Scanning electron microscopy
- 1. PRESENTED BY, MUGILAN N M.TECH NAST 1st year Reg no:16305012 NAST-614: Synthesis and Characterization of Nanostructured Materials TITLE: SCANNING ELECTRON MICROSCOPE COURSE INSTRUCTOR; A.VADIVEL MURUGAN Centre for Nano Sciences & Technology Madanjeet School of Green Energy Technologies
- 2. Contents • Introduction • Working of SEM • Advantages of SEM • Disadvantages of SEM • Applications • Conclusion
- 3. ELECTRON MICROSCOPE Electron microscopes are scientific instruments that use beams of energetic electrons to examine objects on a very fine scale. Electron microscopes magnifications over 1,000,000× Electrons cannot travel freely in air, electron microscopes are built into airtight metal tubes or “columns” and use vacuum pumps to remove all the air from within the microscope.
- 4. Scanning electron microscope (SEM). SEM Electron beam scans over surface of sample. Image shown on TV monitor. Image is of the surface of the sample. SEM FEATURES Is a microscope that uses electrons rather than light to form an image and to examine objects on a very fine scale. This examination can yield the following information: Topography – surface features. Morphology – shape and size. Composition – elements and compounds/ relative amount of them. Crystallographic information - how the atoms are arranged in the sample.
- 5. SETUP OF A SCANNING ELECTRON MICROSCOPE EQUIPMENT
- 7. DEFINITION Scanning electron microscope is a type of electron microscope that images a sample by scanning it with a beam of electrons in a raster scan pattern. WORKING PRINCIPLE In SEM, an electron beam is emitted from an electron gun fitted with a tungsten filament and passed towards the anode. This makes the electron to travel in vertical path through the column of the microscope in a concentrated manner The electrons to pass through electromagnetic lenses which focus and direct the beam down towards the specimen The detectors detect the back scattered electrons and secondary electrons and convert them to a signal that is sent to a viewing screen. The signals from the detectors are received and converted into images and displayed on the screen.
- 9. Secondary electrons (SE) Generated when a primary electron dislodges a specimen electron from the specimen surface. Produced by inelastic interactions. SE energy level is only 3 ~ 5 eV. Can be easily collected The maximum escape depth is about 5 nm in metal and 50 nm in insulators. BACKSCATTERED ELECTRONS(BSE) Produced by elastic interactions. BSE energy level is about 60 ~80 % of incident one. Special detector is required to collect BSE. The maximum escape depth varies inversely with the average atomic number, The range from a fraction of micrometer to several micrometers. (Larger atomic number, larger escape depth). High energy electrons that are reflected or back scattered out of the specimen. BSE images provide information about distribution of different elements in the sample
- 10. X-rays Generated when electron from incident beam knocks off electron from inner shell leaving hole. Electron from higher energy level jumps to inner shell emitting the excess energy as X rays. Energy and wavelength of x-ray can be used for chemical analysis. Electron Detectors
- 11. Detecting electrons Detectors used to amplify the signal rather than detected secondary andbackscattered electrons directly. i.e. Electrons Photons Electrons. Backscattered electrons are high energy (i.e. 1 - 40 keV) Secondary electrons are low energy (~0 - 50 eV) Must use different detectors to collect the backscattered and secondary electron signals. Detecting backscattered electrons Two specialized detectors are used to detect BSE. Detector sits directly beneath the objective lens. Large surface area to maximize collection efficiency. Scintillator detectors These comprise a scintillator (phosphor) with a light pipe and a photomultiplier Fast response time and high gain, which makes them suitable for use at TV rates. Bulky and may restrict the working distance of the microscope.
- 14. Interaction between electron beam and sample; The interactions may be elastic or inelastic. The elastic interactions: Only trajectory change KE and velocity remains constant little energy loss The inelastic interactions: Incident electrons will collide and displace electrons from their orbit heavy energy loss
- 15. The photo shows a lung cancer cell during cell division – taken with a scanning electron microscope.
- 16. Advantages of SEM The SEM is routinely used to generate high- resolution images of shapes of objects. SEMs can magnify objects at upward of 300,000 times the size of the object . fast speed. Higher magnification. Depth of field will be more. 3D image of the object can be viewed. Image can be directly viewed on the screen.
- 17. Disadvantages Samples must be solid and they must fit into the microscope chamber. Very costly and large size. Scanning electron microscopes are radiation- generating devices and should be well protected. Some may loss their structural property due to the interaction of the electron.
- 18. Wide range of applications in scientific and industry related field. Forensic science. Structural analysis Small feature measurements Fracture mode preliminary identification Grain size Corrosion failure inspection Surface contamination evaluation Chemical Composition Crystallographic information Applications
- 19. Conclusion Although SEMs are large, expensive pieces of equipment, they remain popular among researchers due to their wide range of applications and capabilities, including the high-resolution, three-dimensional, detailed images they produce.