1. Nanomaterial Synthesis Method
Ri-ichi Murakami
Nanoscience and nanotechnology

2. Nanomaterial Synthesis Method
There's Plenty of Room at the Bottom
By Richard Feyman in 1959
Nanotechnology application in nowadays
Targeted drug delivery
Super nano-capacitors
CNT Transistor

3. 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

4. Emergence of Nano Moore’s Law
Original contact transistor
1947
~cm
Transistor
in Integrated circuit
Nowadays
~micrometer
CNT Transistor
Future
~nanometer
Moore’s Law plot of transistor size versus year
To meet the Moore’s Law, the size of transistor should be decreased

5. Emergence of Nano In our life
LED for display
PV film
Self-cleaning window
Temperature control fabrics
Health Monitoring clothes
CNT chair
Biocompatible materials
Nano-particle paint
Smart window
Data memory
CNT fuel cells
Nano-engineered cochlear
The nanotechnology is changing our life, but not enough.
Energy crisis, environmental problem, health monitoring, Artifical joints

6. Challenges in Nano Atomic scale imaging
TEM in biology
LaSrMnO and SrTiO superlattice
Understand and manipulate the target in nano scale

7. Challenges in Nano Interdisciplinary Investigation Protein TEM image
Biology
&
Medicine
Physics
&
Chemistry
Materials
Mechanics
&
Electronics
Nano drug delivery
Nano
Nano mechanics

8. Approaches 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

9. Approaches
Obviously there are two approaches to the synthesis of nanomaterials and the fabrication of nanostructures:
Top-down
Bottom-up
Lithography
Self-assembly

10. Synthesis of Nanoparticles
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

11. 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: Gibbs free energy
△G: Driving force for solidication △G G
GVS
GVL △T T* Tm
At any temperature below Tm there is a driving force fro solidification.

12. Synthesis of Nanoparticles
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.

13. Synthesis of Nanoparticles
Synthesis of metallic nanoparticles
Influences factors A B
Concentration
A: 0.25M AgNO3
B: 0.125M AgNO3
A large precursor concentration induces a large critical radius and favors relarively larger particles. A B
A weak reduction reagent induces a slow reaction rate and favors relatively larger particles.
Differenct reagents
A:sodium citrate
B: citric acid
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.

14. Evaporation and Condensation
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

15. 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.

16. Evaporation and Condensation
The Kinetic theory Let’ s start with the equilibrium process.
Adsorption
Condensation
Substrate
Supersaturation condition:
ji, incident flux
Tsub, temperature of 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
P, the gas pressure;m, the particle mass; k, Boltzmann’s constant, 1.38×10-23 J/K; T, the temperature
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.

17. 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.
J: flow density
A: area of the leak
z: implingement rate
Tsource Peq
On a certain angle
Source
The angle distribution is important for a co-sputtering condition.
substrate
Co-sputtering

18. Evaporation and Condensation
The vapor source
If 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
Alloy source
Al, Mg, Mn have different atomic mass.
Al: %
Mn: 0.01%
Mg:99.99%
Mn atom
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.
Al atom
Mg atom
beer can
This result is obtained under consideringt the adsorption and desorption effect.
Diffusion cell at 900 K

19. Evaporation How to get the stoichiometric vapor 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.
substrate
The substrate was placed at a temperature that was a supersaturation temperature for each component.
Heater
Flash Evaporation

20. Evaporation How to get the stoichiometric vapor E-Gun substrate 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.
substrate e-
Molten End
E-Gun
Rod-Fed Source

21. Evaporation How to get the stoichiometric vapor Coevaporation
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).
substrate T3
Effusion
Cells T1 T2 A B
Co-evaporation

22. Evaporation How to get the stoichiometric vapor Sputtering
Sputtering 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

23. Evaporation The evaporation source Evaporation Sources
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
Evaporation
Sources
Effusion cell
Effusion cell

24. Evaporation source Ideal Effusion Cell How to design a effusion cell L
aorifice L
Liquid
Gas, Peq
Lbody
How to design a effusion cell
The liquid and vapor are in equilibrium within the cell. Pliq=Pvap, Tliq=Tvap, Gliq=Gvap
The mean free path inside the cell is much greater than the orifice diameter.λ>>aorifice
The orifice is flat.
The orifice diameter is much less than the distance to the receiving surface.
The wall thickness is much less than the orifice diameter. L<<aorifice

25. Evaporation source Near-ideal Effusion cell
It is impossible to design an ideal effusion cell
Direct
Re-emitted
With a thick orifice lid, diffuse and specular reflection off the sidewalls are possible. L
Lbody
Lbody
It is the restriction due to the long cell body that cause a nonequilibrium behavior of vapor.
Gas, Peq
Liquid

26. Evaporation source Open-Tube Effusion Cell
The relative beam intensity of the open-tube effusion cell calculated for various tube
length-to-tube radius ratios (L/a) a L
A quasiequilibrium source
An open-tube effusion cell
Figure 2.56
Chapter Ⅱ

27. 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.

