Chemistry in Nanomanufacturing

(4th Year Undergraduate Module)

Erjia Liu

School of Mechanical & Aerospace Engineering

Nanyang Technological University (Singapore)

210th 2YC3, Kaneohe, Hawaii, 22 May 2015

About NTU, Singapore

• 6 colleges (14 schools)• 23,500 undergraduates

• 4,300 teaching & research staff• 10,000 graduate students

http://www.ntu.edu.sg/ 2

School of Mechanical & Aerospace Engineering (MAE), College of Engineering

• 230 teaching & research staff • 650 graduate students

• 3,000 undergraduates• 30 laboratories/centres

http://www.mae.ntu.edu.sg/ 3

MAE Undergraduate Programmes

Bachelor of Engineering (Mechanical Engineering)

Bachelor of Engineering (Aerospace Engineering)

Full-Time Undergraduate Programme

BEng(Mech) – Mainstream

BEng(Mech) – Design stream

BEng(Mech) – Mechatronics stream

Part-Time Undergraduate Programme

BEng(Mech) – Mainstream

Full-Time Undergraduate Programme

BEng(Aero) – Mainstream only

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ManufacturingEngineering

Biomedical Engineering

SystemsEngineering

AeronauticalEngineering

Marine & OffshoreEngineering

InnovativeDesign

Mechatronics & Control

Energy & theEnvironment

Mechanical Engineering (Mainstream)

Mechanical Engineering (Mainstream)

Final Year Specialisations

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• Production of nanoscaled materials, e.g. powders or fluids.

• Manufacturing of parts via "bottom up" from nanoscaled materials or "top down" in smallest steps for high precision.

• Industrial-scale manufacturing of nanotechnology-based objects with the emphasis on low cost and reliability.

• Another related term: Nanofabrication - to fabricate, by directed or self-assembly

methods, functional structures or devices at the atomic or molecular level.

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What Does Nanomanufacturing Do?

Scope of Module

• Designed to give the students an appreciation of the latest nanomanufacturing technologies.

• For examples:- In the medical device industry, it enables extreme

miniaturization and the development of new types of product.- In the aerospace industry, it enables the development of

products based on novel nanomaterials.• This module covers following topics:

- Photolithography- Nanolithography- Self-assembly- Chemical vapour deposition, and- Synthesis of zeolite

7

1 m 1 nm 1 Å0.13 m

He-

Ne

lase

r (6

33 n

m)

Res

olut

ion

of S

PM

Scale Ranging from Micrometer to Angstrom

Diameter of the earth: 12,756 km 10-9 = 1.3 cm 8

Bacteria VirusDNA

Compound

Structure

Molecule

Atoms

Research Areas in Nanotechology

• Nano/quantum electronics • Nanostructured materials • Scanning probe microscopy • Molecular nanoelectronics • Molecular nanotechnology • Computer modeling • Mesoscopic physics/technology• Supramolecular chemistry • Cluster/mesoscience • Nanoelectromechanical system (NEMS)• Nanomedicine• Nanometrology (nanoruler/guide/drive/probe)• Nanoimprinting• Self-assembly• Others…

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Nanomanufacturing Processes

Nanolithography Vapor deposition • E-beam (1.2 nm).

• Atom beam, e.g. Li, Na, K, Rb, Cs.

• Ion beam, e.g. Ga+.• Electromagnetic

radiation, e.g., X ray (=0.1-10 nm),

EUV (extreme UV, =10-70nm),

DUV (deep UV, =193 or 248 nm).

• Laser beam (frequency doublers and quadruplers, ~150 nm).

• Chemical vapor deposition.

• Physical vapor deposition.

• Epitaxial growth of single crystal layers.

• Synthesis of carbon nanostructures.

• Synthesis of ceramic coatings.

Self-assembly & Scanning

Probe• SPM.• Dip pen

lithography.• Self-

assembled monolayer.

• Self-assembled quantum dot.

Other Methods

• Compaction.• Consolidation.• Melt spinning.• Multilayer.• Etching.• Synthesis of

zeolite.

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Two Approaches in Nanomanufacturing

(1) Top-down approach:

• Starts with a large-scale object or pattern and gradually reduces its dimension(s).

