Sem mahfooz
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The document provides an overview of scanning electron microscopes (SEM). It discusses the history and development of SEMs, including key inventors. The main components of an SEM are described, including the electron gun, electromagnetic optics, scan coils, sample stage, detectors, and vacuum system. The document explains how SEMs form images by scanning a focused electron beam over a sample and detecting signals from electron-sample interactions, including secondary electrons, backscattered electrons, and X-rays. Resolution can be improved by increasing the beam voltage and current and decreasing the spot size and working distance. SEMs allow examination of surface topography, morphology, chemistry, and crystallography of samples.
![Effect of Accelerating Voltage
Figure 4. The above images were taken at different accelerating voltages, notice
how the resolution is improved with increased accelerating voltage [6].](https://image.slidesharecdn.com/semmahfooz-180712071734/85/Sem-mahfooz-22-320.jpg)
![Figure 5. The above images were taken at different accelerating voltages, notice
how the resolution is improved with increased accelerating voltage [6].
Effect of accelerating voltage](https://image.slidesharecdn.com/semmahfooz-180712071734/85/Sem-mahfooz-23-320.jpg)
![Figure 6. The above images were taken at different accelerating voltages, notice how the resolution is
improved with increased accelerating voltage [6].
Cont..](https://image.slidesharecdn.com/semmahfooz-180712071734/85/Sem-mahfooz-24-320.jpg)
![Figure 7. The above images were taken at two different working distances, notice how the depth of field
increases with working distance [6].
Effect of working distance](https://image.slidesharecdn.com/semmahfooz-180712071734/85/Sem-mahfooz-25-320.jpg)
![Effect of spot size/ probe dia
Figure 8. The above images were taken with two different size spot sizes.Notice how much the depth of field is
improved in the 200 micron spot size image [6].](https://image.slidesharecdn.com/semmahfooz-180712071734/85/Sem-mahfooz-26-320.jpg)
![References
Fundamentals of materials Science and Engineering – William D. Callister
Physical Metallurgy – Robert W. Cahn
Physical Metallurgy and Advanced Materials – R. E. Smallman
Physical Metallurgy Principles – Robert E. Reedhill
http://en.wikipedia.org/scanning_electron_microscope
JEOL. Guide to scanning electron Microscopy [Online]. Available at
http://www.jeol.com/sem/docs/sem_guide/tbcontd.html (accessed 1 Feb
2005).](https://image.slidesharecdn.com/semmahfooz-180712071734/85/Sem-mahfooz-27-320.jpg)
Sem mahfooz
- 1. SCANNING ELECTRON MICROSCOPE Course instructor Dr Venkata Girish Kotnur Assistant Professor University of Hyderabad Presented by Mahfooz Alam M.TECH. 17ETMM10
- 2. History TEM constructed in 1931 by Max Knoll and Ernst Ruska Ruska was awarded the Nobel Prize in physics for the development of transmission electron microscopy Von Ardenne first STEM in 1938 by rastering the electron beam in a TEM Zworykin et al. 1942, first SEM for bulk samples Manfred von Ardenne who in 1937 invented a true SEM 1965 first commercial SEM by Cambridge Scientific Instruments Resolution at that time ~ 50 nm : Today < 1 nm Morphology only at that time : Today analytical instrument
- 3. What is SEM? It is a microscope that produces an image by using an electron beam that scans the surface of a specimen inside a vacuum chamber. What can we study in a SEM? • Topography and morphology • Chemistry • Crystallography • Orientation of grains • In-situ experiments: – Reactions with atmosphere – Effects of temperature “Easy” sample preparation!! “Big” samples!
- 4. What does it looks like…. AFM Cantilever Tip Ant Head Blood Cells Diamond Thin Film (Numerous Multifaceted Micro- crystals) Microstructure of a plain carbon steel that contains 0.44 wt% of carbon Calcium Phosphate Crystal
- 5. The Instrument in Brief
- 6. Components of the Instrument • electron gun (filament) • electromagnetic optics • scan coils • sample stage • detectors • vacuum system • computer hardware and software
- 7. Electron Guns We want many electrons per time unit per area (high current density) and as small electron spot as possible Traditional guns: thermionic electron gun (electrons are emitted when a solid is heated) W-wire, LaB6-crystal Modern: field emission guns (FEG) (cold guns, a strong electric field is used to extract electrons) Single crystal of W, etched to a thin tip
- 8. Detectors Secondary electron detector: (Everhart-Thornley) Backscattered electron detector: (Solid-State Detector) Secondary electrons: Everhart-Thornley Detector Backscattered electrons: Solid State Detector X-rays: Energy dispersive spectrometer (EDS)
- 9. HOW THE SEM WORKS? The SEM uses electrons instead of light to form an image. A beam of electrons is produced at the top of the microscope by heating of a metallic filament. The electron beam follows a vertical path through the column of the microscope. It makes its way through electromagnetic lenses which focus and direct the beam down towards the sample. Once it hits the sample, other electrons ( backscattered or secondary ) are ejected from the sample. Detectors collect the secondary or backscattered electrons, and convert them to a signal that is sent to a viewing screen similar to the one in an ordinary television, producing an image.
