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Nanofabrication
•Top-down Approach; ‘Top-down’ techniques involve starting with a block of material,
and etching or milling it down to the desired shape and size
•Bottom-up Approach: ‘bottom-up’ involves the assembly of smaller sub-units (atoms
or molecules) to make a larger structure
Microelectronics
Self-Assembly
sculptor
LEGO
•Hybrid Approach combines elements of top-down and bottom-up aproaches.
NanotechnologicalProducts
Bottom-upnanofabrication
Atoms, moleculesor nanoparticles
as bulding blocks
Top-downnanofabrication
Breaking downbulk
Hybridnanofabrication
Combining top-down
and bottom-up
Lithography
Nanoimprinting
Mechanical attrition
Micro-machining/MEMS
Etc.
Self-assembly
Atomic Layer Deposition
Electrodeposition
Chemical Vapor Deposition
Etc.
Methods of synthesis
Basic top-down approaches in nanofabrication
Lithography
Patterntransfer
Deposition
(film growth)
Modification
Etching
(removal of material)
The Basic Steps of
Top-down
Nanofabrication
(in any sequence)
Lithography comes from two Greek words, “lithos” which means
stone and graphein which means write. “ writing a pattern on stone”
PHOTOLITHOGRAPHY
A light sensitive photoresist is spun onto the wafer forming a thin layer on the
surface. The resist is then selectively exposed by shining light through a mask
which contains the pattern information for the particular being fabricated. The
resist is then developed which completes the pattern transfer from the mask to
the wafer.
Spin Coating
Photoresist materials are organic compounds whose chemical properties change when
exposed to the UV light.
6
The most commonly used positive resist consist of diazonaphtoquinone (DQ), which is the photoactive compound (PAC), and novolac (N), a matrix material called resin. Upon absorption of UV light, the PAC undergoes a structural transformation which is followed by reaction with water to form a base soluble carboxylic acid, which is readily soluble in basic developer (KOH, NAOH, TMAH etc.)
Spin on photoreist
Align photomask
Expose to UV light
Dissolve exposed photoresist in
liquid developher
Pattern transfer and Substrate Modification
Remove the photoresist (Etch
or Ion Implantation
When the exposed
region become less
soluble in the developer,
the compound is called
negative resist
When the exposed
region become more
soluble in the developer,
the compound is called
positive resist
Wet Etching
– Uses liquid chemistry to chemical react with substrate materials
– For patterned amorphous materials wet etchants produce isotropic etch profiles
– Isotropic features are just as wide as they are deep
Reactive Ion Etching
– Use plasma to ionize gas
– Processing gas is selected for chemical etching of substrate materials
– A negative bias is place on substrate to allows for physical etching from positively
charged gas species.
– The pressure of the system determines the etch profile of the sample
Dry etching
– Use high energetic ions in vacuum.
– High pressure etching or Low pressure etching
Projection printing provides high
resolution and low defect densities and
\ dominates today.
Exposure Systems
Contact printing is capable of high
resolution but has unacceptable
defect densities. Inexpensive,
diffraction effects are minimize
Proximity printing cannot easily print
features below a few m (except for x-
ray systems). Poor resolution due to
diffraction effects, required 1 X mask.
Typical projection systems use reduction optics (2X - 5X), step and repeat or step and
scan mechanical systems, print 50 wafers/hour and cost $5 - 10M.
R < 0.2 m Expensive and complex optic systems
R < 0.5m Inexpensive and mask causes surface defects
Resolution is limited by diffraction
λ is the exposure
wavelength,
NA is the numerical
aperture (NA=n sinθ) of
the projection optics
(collected light by lens)
k1 is a dimensionless scaling
parameter (depends on resist and
procedure (0.5-1.0 in according to
raighleigh scattering) .
k is function of resist & ‘optical
engineering’
AFM cantilevers
Diameter with 40 nm
Why is optical lithography so widely used and what makes it such a
promising method?
• It has high throughput, good resolution, low cost and ease in
operation.
However;
• Due to deep submicron IC process requirements, optical lithography
has limitation that have not yet been solved.
Therefore,
It is required to find alternatives to optical lithography. The possible
promising techniques are:
• Electron beam lithography
• X-ray lithography
• Ion beam lithography
• Extreme Ultraviolet Lithography
10-100 keV electron beam
E-beam lithography
Advantages:• Generation of submicron resist geometries• Greater depth of focus• Direct patterning without using a mask. • Highly automated and precisely controlled
operation (computer control).• Its ability to accurately define small features
(10-20 nm of resolution)• Currently EBL is the Technology of choice for
Mask generation
Disadvantage:• Low throughput (slow writing)• Need high vacuum (10-6-10-10 torr)• Electron scattering on resist and substrate.• Very expensive system (not commercially
viable except a few applications).
