Fachgebiet 3D-Nanostrukturierung, Institut für Physik

Contact: yong.lei@tu-ilmenau.de; yang.xu@tu-ilmenau.de Office: Heliosbau 1102, Prof. Schmidt-Straße 26 (tel: 3748)

www.tu-ilmenau.de/nanostruk

Vorlesung: Mittwochs (G), 9 – 10:30, C 108 Übung: Mittwochs (U), 9 – 10:30, C 108

Prof. Yong Lei Dr. Ynag Xu and Mr. Grote Fabian

Techniken der Oberflächenphysik (Techniques of Surface Physics)

Contents of Class 6

Short review of the contents in the pervious 5 classes (how to fabricate surfaces especially in nano-sized range) Nano-fabrication: lithography and soft lithography,

nano-imprinting (how to characterize surfaces) PVD, ALD, CVD; STM and AFM

• Objects are contacted via their surface. •  Chemical reactions: Catalysis, electrodes of batteries •  Many properties are related: Friction and Lubrication •  Nanotechnology - Surface Physics Surfaces become more important for smaller objects

Surface Physics - Why?

Kai M. Siegbahn (Swedish) Nobel Prize 1981 Physics Developing the method of Electron Spectroscopy for Chemical Analysis, now usually described as X-ray photoelectron spectroscopy (XPS)

G. Binnig (German) & H. Rohrer (Swiss) Nobel Prize 1986 Physics Designing of the scanning tunneling  microscope  (STM)  →  SPM systems

Nobel Prizes with researches related to surface physics and structures:

Gerhard Ertl (German) Nobel Prize 2007 Chemistry for his studies of chemical processes on solid surfaces

Albert Fert (French) & Peter Grünberg (German) Nobel Prize 2007 Physics Interfaces - Giant magnetoresistance effect (GMR) which is a breakthrough in gigabyte hard disk drives.

Konstantin Novoselov & Andre Geim (Russian) Nobel Prize 2010 Physics for groundbreaking experiments regarding the two-dimensional graphene

SEM: Scanning Electron Microscope; STM/AFM: Scanning Tunneling Microscope/Atomic Force Microscope TEM: Analytical Transmission Electron Microscope X-Ray: X-ray Morphology; IP: Image Processing; LM: Lightweight Morphology; RBS: Rutherford Backscattering Spectrometry (Kelsall et al., Nanoscale science and technology. 2005)

TEM: Analytical Transmission Electron Microscopy; AES: Auger Electron Spectrometer; XRD: X-ray Diffraction; RBS: Rutherford Backscattering Spectrometry; XPS: X-ray Photoelectron Spectrometer; (Kelsall et al., Nanoscale science and technology. 2005)

SEM: Scanning Electron Microscopy; ATEM: Analytical Transmission Electron Microscopy; AEM: Auger Electron Microscopy. XRD: X-ray Diffraction; LEED: Low-energy electron diffraction; RBS: Rutherford Backscattering Spectrometry (Kelsall et al., Nanoscale science and technology. 2005)

Highly ordered CdS nanodot arrays, UTAMs and CdS top layer on the surface of the UTAM.

CdS replicated mask

Alumina

CdS nanodots

UTAM surface nano-patterning technique

Schematic of the addressing system (only shows an array of 3 × 3)

Addressing System for 3-D surface nanostructures with nano-scale resolution

3D Surface Nano-Patterning: Addressing

nanowire  ‘1A’

Templates with large-scale (1 mm2) perfect rectangular pore arrays without defect

A B

Binary nanotube/nanowire arrays realized by ALD technique via template

Binary nanotube/wire array Binary nanotube/tube array

The core material: Nanotube opening

Partial etching and mechanical removal

Supercapacitors

Organic memory device

C.L. Wang, Y. Lei et al., J. Mater. Chem. C 2013, (backside cover paper), 1, 8003.

3D Ordered Macro-mesoporous Mo:BiVO4 Photoelectrochemical Water Splitting

PS template BiVO4 Mo:BiVO4

Adjustable template with interconnected area

Suitable infiltration with high infiltration fraction

Controllable dual

pore diameter in resulting architectures

Applicable to various attractive materials

Nanoscale, 2014, 6, 3536.

3D Porous AgVO3/graphene Aerogels Lithium Ion Storage

a) Photography of a device for photocurrent responses test; b) Top-view SEM images of Au nanoparticle array after coating a thin shell of TiO2 with thickness of ~22 nm by ALD (500 cycles); and photocurrent responses results of c) Au and d) Ag samples

Introduction of the fundamentals of surface physics and their most important points

(what the main properties of surfaces) SPR, quantum-confinement effect, sensing

(how to characterize surfaces) SEM, TEM, XPS; STM and AFM (how to fabricate surfaces especially in nano-sized range) Template-based processes, PVD, ALD, CVD

Nano-fabrication: lithography and soft lithography, nano-imprinting

(what’s  the  main  applications  of  surfaces) Solar water splitting, gas sensor, supercapacitor, etc.

