SEM and TEM Notes [PDF]

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Electron Microscope Electron microscopes have emerged as a powerful tool for the characterization of a wide range of materials. Their versatility and extremely high spatial resolution render them a very valuable tool for many applications. The two main types of electron microscopes are the Transmission Electron Microscope (TEM) and the Scanning Electron Microscope (SEM). Principle of electron microscope The interaction between the primary beam of high energy electrons and the sample leads to number of detectable signals as summarized in following scheme.



Schematic of electron beam interaction. In SEM, mainly secondary and back scattered electrons are used for imaging. These electrons have a very low energy (around 50 eV) compared to the energy of primary electrons (up to 30keV). Due to the low energy, these electrons can escape only from the surface area of the specimen and provide the information about the surface topography. In TEM analysis transmitted electrons like elastically scattered electrons are used for imaging. These transmitted electrons provide a two-dimensional image of the object. As a result, TEM offers invaluable information on the inner structure of the sample, such as crystal structure, morphology and stress state information



Scanning Electron Microscopy The scanning electron microscope (SEM) produces images by scanning the sample with a highenergy beam of electrons. The electrons in the beam interact with the sample, producing various signals that can be used to obtain information about the surface topography and composition. Principle: Accelerated electrons in an SEM has significant amounts of kinetic energy, and this energy is dissipated as a variety of signals produced by electron-sample interactions when the incident electrons are decelerated in the solid sample. As the electrons interact with the sample, they produce secondary electrons, backscattered electrons, and characteristic X-rays. These signals are collected by one or more detectors to form images which are then displayed on the computer screen. Secondary electrons and backscattered electrons are commonly used for imaging samples: secondary electrons are most valuable for showing morphology and topography on samples and backscattered electrons are most valuable for illustrating contrasts in composition in multiphase samples (i.e. for rapid phase discrimination). Instrumentation The main SEM components include: ▪



Source of electrons (Electron gun)



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Electromagnetic lenses



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Electron detector



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Sample chamber



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Computer and display to view the images



1. Electron gun Electrons are emitted from a metal by two methods: 1. Thermionic emission: In this method the electrons are emitted from the metals by heating them. 2. Field emission: In this method the electrons are emitted from metals, under strong electric fields. Thermionic electron gun The filament is made from a high melting point material or low work function, in order to emit many electrons. Tungsten filament or lanthanide hexaborate are commonly used in thermionic



electron gun. Tungsten wire used as thermionic cathodes are of 0.1-0.2mm in diameter bent like a hairpin and soldered on contacts. The wire is heated by a current of a few amperes. Field emission electron gun In field emission electron gun, a very strong electric field is used to extract electrons from a metal filament. Temperatures are lower than that needed for thermionic emission. This gives much higher source brightness than thermionic guns, but requires a very good vacuum.



Blog diagram of SEM



2. Magnetic lens system The magnetic lens system consists of a: 1. Condenser lens 2. Objective lens/aperture



3. Scanning coils Condenser lens The condenser lenses are made up of magnets capable of bending the path of electrons. These lens controls the intensity of the electron beam reaching the specimen. Objective lens The objective lens brings the electron beam into focus (de-magnifies) on the specimen. Objective lens (OL) aperture This aperture is used to reduce or exclude extraneous (scattered) electrons. An optimal aperture diameter should be selected for obtaining high resolution secondary electron images. Scanning Coils The scanning coils deflect the electron beam horizontally andvertically over the specimen surface. This is also called rastering. The scanning coils consist of two solenoids oriented in such a way as to create two magnetic fields perpendicular to each other. Varying the current in one solenoids causes the electrons to move left to right and that of in other solenoid forces these electron to move right to left and downwards.



