Nanofabrication...Nanofabrication •Top-down Approach; ‘Top-down’techniques involve starting...

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