Nanomaterial Synthesis Method

Nanomaterial Synthesis Method

Nanoscience and nanotechnology

Ri-ichi Murakami

The University of Tokushima

Nanomaterial Synthesis Method

There's Plenty of Room at the BottomBy Richard Feyman in 1959

Nanotechnology application in nowadays

Targeted drug deliverySuper nano-capacitors

CNT Transistor


Outline

Emergence and Challenges in Nanotechnology

Bottom-Up and Top-Down Approaches

Introduction to synthesis of nanoparticles

Evaporation and Condensation growth

Lithography technology

Method to nano composite structure


Emergence of Nano

• Moore’s Law

Moore’s Law plot of transistor size versus year

Original contact transistor1947~cm

Transistor in Integrated circuitNowadays~micrometer

CNT TransistorFuture~nanometer

To meet the Moore’s Law, the size of transistor should be decreased


Emergence of Nano

• In our life1. LED for display2. PV film3. Self-cleaning window4. Temperature control fabrics5. Health Monitoring clothes6. CNT chair7. Biocompatible materials8. Nano-particle paint9. Smart window10. Data memory11. CNT fuel cells12. Nano-engineered cochlear

The nanotechnology is changing our life, but not enough.Energy crisis, environmental problem, health monitoring, Artifical joints


Challenges in Nano

• Atomic scale imaging

Understand and manipulate the target in nano scale

LaSrMnO and SrTiO superlattice

TEM in biology


Challenges in Nano

• Interdisciplinary Investigation

Nano drug delivery

Protein TEM image

Nano mechanics

Biology&

Medicine

Physics&

Chemistry&

Materials

Mechanics&

Electronics

Nano



Approaches







Approaches

• Obviously there are two approaches to the synthesis of nanomaterials and the fabrication of nanostructures:

• Top-down

• Bottom-up

Lithography

Self-assembly



Synthesis of Nanoparticles








• Homogeneous nucleation A solution with solute exceeding the solubility or supersaturation possesses a high Gibbs free energy, the

overall energy of the system would be reduced by segregating solute from the solution.

G △G

△T

GVS

GVL

TmT*

At any temperature below Tm there is a driving force fro solidification.

G: Gibbs free energy

△G: Driving force for solidication



• Homogeneous nucleation

For nucleus with a radius r > r*, the Gibbs free energy will decrease if the nucleus grow. r* is the critical nucleus size, G* is the nucleation △barrier.



• Synthesis of metallic nanoparticlesInfluences factors

Differenct reagentsA:sodium citrate B: citric acid

A B

A weak reduction reagent induces a slow reaction rate and favors relatively larger particles.

ConcentrationA: 0.25M AgNO3

B: 0.125M AgNO3

A B

A large precursor concentration induces a large critical radius and favors relarively larger particles.

Other factors: the surfactants, polymer stabilizer, temperature, ect

The details about the synthesis of nanoparticles via chemical method would be introduced by other professors in this lecture.



Evaporation and Condensation








• The evaporation and condensation are the fundamental phenomena in preparing thin films with nano meters thickness.

Substrate

Condensation

Source

vapor

energy

Evaporation

If a condensible vapor is produced by physical means and subsequently deposited on a solid substrate, it is called physical vapor deposition.If a volatile compound of a material react, with or without other gases, to produce a nonvolatile solid film, it is called the chemical vapor deposition.Although both are nonequilibrium processes, the kinetics and transport phenomena are the fundamental theory.



• The Kinetic theory Let’ s start with the equilibrium process.

2

Pz

mkT

AdsorptionCondensation

Substrate

The impingement rate:

the number of collisions per unit area per second that a gas makes with a surface, such as a chamber wall or a substrate

Supersaturation condition:

1 0( )

i

eq sub

jS

z T

P, the gas pressure;m, the particle mass; k, Boltzmann’s constant, 1.38×10-23 J/K; T, the temperature

ji, incident fluxTsub, temperature of substrate

The substrate should be placed at relactively low temperature to meet the supersaturation condition.The impingement rate indicates the equilibrium process between evaporation and condensation.



