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Procese tehnologice avansate
Titularul de disciplin:Prof. Dr. Ing. Dan Dasclu
Cerc.st. gr. I, dr. Ing. Mircea Dragoman
Dr. Emil Mihai Pavelescu (IMT).
Cursul 6
Metode de autoasamblare
Metode de crestere prin matrite (template)
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Semiconductor doping is a key technological process in microelectronics andaccompanies the deposition processes. It is well known that p- or n-doping of asemiconductor changes significantly the electrical, chemical and mechanicalcharacteristics of semiconductors. These changes are often referred to as thefunctionalization or engineering of the material. The electrical changes areused to produce almost any active electronic component, such as transistors anddiodes, while the mechanical changes create MEMS devices with prescribed
mechanical characteristics. Impurities ofp- or n-type are controllable introducedin intrinsic silicon via diffusion into furnaces at high temperatures from liquid or solidsources or via ion implantation techniques, which are more accurate with respect tothe amount of impurities/area and the prescribed impurity profile.
The variety of functionalization techniques at nanoscale is much richercompared to that of microscale transistors, diodes and integrated circuits.The doping is still used, for example, for nanotube transistors, but the
functionalization of nanowires or dots can be done via oxygenation,hydrogenation, adsorbing of molecules, biomolecules etc
Semiconductor doping
Doping by diffusion
Typical doping
concentration
1015-1019atoms/cm3
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Feature size (nm)
1000 100 10 1 0.1
SUBMICRON
TECHNOLOGYNANOTECHNOLOGY ATOM TECHNOLOGY
EB
FIB
EUV
Optical
AFM, STM
Atom lithography
Nanoimprint
Next lecture
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Self-Assembly Techniques
The self-assembly process refers to the spontaneous organization of various components (molecules or
various nanosize objects such as nanoparticles) into a single, ordered aggregate. The organization process is
made into a desired structure via physical, chemical, or biochemical interactive processes involving, for
example, electrostatic and surface forces, hydrophobic and hydrophilic chemical interactions. All these
processes, irrespective of their origin, are very selective and reject defects so that the resulting desired
structure is characterized by a high degree of perfection
The Langmuir-Blodgett (LB)technique dedicated to thin film realization is a well spread self-
assembly technique in which the organized aggregate is built by growing one monolayer at a
time. A monolayer of a desired material, which is initially adsorbed at a gas-liquid interface, is
transported to a substrate on which the self-assembly structure will be grown. For example, a
monolayer of some molecular species, such as a fatty acid, is spread over the surface of the water.In the water there is a glass microscope slide, which plays the role of the self-assembly substrate.
If we pull out the glass slide the monolayer will be attached to it. If we then pass the glass slide
several times through the water surface we will deposit on it a monolayer at each passing .The
LB technique is implemented with specialized instruments comprising a Langmuir trough, a
dipping device for the substrate that will be raised or lowered, and a movable barrier controlled
by a pressure sensor, which slides on the gas-liquid interface to maintain a certain surfacepressure
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Water
Fat acid
Monolayer Glass plate
LB technique
Although 2D gold, nanoparticle arrays,
semiconducting quantum dots and
polymeric films were realized usingthe LB technique, the method is quite
difficult and requir esexpensive
instrumentation
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The electrostatic self-assemblyis realized through the electrostaticinteraction between molecules or nanoparticles. This self-assembly technique
originates from the fabrication of multilayer films where each layer is composed of
positive- and negative-charged colloid particles such as Si and Al. The method is
widespread in the implementation of nanostructured films containing metals,
semiconductors, magnetic materials, polymers, or organic molecules. The resulting
film is uniform and stable due to strong ionic bonds between negative- andpositive-charged particles, and the defects are minimized due to the repulsive
force between the particles involved in the process. The utilization of polymers
in combination with layers of charged nanoparticles is the best way to minimize
the defects. The electrostatic self-assembly process starts with the immersion of
a clean substrate into a cationic solution, followed by a dip of the substrate
coated with cations into an anionic solution where the adsorption of anionicmolecules takes place at a molecular level.For example, positively charged gold
nanoparticles are self-assembled on a negative glass substrate by immersion into 4-
ATP(aminotiophenol)-capped gold solution with pH = 4. A layer of negatively charged
Ag nanoparticles can be further deposited by immersion of the glass coated with Au
particles into a 4-CTP(carboxythiophenol)-capped silver solution with pH = 8.5 Thus, it
is possible to obtain a heterostructure consisting of successive layers of Au and Agnanoparticles.
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Cationic
solutionAnionic
solution
Water
water
RINSE DRY RINSE DRY
Water
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The chemical self-assembly methods are the most commonly used
techniques. Among them, SAM (self-assembled monolayers) and MPC(monolayer-protected clusters) are the most prominent. SAM deals withthe spontaneous formation of monolayers via immersion of a suitablesubstrate into a solution, while MPC refers to nanoclusters whose surface isderivatized by ligand molecules with the help of chemisorption
Biomolecular self-assembly uses DNA or proteins as basic constituents to
realize: 1) biomolecular-metal complexes such as DNA-Au complexes, 2)self-assembly of semiconducting nanoparticles such as CdSe quantum dotensembles, or 3) functionalized metallic nanoparticles.