Nanofabrication

1. Photon-based lithography: DUV (deep UV), EUV (extreme UV), X-ray

2. Charged-beam lithography: electron beam, focused ion beam

3. Soft lithography and nano-imprinting

Nanofabrication - two principals

Bottom up: Atomic and molecular (or nano-) scale directed assembly to create larger scale structures e.g., chemical self -assembly

Top Down: Using techniques to remove, add or distribute atoms or molecules in a bulk material to create a final structure. Miniaturizing existing processes at the macro/micro/nano-scale

Assembled Machined

Bottom-up nanofabrication

Synthesizing nano- or molecular-units: • Nanotubes and nanowires • Quantum dots and nanoparticles Functional arrangement • Self assembly oNano-sphere lithography oBlock copolymers o Functionalized nanoscale structures

• Template-based growth • Scanning probe manipulation oAFM, STM with atomic resolution

Carbon nanotube

Anodized aluminum oxide

Nanosphere lithography (bottom up, self assembly)

Top-down nanofabrication

Top down approach: three components Lithography (lateral patterning): generate pattern in a material called resist photolithography, electron-beam  lithography,  nanoimprint  lithography… Thin film deposition (additive): spin coating, chemical vapor deposition, molecular

beam  epitaxy,  sputtering,  evaporation,  electroplating… Etching (subtractive): reactive ion etching, ion beam etching, wet chemical etching,

polishing…

Nanofabrication

1. Photon-based lithography: DUV (deep UV), EUV (extreme UV), X-ray

2. Charged-beam based lithography: electron beam, focused ion beam

3. Soft lithography, nano-imprinting

Photolithography for IC manufacturing

Patterning process consists of:

Mask design Mask fabrication Wafer exposure

• In IC manufacturing, lithography is the single most important technology.

• 35% of wafer manufacturing costs comes from lithography.

• 70% dimension shrink every 3 yr.

Photomask

Positive and Negative photoresist

Light source: mercury arc lamp

High pressure Hg-vapor lamps Order $1000, lasts 1000 hours.

Traditional Hg vapor lamps have been used which generate many spectral lines.

g line =436 nm i line =365 nm (for  0.5  and  0.35μm  lithography  process)

Light source: excimer laser Decreasing feature size (to < 0.35 m) requires shorter .

Kr NF3 energy KrF photon emissionKrF = 248 nm (used for 250 nm lithography generation) ArF = 193 nm (currently used for 45 nm node/generation production)

Advantages: • Good resolution (down to 30 nm) • No interference from dust • Relatively fast • Deep penetration to resist, high aspect ratio

80μm resist structure with aspect ratio > 10. White, APL, 66 (16) 1995.

Three-cylinder photonic crystal structure in ceramic. Exposed by repeated exposures at different tilt angles. G. Feiertag, APL, 71 (11) 1997.

High aspect ratio micro-structures by XRL

X-ray lithography (XRL) masks

Nanofabrication

1. Introduction. 2. Photon-based lithography: DUV (deep UV), EUV

(extreme UV), X-ray 3. Charged-beam based lithography: electron beam,

focused ion beam 4. Soft lithography, nano-imprinting

Lithography using charged particles I: electron beam lithography (EBL)

Finely focused electron beam, = 2-5 nm

Resist (PMMA…)

Metal patterning by EBL and liftoff

Electron beam lithography (EBL)

• Electron beam has a wavelength so small that diffraction no longer defines the lithographic resolution.

• Like a SEM with on-off capability. • Accurate  positioning,  “see”  the  substrate  first,  then  exposure. • Most popular prototyping tool for R&D, but too slow for mass production.

Wavelength of electrons

)(226.1 nmV

Where V is electron energy in eV unit. For  example,  30  keV  →   = 0.007 nm!

• Ga+ ion beam (down to 5 nm) to raster over the surface. • FIB can cut away material (electron is too light for this). • By introducing gases, FIB can selectively etch or deposit a metal or oxide.

Lithography using charged particles II: focused ion beam (FIB)

• Like electron beam lithography, direct write technique – no masks necessary. • Can expose a resist with higher sensitivity than EBL, but very low penetration

depth (resist << 100nm, pattern transfer difficult). • In summary, very versatile (deposition, etching, lithography, all in one tool);

but slow and expensive, more complicated than EBL.