• Relies on the removal, division and assembly of bulk materials.

Top-down approach

Lithography

0.1 μm

1 nm

1 mm

1 μm

0.1 nm

Methods for top-down approach:• Nanomachining• Nanolithography:

- X-ray

- E-beam

- Ion-beam

- Soft11

Methods for bottom-up approach:

• Self-assembly• SPM based methods• Molecular beam epitaxy

(Cont’d)

(2) Bottom-up approach:

• Widespread in biology.

• To collect, consolidate and fashion individual atoms, molecules or even nanoparticles as building blocks to create complex nanostructures.

Bottom-up approach

Self-assembly

0.1 μm

1 nm

1 mm

1 μm

0.1 nm

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Prepare surface

Apply photoresist

Soft bake Align mask and expose

Develop resist Hard-bake Etch Strip resist

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Photolithography Process

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Photoresist Spin Coating• A wafer is held on a spinner chuck by vacuum and a photoresist layer is

coated on the wafer to a uniform thickness by spin coating.• Typically 3,000-6,000 rpm for 15-30 s.

Resist thickness, t = kp2/1/2

Photoresist

A liquid mixture, consisting of 3 parts:• Resin:

“Plastic-like” or “glue-like” compound that is solid in its undiluted state, e.g., novolac, methyl-methacrylate.

• Solvent:Chemical used to dissolve the resin, allowing the resin to be applied in a liquid state, e.g., KOH, C3H6O (acetone).

• Photoactive Compound (PAC):Inhibitor or promoter to dissolution of the resin in developer, e.g., DNQ (diazonaphthoquinone). E.g., it inhibits dissolution in positive resists before light exposure. After exposure the PAC promotes dissolution of the resin.

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Methyl-methacrylate

Diazonaphthoquinone

Novolac

Two Types of Photoresist

• Positive: exposed area is removed by developer.• Negative: unexposed area is removed by developer.

Mask

Positive photoresist

Negative photoresist

Cr

Resist

Resist

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(Cont’d)

Prebake

• To evaporate the coating solvent and densify the resist layer.

• 75-85 °C for 45 s on a hot plate.• The resist thickness is usually decreased by

25%.

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Alignment and Exposure

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2 operation modes:• contact for

exposure• separate for

alignment

Less wear on mask, but poorer image than from a contact aligner

Image optics is used in between mask and wafer

Postbake

• To stabilize and harden the developed photoresist layer.

• Photoresist undergoes plastic flow with time and temperature.

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Etch-back20

(a)

(b)

(c)

(d)

Two Primary Techniques for Patterning

(a)

(b)

(c)

(d)

Lift-off

• The art and science of etching, writing, or printing at the microscopic level.

• Used during the fabrication of leading-edge semiconductor integrated circuits at the atomic level (nanocircuitry) and NEMS.

Nanolithography

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Electron Beam Lithography

Used to create a pattern with line widths of a few nm on a polymer thin film with E-beam energy = 1-20 keV, wavelength 1.2 nm, and width 5 nm.

Challenges:• E-beam instruments are expensive (a few M$ per system)• Impractical for large-scale manufacturing, because it works

just like copying a manuscript by hand, one line at a time, e.g.- a few hours per Si wafer with E-beam lithography, but - a few minutes per Si wafer with photolithography.

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X-ray or EUV Lithography

Used to create a nanoscale pattern with minimised blurring of nanofeatures due to shorter wavelengths (X-ray: = 0.1-10 nm; EUV radiation: = 10-70 nm)

Challenges:• Conventional lenses used for UV light are not

transparent to EUV light and cannot focus x-ray either.• Energetic X-ray and EUV radiations (E = hc/) rapidly

damage many of the materials used in masks and lenses.

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Initial quantum well

Irradiation through template

After dissolving irradiated resist

Deposition of metal mask

After removal of remaining photoresist

After removal of unwanted Q-well material

After removal of metal mask

Positivephotoresist

(1) (2) (3)

(4) (5) (6) (7)

Quantum Dot or Wire by Nanolithography Template

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(b) After lithography: A 24-Q-dot array consisting of six columns, each containing four stacked Q-dots.

