discharge with dc voltage.
• The chamber is supplied with Ar and a constant
gas pressure of some 100 mPa is built up.
• The target, being attached a few centimeters above the
substrate, is raised to a negative dc potential from -500 to
- 5000 V, while both chamber and substrate are grounded.
• When the voltage is slowly increased, a small current
flows over the two electrodes.

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• At higher voltages across the anode and cathode, the
gas in the chamber become ionized.
• The freed carriers can gain enough energy so that
additional carriers are freed during collisions; the
process is a Townsend avalanche or multiplication.
• The ionised atom will hit the target and knock out
atoms.
• This will be deposited on a solid state substrate.

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High frequency discharge
• It replaces the dc voltage source to a high frequency
generator.
• during DC sputtering, the working gas will be ionized. As a
result, many positive ions will be produced within the
chamber and accumulate on the surface of target.
• In order to avoid this problem, we can use AC power instead
of DC.
• In the first cycle,the source or target material is given -ve
charge which causes polarization of atoms and the
sputtering gas atoms are attracted to the source where they
knock off source atoms.
• However, the source atoms may,remain at the target surface
• During the second cycle, when target is given positive
charge,the sputtering gas ions are ejected due to reverse
polarization.

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Ion plating.
• This process is a combination of resistance evaporation
and glow discharge
• A negative voltage is applied to the substrate, while the
anode is connected with the source of the metal
vaporization.
• The chamber is subsequently filled with Ar with a
pressure of a few Pa, and the plasma is ignited.
• The material is vaporized with the help of e-gun .
• Thegrowing of the layer on the
substrate is improved by the plasma.
• The advantages of ion plating are higher energies of the
vaporized atoms and therefore better adhesion of the
produced films.
• The disadvantage is heating of the substrate.

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Laser ablation.
• A high-energy focused laser beam is capable of
gradually remove the surface of a target rotating with a
velocity of one revolution per second.
• The material is vaporized on the substrate, and as a
result, a film is produced on it.
• The growth can be supported by heating the substrate
(750 °C) and by chemical reactions.

Reflection high-energy electron
diffraction (RHEED)
• Reflection high-energy electron diffraction (RHEED) is a
technique used to characterize the surface of crystalline
materials
• A RHEED system consist an electron source (gun), detector
screen and a sample with a clean surface.
• The electron gun generates a beam of electrons which strike
the sample at a very small angle relative to the sample
surface.
• Incident electrons diffract from atoms at the surface of the
sample and form regular patterns on the detector.
• The electrons interfere according to the position of atoms on
the sample surface, so the pattern at the detector is a
function of the samplesurface 22

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Advantages
• Deposition of materials of high-melting points
• Good control over impurities.

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Chemical Vapor Deposition (CVD)
• Chemical vapor deposition (CVD) is a deposition method used
to produce high quality, high-performance, solid materials,
typically under vacuum.
• The wafer is put on a carrier and heated to a temperature
between 350 and 800C.

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Types of CVD
CVD is practiced in a variety of formats.
• Atmospheric pressure CVD (APCVD)
– CVD at atmospheric pressure.
• Low-pressure CVD (LPCVD)
– CVD at Reduced pressures tend to reduce
unwanted gas-phase reactions and improve film
uniformity across the wafer.
• Ultrahigh vacuum CVD (UHVCVD)
– CVD at very low pressure, typically below 10−6 Pa
• Plasma-Enhanced CVD (PECVD)
– CVD that utilizes plasma to enhance chemical
reaction rate.

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• One or several species of gases are let in so that a gas
pressure is formed between very low and normal
pressure.
• The gas flow hits the wafer at a normal or a glancing
incidence.
• Now a dissociation (in the case of a single gas species)
or a reaction between two species takes place.
• In both cases, a newly formed molecule adheres to the
wafer surface and participates in the formation of a new
layer.

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Examples
• Silane (SiH4)
On impact, it disintegrates into elementary silicon,
which partly adheres to the surface, and to hydrogen,
which is removed by the pumps.
• SiH4 & N2O
SiH4, reacts with N2O to form SiO2.

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PECVD
• CVD deposition can be supported by an RF
plasma.
• An example of an amorphous or micro-crystalline silicon
deposition.
• The major difference to the conventional CVD is the
addition of Ar for the ignition of the plasma and of H2.
• The degree of the SiH4 content in H2 determines
whether amorphous or microcrystalline silicon is
deposited

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Epitaxy
• The term epitaxy comes from the Greek roots epi,
meaning "above", and taxis, meaning "in ordered
manner". It can be translated "to arrange upon.
• Epitaxy means the growth of a single crystal film on top
of a crystalline substrate.