• The vapor source The vapor is usually produced from a effusion cell, rather than a open system, therefore, we can solve the flow

density from the implingement rate.

zAJ

Tsource Peq

J: flow densityA: area of the leakz: implingement rate

On a certain angle cos

4avn v

J

Source

substrate

The angle distribution is important for a co-sputtering condition.

Co-sputtering



• The vapor sourceIf we use a beer can as source material, what vapor will we obtain? Al 97.7%

Mg 1%Mn 1.3%Consider the the implingement rate

2

Pz

mkT

beer can

Diffusion cell at 900 K

Mn atomAl atom

Mg atom

Alloy source

Al, Mg, Mn have different atomic mass.Al: 0.0001%Mn: 0.01%Mg:99.99%

It is not practical to use a congruent evaporation temperature to deposit a compound (or alloy) film from a compound (or alloy ) film with a certain stoichiometric.

This result is obtained under consideringt the adsorption and desorption effect.


Evaporation

• How to get the stoichiometric vapor

Flash Evaporation

AC

Heater

substrate

Flash Evaporation

Flash evaporation utilizes very rapid vaporization, typically by dropping powders or grains of the source material onto a hot surface. The vapor condenses rapidly onto a relatively cold substrate, usually with the same gross composition as that of the source material.

The substrate was placed at a temperature that was a supersaturation temperature for each component.


Evaporation

• How to get the stoichiometric vaporE-Gun

AC

substrate

Molten End

E-Gun

Rod-Fed Source

e-

In a rod-fed source, typically an electron-beam-heated evaporator, the source material evaporators from the molten end of the rod. The rod advances as material is lost from the molten end. In steady state, the composition of the vapor stream must equal that of the rod. This requires that the molten end be enriched in the less volatile component. The adjustment is automatic, since diffusion in the liquid state is rapid.


Evaporation


Coevaporation

substrate

A B

EffusionCells

Co-evaporation

T1 T2

T3

The covaporation with the three-temperature method has been an effective technique for the compositionally accurate deposition of compound semiconductor films whose components’ vapor pressure differ greatly. It was the forerunner of molecular beam epitaxy (MBE).


Evaporation


SputteringSputtering of certain materials, whose ejected particles are molecules, was utilized to obtain a stoichiometric vapor.• Direct current sputtering• Direct current reactive sputtering• Radio-frequency sputtering


Evaporation

• The evaporation source The simplest sources to produce vapors of materials may be thermal sources. These are sources

where thermal energy is utilized to produce the vapor of the evaporant material. Even when the energy that is supplied to the evaporant may come from electrons or photons, the vaporizing mechanism may still be thermal in nature.

quasiequilibrium

nonequilibrium

EvaporationSources

Effusion cell

Effusion cell


Evaporation source

• Ideal Effusion CellδA

aorificeL

Liquid

Gas, Peq

Lbody

1. The liquid and vapor are in equilibrium within the cell. Pliq=Pvap, Tliq=Tvap, Gliq=Gvap

2. The mean free path inside the cell is much greater than the orifice diameter.λ>>aorifice

3. The orifice is flat.4. The orifice diameter is much less than the

distance to the receiving surface.5. The wall thickness is much less than the

orifice diameter. L<<aorifice

How to design a effusion cell


Evaporation source

• Near-ideal Effusion cellIt is impossible to design an ideal effusion cell

Direct

Re-emitted

L

Liquid

Gas, Peq

Lbody Lbody

With a thick orifice lid, diffuse and specular reflection off the sidewalls are possible.

It is the restriction due to the long cell body that cause a nonequilibrium behavior of vapor.


