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University of Vaasa – AUTO3160 Optics Electron Microscopy (SEM & TEM) By Thomas Höglund 15 May 2012 1 Scanning Electron Microscopy (SEM) 15 May 2012 2 The scanning electron microscope • Versatile • High resolution • Many imaging modes • Digital image processing Example applications: • Failure analysis and quality control in metals • Scientific research • Comparison of blood and tissue samples • Comparison of samples as evidence in forensics Hitachi SU-70 Schottky field emission SEM 15 May 2012 3 What is the SEM used for? • • • • Microscopic imaging Failure analysis Semiconductor development Quality control – troubleshooting and finding production problems – evaluating process parameters • • • • Morphology Metrology Crystallography Criminalistics 15 May 2012 4 SEM in development and fabrication • Infrastructural requirements: – – – – – • Sample requirements – – – – – • • • • Power Supply Vacuum System Cooling system Vibration-free floor Room free of ambient magnetic and electric fields Has to be solid Has to fit into specimen chamber Cannot be a living organism and biological samples require special treatment Must be stable in vacuum The surface has to be clean and clear Possibility to image objects several centimeters in size Shadows give the impression of 3D Depending on the sample, it can be time consuming to prepare it for SEM Low vacuum and environmental SEMs exist for more demanding samples 15 May 2012 5 The electron gun 15 May 2012 6 Scanning the beam 15 May 2012 7 Optics d0 = filament diameter d1 = diameter of intermediate image d = probe diameter v2 = working distance (WD) I0 = initial current I1 = probe current r = resolution λ = wavelength μ = refractive index of the vacuum 15 May 2012 8 Aberrations Coma 15 May 2012 9 Depth of focus/field Ligth microscope SEM 01.12.2009 10 Resolution 15 May 2012 11 Electron beam – specimen interaction • Samples must be – appropriately sized – mounted rigidly on a specimen holder (specimen stub) – electrically conductive – electrically grounded – coated (nonconductive specimens) with an ultrathin coating of electricallyconducting material 15 May 2012 Ref: Modified Thiocarbohydrazide Procedure for Scanning Electron Microscopy: Routine use for Normal, Pathological, or Experimental Tissues by “Linda E. Malick; Richard B. Wilson; David Stetson” Figure. The overview of Electron Beam’s path in SEM 12 Electron Interaction Volume The size of the interaction volume depends on: • Angle of incidence • Accelerating voltage • Atomic number (Z) 15 May 2012 13 Electron Beam – Specimen Interaction • • • • • • 15 May 2012 Secondary electrons (SE) Backscattered electrons (BSE) Auger electrons Cathodoluminescence (CL) Bremsstrahlung Characteristic x-ray radiation 14 Electron Interaction Volume 15 May 2012 15 Electron Beam – Specimen Interaction Figure. a) Elastic scattering 15 May 2012 Figure. b) Inelastic scattering 16 Electron Beam – Specimen Interaction • The Auger effect, Auger electrons – Energies between 280 eV (carbon) and 2.1 keV (sulfur) 15 May 2012 17 Energy Distribution of Emitted Electrons 15 May 2012 18 SE Detector • The Everhart-Thornley detector – Converts electrons into light, then back into electrical signals – Positive Faraday cage attracts low E SEs – Also collects BSEs present within line of sight The SEs emitted from the specimen are collected by a grid of +200V and accelerated onto the scintillator with a high voltage (10kV). Photomultiplier Scintillator 15 May 2012 19 BSE Detectors The generation region of backscattered electrons is larger than that of secondary electrons, i.e. several tens of nm. Therefore, backscattered electrons give poorer spacial resolution than secondary electrons. But because they have a larger energy than secondary electrons, they are less influenced by charge-up and specimen contamination. 15 May 2012 20 Imaging with Secondary Electrons 15 May 2012 21 Electron Beam – Specimen Interaction • Secondary electron (SE) contrast and yield depends on sample surface tomography and tilt, as well as on the position of the SE detector. • By detecting the SE from different angles, shadows are created and this gives an impression of threedimensionality. • Example: Cracks in the sample appear bright in the SE image. 