3. Detectors: Secondary electron detector (SED) – Everhart-Thornley Detector Due to the low energies of secondary electrons (SE) (~2 to 50 eV) they are ejected only from near-surface layers. Therefore, secondary electron imaging (SEI) is ideal for recording topographical information. To attract (collect) theselow-energy electrons, a small bias (often +/- ve selectable but usually around +200 to 300V) is applied to the cage at thefront end of the detector to attract the negative electrons towards the detector. [If the cage is negatively biased it functionsas a BS detector]. A higher kV (e.g. 7 to 12kV) is applied inside the cage i.e. to the scintillator, to accelerate the electronsinto the scintillator screen. Backscattered electron detector (BSD) – solid state diode detector



The BSD is mounted below the objective lens pole piece and centered around the optic axis. As the specimen surface isscanned by the incident electron beam, backscattered electrons (BSE) are generated, the yield of which is controlled by thetopographical, physical and chemical characteristics of the sample. Both compositional or topographical backscatteredelectron images (BEI) can be recorded depending on the window of electron energies selected for image formation.



4. Sample chamber The specimen chamber is maintained at high vacuum that minimizes scattering of the electron beam before reaching thespecimen. This is important as scattering or attenuation of the electron beam will increase the probe size and reduce theresolution, especially in the SE mode. A high vacuum condition also optimizes collection efficiency, especially of thesecondary electrons. Specimen stage The specimen holder is fixed to the specimen stage by the dovetail locating slide. The stage can be moved manually alongthe X, Y (in the specimen plane), and Z directions (at right angles to the specimen plane). The Z adjustment is also knownas the specimen height. The specimen stage can also rotate continuously. Sample preparation The samples for SEM analysis require a coating of electrically conducting layer (e.g., gold, graphite, platinum, etc.) particularly if they are electrically non-conducting. However, metal samples do not require such coatings. Biological samples require fixation to preserve their structure by incubation in a fixation such as formalin or glutaraldehyde and also require dehydration to remove water.



Working: Electrons are produced at the top of the column, accelerated down and passed through a combination of lenses and apertures to produce a focused beam of electrons which hits the surface of the sample. The sample is mounted on a stage in the chamber area. The position of the electron beam on the sample is controlled by scan coils situated above the objective lens. These coils allow the beam to be scanned over the surface of the sample. This beam rastering or scanning, as the name of the microscope suggests, enables information about a defined area on the sample to be collected. As a result of the electron-sample interaction, a number of signals are produced. These signals are then detected by appropriate detectors.



Transmission Electron Microscopy Instrumentation The main components are ➢ An electron source; ➢ A series of electromagnetic and electrostatic lenses to control the shape and trajectory of the electron beam; ➢ Electron apertures.



Block diagram of TEM Working: The beam of electrons from the electron gun is focused into a small, thin, coherent beam by the use of the condenser lens. This beam is restricted by the condenser aperture, which excludes high angle electrons. The beam then strikes the specimen and parts of it are transmitted depending upon the thickness and electron transparency of the specimen. This transmitted portion is focused by the objective lens into an image on phosphor screen or charge coupled device (CCD) camera. Optional objective apertures can be used to enhance the contrast by blocking out high-angle diffracted electrons. The image then passed down the column through the intermediate and projector lenses, is enlarged all the way. The image strikes the phosphor screen and light is generated, allowing the user to see the image. The darker areas of the image represent those areas



of the sample that fewer electrons are transmitted through while the lighter areas of the image represent those areas of the sample that more electrons were transmitted through.



The difference between SEM and TEM The main difference between SEM and TEM is that SEM creates an image by detecting reflected or knocked-off electrons while TEM uses transmitted electrons (electrons which are passing through the sample) to create an image. As a result, TEM offers valuable information on the inner structure of the sample, such as crystal structure, morphology and stress state information, while SEM provides information on the sample’s surface (topographical information) and its composition.



Type of electrons High tension Specimen thickness Type of info



SEM Scattered, scanning electrons ~1 – 30 kV Any 3D image of surface



Max. magnification



Up to ~1 – 2 million times



Max. Field of View Optimal spatial resolution Image formation



Large ~0.5 nm Electrons are captured and counted by detectors, image on PC screen Little or no sample preparation, easy to use



Operation



TEM Transmitted electron ~60 – 300 kV typically,