Introduction to SEM


Intro SEM, set up and magnification

  • SEM ~ 30kV, 10 nm resolution , use uncorrected wavelength equation
  • TEM ~ 300kV, 1nm resolution, sample is thin (~< 200 nm)
  • Need to account for relativistic change in the wavelength]

Field Emission gun

  • Produces electron beam (filament hetaed to 1800K)
  • Beam focussed using electromagnetic lenses
  • Use aperture to limit number of electrons
  • Working distance: distance between where electrons leave column and where they hit the sample 
  • Use ultra high vacuum and high vacuum 
  • Sample stage is ground to Earth to limit charging (build-up of electrns on the surface)
  • SE detector --> side of the chamber 
  • Back scatter detector --> above sample , below electron column
  • Magnification depends on the sizes of displayed image 
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Depth of field and Aberrations

Depth of field

  • distance that electron field is still in focus
  • features in image can be resolved 
  • depends on beam size and working distance
  • Larger at long working distance and beam diameters
  • High resolutions: acheieved with smaller beam diameter and working distance


  • Spherical: diffrence in focal length of edge and centre of lens
    • Use small aperture to bget rid
  • Differaction: wave nature of electrons; use large aprture
  • Chromatic: energy spread of electrons ; use monochromatatic beam
  • Astigmatism: physical imperfections in lens
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Charging, SEM information, Interaction volume

Drift, beam and sample interaction: Beam deflected from intended postion due to charging

SEM information

  • Back scattered : elastically scattered
  • SE: inelasically scattered 
  • Cathodoluminescence: loss of energy via visible light photons
  • Energy dispersive spectroscopy: detect emitted X-rays

Interaction volume

  • Pear shape drop
  • BS and SE --> ~100 nm
  • X-rays emitted ~1 micron
  • Increases with increasing energy
  • Increases with decreasing number of atoms
  • Resolution decreases with increasing energy as IV(power) increases
  • Signal comes from larger volume
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X-ray generations and Electron yield

X-ray generation

Electron interacts wwith matter, ionises atom, electron fills space and produces characteristic x-rays

Electron yield

  • Highest portion--> secondary electrons
    • Independent of atomic number 
  • Smaller portion --> Back scatter electrons ;
    • increases with increasing atomic number 
    • Better contrast in image than SE if have varying atomc number in sample
    • As Some BS electrons form diffraction patterns , based on crystaollographic arrangemnet
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