Focused ion beam (FIB)

Nanofabrication

1. Introduction. 2. Photon-based lithography: DUV (deep UV), EUV

(extreme UV), X-ray 3. Charged-beam based lithography: electron beam,

focused ion beam 4. Soft lithography, nano-imprinting

Nanoimprint lithography: patterning by mechanical replication

mold

substrate

Waffel

Lithography by molding/material transferring I: soft lithography (pattern duplication)

Stamp (mold) production

• A master mold is made by lithographic process and a stamp is cast from the master. • Poly di-methyl siloxane (PDMS) is most popular material for stamps.

PDMS properties: • Soft and flexible. • Can be cured to create a robust PDMS stamp. • Chemically inert, non-hygroscopic, good thermal

stability. • Can be bonded to a glass slide to create micro-

fluidic components. (hygroscopic: readily taking up and retaining moisture)

PDMS stamp (mold) after peel off from SU-8 master Photolithography pattern SU-8 Cast PDMS pre-polymer and cure

Lithography by molding/material transferring II: nanoimprint lithography (thermal/hot embossing)

Mold = mask = template = stamp

Heat-up polymer resist and press down

mold

Cool-down and remove mold

Pattern transfer to substrate

(how to characterize surfaces) STM and AFM

SPM Scanning Tunneling Microscopy (STM)

Atomic Force Microscopy(AFM) Scanning Probe Microscopy (SPM)

Gerd Binnig Heinrich Rohrer

(born 20 July 1947) German physicist (born June 6, 1933) Swiss physicist

They shared half of the 1986 Nobel Prize in Physics with for the design of STM (the other half of the Prize was awarded to Ernst Ruska).

Scanning Tunneling Microscopy

• STM

The principle of STM

Probe Sample

http://www.youtube.com/watch?NR=1&v=lR9-O_uwomc&feature=endscreen

It ~ e-2kd

The structure of STM

http://www.youtube.com/watch?v=47UgMpXFVj4

The manipulation of STM

Constant current image (topography) of an atomic layer iron on W(001) with defects and atoms.

The Application of STM

• 1. Atomic Microscope

Nickel (110)

Platinum (111)

High performance STM image showing atomic resolution on Si(111) 7nm x 7nm

cobalt sulfide "nanoflower" structure synthesized on a Au(111) surface 9nm x 9nm

• 2. Manipulation of single atoms or single molecules

Desorption: Similar to vertical manipulation, but desorp individual absorbate directly into surrounding gas phase.

Lateral manipulation: Transfer of atoms/molecules along surface using attractive/ repulsive forces between tip and absorbate.

Vertical manipulation: The reversible transfer of atoms/molecules between surface and STM tip employing additional electronic/ vibrational excitation of absorbate.

positioned 48 iron atoms into a circular ring in order to "corral" surface state electrons and force them into "quantum" states.

• 3. Single-molecular chemical reactions

Synthesis: Selective bond formation between two molecular units employing lateral manipulation, followed by electronic/vibrational excitation.

Dissociation: Selective bond breaking within a molecule

The advantages and disadvantages of STM

• Advantages: 3D profile of a surface, to examine roughness, surface defects and determining molecules such as size and conformation. Other advantages of STM include: • much more details than many other microscopes, better

understand on a molecular level. • Versatile. STM can be used in ultra high vacuum, air, water

and other liquids and gasses. • STM can be operated in temperatures as low as zero Kelvin

up to a few hundred degrees.

• Disadvantages:

3 major downsides to using STMs: • Less effectiveness. STM is a very specific technique that

requires a lot of skill and precision.

• STM require very stable and clean surfaces, excellent vibration control and sharp tips. And STM only can be used to scan good conductor samples (no easy surface oxidized)

• STMs use highly specialized equipment that is fragile and expensive.

Atomic Force Microscopy (AFM)

• 1986 --- Binnig, Quate and Gerber invented the first atomic force microscope

The principle of AFM

•When tip closes to sample, mainly 2 forces operate.

•Typically forces contributing to the movement of AFM cantilever are coulombic and van der Waals interactions.

Coulombic force: This strong ,short range repulsive force arises from electrostatic repulsion by the electron clouds of tip and sample. This force increases as the separation decreases.

•Van der Waals force: longer range attractive force, which is felt at separations of up to 10 nm or more. As tip gets closer to the sample, this attraction increases.

The combination of the 2 forces. The repulsive force causes cantilever to bend as tip is very close to surface.

