Techniken der Oberflchenphysik Techniques of Surface Physics 3

Techniken der Oberflächenphysik (Techniques of Surface Physics) 3. VL im WS 15/16, 11. 2015 Prof. Yong Lei & Stefan Boesemann (& Liying Liang) Fachgebiet 3 D-Nanostrukturierung, Institut für Physik Contact: yong. lei@tu-ilmenau. de stefan. boesemann@tu-ilmenau. de; liying. liang@tu-ilmenau. de Office: Gebäude V 202, Unterpörlitzer Straße 38 (tel: 3748) www. tu-ilmenau. de/nanostruk Vorlesung: Übung: Mittwochs (G), 9 – 10: 30, C 108 Mittwochs (U), 9 – 10: 30, C 108 1

Outline for today 1. Molecular Beam Epitaxy (MBE) 2. Electrochemical Deposition 3. Spin Coating 4. Wet and Dry Etching 5. SEM / TEM / EDX 2

1. Molecular Beam Epitaxy - A kind of PVD - Ultra-High Vacuum (10− 8 Pa) - Single crystal deposition 3

1. Molecular Beam Epitaxy - Used for multi-layers - Highest purity - Low deposition rates ~1 nm/s - In-situ layer control (e. g. RHEED) SOURCE: Uni Würzburg 4

2. Electrochemical Deposition … is a process to form metal, metal oxides or metal alloy coatings on an electrode by reducing dissolved metal ions from an electrolyte. Dissolved metal ions in the electrolyte Electrical circuit Conductive surface, pre-structuring or templates are possible 5

2. Electrochemical Deposition Two kinds of metal ion sources: Left: reduction of consumeable anode Right: non-consumable anode, ions are in the electrolyte 6

2. Electrochemical Deposition Redrawn from: Pletcher, Horwood Publishing, 2001. 7

2. Electrochemical Deposition Tuning the features of the layer: - electrolyte - applied voltage/ current - p. H value of the electrolyte - process temperature 8

2. Electrochemical Deposition Quelle: Lodermeyer, Uni Regensburg, 2006 9

2. Electrochemical Deposition Advantages: - possible to operate at room temperatures - possible to use water-based electrolytes - easily scale up from atomic dimensions to large areas - fast growth rates - cheap - many materials possible - complex 3 D masks can be used as templates 10

2. Electrochemical Deposition Disadvantages: - substrate has to be conductive - substrate need a good adhesion - not easy to control - very thin layers are not possible - uniformity 11

3. Spin Coating - Methode to deposit uniform thin films from solution - Non-volatile substance (e. g. poymer, resist, liquid crystals) in highly volatile solvent SOURCE: http: //www. sneresearch. com, 2015 12

3. Spin Coating Tuning the film: - composition of solution (viscosity) - rotation speed - rotation duration - heating temperatur 13

3. Spin Coating Advantages: Disadvantages: - easy to handle - single batch, low throughput - cheap - low material usage <10% - high uniformity - very thin filmes (<30 nm) not possible - thick filmes (>200 nm) not possible - templates can not be used 14

4. Wet and Dry Etching Anisotropic and Isotropic etching: Uniform in vertical direction Uniform in all directions SOURCE: Wet and Dry Etching , Avinash , Logeeswaran , Islam; University of California 15

4. 1 Wet Etching … is a process with liquid chemicals or reactants to remove material from a substrate. - Multiple chemical reactions that consume the original reactants - New reactants are produced - 3 general steps 1) Diffusion of the reactants to the structure 2) Reaction between etchant and material (redox reaction) oxidation of material dissolving of the oxidzed material 3) Diffision of the by-productes - Masks can be used to fabricate patterns, masks can protect material only unprotected materials are echted away 16

4. 1 Wet Etching Tuning the process: - Composition of solution - Concentration of solution - p. H-value - Temperature - Time - Crystalline structure of substrate Wet etched silicon (100) mikro and nano scale SOURCE: Wet and Dry Etching , Avinash , Logeeswaran , Islam; University of California 17

4. 2 Dry Etching … is a process with etchant gasses to remove material from a substrate. - Different kinds of dry etching physical and chemical reaction and combination of both - Widly used in semiconductor industry (Si-etching) 18

4. 2 Dry Etching Physical dry etching: - Requires high kinetic energy of particle beams (ion, electron, or photon) to etch off the substrate atoms - No chemical reactions - Utilize RF-plasma to provide energy to detach surface atoms, like sputtering (vacuum is require) - High energy particles knock out atoms from the substrate material evaporates in the process chamber SOURCE: Wet and Dry Etching , Avinash , Logeeswaran, Islam; University of California 19