Electron Beam Lithography
At 30 keV, electrons travel >14
mm deep into a resist layer
Silicon pillar 60 nm in dimeterand 600nm tall
E-beam resist still on top of the pillars
16
X-ray lithography
Advantages:•No vacuum environment required (no charged particles involved)•Very small wavelength (< 14Å) - can produce 0.15 µm features •High reproducibility (exposure independent of substrate type, surface reflections)•Low diffraction
Disadvantages:•No optics involved – limited to 1:1 shadow printing (no image reduction is possible) •Very expensive and complex mask fabrication (~10 days, cost is $4k-$12k)•Low sensivity of the resists•High cost of sufficiently bright X-ray sources•High energy x-ray destroy conventional optics
Ion beam
Step-and-scan wafer
stage
Electrostatic lens system(4:1 reduction)
Vacuum chamber
Ion source, Ga, Au-Si-Be alloys
Mask
Reference plate
Ion Beam lithographyAdvantages:•Computer-controlled beam •No mask is needed •Can produce sub-1 µm features •Resists are more sensitive than electron beam resists •Diffraction effects are minimized •Less backscattering occurs (Negligible ion scattering in the resist) •Higher resolution•Can also be used as direct deposition or chemical assisted deposition, or doping (etching).
Disadvantages:• Reliable ion sources needed (e.g Gallium)• Swelling occurs when developing negative ion
beam resists, limiting resolution • Expensive as compared to light lithography
systems • Slower as compared to light lithography systems• Tri-level processing required• Charge-space effect• Lower throughput, extensive substrate damage.
Ni nanodots
Interference Lithography
Advantages:• Large area, up to 10 wafer• Regular arrays.
Disadvantages:• Wave lenght limited, like UV• High symetry patters
Line widths of 0.1 mLine widths of 0.2 m
Line widths of 0.1 mRoutinely line widths of 2-3 m
Scanning Probe Lithography
Probe STM, AFM Techniques; Voltage pulse, CVD, Local electrodeposition, Dip-pen
Voltage Plus
STM or AFM CVD
Local anodic oxidation, passivation, localized chemical vapor deposition,
electrodeposition, mechanical contact of the tip with the surface, deformation of
the surface by electrical pulses
Local Electrodeposition
Dip-pen
Diagram illustrating thermal dip pen nanolithography. When the
cantilever is cold (left) no ink is deposited. When the cantilever is
heated (right), the ink melts and is deposited onto the surface.
(Journal of the American Chemical Society, 128(21) pp 6774 -
6775 , 2006)
• Thermal Dip Pen Lithography
Umit Demir, K. K. Balasubramanian, Vince Cammarata and Curtis Shannon,
J. Vac. Sci. Technol. B, 1995, 13, 1294.
AFM Tipi
Soft lithography (Microcontact printing)
Simple and suitable for non-planar substrates!
Xia, Y. N. and Whitesides, G. M. “Soft Lithography”. Annu. Rev. Mater. Sci. 28, 153-184 (1998)
The concept of microcontact printing is use the relief pattern on the surface of a PDMS stamp to form patterns of SAMs on the surfaces of substrates by contact. For example, alkylthiol on Au and Ag surfaces.
Making metal electrodes by microcontact printing
Gerber, R. W. and Oliver-Hoyo, M. T. “Selective Etching via Soft Lithography of Conductive Multilayered Gold Films with Analysis of Electrolyte Solutions”. Journal of Chemical Education 85, 1108-1111 (2008)
An organic material is used as target, stamp or mold to transfer the pattern
Poly(dimethylsiloxane) (PDMS)
Poly(dimethylsiloxanes):
1.A unique combination of properties resulting from the presence of an inorganic siloxane backbone and organic methyl groups attached to silicon. 2.Very low glass transition temperatures and hence are fluids at room temperature. 3.Can be readily converted into solid elastomers by cross-linking.
PDMS:
Dean J. Campbell, Katie J. Beckman
Mechanism of cross-linking
Advantages/Disadvantages/Applications
◼ Advantages
– Low cost
– Little chemistry
– No radiation damage, no diffraction or scattering etc.
– Easily accessible
– Does well with small details
– 3D printing
◼ Disadvantages
– PDMS is a soft structure
– PDMS has a high thermal expansion
– Low throughput
– Slow
◼ Applications
– MEMS Devices
– Sensors
– Micoreactors
– Microfluidics
Top-down Approach?
UV, X-ray, e-beam, ion etc.
Mask
Resist
Substrate
Etch.
Deposition etc
There are some limitations of top-down fabrication
Diffraction effects, Mask, Feature size limitation, Very low throughput etc.
An alternate method is “bottom-up” fabrication.