28. 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.

29. 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.

30. 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. 

31. 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
adsorption
at special sit
surface
diffusion
nucleation
Inter diffusion
film growth
film
Adsorption of atoms from gaseous phase
Cluster formation
Critical size islands growth
Coalescence of neighboring islands
Percolation of islands network
Continuous film growth

32. Condensation Adsorption gas physisorption Van der Waals force
It is defined as chemisorption coefficient that he fraction of adsorbated atoms transferred from physisorption into chemisorption but not re-evaporated.
physisorption
chemisorption
gas
substrate
Van der Waals force
chemical bond
re-evaporation
transition
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.

33. Condensation Condensation coefficient ji: the incident flux density
substrate
incident flux
re-evaporation
condensation
The fraction of the incident flux that actually condenses
ji: the incident flux density
ac: the condensation coefficient
jc: the condensation flux

34. Condensation Deposition Rate Growth speed Si An example
The deposition rate, or the growth speed Si
jc, the condensation flux
nf, the particle density,
how many particles per volume
An example
8 atoms per conventional unit cell
The volume per unit cell, (5.430 A)3= A3
The particle density, 8/( A3)=0.05 A-3
The growth speed a
5.430A
cubic lattice parameter, A

35. Condensation
Growth mode

36. Condensation Non-epitaxial growth
For most film-substrate material combinations, film grow in the Volmer-Weber (VW) mode which leads to a polycrystalline microstructure.

37. 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.

38. Condensation Epitaxial growth-Atomic layer deposition
based on the sequential use of a gas phase chemical process.

39. 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.

40. Methods for deposition
ALD
MBE
CVD
Sputtering
Evapor
PLD
Thickness Uniformity
good
fair
Film Density
Step Coverage
poor
varies
Interface Quality
Low Temp. Depostion
Good
Deposition Rate
Industrial Application

41. Lithography 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

42. 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.

43. 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

44. 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.

45. 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

46. 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

47. Photolithography Photoresist Spin Coating
• Wafer is held on a spinner chuck by vacuum and resist is coated to uniform thickness by spin coating.
• Typically rpm for 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
– p = resist solids content in percent
– w = spinner rotational speed in rpm/1000
• Most resist thicknesses are 1-2 mm for commercial Si processes

48. Photolithography Prebake
Used to evaporate the coating solvent and to densify the resist after spin coating.
• Typical thermal cycles:
– °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.

49. Photolithography
Align/Expose/Develop

50. Photolithography Etching/remove photoresist
photoresist has same polarity as final film; photoresist never touches the substrate wafer.

51. Photolithography Etching/remove photoresist
photoresist has opposite polarity as final film; excess deposited film never touches the substrate wafer.

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

53. E-beam lithography The theoretical resolution of photolithography is
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.

54. 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 ( μC/cm2 )
-available with high (950K) and low (50k) molecular weight
-contrast: high for 950k-resist, low for 50k-resist

55. 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.

56. 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

57. 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.

58. Nanomanipulation and nanolithography
(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.

59. Scanning tunneling microscopy
STM relies on electron tunneling, which is a phenomenon based on quantum mechanics.
Principle
A famous sample

60. 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.

61. Atomic force microscopy
Local oxidation nanolithography
Schematic diagram for the AFM based local oxidation lithography on both silicon and Ag monolayer.

62. Atomic force microscopy
Effects of tip bias potentials on the lithography patterns.

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

64. Nanomanipulation and nanolithography
Some examples

65. Quiz
How to get the stoichiometric vapor ?

66. Quiz How to get the stoichiometric vapor ? Flash Evaporation E-Gun
Covaporation
Sputtering

67. Quiz
Can we get the vapor with the same stoichimometric as the source materials? Why?

68. 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

69. Quiz
Describe a typical photolithographic process

70. Quiz
Describe a typical photolithographic process

71. Lecture by Ri-ichi Murakami
Thank you !