Nanomaterial Synthesis Method

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. Bottom-Up and Top-Down Approaches Introduction to synthesis of nanoparticles Evaporation and Condensation growth Lithography technology Method to nano composite structure Outline Emergence and Challenges in Nanotechnology

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. Emergence and Challenges in Nanotechnology Introduction to synthesis of nanoparticles Evaporation and Condensation growth Lithography technology Method to nano composite structure Approaches Bottom-Up and Top-Down Approaches

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. Emergence and Challenges in Nanotechnology Evaporation and Condensation growth Lithography technology Method to nano composite structure Synthesis of Nanoparticles Bottom-Up and Top-Down Approaches Introduction to synthesis of nanoparticles

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 Tm T* 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 Concentration A: 0.25M AgNO3 B: 0.125M AgNO3 A large precursor concentration induces a large critical radius and favors relarively larger particles. B A 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 Lithography technology Method to nano composite structure Bottom-Up and Top-Down Approaches Introduction to synthesis of nanoparticles Evaporation and Condensation growth

15. Substrate Condensation vapor Evaporation energy Source Evaporation and Condensation • The evaporation and condensation are the fundamental phenomena in preparing thin films with nano meters thickness. 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. Condensation Adsorption Substrate Evaporation and Condensation • The Kinetic theory Let’ s start with the equilibrium process. 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. Tsource Peq 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 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: 0.0001% 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 beer can Mg atom 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 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. quasiequilibrium Evaporation Sources nonequilibrium 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. Effusion cell Effusion cell

24. δA L aorifice Lbody Gas, Peq Liquid Evaporation source • Ideal Effusion Cell 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. Direct Re-emitted Evaporation source • Near-ideal Effusion cell It is impossible to design an ideal effusion cell 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. a 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) L An open-tube effusion cell A quasiequilibrium source 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 re-evaporation film growth adsorption at special sit surface diffusion nucleation Inter diffusion 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. 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. gas physisorption Van der Waals force re-evaporation transition chemical bond chemisorption substrate Condensation • Adsorption 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.

33. incident flux re-evaporation condensation substrate Condensation • Condensation coefficient 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 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=160.10 A3 The particle density, 8/(160.10 A3)=0.05 A-3 The growth speed a 5.430A cubic lattice parameter, 5.430 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 oneor several molecular or atomic beams that occurs on a surface of a heated crystallinesubstrate.

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

41. Lithography Emergence and Challenges in Nanotechnology Method to nano composite structure Bottom-Up and Top-Down Approaches Introduction to synthesis of nanoparticles Evaporation and Condensation growth Lithography technology

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 tophotoresist 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) ortrichloroethylene (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 iscoated 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 Siprocesses

48. Photolithography • Prebake Used to evaporate the coating solvent and to densify theresist 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 alsoused 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.