
Electron Spot/Beam Size and Shape Limited by Diffraction
 Practical Electron Microscopy and Database 
 An Online Book 

http://www.globalsino.com/EM/

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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Because of the dependence of beam size on diffraction and aberrations as shown in Figure 2752, in order to achieve the optimum performance the diffraction and aberrations should be balanced as indicated by the optimum convergence semiangles. For big electron sources, at low convergence semiangles (α), the diffraction mainly determines the beam size, while at high α the spherical aberration is the dominant factor.
The diffraction term is given by,
d_{d}=0.61λ/α  [2752a]
The spherical aberration term is given by,
d_{s}=1/2C_{s}α^{3}  [2752b]
where,
λ  The wavelength of the incident electron beam,
C_{s}  The spherical aberration coefficient,
α  The convergence semiangle.
Figure 2752. Diameter of the electronbeam as a function of beamconvergence semiangle. 1: big electron source. 2: small electron source.

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