电镜分析

1 Electron Microscopy Jin Zou (邹进） School of Engineering and Centre for Microscopy and Microanalysis The University of Queensland, Australia Email: j.zou@uq.edu.au 2 Light Microscopes Compound Stereo 3 Comparison of Resolution Optical image SEM image 25 µm SEM image has much better resolution than the optical image. Large depth of field 4 Why Electrons Better Than Light? The resolution (resolving power) of any microscope is fundamentally limited by the wavelength of the illumination used, according to following equation δ = 0.61λ/µsinβ or δ ≅ λ/2 � For light in visible spectrum, the resolution is approximately 200nm � It is difficult to work with X-rays (problems with focusing) � Need to find something else reflective index of the view medium Semiangle of collection of the magnifying lens 5 Why Electrons? Electrons have wave-like properties (de Broglie) with wavelength dependent upon their energy A 100keV electron has λ ≅ 4pm. Other effects limit the resolution, 0.1 ~ 0.2 nm is achievable, but sufficient to image atoms, because the distance between the first nearest neighbors of atoms is about that range. Also, the electron-specimen interaction provides many useful signals that can be used to understand specimens 6 Similarity: Light via Electron Illumination Condenser Sample Imaging Recording 2 7 Difference: Light via Electron Source: Light Electrons Lens: Glass Magnetic Medium: Air Vacuum Importantly: Significant difference in resolution 8 Electron-Specimen Interactions e- Specimen Inelastic scattered electrons Elastic scattered electrons Heat Whether electrons are reflected or transparent depends upon: • Power of incidental electrons (eV) • Specimen thickness • Chemical composition of the specimen Inelastic scattered electrons Elastic scattered electrons 9 Types of Electron Microscopes thick thin 10 Scanning Electron Microscope Electronics Specimen chamber Electron source Column TV screen Scanning Electron Microscope 11 Scanning Electron Microscopes 12 Essential Components in Electron Microscope Electron gun: In order to extract a lot of electrons from a material, the material we used should have a low work-function, e.g. tungsten. [Work-function: energy (or work) required to withdraw an electron completely from a metal surface. This energy is a measure of how tightly a particular metal holds its electrons.] Lenses for electrons: As negatively charged particles, electrons can be bend in the magnetic field. For this reason, electron microscopes use magnetic lenses to bend electrons. Vacuum system: Electrons, very light in mass, will be strongly scattered by gas molecules when they travel in air. In order to avoid it, a vacuum system needs to be built. 3 13 Working Principle of SEM A electron beam is produced by the electron gun and accelerated by the anode. Magnetic lenses focus the electron beam down the sample. Focused electron beam scans across the sample by the scanning coils. Once the electron beam hits the sample, other electrons originated from the sample are ejected. The detectors collect these ejected electrons and convert them to signals that are sent to a viewing screen (CRT). The CRT and the scan are synchronous, so that an image of the entire sample is built up on the CRT. 14 Main Signals Collected in a SEM e- A thick specimen Backscattered electrons Inelastic scattered electrons Elastic scattered electrons Secondary electrons Characteristic x-rays 15 e-This is due to inelastic scattering between incident electron beam with the outer shelled electrons in the specimen. Mechanism of Secondary Electrons Secondary electrons are weak in energy, typically < 50eV. 16 Secondary Electrons 17 Contrast Raised by Secondary Electrons Shadow and edge contrast Secondary electrons Secondary electron Detector To see details of surface morphology, the low voltage should be used, however, the resolution is reduced at the low voltage. 18 Signal of an Inclined Sample 4 19 GaAs Nanostructures 20 e- Mechanism of Backscattering Electrons This is due to elastic scattering between incidental electrons with nucleus. Backscattered electrons are bounced by nucleus in the sample, so that they have high energy. nucleus 21 Contrast Raised by Backscattered Electrons e- e- Nucleus High density (high Z) materials cause more backscattered electrons, so that the high density materials show brighter contrast. 