Advantages and Disadvantages of Low/High Voltage TEM and STEM
-- Comparison between Low and High Voltage TEM/STEM --
- Practical Electron Microscopy and Database -
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This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.

 

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High voltage TEMs have been widely used in all the scientific and industrial fields because they reduce spherical and chromatic aberration effects of the imaging lenses, but further development of aberration correctors can allow even atomic resolution at 60 kV and lower voltages [1].

Low voltage TEM (transmission electron microscope) can be used for avoiding atomic sputtering caused by the incident electron beam because the sputtering thresholds of the surface atoms are normally higher than 60 keV for most common materials. Lower voltage TEMs have also the advantages of achieving high energy stability (e.g. good for electron energy loss spectroscopy (EELS)) and of reducing overall cost. Low accelerating voltage electron beams can also enhance the sensitivities for EELS and energy-dispersive X-ray spectroscopy (EDS) because the reduced the acceleration voltage increases the ionization cross-section, which is determined by the overvoltage ratio.

However, electrons at low energies are more strongly scattered so that the TEM samples must be very thin, especially for heavy elements, in order to provide sufficient transmitted intensity and simple interpretation of the image contrast. On the other hand, strong scattering for low energy electrons leads to higher contrast, so voltages of 60 to 80 kV are preferred for low-contrast biological specimens.

Since the wavelength (λ) of the incident electrons increases at lower acceleration voltages,  the convergence semi-angle (α) should be increased to achieve an Airy disc dd equivalent to that obtained at higher acceleration voltages.

The focal length at low accelerating voltages is much smaller than that at high accelerating voltages.

The Ronchigrams in STEMs operated at lower accelerating voltages have smaller semi-angles than those operated at higher accelerating voltages. These smaller semi-angles represent smaller flat-contrast areas where the probe coherently converges on specimens in STEMs.

Table 4383. Comparison between low- and high-voltage TEM/STEM.  

Low-voltage TEM/STEM
High-voltage TEM/STEM

Higher (disadvantage)

Lower (advantage)
Higher (disadvantage) Lower (advantage)
Atomic sputtering by incident beam
Lower (advantage) Higher (disadvantage)
Higher (advantage) Lower (disadvantage)
Overall cost
Lower (advantage) Higher (disadvantage)
Sensitivities for EELS and EDS
Higher (advantage) Lower (disadvantage)

Thinner: Difficult to prepare & Less similar to bulk sample (disadvantage)

Thicker: Easy to prepare & More similar to bulk sample (advantage)

Less (advantage) More (disadvantage)
Longer Shorter
Smaller Bigger
Smaller semi-angles (disadvantage) Bigger semi-angles (advantage)
Lower (disadvantage) Higher (advantage)
Weaker (disadvantage) Greater (advantage)
Worse (disadvantage) Greater (advantage)

Image contrast

Higher contrast in specimens with a given thickness (advantage)

Lower contrast in specimens with a given thickness (disadvantage)

Dimension of lenses
Smaller (advantage) Larger (disadvantage: e.g. focusing properties of objective lens)
Weaker Stronger
X-radiation
Less danger (advantage) Greater danger (disadvantage)
Thinner Thicker
Thinner (advantage) Thicker (disadvantage)

 

 

 

 

[1] O. L. Krivanek, N. Dellby, R. J. Keyse, M. F. Murfitt, C. S. Own, Z. S. Szilagyi,in: P. W. Hawkes (Ed.), Advances in Imaging and Electron Physics, Elsevier, Amsterdam, 2008.

 

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