15 May 2012 22 Surface Tilt Contrast • By combining images of the specimen taken in different angles, it is possible to construct a 3D image. 15 May 2012 23 Influence of accelerating voltage on SE Image Quality 15 May 2012 24 Influence of accelerating voltage on SE Image Quality 15 May 2012 25 Summary • • • • • • The SEM is a very versatile technique that can be used to image at higher resolution and deeper DOF than a light microscope. The SEM has many imaging modes making it useful for many purposes. Electron interaction volume increases with an increase in angle of incidence, accelerating voltage, and a decrease in average atomic number. Secondary electrons have lowest energy in a maximum of 50 eV, Auger electrons range between 50-2000 eV, while backscattered electrons can have an energy range of 50 eV to much higher values when compared to those. Beam damage occurs when a specimen area is irradiated with an electron probe for a long time at high magnification which can be eliminated by use low accelerating voltage, decrease electron beam intensity and shorten exposure time. When a nonconductive specimen is directly illuminated with an electron beam, its electrons with a negative charge collect locally called specimen charge-up, which can be decreased by properly selecting the accelerating voltage, or by using low accelerating voltage. • SEs are caused by inelastic interactions and BSEs due to elastic interactions. • The image contrast depends on many things and can be adjusted to reveal different features of the specimen. • The contrast also depends on atomic number of the specimen and the angle of tilt. • SEM is a very important technique for investigation of semiconductor materials 15 May 2012 26 References • • • • • • • • • • • Maryland Nanocenter http://www.nanocenter.umd.edu/new_facilities/hitachi_su-70_sem.jpg http://www.pvdproducts.com/services/brochures/sem.pdf Susan Swapp, University of Wyoming: http://serc.carleton.edu/research_education/geochemsheets/techniques/SEM.html Electron Microscopy and Analysis Third Edition, Goodhew, Humphreys, Beanland http://www.youtube.com/watch?v=EXmaY2txEBo Dr. Andriy Lotnyk, PTEM lecture slides 4. http://bellaitalianaphotography.files.wordpress.com/2008/04/chicory-sem.jpg http://www.engineering.arizona.edu/news/media/image/matsci_pollen.jpg http://www.nhml.com/resources/1998/10/1/scanning-electron-microscopes-sem Particle Beam Microanalysis E. Fuchs, H. Oppolzer, H. Rehme http://www4.nau.edu/microanalysis/Microprobe-SEM/Signals.html 15 May 2012 27 High-Resolution Transmission Electron Microscopy HRTEM 28 29 Introduction to HRTEM • HRTEM is used for imaging samples at atomic resolution • The accuracy of spatial measurements has reached a few picometers • Relies on quantum mechanical phase shifts caused by the interaction between the electrons and the interatomic potential for contrast • A high acceleration voltage of a few hundred kV is needed 30 Dynamical Scattering • The scattering of electron waves in the crystal potential • An incident plane wave is modified in amplitude and phase by the potential of the atoms in the object. • Electron interactions with the scattering potential is very strong so the sample thickness plays a very important role. • The same crystal structure under the same microscope parameters lead to different exit wave funtions and different images when the sample thickness is altered. • The Bloch-wave and multislice methods allow us to know object parameters to compute exit plane wave functions for varying sample thicknesses. 31 The multislice method The multislice approximation mathematically converts a specimen (left) into many thin slices (right). The electron wave is transmitted through the specimen and exits on the bottom. Each slice is thin enough to be approximated as a simple phase shift of the electron wave which is propagated between slices as a free space wave. 32 Contrast Transfer Function CTF, H(u) • The relationship between the exit wave and the image wave is a highly nonlinear one and is a function of the aberrations of the microscope described by the contrast transfer function • CTF: • The CTF tells us how information in u space is transferred to