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Types of Epitaxy
• Homoepitaxy
The film and the substrate are the same material.
Often used in Si on Si growth.
Epitaxiallygrown layers are purer than the
substrate and can be doped independently of it.
• Heteroepitaxy
Film and substrate are different materials.
Eg: AlAs on GaAs growth
Allows for optoelectronic structures

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Epitaxy is also classified based on the phase from which the film is
made:
• vapor phase epitaxy
– In vapour phase epitaxy the deposition atoms come from a
vapour, so that growth occurs at the interface between gaseous
and solid phases of matter.
– A subclass of vapor phase epitaxy is molecular beam epitaxy
(MBE).
• liquid phase epitaxy (LPE)
– Liquid-phase epitaxy (LPE) is a method to grow semiconductor
crystal
layers from the melt on solid substrates.
• solid state epitaxy
– Solid-phase epitaxy (SPE) is a transition between the amorphous
and crystalline phases of a material.
– a thin amorphous (noncrystalline) film layer is first deposited on a
crystalline substrate, which is then heated to convert the film into
a crystalline layer.

molecular beam epitaxy (MBE)
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• The constituents of the deposited film are contained in
mini furnaces as elements, the so-called Knudsen cells.
• During heating some vapor pressure develops and an
atom beam is emitted, which is bundled by successive
apertures.
• The beam hits the wafer surface to which the atoms
remain partially adhered.
• There, they can react with atoms of a second or third
beam, which is also directed towards the wafer surface.
• A favorable reaction and finally the film deposition
depend on the selection of the parameters.
• The parameters are: wafer temperature, the ratios of the
beam densities, the purity of the surface, ..etc

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effusion (Knudsen) cell
• The material to be deposited is contained in the
innermost cell which is heated up.
• Its temperature is controlled by a set of thermocouples
and resistance heaters.
• A screen cooled with liquid air is installed around the
internal cell to avoid high thermal flows to the furnace.

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Liquid Phase Epitaxy (LPE)
• The mixture of Ga and As in a ratio of 50:50 takes over
the role of a constituent.
• In other words: GaAs forms a stable compound which is
not subjected to any chemical change during the change
of temperature.
• Pure GaAs freezes out on the already available solid
GaAs by decreasing the temperature and the melt
becomes richer in Ga until finally only pure Ga remains.

Comparison
• LPE VS VPE vs MBE
Do as homework
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Ion Implantation
• The depth of doping and the quantity of doping can precisely control.
• Ion implantation is a doping technique with which ions are shot into a
substrate (e.g., a silicon wafer) using an accelerator.
• The desired ion species is let in as a gaseous compound through a needle
valve
• The compound is ionized with an electron beam.
• The arising ions are pulled out of the source area and brought to an
energy of 30 keV by a preliminary acceleration .
• Then, the ions pass through a magnetic field filter which is adjusted in such
a way that only the desired ion type can run into the acceleration tube.
• At the end of the accelerator tube the ions acquire a total energy of 360
keV.

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• The beam can be scanned upwards and
downwards or left and right.
• The beam current is measured by an ammeter, which is
connected to the substrate.

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Equipment can devide into four sections
• Ionizing chamber
• Ion selection chamber
• Acceleration chamber
• Doping chamber

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Ionizing chamber
• For doping boron, boron trifluoride (BF3) is used. It has
to be heated and evaporated.
• The atoms in BF3 must be converted to ions. In the
ionizing chamber, a low pressure is maintained and a
tungsten filament is heated using electricity
• When it is very hot, electrons are emitted from the
filament in a process called thermionic emission.
• Some of the electrons hit the BF3 molecules and
create ions such as BF3+, BF2+, BF+ and B+.
• Thus ions are created from the gas.

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Ion selection chamber
• We would like to dope the wafer with boron only and not with
fluorine.
• Hence only B+ must be selected from the mixture of ions and
added to the wafer.
• In order to select the required ion (B+), electric field and
magnetic field are used.
• If we send the stream of ions between two electric plates, which
have positive and negative connection, the ions will be attracted to
the negative plate and move towards the negative plate.
• If a magnetic field is also applied at the same time, the
direction of the movement will change.
• The speed and the direction of the movement will depend on the
charge and the mass of the ions.
• A plate with a hole in the appropriate place can be used to block all
the ions except the desired one.
• Thus B+ ion can be separated from the mixture of ions.

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Accelerating chamber;
• In order to ensure that they penetrate the wafer, they
must be accelerated.
• For this, a set of ring electrodes are used .
• If negative charge is applied on the plates, the ions will
get accelerated considerably.
• They formwhat is called as “ion beam”.
• The ionizing chamber must be kept under high vacuum.

