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Materials characterization (SEM, TEM, XRD, XPS)

What is the main reason specimens for TEM (transmission electron microscopy) must be thinned to about 100 nm or less?

  1. AElectrons interact strongly with matter, so they cannot pass through thick specimens, and multiple scattering blurs the image
  2. BThe electron wavelength is longer than that of X-rays, so diffraction does not occur unless the specimen is thin
  3. CThinning the specimen increases charging by the electron beam and raises the contrast
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Answer: A. Electrons interact strongly with matter, so they cannot pass through thick specimens, and multiple scattering blurs the image

Because electrons are charged, they interact with matter far more strongly than X-rays, and even at an accelerating voltage of about 200 kV a thick specimen transmits almost nothing or scatters the electrons many times, blurring images and spectra. Specimens are therefore thinned to about 100 nm or less by ion milling or FIB (focused ion beam) for observation. The wavelength of 200 kV electrons is about 0.0025 nm, far shorter than the roughly 0.154 nm of Cu Kα radiation, so the choice saying the wavelength is longer is backward. Charging is a source of image disturbance, not the purpose of thinning. In failure analysis, the standard procedure is to use FIB to thin a lamella targeting the defective site and observe the cross section by TEM.

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