Techniken der Oberflächenphysik (Techniques of Surface Physics)

Techniken der Oberflächenphysik (Techniques of Surface Physics) Prof. Yong Lei Dr. Ynag Xu and Mr. Grote Fabian 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

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

Surface Physics - Why? 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

Nobel Prizes with researches related to surface physics and structures: 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

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: Lowenergy electron diffraction; RBS: Rutherford Backscattering Spectrometry (Kelsall et al., Nanoscale science and technology. 2005)

UTAM surface nano-patterning technique CdS replicated mask Alumina CdS nanodots Highly ordered CdS nanodot arrays, UTAMs and CdS top layer on the surface of the UTAM.

3D Surface Nano-Patterning: Addressing Addressing System for 3-D surface nanostructures with nano-scale resolution nanowire 1A Schematic of the addressing system (only shows an array of 3 3)

Templates with large-scale (1 mm 2 ) perfect rectangular pore arrays without defect

A B

Binary nanotube/nanowire arrays realized by ALD technique via templat Binary nanotube/wire array Binary nanotube/tube array

Supercapacitors The core material: Nanotube opening Partial etching and mechanical removal

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:BiVO 4 Photoelectrochemical Water Splitting PS template BiVO 4 Mo:BiVO 4 Adjustable template with interconnected area Suitable infiltration with high infiltration fraction Controllable dual pore diameter in resulting architectures Applicable to various attractive materials

3D Porous AgVO 3 /graphene Aerogels Lithium Ion Storage Nanoscale, 2014, 6, 3536.

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

Bottom-up nanofabrication Synthesizing nano- or molecular-units: Nanotubes and nanowires Quantum dots and nanoparticles Functional arrangement Self assembly o Nano-sphere lithography o Block copolymers o Functionalized nanoscale structures Template-based growth Scanning probe manipulation o AFM, 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 In IC manufacturing, lithography is the single most important technology. 35% of wafer manufacturing costs comes from lithography. 70% dimension shrink every 3 yr. Patterning process consists of: Mask design Mask fabrication Wafer exposure

Photomask

Positive and Negative photoresist

Light source: mercury arc lamp 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) High pressure Hg-vapor lamps Order $1000, lasts 1000 hours.

Light source: excimer laser Decreasing feature size (to < 0.35 m) requires shorter. Kr NF 3 KrF photon emission energy KrF = 248 nm (used for 250 nm lithography generation) ArF = 193 nm (currently used for 45 nm node/generation production)

X-ray lithography (XRL) masks Advantages: Good resolution (down to 30 nm) No interference from dust Relatively fast Deep penetration to resist, high aspect ratio High aspect ratio micro-structures by XRL 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.

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 1.226 ( nm) V Where V is electron energy in ev unit. For example, 30 kev = 0.007 nm!

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

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.

Nanoimprint lithography: patterning by mechanical replication Waffel mold substrate

Lithography by molding/material transferring I: soft lithography (pattern duplication) 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. Stamp (mold) production 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 microfluidic components. (hygroscopic: readily taking up and retaining moisture) Photolithography pattern SU-8 Cast PDMS pre-polymer and cure PDMS stamp (mold) after peel off from SU-8 master

Lithography by molding/material transferring II: nanoimprint lithography (thermal/hot embossing) mold Heat-up polymer resist and press down Cool-down and remove mold Pattern transfer to substrate Mold = mask = template = stamp

(how to characterize surfaces) STM and AFM SPM Scanning Tunneling Microscopy (STM) Atomic Force Microscopy(AFM) Scanning Probe Microscopy (SPM)

Scanning Tunneling Microscopy 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).

STM

The principle of STM Probe Sample http://www.youtube.com/watch?nr=1&v=lr9-o_uwomc&feature=endscreen

http://www.youtube.com/watch?v=47ugmpxfvj4 The structure of STM I t ~ e -2kd

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

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

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

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. The combination of the 2 forces. The repulsive force causes cantilever to bend as tip is very close to surface. 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 structure of AFM Position Sensing Part Position Sensing photodetctor Force Sensing Part Feedback System

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

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.

Advantages and Disadvantages of AFM Modes Contact Mode Advantage - High scan speeds - Atomic resolution possible - Easier scanning of rough samples (with large changes in vertical topography). Disadvantage Lateral forces can distort the image Combination of these forces reduces spatial resolution and can cause damage to soft samples. Tapping Mode Noncontact Mode - Higher lateral resolution (1 to 5 nm). - Lower forces and less damage to soft samples in air. - Both normal and lateral forces are minimised, so good for very soft samples - Can get atomic resolution in a UHV environment Slower scan speed than in contact mode Slower scan speed Lower lateral resolution, limited by tip-sample separation. Usually only applicable in extremely hydrophobic samples.

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.

PMMA spheres scaning range 45x45 μm NCAFM image of the Ge/Si(105) surface, 4.2 nm x 4.2 nm 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).

The advantage and disadvantage of AFM 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

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

Thank you and have a nice day!