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Doping chamber
• Ions will fall on the wafer
• using electrodes, the beam is moved by
applying suitable positive and negative
voltages.

Calculation of the Implantation Time
• Typical required dose values NI lie between 1012 and 1016
cm2.
• The necessary implantation time depends on the available
beam current, I, substrate surface, A, and on the charge of
the ions.
Q = q NI A,
• As an example, an ion current of 1 μA, a substrate surface
of 100 cm2, and a dose of 1013 cm2 deliver a time of 160 s.
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Radiation Damage
• The penetrating ions pass through the lattice depending
on the ion mass/lattice atom mass ratio and the
momentary velocity in a zigzag path.
• At the path, the ions collide on host lattice atoms, which
leave their places and go to the interstitial site. Thus,
lattice defects of different nature develop.
• The defect produced by the displacement of a lattice
atom into an interstitial site is called Frenkel defect.
• Double vacancies can be formed if the ion dislodges two
nearest neighbor lattice atoms
• Dislocation lines anneal only at high temperatures (1000
°C), and very often they do not anneal at all in implanted
layers.

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• If many lattice atoms are displaced by an impact ion in a
considerably small volume, a locally amorphized area
develops. Often, this area is known as cluster.
• Annealing is applied for both the annealing of lattice
defects and for the relocation of the doping atoms from
interstitial sites into lattice sites.
• Annealing is the process of thermal treatment of the
implanted samples at approximately 900–1100 °C in a
suitable atmosphere like nitrogen or hydrogen gas.

Implantation Profile
• The distribution of the implants is described
by a Gaussian curve:
• Rp is the position of the center of the
distribution, Rp is the full width.
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• If one shoots toward a low indexed crystal orientation,
then the ion beam runs without resistance through the
lattice channels and reaches substantially large depths
(channeling).
• Channeling is avoided by tilting the crystal, which is
aligned in (110) direction against the ion beam, usually
by 7°.

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Formation of Silicon Oxide
• Thin oxide layers are contained in almost all
electronic devices.
• SiO2 can create in different ways
– Thermal Oxidation
– CVD : PECVD & TEOS
– Anodic Oxidation
– SOI(Silicon on oxide)

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Thermal oxidation.
• Si wafer is cleaned so that any impurity on the surface is
removed.
• Then, the wafer is heated to a temperature of about 1100°C.
• A flow of an oxidizing gas, either pure oxygen (dry oxidation)
or nitrogen driven through water (wet oxidation), is
maintained.
• When oxygen penetrates into the substrate, the Si surface
reacts with the oxygen and forms silicon dioxide.
• Above process requires highest cleanliness, which is the
critical step for the production

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• The chemical reaction occurring at the silicon surface
during dry oxidation is
• Si + O2  SiO2 -----------------DRY OXIDATION
• And for wet oxidation is
• Si + 2H2O  SiO2 + 2H2--------WET
OXIDATION
• Dry oxidation has a lower growth rate than wet oxidation
although the oxide film quality is better than the wet
oxide film.
• wet oxidation process has a significantly higher oxidation
rate than the dry oxidation. Hence it is used to grow thick
oxides

64
TEOS
• A feed gas (usually nitrogen) is driven through a
container filled with tetraethylorthosilicate (TEOS).
• The enriched nitrogen flows to the wafers where SiO2
is deposited on their surfaces.
• The deposition is maintained at a temperature of about
650 to 850 °C.
• deposition temperature (>650 °C) prevents use
• In order to obtain lower deposition temperatures, ozone,
is required.
• After adding some few molar per cent of ozone, the
optimum deposition temperature is reduced to 400 °C.

anodic oxidation
• The wafer is immersed into a 0.04 M solution of KNO3 in
ethylene glycol (CH₂OH)₂. with a small addition of water.
• After mounting it to a holder with a vacuum, it is
positively charged, while a platinum disk acts as a
backplate electrode.
• The quality of the oxide is too low, and the process is
time-consuming. 66

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SOI
• The implantation of oxygen up to the dose and its
reaction with silicon to form SiO2.
• SOI has gained in importance because of better radiation
resistance and heat distribution compared to
conventional technologies

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Nanoparticles
• A nanoparticle is defined as a ball or a ball-like molecule
which consists of a few 10 to some 10,000 atoms
interconnected by interatomic forces.
• The deposited nanoparticle layers are differ from uniform
layers, particularly due to the presence of grain(rough)
boundaries.