More Complex Quantum Structure by Nanolithography

(a) Before lithography:A multiple Q-well structure having a resist atop it and a template with 6 holes located above it.

(a)

(b)25

Soft Lithography – Nanotemplate

• Firstly, a bas-relief master is directly produced using E-beam lithography, in which islands of photoresist layer stand out from Si substrate: BAS-RELIEF MASTER

• A liquid PDMS (polydimethylsiloxane) is poured over the master and cured into a PDMS stamp that matches the original pattern with astonishing fidelity.

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Soft Lithography – Microcontact Printing• The PDMS stamp is inked with a reagent solution consisting of

organic molecules, e.g. thiols.• The inked stamp is brought into contact with an Au thin film

deposited on a glass, Si or polymer substrate.• The thiols react with the Au surface, forming a highly ordered

self-assembled monolayer (SAM) film that replicates the stamp’s pattern.

• Features < 50 nm can be fabricated using this method.

(Cont’d)

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• The PDMS stamp is placed on a hard surface and a liquid polymer flows by capillary action into the recesses between hard surface and stamp.

• The polymer then solidifies into the desired pattern. This technique can replicate structures < 10 nm.

Soft Lithography – Micromolding

(Cont’d)

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Dip-pen Lithography• A metal nanoprobe is surrounded by a meniscus of water

(capillary action) that contains chemical species. • Alternations to the sample surface are accomplished via specific

or controlled- either oxidation of a Si surface if the solution contains

hydroxyl groups (OH-), - or self-assembly (SA) to form a monolayer on a gold surface

if the solution contains alkane thiols (CH3(CH2)nSH). • In either case, the surface is altered to form a nanostructure.

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Dip-pen Lithography – Oxidation

• A scanning probe is operated in air and biased (between tip and sample) at 2-10 V.

• Moisture in air forms a water meniscus at the contact point, which serves as an electrolyte.

• The biased tip anodically oxidizes a small region of the sample surface.

Water electrolysis at tip (cathode):2H+(aq) + 2e- H2(g)

Water electrolysis at sample (anode):2H2O(l) O2(g) + 4H+(aq) + 4e-

Ti sample is locally oxidized as:Ti(s) + 2H2O(l) TiO2(s) + 4H+(aq) + 4e-

Ti

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Oxidation conditions: • voltage on tip: -10 V• impulse duration: 20 ms

AFM images of 10 nm Ti film on Si substrate after local anodic oxidation

(Cont’d)

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Dip-pen Lithography – Self-assembled Monolayer (SAM)

Dip-pen lithography can use different types of molecules as “inks” and thus brings great chemical flexibility to nanoscale writing.

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• Spontaneous formation of a desired structure of molecules in a thermodynamic equilibrium state with or without a catalyst.

• Weak, non-covalent bonding (e.g. H bonds and van der Waals forces) between parts of molecules without any central control.

• Self error-checking during the growth.• Initial molecules or subunits are small in size and easy

to synthesize.

Self-assembly (SA)

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Self-assembled Monolayer (SAM)

3 building groups: • head group: binds strongly

to the substrate surface, • surface terminal group:

constitutes the outer surface of the layer,

• spacer chain: connects the head and tail groups.

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• Head groups (S) bind strongly to the Au surface (S-Au chemical bond: ~44 kcal/mol).

• Tail groups (CH3) constitute the outer surface of the monolayer. • Backbones ((CH2)n) connect the head & tail groups (van der

Waals forces: ~1.75 kcal/mol).

CH3(CH2)nSH + Aum CH3(CH2)nS-(Au+3).Aum-3 + (½)H2

Au (111) surface

CH3(CH2)nS(thiolate)

Au

CH3

S

(CH2)n

Au (111) surface

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SAM of Thiol on Au (111) Substrate

SA of Quantum Dot

(1) Cadmium ions, selenium ions and organic molecules are brought together.

(2) Organic molecules act as surfactants, binding to the surface of CdSe crystal as it grows.

(3) When CdSe crystal reaches its optimum size, organic molecules coat its surface in a stable packing.