The structure of AFM

Feedback System

Position Sensing Part

Force Sensing Part

Position Sensing photodetctor

Three primary imaging modes:

• 1. Contact AFM < 0.5 nm probe-surface separation • 2. Tapping mode AFM (Intermittent contact ) 0.5-2 nm probe-surface separation • 3. Non-contact AFM 0.1-10 nm probe-surface separation

• 1. Contact AFM •In contact mode the tip contacts the sample surface.

•The detector monitors the changing cantilever deflection and the force is calculated using Hooke’s law:

•The feedback circuit adjusts the probe height to try and maintain a constant force and deflection on the cantilever. This is known as the deflection setpoint.

F = − k x (F = force, k = spring constant, x = cantilever deflection)

• 2. Tapping mode AFM

•In tapping mode cantilever oscillates at or slightly below its resonant frequency. The amplitude of oscillation typically ranges from 20 to 100 nm. Tip slightly “taps” on sample surface during scanning.

•The oscillation is also damped when the tip is closer to the surface. Hence changes in the oscillation amplitude can be used to measure the distance between the tip and the surface. The feedback circuit adjusts the probe height to try and maintain a constant amplitude of oscillation i.e. the amplitude setpoint.

• 3. Non-contact AFM •In non-contact mode cantilever oscillates near sample surface, but does not contact it. The oscillation is at slightly above the resonant frequency. Van der Waals and other long-range forces decrease the resonant frequency.

•In ambient conditions the adsorbed fluid layer is often much thicker than the region where van der Waals forces are significant. So the probe is either out of range of van der Waals force, or becomes trapped in the fluid layer. Therefore non-contact mode AFM works best under ultra-high vacuum conditions.

The Properties of the different operation modes in AFM.

Advantage Disadvantage

Contact Mode

- High scan speeds - “Atomic  resolution” possible - Easier scanning of rough

samples (with large changes in vertical topography).

• Lateral forces can distort the image •Combination of these forces reduces spatial resolution and can cause damage to soft samples.

Tapping Mode

- Higher lateral resolution (1 to 5 nm). - Lower forces and less damage to soft samples in air.

• Slower scan speed than in contact mode

Non-contact Mode

- Both normal and lateral forces are minimised, so good for very soft samples

- Can get atomic resolution in a UHV environment

• Slower scan speed • Lower lateral resolution, limited by tip-sample separation. •Usually only applicable in extremely hydrophobic samples.

Advantages and Disadvantages of AFM Modes

• 1. Imaging

The application of AFM

AFM 3D image of a detail of artificial opal The figure is 800 nm wide and 10 nm high Pd/Fe/Pd thin film dots.

NCAFM image of the Ge/Si(105) surface, 4.2 nm x 4.2 nm

PMMA  spheres  scaning  range  45x45  μm

AFM image of human plasma fibrinogen

• 2. Measuring forces (and mechanical properties) at the nanoscale

An AFM tip measuring force, and to move a cobalt atom on a crystalline surface. The ability to measure the exact force and to move individual atom is one of the keys to design and construct small structures. (Credit: Image courtesy of IBM) http://www.youtube.com/watch?v=BUq2bQkL1zo

• 3. As a nanoscale tool Bending, cutting and extracting soft materials (Polymers, DNA,

nanotubes) under high-resolution image controlling

grabbing and holding a nanoparticle in position

Manipulation of a nanotube on a silicon substrate. The AFM tip is used to create the Greek letter "theta" from a 2.5 micronmeter long nanotube

A single nanotube (in red) originally on an insulating substrate (SiO2, in green) is manipulated in a few steps onto a tungsten film (in blue), and finally is stretched across an insulating tungsten oxide barrier (in yellow).

Advantages :

1) True and high-resolution 3D surface images; 2) not require special sample treatments; 3) not require a vacuum (can be in both air and liquid); 4) could be used for organic materials. Disadvantages:

1) imaging feature size much smaller than electron microscopes; 2) slow in scanning an image, unlike an electron microscope which does it in almost real-time. 3) not true sample topography, but the interaction of the probe with the sample surface 4) expensive tips

The advantage and disadvantage of AFM

Tip convolution----Tip-Related Artifacts

protrusions (dots) appear wider, pores (depressions) narrower than the real size.

Radius of tip end determine the resolution of the scan

http://www.youtube.com/watch?v=fivhcWYEtkQ

Introduction of the fundamentals of surface physics and their most important points

(what the main properties of surfaces) SPR, quantum-confinement effect, sensing

(how to characterize surfaces) SEM, TEM, XPS; STM and AFM (how to fabricate surfaces especially in nano-sized range) Template-based processes, PVD, ALD, CVD

Nano-fabrication: lithography and soft lithography, nano-imprinting

(what’s  the  main  applications  of  surfaces) Solar water splitting, gas sensor, supercapacitor, etc.

Thank you and have a nice day!