Au substrate Au
nanoparticleAlkanethiol
molecules
SAM MPC
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DNA origami
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Nanoparticles
The physical properties, and especially the electronic structure of metal nano-
particles, are very different from those of bulk metals and are strongly dependenton their size. For example, the conduction band of bulk metals is replaced by
discrete energy states in a metal nanoparticle. Thus, the metal particle behaves like
a quantum dot, in which electrons are confined in all directions, in contrast with thefree electrons of bulk metals. Metal particles covered with organic molecules such as
thiols are able to self-organize in 1D, 2D and 3D arrays. The discreteness of
electronic states in a metal nanoparticle is characterized by the Kubo gap (i.e., the
average spacing between successive quantum levels)
nE 3/4 Fwhere EFis the Fermi energy, and nis the number of electrons in the nano-
particle. The nanoparticle is metallic if :
TkB
For example, a silver particle with a diameter of 3 nm, for
which = 10 meV, is metallic at room temperature sincekBT
=25 meV for T= 300 K.
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CdS (semiconductor nanoparticles)
Gold substrate
CdS (semiconductor nanoparticles)
Gold substrate
There are not only metal nanoparticles, but also semiconductor nanoparticles
such as silicon nanoparticles, III-V nanoclusters, elemental II-VI semiconductorquantum dots in solution or gaseous phases.
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Nanowires
A nanowire is a metallic, semiconducting, superconducting or magnetic physical system,which is confined in two dimensions. The nanowire has a transverse area of a few nm and
can attain 100300 nm in length, although longer nanowires, up to 1 m, have also been
fabricated. Nanowires can be realized via many methods.
Aluminium
Anodized aluminaSourcematerial
Template synthesis
SiO2
Si Si Si
MetalCracksNanowire array Top
view
Self assembly
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Nanowires with ultra-small diameters are grown using CVD or MOCVD. The nanowire
precursor material is heated to produce vapors that penetrate the nanopores of thetemplate, which is then cooled to get the solidified nanowires. Nearly single-crystal
nanowires are obtained with the CVD method, while in the rest of the above-mentioned
methods mainly polycrystalline nano-wires are fabricated. Single-crystal nanowires of Bi,
GaN, GaAs, and InAs, with diameters less than 10 nm, can be grown with the CVD
techniques. Carbon nanotubes (CNTs), are also grown in alumina templates via CVD
techniques. CNTs with very small diameters, of a few nm, can be grown by replacing the
alumina template with a zeolite template.The vapor-liquid-solid (VLS) nanowire growthmethod is based on the fact that the vapors (V) of the source material can be absorbed into
a liquid (L) droplet of a catalyst. The nanowire is obtained due to the solidification (S) of the
source material as a result of the saturation of the liquid alloy followed by a nucleation
process, which creates a preferential site for further deposition at the liquid boundary.
Growth
direction
Vapors (V)
Liquid catalyst
(L)
Nanowire (S)
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Millions of CNTs can be aligned and integrated using a large-scale assembly technique
inspired from biomolecular self-assembly processes . In this method, chemically
functionalized patterns on a surface are first realized, on which millions of CNTs spread
in a solution are then aligned. Two distinct regions coated via direct deposition with polar
groups and nonpolar groups, respectively, create the functionalization of the surface. By
placing the functionalized surface into a liquid suspension containing millions of CNTs
the nanotubes are attracted by the polar regions and millions of CNTs are aligned in less
than 10 s, The electrostatic attraction force rotates the CNTs towards the polar regionand confines them only in this region. The efficiency of alignment is very high, of about
90%.
Polar region
Gold surface
Rotationtowards polar
region
Nonpolar
region
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Dielectrophoresis(or DEP) is a phenomenon in which a forceis exerted
on a dielectricparticle when it is subjected to a non-uniform electricAll
particles exhibit dielectrophoretic activity in the presence of electric fields.
However, the strength of the force depends strongly on the medium and
particles' electrical properties, on the particles' shape and size, as well as
on the frequency of the electric field. Consequently, fields of a particular
frequency can manipulate particles with great selectivity. This has allowed,
for example, the separation of cells or the orientation and manipulation ofnanoparticles and nanowires.
Procese tehnologice avansate
http://en.wikipedia.org/wiki/Forcehttp://en.wikipedia.org/wiki/Dielectrichttp://en.wikipedia.org/wiki/Electric_fieldhttp://nanotechweb.org/articles/journal/5/10/2/1http://nanotechweb.org/articles/journal/5/10/2/1http://en.wikipedia.org/wiki/Electric_fieldhttp://en.wikipedia.org/wiki/Dielectrichttp://en.wikipedia.org/wiki/Force -
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CNT
interconnect
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Al template
Si nanowire
ZnO
GaN CNT
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