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Grinding with Iron Balls
• Container is filled with stainless steel balls of a few
millimeters in diameter.
• The material to be crushed is added in the form of a
powder of about 50 μm diameter grain size.
• After filling the container with liquid nitrogen, a rotating
shaft grinds the material.
• The grinding periods are within the range of minutes to
some 100 hours. This process is simple

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Gas Condensation
• The operation occurs in an evacuated chamber with a
pressure of 10-5 Pa.
• After mounting the raw material on one or more crucibles,
it is evaporated thermally, by an electron gun, or by ion
sputtering.
• The evaporated atoms or molecules unite and form
particles of different sizes.
• Finally they are captured and collected with a funnel.
• The particle diameter is usually within the range of 5 to
15 nm.

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Laser Ablation
• The raw material is provided as a solid.
• Its dissolution is achieved by a focused laser beam—
similar to the cutting of a metal or a semiconductor.
• The advantage of this procedure is a 1:1 transfer of the
material composition from the raw material to the
particles.

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Thermal and Ultrasonic
Decomposition
• As an example of thermal decomposition, the starting
material iron pentacarbonyl, Fe(CO)5, is considered.
• It decomposes in a polymeric solution, e.g.,
polybutadiene, and thus produces iron particles of 7 to 8
nm diameter.
• The material can also be decomposed as immersion in
decane by ultrasonic irradiation, resulting in particle
sizes roughly between 6 and 240 nm.

Reduction Methods
• Some metal compounds (e.g., chlorides) can be
reduced to elementary metallic nanopowder by the
application of NaBEt3H, LiBEt3H, and NaBH4.
• For example,the reaction equation can be written as
(M: metal, Et: ethyl):
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Self-Assembly
• Three-dimensional islands are formed with a rather
surprising regularity on a substrate.
• Two extremes can occur: regular layer-on-layer growth
and cluster formation.
• The first case is comparable with the picture of butter on
bread, and the second one with water drops on butter.
• With sufficient film thickness the film will form three-
dimensional islands as shown

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Low-Pressure, Low-Temperature
Plasma
• A conventional capacitively coupled RF plasma device.
• A gas (e.g., silane) is let in at a pressure of 1 to 200 Pa.
• An inert gas is ionized by the applied field. The electrons
acquire kinetic energy and, in return, ionize neutral
molecules of silane upon impact.
• Dissociation of silane happen after impact with electron
of higher energy.
• These ions form the nanoparticles

Thermal High-Speed Spraying of
Oxygen
• Thermal spraying is a procedure for Nano crystalline
cover layers.
• A burn reaction produces high temperature and high
pressure within a spray gun.
• The pressure drives particles of a Nano powder
contained in the pistol through a nozzle onto the surface
which is to be covered.
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Atom Optics
• An atom beam which is produced by heating up a material in
a crucible is assumed.
• The beam is bundled by one or more apertures and steered
onto the substrate
• The beam is subjected to the dipole forces of a standing wave
which is produced by a laser beam.
• The atoms will deviate to the nodes of the standing wave
where they are subjected to the smallest forces.
• A lattice pattern reflecting the wavelength of the laser is
produced.
• A second standing wave may be used perpendicularly to the
first and to the direction of the beam for a two-dimensional
pattern of deposited atoms

Sol Gels
• A sol (hydrosol) is a colloidal dispersion in liquid.
• A gel is a jelly-like substance formed by coagulation of a
sol into a gel.
• The best known example of a sol gel process is probably
the production of SiO2.
• A catalyst (acid or base) is added to a solution of
tetramethoxysilane (TMOS), water, and methanol. This
leads to the formation of Si–OH groups:
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• Further dehydration reduces the “Si(OH)4” to SiO2 gel.
• During the reaction, the gel reaches a viscosity so low that
it can be applied onto a centrifuge and distributed over the
wafer.
• Then SiO2 layers are produced by annealing over 800 °C.

Precipitation of Quantum Dots
• Quantum dots are three-dimensional semiconductor materials
and falls in the category of nanopartilces.
• Precipitation is the creation of a solid
from a solution.
• When the reaction occurs in a liquid solution, the solid formed
is called the 'precipitate'.
• The chemicalthat causesthe solid to form is called the
'precipitant'.
• Precipitation is still used in the manufacturing GaAs, and Si
nanocrystallites in silica glasses.
• The contaminants are added to the melt, and after a further
annealing step from 600 to 1400 °C, they form precipitates of
controllable size.
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• Quantum dots can also be manufactured by means of
electron beam lithography.
• The material which is to be converted into quantum dots
is deposited on a substrate like GaAs via MBE.
•
• The size of the electron beam spot determines the
smallest possible size of the quantum dot.
• Therefore, if the wafer is etched, islands of this size
remain. They are further reduced by etching so that
quantum dots of a few 10 nm can be produced.

ect 292 nanoelectronics

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