- 10. How do we get an image? 156 electrons! Image Detector Electron gun 288 electrons!
- 11. Electron beam-sample interactions The incident electron beam is scattered in the sample, both elastically and inelastically This gives rise to various signals that we can detect (more on that on next slide) Interaction volume increases with increasing acceleration voltage and decreases with increasing atomic number
- 12. Signals from the sample Incoming electrons Secondary electrons Backscattered electrons Auger electrons X-rays Cathodo- luminescence (light) Sample
- 13. Where does the signals come from? • Diameter of the interaction volume is larger than the electron spot resolution is poorer than the size of the electron spot
- 14. Secondary electrons (SE) Generated from the collision between the incoming electrons and the loosely bonded outer electrons Low energy electrons (~10-50 eV) Only SE generated close to surface escape (topographic information is obtained) Number of SE is greater than the number of incoming electrons We differentiate between SE1 and SE2
- 15. SE1 The secondary electrons that are generated by the incoming electron beam as they enter the surface High resolution signal with a resolution which is only limited by the electron beam diameter SE2 The secondary electrons that are generated by the backscattered electrons that have returned to the surface after several inelastic scattering events SE2 come from a surface area that is bigger than the spot from the incoming electrons resolution is poorer than for SE1 exclusively Sample surface Incoming electrons SE2
- 16. Backscattered electrons (BSE) A fraction of the incident electrons is retarded by the electro-magnetic field of the nucleus and if the scattering angle is greater than 180° the electron can escape from the surface High energy electrons (elastic scattering) Fewer BSE than SE We differentiate between BSE1 and BSE2
- 17. BSE vs SE SE produces higher resolution images than BSE By placing the secondary electron detector inside the lens, mainly SE1 are detected Resolution of 1 – 2 nm is possible
- 18. X-rays Photons not electrons Each element has a fingerprint X-ray signal Poorer spatial resolution than BSE and SE Relatively few X-ray signals are emitted and the detector is inefficient relatively long signal collecting times are needed
- 19. Some comments on resolution Best resolution that can be obtained: size of the electron spot on the sample surface The introduction of FEG has dramatically improved the resolution of SEM’s The volume from which the signal electrons are formed defines the resolution SE image has higher resolution than a BSE image Scanning speed: a weak signal requires slow speed to improve signal-to- noise ratio when doing a slow scan drift in the electron beam can affect the accuracy of the analysis
- 20. Resolution We can also improve the resolution by: 1. Beam Accelerating Voltage(kv) 2. Probe convergence angle ap 3. Probe current ip 4. Probe diameter or spot size dp 5. Increasing strength of condenser lens 6. Decreasing the size of objective aperture 7. Decreasing the working distance
- 22. Effect of Accelerating Voltage Figure 4. The above images were taken at different accelerating voltages, notice how the resolution is improved with increased accelerating voltage [6].
- 23. Figure 5. The above images were taken at different accelerating voltages, notice how the resolution is improved with increased accelerating voltage [6]. Effect of accelerating voltage
- 24. Figure 6. The above images were taken at different accelerating voltages, notice how the resolution is improved with increased accelerating voltage [6]. Cont..
- 25. Figure 7. The above images were taken at two different working distances, notice how the depth of field increases with working distance [6]. Effect of working distance
- 26. Effect of spot size/ probe dia Figure 8. The above images were taken with two different size spot sizes.Notice how much the depth of field is improved in the 200 micron spot size image [6].
- 27. References Fundamentals of materials Science and Engineering – William D. Callister Physical Metallurgy – Robert W. Cahn Physical Metallurgy and Advanced Materials – R. E. Smallman Physical Metallurgy Principles – Robert E. Reedhill http://en.wikipedia.org/scanning_electron_microscope JEOL. Guide to scanning electron Microscopy [Online]. Available at http://www.jeol.com/sem/docs/sem_guide/tbcontd.html (accessed 1 Feb 2005).
- 28. Thank you