Bottom-up Approachs
Atoms, ions and molecules are
manipulated and combined to form larger
nanoscale structures as in
chemical and biological systems
More extreme example: Self-replicating robots !!
Building Block Fabrication(molecules, macro molecules,
particles, layers etc.)
Molecular Synthesis
Colloidal Chemistry
Physical Fabrication Methods
Chemical Vapor Deposition
Electrodeposition
Assembly
Chemical assembly
Physical assembly
Biological assembly
Sequence may be repeated many times
Molecules Synthesized for Linking
Molecules made with tail group (SH)
to give functionality; i.e., used to
dictate what molecule will bound.
Head group can then
dictate what molecule links
to at its other end. Body of
molecules
Longer alkanethiol
molecules have greater
thermodynamic stability
What drives and governs
self assembly?
• Forces of chemical bonding
• covalent, ionic, van derWaals, hydrogen
• Other forces (magnetic, electrostatic, fluidic, ...)
• Polar/Nonpolar (hydrophobicity)
• Shape (configurational)
• Templates (guided self assembly)
• Kinetic conditions (e.g., diffusion limited)
Tile assembly example
5
4
3
2
1
Self assembly mechanisms are inherent within the structures
Self assembly occurs without any external forces or controls
i.e. crystals
Macromolecules Are Built By Linking a Set Of Building Blocks
(Monomers) Together Into Long Chains (A Polymer).
In Biology, Shape Matters
Its not just chemical formula, it’s the shape of the molecule that lets it
do its “job”.
Never forget the axiom – structure dictates function.
Some biological molecules.
Surfactant: Hydrophilic Head Example: Phospholipid
+ Hydrophobic Tail
Micelle: Inverse Micelle:
Heads outside, Water outside Heads inside, Water inside
A nanoscale chemical beaker
with aqueous solution inside
DNA Self-assembly
Biological Self Assembly
DNA is used to link gold nanoparticles together
F. Huo et al., Adv. Mater. 2006, 18, 2304–2306
Colloidal Chemistry
The general procedure involves chemical reactions in aqueous or nonaqueous solutions
containing soluble or suspended salts, which are one of the precursors.
Once the solution becomes supersaturated with the product of the chemical reaction
involving the precursors, the precipitate (nanoparticles) start to form by nucleation and
growth.
By setting up carefully controlled reactions in a solution, atoms and molecules can be
assembled, using colloidal chemistry, to make nanoparticles.
It is a great way to make nanoparticles such as metal nanoparticles and quantum dots.
Colloidal chemistry methods are by far the most commonly used to produce low cost
bulk quantities.
Electrodeposition
The deposition of a substance on an electrode by the action of electricity
Molecular Beam Epitaxy
Epitaxy: Deposition and growth of monoctystalline structures/layers
Epitaxial growth results in monocrystalline layers differing from deposition which gives rise to
polycrstalline and bulk structures
• Vapor-Phase Epitaxy (VPE)
• Liquid-Phase Epitaxy (LPE)
1. Trimethyl Aluminum (TMA) react with the adsorbed hydroxyl
groups.
2. The excess TMA is pumped away with the methane reaction
product
3. Water vapor (H2O) is pulsed into the reaction chamber.
4. H2O reacts with the dangling methyl groups on the new surface
forming aluminium-oxygen bridges and hydroxyl surface groups.
5. The reaction product methane is pumped away.
6. One TMA and one H2O pulse form one cycle and one monolayer
of Al2O3
Al2O3 growth
Atomic Layer Deposition (ALD)
ALD is a method of applying thin films to various substrates with atomic scale precision.
1. Self-Assembling 2. Patterning 3. Electrodeposition
400nm 400nm
Madueno R, Raisanen MT, Silien C, and Buck M (2008), Nature 454: 618–621.
3-butoxy-4-methylthiophene, or BuOMT,
Diamond
3D
sp3
Graphite, Graphene (= single
sheet)
2D
sp2
Nanotube
1D
Fullerene
0D
Carbon Structures
Formation of fullerenes during cooling of the plasma.
Carbon clusters smaller than C60 are often short chains.
C60 solution
in toluene
Buckminsterfullerene C60 has the same hexagon + pentagon pattern
as a soccer ball. The pentagons (highlighted) provide the curvature.
Fullerenes
Buckminster Fuller,
father of the geodesic dome
Top-Down ve Bottom-up
Electrochemical etching – oxide
2H2O → O2 + 4e- + 4H+
Ti + O2 → TiO2
Chemical dissolution – formation of pits
TiO2 + 6F- + 4H+→ TiF6
2- + 2H2O
Top-Down ve Bottom-up Methot
palindromic bistable rotaxane
Oxynaphthalene (ONP)
Molecular Muscle