4. 2 Dry Etching Chemical dry etching: - Untilize etching gasses (no liquid chemicals) - Chemical reaction to attack the substrate surface - Common etching chemicals: tetrafluoromethane (CH 4), sulfur hexafluoride (SF 6), nitrogen trifluoride (NF 3), chlorine gas (Cl 2), or fluorine (F 2) *often very toxic * - Interaction of reactive ions and surface atoms bond between reactive ions and surface atoms chemical removing of surface atoms SOURCE: Wet and Dry Etching , Avinash , Logeeswaran, Islam; University of California 20

4. 2 Dry Etching Reactive ion etching (RIE): - Combination of physical and chemical mechanisms - Cations are produced from the reactive gases accelerated with high energy (RF-plasma) to the surface high energy collision + chemical reactions remove specimens from surface - Very fast, highly selective, high resolution, versatile, high aspect ratio - widely used in industry SOURCE: Wet and Dry Etching , Avinash , Logeeswaran, Islam; University of California 21

4. 2 Dry Etching Reactive ion etching (RIE): SOURCE: Wet and Dry Etching , Avinash , Logeeswaran, Islam; University of California 22

5. SEM / TEM / EDX • SEM – Scanning Electron Microscopy REM – Rasterelektronenmikroskopie • TEM – Transmission Electron Microscopy 23

Why Electron microscopy? 24

Why Electron microscopy? Kinetic energy of electrons De Broglie equation 25

Why Electron microscopy? 26

Resolution of TEM, RTM/AFM in comparison– Stand 1993/2008 REM/SEM 27

SEM Electron gun Lens system SEM Hitachi S 4800 im Feynmanbau Sample/Chamber Vacuum lock 28

SEM Difference to ordinary light microscope • Higher magnification due to the use of electrons compared to photons • Higher field depth (Schärfentiefe) due to scanning principle • Only black and white images Application: Images of surfaces, nanostructures, material composition 29

SEM Vacuum 1. Rotary pump for vacuum lock: 10 -3 mbar 2. Turbo molecular pump for chamber: 10 -5 mbar 3. Ion getter pump for electron gun: 10 -7 mbar Course of electron beam in a SEM 30

Heizung Filament Kathodenspannung (z. B. -30 k. V) Cathodes for electron microscopy Wehneltspannung (z. B. -30, 5 k. V) Wehnelt "Crossover" Anode (0 V) 31

Estimation of the electric field in a FE-Cathode • 32

Interaction of electrons with a bulk material in SEM Primärstrahl • Back scattered electrons (BSE) Sekundärelektronen SEM max. Tiefe 50 nm max. Energie 50 e. V • Secondary electrons (SE) SEM • Fluorescent X-ray radiation Rückstreuelektronen EDX 3 - 1000 nm Durchmesser E 0 = 3 - 30 k. V max. Tiefe 200 nm max. Energie E 0 Röntgenstrahlung Tiefe 500 nm - 10 µm 33

Signal detection sample BSE detector Back scattered electrons Material contrast chamber Everharth-Thornley. Detector Secondary electrons Surface sensitive 34 technique

SE - Everharth-Thornley-Detector Simulation under following link http: //www. materials. ac. uk/elearning/matter/Introduction. T o. Electron. Microscopes/SEM/everhart. html 35

BSE - BSE Detector • Electrons with energy higher than 50 e. V • Resolution depends on acceleration voltage and atomic number of the material Advantage of dependence of atomic number • Material contrast grey shades (Graustufen) indicate different materials 36

Contrast in BSE Cross-section SE-SEM image, showing UTAM filled with Sn. O 2. The present of two different materials can not be observed clearly. BSE detection proves the existance of 2 materials. Cross-section BSE image, showing pore opening, pore wall, and Sn. O 2 layer. Al 2 O 3 membrane and Sn. O 2 show different contrast. Tin dioxide is brighter compared to Al 2 O 3 because of higher z-value 37

Bild – Abhängigkeit von Beschleunigungsspannung und Probenstrom Stapel Bariumglas – Nickel – Platin – Aluminiumoxid - Kohlenstoffbeschichtet 1 k. V 5 µA 1 k. V 20 µA 6 k. V 5 µA 15 k. V 10 µA 30 k. V 10 µA 30 k. V 25 µA 38

Sample preparation • Sample need to be electrically conductive • Sample with low conductivity need to be coated with a thin film (> 10 nm) of a conductive material (e. g. gold or carbon) • It should be noted, that gold is not a good choice if BSE or EDX measurement are done (high atomic number (z-value)) 39

Shading effect Schattenraum Signal Kollektor/ Detektor SE-Elektronen BSE-Elektronen 40