the image. H(u)=A(u)E(u)B(u) (A=aperture, E=envelope, B=aberration functions) • u=reciprocal-lattice vector, the spatial frequency for a particular direction [1/length] 33 Approximations for thin objects • Phase grating approximation 34 Image forming by a lens without aberrations 35 Interpretation of the interference image Distribution of maxima and minima of interference at the exit of the crystal 36 Problems in image interpretation • Three major problems make it difficult to interpret the images: – The wavefunction is influenced by inteference, which encrypts the object information – The exit-plane wave function is distorted due to optical aberrations – The phase information is lost due to the quantum mechanical properties of the travelling electron 37 Lattice image 38 • Zeros in the CTF represent gaps in the image transfer. • Oscillation causes contrast inversion. 39 Lattice imaging of crystals 40 Micrographs and diffractograms 41 Passbands • One can find special defocuses that allow higher spatial frequencies to contribute to the image, while on the other hand introducing zeros in the CTF at low u. • Passbands occur periodically: 42 Passbands (cont.) 43 Computer simulation • The image of the crystal structure is very sensitive to the thickness of the specimen and the defocus. • Computer simulation of lattice images is often required for image interpretation. • In order to compare experimental and calculated images the thickness and the defocus have to be known. 44 Computer simulation (cont.) • When calculating an image, three steps are reqired: – A model of the crystal structure or the defect must be set up. – Based on dynamic electron scattering, the wave function at the exit surface of the specimen is then calculated. – The optical transfer characteristic of the system is taken into account. • The most common method for calculating the wave function is the multi-slice method. – Weak-phase approximation is utilized by mathematically splitting the crystal into numerous thin slices. The transmission function for each slice can then be expressed by the weak-phase approximation. – The wave functions for each slice are then iterated to get the function at the exit surface of the specimen. 45 Example simulations Tecnai F30 S-Twin Titan T 80-300 kV 46 Spherical aberration • The objective lens compensates the distortion of the electron wave due to the sperical aberration with a negative defocus as a counter-distortion. • Phase shift is introduced. 47 Aberration correction • Using multipole (hexapole) lenses it is possible to get a combined effect of zero spherical aberration. • Even though it is possible to as good as remove all spherical aberration, a small amount of spherical aberration has to be used in order to achieve optimum contrast. 48 Applications • “It is possible to observe how a nanocrystal grows or melts or changes its structure atom by atom, or to investigate the structure of nanocrystals embedded in microcrystals.” (U. Dahmen, 2007) • Characterizing the atomic-scale structure, chemistry, and dynamics of individual nanometer-scale structures • Viewing grain boundaries • Viewing lattice inhomogeneities • Atomic resolution tomography • Single atom spectroscopy • Local electronic structure and bonding • Dynamics and mechanisms of reactions 49 50 Summary and outlook • • • • Relies on quantum mechanical phase shifts for contrast Lattice structure and inhomogeneities can be imaged Defocus and aberration greatly affect the image Computer simulation is important for understanding the images • Aberration correction highly enhances the resolution • The TEAM project has recently pushed the resolution down to 0.5 Å 51 Sources • Fuchs, Oppolzer, Rehme (1990), Particle Beam Microanalysis • Williams, Carter (2009), Transmission Electron Microscopy Second Edition • Urban, Jia, Houben, Lentzen, Mi and Tillmann (2009), Negative spherical aberration ultrahigh-resolution imaging in corrected transmission electron microscopy (Article) • U. Dahmen (2007), Aberration-corrected HRTEM and the TEAM project (Article) • http://www.fei.com/resources/image-gallery/ • http://www.fei.com/uploadedFiles/Documents/Content/20 07_10_semimarket_bro.pdf • http://ncem.lbl.gov/TEAM-project 52 Thank you 53