Se2-

Organicmolecule

Cd2+

CdSe36

heated

RF driven

• A plasma (RF or DC) is created in the vicinity of the substrate.• Energetic ions impart their energy and momentum on the reactant gas

molecules and atoms.• Energy transfer breaks up the molecules and aids the chemical reactions.

Chemical Vapor Deposition (CVD)It involves interaction between mixed gases and heated substrate surface, causing chemical decomposition of gas constituents and formation of a solid film on the substrate.For example, PECVD (plasma-enhanced CVD):

• Substrate sits directly on heated electrode.

• Operating pressure: 0.5-1.0 Torr.• Power density: 0.02-0.1 W/cm2.

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Synthesis of Carbon Nanotube (CNT) by CVD

• CVD is the most favourable method for scale-up production of CNTs.

• SWCNT (single-walled CNT)– CH4 (methane) as the

hydrocarbon– Temperature: 850 – 1000 °C– Ni, Co or Fe nanoparticles as

catalyst• MWCNT (multi-walled CNT)

– C2H4 (ethylene) or C2H2 (acetylene) as the hydrocarbon

– Temperature: 550 – 700 °C– Ni or Co nanoparticles as catalyst

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(a)

Types of SWCNT:a) zig-zagb) armchairc) chiral

• Generally closed at both ends. • Almost all C atoms are only

involved in hexagonal aromatic rings in cylinder walls.

• Some pentagonal rings are found at the ends.

• C=C bonds in aromatic rings are no longer planar as they should ideally be.

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SWCNT

MWCNT

An easiest form to imagine is the concentric type, in which SWCNTs with regularly increasing diameters are coaxially displayed.

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• Porous materials, like cubic mineral faujasite: (Na2,Ca)(Al2Si4)O12•8H2O.

• The pores have a regular arrangement in space.

• The pores are large enough to accommodate small clusters by injection of guest clusters in molten state.

• The filled clusters are stabilized in pores by van der Waals interactions between clusters and zeolites. Array of nanoparticles in zeolite

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Synthesis of Zeolite

• It has long parallel channels running through it.

• Selenium atoms (dSe = 0.28 nm) can be incorporated into these channels, forming the chains of single atoms.

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1D or 2D Zeolite - Mordenite

a) assembly of colloidal particles into a regular array (template),

b) impregnation of the template with monomers followed by polymerization,

c) removal of the template.

(Wiley-VCH)

Synthesis of Zeolite from Rigid Colloidal Template

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• Zeolite is nucleated between C particles.

• Mesopores are sufficiently large, and the gel is sufficiently concentrated to allow the growth to continue within the pore system until the mesopores are fully filled.

Growth of Zeolite from Pore System of C Nanoparticles

Carbon particles, ~12 nmPores created by combustion of

C particles

Mesoporous zeolite single crystal

Zeolite crystal, ~1 µm, grown in pore system of C

O2

+ CO2

823 K

• By oxidation of C at high T, C particles are removed and zeolite single crystal is formed.

(ACS)

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Summary

• Scope of module• Two approaches in nanomanufacturing• Photolithography• Nanolithography• Self-assembly (SA)• Chemical vapour deposition (CVD)• Synthesis of zeolite

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References

• C.P. Poole and F.J. Owens, Introduction to nanotechnology, WILEY-Interscience, 2003.

• M.P. Groover, Fundamentals of Modern Manufacturing, John Wiley & Sons, Inc., 2nd ed. (2002).

• Silicon VLSI Technology by Plummer, Deal and Griffin.• Lecture Notes: MA4838 Non-conventional Manufacturing

Processes, E. Liu, NTU.• Lecture Notes: MP4004: Advanced Manufacturing and

Nanotechnology E. Liu, NTU.• Lecture Notes: EE-527: MicroFabrication, R.B. Darling.• Lecture Notes: ECE 6450: Photoresists and Non-optical

Lithography, A. Doolittle, Georgia Institute of Technology.• http://en.wikipedia.org/wiki/Photolithography• http://en.wikipedia.org/wiki/Chemistry_of_photolithography• http://en.wikipedia.org/wiki/Chemical_vapor_deposition• http://en.wikipedia.org/wiki/Nanolithography• http://en.wikipedia.org/wiki/Nanomanufacturing

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Thank You!