22 Backscattering Electrons Images 23 Secondary via Backscattered Electrons Secondary electrons shows sample topography, whereas backscattered electrons shows the composition. Secondary electrons Backscattered electrons 24 e- Photon emission characteristic x-ray E=hν Mechanism of Characteristic X-ray This is due to inelastic scattering between incident electron beam with electrons in the inner shells of atoms in the specimen. Typical energy of characteristic x-ray is 0.5 ~ 20 keV. 5 25keV co un ts Energy Dispersive Spectroscopy 26 Example of X-ray Microanalysis Low average atomic number High average atomic number Zr50.5Cu21.1Ni11.9Al16.5 Zr46.4Cu19.0Ni11.6Al19.6O1.4Zr37.2Cu23.3Ni9.4Al30.1 27 Example of X-ray Mapping Si SiO2 Al TiNx CoSiy W 28 e- Increasing voltage Increasing atomic number Surface Source of secondary electron signal Source of back- scattered electrons Source of X-rays Interactive Volume and Signals Interactive volume E=eV Accelerating voltage Energy of electron 29 Typical Operating Parameters of SEM Accelerating voltage: V = 1 ~ 50 kV, usually 5 ~ 40 kV Probe size of electron beam: 1 ~ 50 µm Energy of secondary electrons: < 50 eV Signals of backscattered electrons: > 70% incidental electrons Energy range of X-ray: 0.5 ~ 20 keV 30 Major Applications of SEM Topography � The surface features of an object and its texture (hardness, reflectivity… etc.) Morphology � The shape and size of the particles making up the object (strength, defects in IC and chips...etc.) Composition � The elements and compounds that the object is composed of and the relative amounts of them (melting point, reactivity, hardness...etc.) Crystallographic Information How the grains are arranged in the object (conductivity, electrical properties, strength...etc.) 6 31 Advantages of the SEM Long depth of focus - 3D effect Large specimen size Simpler column design Simple and rapid specimen preparation Large range of magnification: 3X - 150,000X Large depth of field 32 Limitations of SEM High vacuum environment for specimen Specimens have to be conductive Only surface region of specimen can be viewed Less resolution than TEM 33 Types of Electron Microscopes thick thin 34 Transmission Electron Microscopes 35 Transmission Electron Microscopes The electron beam illuminates specimen and produces an image on phosphor screen Image formed as a projection of the specimen Contrast observed depends upon imaging mode used Sample limitations 36 Main Signals Collected in a TEM Elastic scattered electrons A thin specimen e - 2nm 50nm Characteristic x-rays Inelastic scattering Diffraction Electron HREM STEM EELS Energy filtered contrast diffraction imaging Imaging Spectroscopy TEM 7 37 Elastic Scattering Atomic planes e- d θ Wave length of incidental electrons: λ Diffraction occurs when 2dsinθ = nλ ELASTIC scattering process: • Changing direction with no energy loss • Phonon excitation through Coulomb interaction between the incident beam and atomic nucleus in the sample • Basis for electron diffraction 38 Electron Optics O bj ec t p la ne B ac k fo ca l P la ne Im ag e Pl an e Le ns 2nm Crystalline specimens 39 Electron Diffraction Crystalline specimen e- B ac k Fo ca l Pl an e Im ag e Pl an e Le ns 40 Kinds of Electron Diffraction Patterns Single Crystal Polycrystalline Amorphous Spot pattern Ring pattern Diffused Ring pattern 41 Index of Electron Diffraction Pattern Si [111] diffraction pattern 220 022 _ _ g1 g2 [111] //g1× g2 Diffraction pattern of poly- Au 111 220 200 222 311 Showing all reflections Each diffraction spot corresponds to a particular set of atomic planes Each diffraction line corresponds to a family of atomic planes 42 Phase Identification matrix 200nm 8 43 Inelastic scattering with energy loss Photon emission characteristic x-ray E=hν e- Inelastic Scattering INELASTIC scattering process: • Incidental electrons suffer energy loss • Coulomb interaction with electrons in the sample 44 Major TEM Signals Conventional Imaging Modes: �Diffraction Contrast �Phase Contrast �Mass/Thickness Contrast Diffraction Modes: �Electron Diffraction Inelastic Signals �Characteristic X-ray (also called Energy Dispersive Spectroscopy – EDS) �Electron Energy Loss Spectroscopy (EELS + EFTEM) Scanning Transmission Electron Microscopy (STEM) 45 Diffraction Contrast Crystalline specimens e- B FP IP Bright Field Dark Field 46 Origin of Diffraction Contrast 47 Diffraction Contrast & Its Applications Diffraction contrast is used for crystalline specimens Diffraction contrast is formed with a SINGLE diffracted (can be transmitted) electron beam and realised with an aperture at the back focal plane of the objective lens Diffraction contrast is mainly used for study of � Defects � Precipitates � Grain sizes and distributions �…… 48 Examples of Diffraction Contrast Ge islands grown on Si substrate <110> <100> Top view Side view 9 49 Phase Contrast Crystalline specimens e- B FP IP 50 Phase Contrast & Its Applications