Evaporation source

• Open-Tube Effusion Cell

Figure 2.56

a

A quasiequilibrium source An open-tube effusion cell

L

The relative beam intensity of the open-tube effusion cell calculated for various tubelength-to-tube radius ratios (L/a)


Evaporation source

• E-Gun

A target anode is bombarded with an electron beam given off by a charged tungsten filament under high vacuum. The electron beam causes atoms from the target to transform into the gaseous phase. These atoms then precipitate into solid form, coating everything in the vacuum chamber (within line of sight) with a thin layer of the anode material.


Evaporation source

• Pulsed Laser Deposition

A high power pulsed laser beam is focused inside a vacuum chamber to strike a target of the material that is to be deposited. This material is vaporized from the target (in a plasma plume) which deposits it as a thin film on a substrate.


Evaporation source

• Sputtering• In sputtering, energetic ions

from the plasma of a gaseous discharge bombard a target that is the cathode of the discharge. Target atoms are ejected and impinge on a substrate, forming a coating.


Evaporation source

• Plasma-enhanced chemical vapor deposition

Plasma-enhanced chemical vapor depostion is a process used to deposit thin films from a gas state (vapor) to a solid state on a substrate. Chemical reactions are involved in the process, which occur after creation of a plasma of the reacting gases. The plasma is generally created by RF (AC) frequency or DC discharge between twoelectrodes, the space between which is filled with the reacting gases. A plasma is any gas in which a significant percentage of the atoms or molecules are ionized. Fractional ionization in plasmas used for deposition and related materials processing varies from about 10−4 in typical capacitive discharges to as high as 5–10% in high density inductive plasmas.


Condensation

• Condendation is the change of the physical state of matter from gaseous phase into liquid phase or solid phase, and the reverse is vaporization.

condensation re-evaporation

adsorptionat special sit

surface diffusion

nucleation

Inter diffusion

film growth

Adsorption of atoms from gaseous phase Cluster formation Critical size islands growth Coalescence of neighboring islands Percolation of islands network Continuous film growth

film


Condensation

• Adsorption

physisorption

chemisorption

gas

substrate

Van der Waals force

chemical bond

re-evaporationtransition

It is defined as chemisorption coefficient that he fraction of adsorbated atoms transferred from physisorption into chemisorption but not re-evaporated.

An critical condition is that the adsorption is equall to the reevaporation.Only the atoms adsorpted on the substrate and condensed, grow bigger the critical radius, then the film would be deposited.


Condensation

• Condensation coefficient

substrate

incident flux

re-evaporation

condensation

The fraction of the incident flux that actually condenses

c c ij a j

ji: the incident flux densityac: the condensation coefficientjc: the condensation flux


Condensation

• Deposition RateGrowth speed

a5.430A

Si

cubic lattice parameter, 5.430 A

8 atoms per conventional unit cellThe volume per unit cell, (5.430 A)3=160.10 A3

The particle density, 8/(160.10 A3)=0.05 A-3

The growth speed2

3

0.703 /14.06 /

0.05c

nf

j A sv s

n A

cn

f

jv

n

The deposition rate, or the growth speed

jc, the condensation fluxnf, the particle density, how many particles per volume

An example


Condensation

• Growth mode


Condensation

• Non-epitaxial growth For most film-substrate material combinations, film grow in the Volmer-

Weber (VW) mode which leads to a polycrystalline microstructure.


Condensation

• Epitaxial growth---molecular beam epitaxy Molecular beam epitaxy is a technique for epitaxial growth via the interaction

of one or several molecular or atomic beams that occurs on a surface of a heated crystalline substrate.


Condensation

• Epitaxial growth-Atomic layer deposition

based on the sequential use of a gas phase chemical process.


Condensation

• Monolayer monitoring---RHEED Reflection high energy electron diffraction, an extremely popular technique

for monitoring the growth of thin films.

In RHEED, electrons beam strikes a single crystal surface at a grazing incidence, forming a diffraction pattern on a screen. Electrons with tenth of KeV order energy are focused and incident onto the surface. Then, electrons are scattered by the periodic potential of the crystal surface, which results in a characteristic diffraction pattern on the screen. The diffracted intensity is displayed directly on a screen, so the information is available instantly, i.e, real-time analysis is possible. Further, RHEED arrangement in UHV chamber allows it to be used for in-situ observation of MBE thin film growing process.


