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Fachgebiet 3D-Nanostrukturierung, Institut für Physik Contact: Office:

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Präsentation zum Thema: "Fachgebiet 3D-Nanostrukturierung, Institut für Physik Contact: Office:"—  Präsentation transkript:

1 Fachgebiet 3D-Nanostrukturierung, Institut für Physik Contact: Office: Gebäude V 202, Unterpörlitzer Straße 38 (tel: 3748) Vorlesung:Mittwochs (G), 9 – 10:30, C 108 Übung:Mittwochs (U), 9 – 10:30, C 108 Prof. Yong Lei & Stefan Boesemann (& Liying Liang) Techniken der Oberflächenphysik (Techniques of Surface Physics) 3. VL im WS15/16,

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

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

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

5 5 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

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

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

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

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

10 10 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 3D masks can be used as templates

11 11 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

12 12 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: 2015

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

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

15 15 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

16 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

17 Wet Etching Tuning the process: -Composition of solution -Concentration of solution -pH-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

18 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)

19 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

20 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

21 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

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

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

24 Why Electron microscopy? 24

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

26 Why Electron microscopy? 26

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

28 SEM SEM Hitachi S4800 im Feynmanbau Electron gun Vacuum lockSample/Chamber Lens system 28

29 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

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

31 Cathodes for electron microscopy Wehnelt- spannung (z.B. -30,5 kV) Wehnelt Filament Heizung Kathoden- spannung (z.B. -30 kV) Anode (0V) "Crossover" 31

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

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

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

35 SE - Everharth-Thornley-Detector Simulation under following link oElectronMicroscopes/SEM/everhart.html Er besteht aus einer Kombination eines Szintillators und eines Photomultipliers. Im Szintillator (Plastikszintillator) erzeugen die Elektronen Photonen aufgrund von Kathodolumineszenz (10-15 Photonen pro 10 keV-Elektron, der Großteil der Elektronenenergie wird in Wärme umgewandelt). Der Szintillatorkopf ist von einem Kollektor mit einem Gitter, dessen Potential sich von -200 V bis +200 V variieren lässt umgeben. Befindet sich das Gitter auf positivem Potential werden die SE angezogen und gesammelt. Bei negativem Potential (<-50 V) können keine SE das Gitter überwinden, nur die energiereicheren RE können dann den Szintillator erreichen. Die Szintillatoroberfläche ist mit einer nm dicken Schicht Aluminium bedampft und befindet sich auf einem Potential von 10 kV ist. Die SE, die das Kollektorgitter passiert haben, werden folglich auf den Szintillator beschleunigt, durchdringen die Metallschicht und erzeugen ca Elektron-Loch-Paare, von denen ca. 1-3 % an Lumineszenzzentren zu Photonen rekombinieren. Ein Teil der Photonen wird aufgrund von Totalreflexion entlang des Lichtleiters (Plexiglas, Quarzglas) zur Photokathode des Photomultipliers geführt, wo sie mit einer Ausbeute von 5-20 % Photoelektronen auslösen (Anmerkung: Pro auf den Szintillator treffendes SE werden nur ca Photoelektronen erzeugt. Diese relativ kleine Konversionsrate zeigt jedoch ein außerordentlich geringes Rauschen). Die Photoelektronen werden anschließend auf die erste Dynode (ca V) beschleunigt wo sie Sekundärelektronen auslösen, die über weitere Dynoden lawinenartig verstärkt werden. Durch Verändern der Photomultiplier- Spannung kann die Verstärkung über mehrere Größenordnungen variiert werden. 35

36 BSE - BSE Detector Electrons with energy higher than 50 eV 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

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

38 Bild – Abhängigkeit von Beschleunigungsspannung und Probenstrom Stapel Bariumglas – Nickel – Platin – Aluminiumoxid - Kohlenstoffbeschichtet 1 kV 5 µA 1 kV 20 µA6 kV5 µA 15 kV 10 µA 30 kV 10 µA30 kV 25 µA 38

39 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

40 Shading effect SE-ElektronenBSE-Elektronen 40

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

42 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 1000x Lichtmikroskop D = 0.8 µm REM D = 50 µm Diagramm zur Bestimmung der Schärfentiefe im REM 42

43 Field depth - Schärfentiefe Light microscope: large aperture to gain high magnification SEM: wave length very small (approx nm at 1 kV) 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

44 Tiefenschärfe, Auflösung und förderliche Vergrößerung 44

45 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

46 EDX-Anregungsbereich Al- Substrat 30 kV Pt- Substrat 30 kV 500 nm Al- Schicht auf Pt – Substrat 30 kV 500 nm Pt- Schicht auf Al – Substrat 30 kV 46

47 EDX 47

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

49 Interaction of electrons with material in TEM 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! Dicke Probe Kohärent einfallender Elektronenstrahl Inkohärent elastisch rückgestreute Elektronen Anregungs- bereich Sekundär- Elektronen Auger- Elektronen Röntgen- strahlen Dünne Probe (~100 nm) Kohärent einfallender Elektronenstrahl Inkohärent elastisch rückgestreute Elektronen Inkohärent inelastisch vorwärtsgestreute Elektronen Inkohärent elastisch vorwärtsgestreute Elektronen Kohärent elastisch vorwärtsgestreute Elektronen Anregungs- bereich Sekundär- Elektronen Röntgen- strahlen Röntgen- strahlen Direkter ungebeugter Strahl 49

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

51 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 HAADF-STEM Image of Ag nano prism [1] [1] L.J. Sherry et al, Nano Letters, Vol 6 51

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

53 Sample preparation for NT and NW D. V. Sridhara Rao, K. Muraleedharan and C. J. Humphreys, TEM specimen preparation techniques 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. Copper grid 53

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

55 Thanks for listening Any questions? Das Übungsblatt wird heute Abend online gestellt 55


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