Scanning transmission electron microscope

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Scanning transmission electron microscope

  1. 1. Scanning Transmission Electron Microscope
  2. 2. Outline • History • Interactions of Electrons • Background • STEM • Components • Specimen Preparation • Mode • Image formation • Comparison
  3. 3. Cont… • The first STEM was built in 1938 by Baron Manfred von Ardenne.(distroyed in air raid) • Not developed further until the 1970. • Then Albert Crewe at the University of Chicago developed the field emission gun. • Then built a STEM able to visualize single heavy atoms on thin carbon substrates
  4. 4. Interactions of electrons
  5. 5. Back ground • Maximum resolution, d • STEM an electron’s velocity approaches the speed of light, c
  6. 6. STEM • The basic principle of image formation fundamentally different from static beam TEM • small spot size is formed on the sample surface with the condenser lenses • This probe is scanned on the sample surface • the signal is detected by an electron detector, amplified and
  7. 7. Cont… • DETECTOR 1. Small disk on the column axis which detects the transmitted beam (BF STEM image) or diffracted beam (DF STEM image) 2. Annular detector (a plate with a hole) which detects all the diffracted beams except the transmitted one (ADF STEM) • Resolution  limited by the spot size have poorer resolution but
  8. 8. Scattered beam electrons • In STEM signal is detected by I. back scattered electrons(BSE) II. Transmitted beam electrons scattered at some angles • In both cases, BSE and TBE, the signal intensity is a function of the average atomic number of the sample volume and also phase contrast that interacted with the beam • Thus providing atomic number and phase
  9. 9. Cont… • In STEM, the electron beam is rastered (scan coil) across the surface of a sample in a similar manner to SEM, however, the sample is a thin TEM section and the diffraction contrast image is collected on a solid- state (ADF) detector. HAADF-high angle annular dark-field
  10. 10. Components • Source formation - Field emission • Electromagnetic Lenses - Condenser lens • Aperture • Specimen stage -1. Single-tilt 2. Double-tilt • Vacuum system • Scanning coils • Detectors -1. BF 2.ADF3.HAADF
  11. 11. Source formation • The STEM consists of an emission source tungsten filament, or a lanthanum hexaboride • High voltage source (typically 100-300kV) • Electrons emit by field emission.
  12. 12. Vacuum system • STEM is evacuated to low pressure 10^ -4 Pa • It consists of multiple pumping systems and air locks. • Low or roughing vacuum is achieved with either rotary vacuum pump or diaphram pumps • For low vacuum turbomolecular pumps are connected to the chamber • Gate valve: for different vacuum levels in
  13. 13. V1 V2 Field emission Electromagnetic Lenses Aperture Specimen stage mesh
  14. 14. Specimen holder
  15. 15. Specimen Preparation • Preparation done in two steps • Pre-Thinning:  Reducing the thickness to about 0.1mm • Final Thinning:  Reducing the thickness to about 100nm involve Ion Milling Electrolytic Thinning Ultramicrotomy
  16. 16. Ion Milling • Uses a beam of energetic ions to bombard specimen surfaces to reduce the thickness by knocking atoms out of a specimen • General procedure a) Dimple grinding b) ion milling  ion beam of 1–10 keV bombarded  specimen is placed in the center at an angle of
  17. 17. Electrolytic Thinning • Reducing specimen thickness to 100nm • General procedure A specimen placed in an electrochemical cell as anode A suitable reduce specimen thickness Common technique is jet polishing Electrolytic thinning completed in 3–15 minutes.
  18. 18. Ione milling Electrolytic thinning Ultramicrot omy
  19. 19. Modes • Transmitted electrons that leave the sample at relatively low angles with respect to the optic axis(bright field (BF).) • Transmitted electrons that leave the sample at relatively high angles with respect to the optic axis(annular dark field (ADF).) • High Angle ADF (HAADF) collects the
  20. 20. Image formation • BF-STEM images are equivalent to TEM (reciprocity principle). • Produced Bragg disks hitting the detector • Give the bright field or phase signal
  21. 21. Image formation Bright field STEM image of Au particles on a carbon film
  22. 22. ADF images • Electrons which have scattered to high angles are collected • Images contain Bragg diffraction
  23. 23. HAADF images • Two (out of several more) ways to simulate HAADF-STEM images are • Incoherent Imaging Model: The Image is the convolution of object potential and probe intensity. Iimage (r )= Iprob (r ) V2proj (r ) • Multiple Scattering Image Simulation: the frozen phonon approximation.
  24. 24. HAADF better Z-contrast than BF • HAADF is much less sensitive to local diffraction conditions than BF. • Its sensitivity mainly to the atomic number
  25. 25. Bright and dark field STEM image of Au particles on a carbon film
  26. 26. Why use STEM? •For DF imaging the annular detector collects more electrons than an aperture. •STEM ADF images are less noisy then TEM DF images as no lenses are used to form them. •Contrast in STEM images is greater than standard DF images.a) b) c) • Comparison of TEM DF and STEM ADF images of the same sample shows clear contrast difference

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