Methods for deposition

Method ALD MBE CVD Sputtering Evapor PLD

Thickness Uniformity good fair good good fair fair

Film Density good good good good fair good

Step Coverage good poor varies poor poor poor

Interface Quality good good varies poor good varies

Low Temp. Depostion good good varies good good Good

Deposition Rate fair fair good good good Good

Industrial Application varies varies good good good poor



Lithography







Lithography

• We have discussed various routes for the synthesis and fabrication of variety of nanomaterials; however, the synthesis routes applied have been focused mainly on the chemical methods approaches, or the physical vapor deposition. Now, we will discuss a different approach: top-down approach, fabrication of nanoscale structures with various physical techniques---lithography.


Lithography

Lithographic techniques

(a)Photolithography

(b)Phase shifting opitcal lithography

(c)Electron beam lithography

(e)Focused ion beam lithography

(f) Neutral atomic beam lithography

Nanomanipulation and nanolithography

(a)Scanning tunneling microscopy

(b)Atomic force microscopy

(c)Near-field scnning optical microscopy

(d)Nanomanipulation

(e)Nanolithography


Photolithography• Typical photolithographic process consists of producing a mask

carrying the requisite pattern information and subsequently transferring that pattern, using some optical technique into a photoactive polymer or photoresist.


Photolithography

• Wafer preparation---cleaning Typical contaminants that must be removed prior to photoresist coating: •dust from scribing or cleaving (minimized by laser scribing) •atmospheric dust (minimized by good clean room practice) •abrasive particles (from lapping or CMP) •lint from wipers (minimized by using lint-free wipers) •photoresist residue from previous photolithography (minimized byperforming oxygen plasma ashing) •bacteria (minimized by good DI water system) •films from other sources: –solvent residue –H2O residue –photoresist or developer residue –oil –silicone Standard degrease: – 2-5 min. soak in acetone with ultrasonic agitation – 2-5 min. soak in methanol with ultrasonic agitation – 2-5 min. soak in DI H2O with ultrasonic agitation – 30 sec. rinse under free flowing DI H2O – spin rinse dry for wafers; N2 blow off dry for tools and chucks• For particularly troublesome grease, oil, or wax stains: – Start with 2-5 min. soak in 1,1,1-trichloroethane (TCA) or trichloroethylene (TCE) with ultrasonic agitation prior to acetone


Photolithography

• Wafer preparation---primers Adhesion promoters are used to assist resist coating. Resist adhesion factors: •moisture content on surface •wetting characteristics of resist •type of primer •delay in exposure and prebake •resist chemistry •surface smoothness •stress from coating process •surface contamination Ideally want no H2O on wafer surface – Wafers are given a “singe” step prior to priming and coating •15 minutes in 80-90°C convection oven Used for silicon: – primers form bonds with surface and produce a polar (electrostatic) surface – most are based upon siloxane linkages (Si-O-Si) •1,1,1,3,3,3-hexamethyldisilazane (HMDS), (CH3)3SiNHSi(CH3)3 •trichlorophenylsilane (TCPS), C6H5SiCl3 •bistrimethylsilylacetamide (BSA), (CH3)3SiNCH3COSi(CH3)3


Photolithography

• Photoresist Spin Coating

• Wafer is held on a spinner chuck by vacuum and resist is coated to uniform thickness by spin coating.• Typically 3000-6000 rpm for 15-30 seconds.• Resist thickness is set by: – primarily resist viscosity – secondarily spinner rotational speed• Resist thickness is given by t = kp2/w1/2, where – k = spinner constant, typically 80-100 – p = resist solids content in percent – w = spinner rotational speed in rpm/1000• Most resist thicknesses are 1-2 mm for commercial Si processes


Photolithography

• Prebake

Used to evaporate the coating solvent and to densify the resist after spin coating. • Typical thermal cycles: – 90-100°C for 20 min. in a convection oven – 75-85°C for 45 sec. on a hot plate • Commercially, microwave heating or IR lamps are also used in production lines. • Hot plating the resist is usually faster, more controllable, and does not trap solvent like convection oven baking.