Phase contrast deals also with crystalline specimens Phase contrast is formed with at least TWO diffracted electron beams through their interference. Phase contrast is realised with (or without) an aperture at the back focal plane of the objective lens Phase contrast is mainly used for study of � Defects � Precipitates � Interfacial structures � Crystal growth �…… 51 Phase Contrast Phase contrast requires very thin specimens (< 20 nm typically) for the interference Crystals should be orientated to the directions where the zone axes should be parallel to the incidental electron beam Sometimes, image simulations are needed for gain correct interpretation as atoms can be black or white in the image 52 Examples of Phase Contrast 53 Diffraction/Phase Contrasts GaAs AlAs GaAs Ge Quantum dots on Si 54 Mass/Thickness Contrast Mass/thickness contrast arises from incoherent elastic scattering at the nucleus (Rutherford cross section) � The interaction between electrons and heavy atoms is stronger than that between electrons and light atoms . If the thickness is homogeneous, areas in which heavy atoms are concentrated appear with darker contrast than such with light atoms (mass contrast). Of course, more electrons are scattered in thick than in thin areas; thus, thick areas appear dark (thickness contrast). However, a thick area with light elements might have the same contrast as a thinner area with heavier atoms. The contrast depends upon atomic number Z and sample thickness t. 10 55 Application of Mass/Thickness Contrast Amorphous materials, such as polymers and amorphous porous Crystalline materials, but with all electron beam participating the formation of the images. Biological/soft materials 56 Small (< 1 nm) electron probe across sample EDS: Quantitative compositional distributions X-ray detector EELS: Compositional information + electronic structure E parallel electron detector Analytic Electron Microscopy STEM Imaging: Atomic resolution structural and compositional information Magnetic Prism 57 Energy Dispersive Spectroscopy Multiple phases Determination of local composition 58 Examples of X-ray Mapping Cu/Co Multilayer Structure InGaN/GaN Quantum Wells In map Cu map Co map ) Elemental mapping can be achieved in a small volume ) The composition may be determined quantitatively 59 ZLP Plasmon Core-loss Optical Prism analogy Electron Energy Loss Spectrum EELS spectrometer EELS Spectrum If this is an electron beam and the prism is magnetic field 60 Electron Energy Loss Spectroscopy 11 61 Example of Using Energy Loss Signals Energy Filtered TEM Electron Energy Loss Spectrum Boron K-edge Nitrogen K-edge 62 Catalysts in BN nanotubes 63 Contrast Raised by STEM e- e- Nucleus High density (high Z) materials cause more elastic scattered electrons at higher angles, so that the high density materials show brighter contrast. Advantages of STEM over convictional TEM images include • Since this is insensitive the strain; • The STEM images provide chemical information at atomic level. 64 Ge Quantum Dots Stacks in Si Rich information regarding to evolution of quantum dots growth can be obtained in one image!!! A STEM image The image tells us: & If the spacer layer is sufficient thin, growth of vertically aligned quantum dots is achievable; & For health stacks, the average size of upper dots is larger than the underlined dots; & If not, the stack will be terminated; & There is competition of dots growth! 65 Examples of STEM Sb doped Si Bi doped Si Cu Cu segregated in Al grain boundary Perovskite Oxides grown epitaxially on Si and Ge 66 STEM via EFTEM 12 67 Electron Tomograpahy e- Build 3D structure by taking Build 3D structure by taking images between images between ±±7070˚˚ Electron 68 Electron Tomography Resolution: 2Resolution: 2--10 nm10 nm 69 Dynamic Electron Microscopy – A new trend 70 Advantages of the TEM Superior resolution 0.1~0.2 nm, compared of 1~3 nm for SEM Ability to obtain compositional and crystallographic information from specimens with high spatial resolution Allowing to get all sorts of signals from same area 71 Limitations of TEM Very small areas to be investigated, so that it is important to make sure that the areas to be studied are representative Changing for insulating materials, problem for ceramic materials The magnetic lens limits the study on magnetic materials, such as steels. Sample preparation can be quite lengthy and involved Electron beams can damage sample and introduce artifacts 72 Conclusion Electron microscopy provides comprehensive information of targets with scales down to sub-nano- scale. Electron microscopy is a very useful characterising technique.