Effect on Edges • More electrons are emitted on edges compared to planar structures Edges appear especially bright 41

Schärfentiefe (Fokusbereich für scharfes Bild) Die mit am bedeutungsvollste Eigenschaft eines REM. Abhängig von Strahlkonvergenz und Vergrößerung für großen Sichtbereich schmalen Strahl kleine Strahlkonvergenz großer Arbeitsabstand Näherungsformel Vergrößerung 1000 x Lichtmikroskop D = 0. 8 µm REM D = 50 µm Diagramm zur Bestimmung der Schärfentiefe im REM 42

Field depth - Schärfentiefe Light microscope: large aperture to gain high magnification SEM: wave length very small (approx. 0. 04 nm at 1 k. V) With small aperture (e. g. 50 µm) and large working distance (10 mm) it is still possible to gain a theoretical resolution of D < 1 nm. Low aperture in SEM leads to low circle of confusion (Zerstreuungskreis) of objects, which are not in focus High field depth (Schärfentiefe) in SEM. 43

Tiefenschärfe, Auflösung und förderliche Vergrößerung Punktauflösung X 10 µm 1 µm 0, 1 µm 10 nm Tiefens chärfe [m m] 104 Rasterelektronenmikroskop 103 102 Lichtmikroskop 10 1 0, 1 20 1000 Förderliche Vergrößerung 10000 44

EDX - Detector • X-ray is transformed into electric charge in a reversed diode • Created charge is proportional to the energy of the x-ray • FET transfers charge into voltage and amplifies it • Detector is cooled by liquid nitrogen to reduce noise 45

EDX-Anregungsbereich Al- Substrat 30 k. V 500 nm Pt- Schicht auf Al – Substrat 30 k. V Pt- Substrat 30 k. V 500 nm Al- Schicht auf Pt – Substrat 30 k. V 46

EDX 47

TEM Electron gun lens system Vacuum lock Sample/Chamber Schematic cross section of TEM CCD camera/ fluorescent screen Philips TECNAI in Feynmanbau 48

Interaction of electrons with material in TEM Kohärent einfallender Elektronenstrahl Inkohärent elastisch rückgestreute Elektronen Auger. Elektronen • durchdringende Elektronen TEM • Energieverlust durchdringender Elektronen EELS • Elektronenbeugung • Rückgestreute Elektronen (BSE) SEM • Sekundärelektronen (SE) SEM • Augerelektronen AES • Röntgenfluoreszenzstrahlung EDX, WDX • Jede Detektionsmöglichkeit kann ein neues Verfahren ergeben! Röntgenstrahlen Sekundär. Elektronen Anregungsbereich Dicke Probe Kohärent einfallender Elektronenstrahl Inkohärent elastisch rückgestreute Elektronen Sekundär. Elektronen Anregungsbereich Röntgenstrahlen Dünne Probe (~100 nm) Röntgenstrahlen Inkohärent inelastisch vorwärtsgestreute Elektronen Kohärent elastisch vorwärtsgestreute Elektronen Inkohärent elastisch vorwärtsgestreute Elektronen Direkter ungebeugter Strahl 49

Bright Field Imaging • Image intensity direct beam intensity • Scattering is proportional to Z 2 • Acceleration voltage 80 -400 k. V (typical >200 k. V) • Higher Z higher acceleration voltage thinner sample • Weakly diffracting regions appear bright • Strongly diffracting regions appear dark • Standard imaging mode of conventional TEM Bright field detector 50

Dark Field Imaging • Image intensity diffracted beam intensity • Weakly diffracting regions appear dark • Strongly diffracting regions appear bright • Typical application: grain size determination, second-phase particles [1] L. J. Sherry et al, Nano Letters, Vol 6 HAADF-STEM Image of Ag nano prism [1]51

Sub-nanometer region • Fringes • Lattice constant • High resolution TEM 52

Sample preparation for NT and NW Copper grid TEM sample preparation can be time consuming and very difficult, but the fabrication with NT and NW samples is easy. The wires are dispersed by ultrasonification in water or ethanol. A drop of the solution is put on the copper grid and dried in air. D. V. Sridhara Rao, K. Muraleedharan and C. J. Humphreys, TEM specimen preparation techniques 53 http: //www. medicine. mcgill. ca/femr/Rao%20 et%20 al%202010%20 TEM%20 Preparation%20 Materials. pdf

Composition – EDX line scan Phys. Chem. Phys. , 2011, 13, 15221– 15226 15221 54

Thanks for listening Any questions? Das Übungsblatt wird heute Abend online gestellt http: //www. tu-ilmenau. de/nanostruk/teaching/ 55