Photolithography

• Align/Expose/Develop


Photolithography

• Etching/remove photoresist

photoresist has same polarity as final film; photoresist never touches the substrate wafer.


Photolithography

• Etching/remove photoresist

photoresist has opposite polarity as final film; excess deposited film never touches the substrate wafer.


Phase-shifting Photolithography

• Photolithography has a resolution limit. In order to improve the resolution in photolithography, the phase-shifting method was developed.


E-beam lithography• The theoretical resolution of photolithography is

)2

(32 min

dsb

The wavelength of the exposing radiation

s The gap width maintained between the masi and the photoresist surface

d The photoresist thickness

The wavelenght of electron beam is much smaller than UV light, electron beams can be focused to a few nanometers in diameter and can be deflected accurately and precisely over a surface.


E-beam lithography

• Resist film Negative resist: After development, the

exposed structure is higher than the surrounding due to crosslinking of polymer chains.

Positive resist: After development, the exposed structure is deeper than the surrounding due to chopping of polymer chains.

PMMA (Poly-methyle-metacrylate)

-one of the first e-beam resists (1968)

-standard positive resist

-resolution<10 nm

-medium sensitivity (150-300μC/cm2 )

-available with high (950K) and low (50k) molecular weight

-contrast: high for 950k-resist, low for 50k-resist


E-beam lithography

• Challenge

Charging effect: Complicate exact focusing ofelectron-beam, displacement and distortion of exposed structures.

Proximity effect: Scattering of electrons in resist film and substrate, unwanted additional exposure.


Focused ion beam lithography

• Advantages

-Ions have heavy mass than electrons.

-Less proximity effect than E-beam

-Less scattering effect

-High resolution patterning than UV, E-beam lithography

-Even smaller wavelength than E-beam


Neutral atomic beam lithography

• In neutral atomic beams, no space charge effects make the beam divergent; therefore, high kinetic particle energies are not required. Diffraction is no severe limit for the resolution because the de Broglie wavelength of thermal atoms is less than 1 angstrom.



(a)Scanning tunneling microscopy

(b)Atomic force microscopy

(c)Near-field scnning optical microscopy

(d)Nanomanipulation

(e)Nanolithography

Nanomanipulation and nanolithography are based on scanning probe microscopy.


Scanning tunneling microscopy

• STM relies on electron tunneling, which is a phenomenon based on quantum mechanics.

Principle

A famous sample


Atomic force microscopy

• In spite of atomic resolution and other advantages, STM is limited to an electrically conductive surface since it is dependent on monitoring the tunneling current between the sample surface and the tip. AFM was developed as a modification of STM for dielectric materials.



• Local oxidation nanolithography Schematic diagram for the AFM based local oxidation lithography on both

silicon and Ag monolayer.



• Effects of tip bias potentials on the lithography patterns.



• AFM and KPFM(Kelvin probe force microscopy) images of the patterned silver nanoparticle monolayer. Shaped patterns were written on to the monolayer.



• Some examples


Quiz

• How to get the stoichiometric vapor ?


Quiz

• How to get the stoichiometric vapor ?

1. Flash Evaporation

2. E-Gun

3. Covaporation

4. Sputtering


Quiz

• Can we get the vapor with the same stoichimometric as the source materials? Why?


Quiz

• Can we get the vapor with the same stoichimometric as the source materials? Why?

No

Because of the different impingement rate for each element at the same vacuum condition


Quiz

• Describe a typical photolithographic process


Lecture by Ri-ichi Murakami

Nanomaterial Synthesis Method

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