=================================================================================
In STEM mode, the radius of the Airy disc d_{d} due to the diffraction limit is written as,
 [2238a]
where,
α  The convergence semiangle;
λ  The wavelength of the incident electron beam.
Since the wavelength (λ) of the incident electrons increases at lower acceleration voltages, the convergence semiangle (α) should be increased to achieve an Airy disc d_{d} equivalent to that obtained at higher acceleration voltages.
On the other hand, the beam spreading caused by chromatic aberration d_{c} can be given by,
 [2238b]
where,
ΔE  The energy spread of the electron beam;
E_{0}  Eenergy of incident electron beam;
C_{c}  The nonrelativistic chromatic aberration and its coefficient.
From Equation 2238b we know that both C_{c} and ΔE should be low enough in order to suppress beam spreading at low acceleration voltages, especially since C_{c} depends strongly on the accelerating voltage.
In STEMrelated measurements, it is possible to reduce the convergence of the electron beam and thus the tail produced by spherical aberration, by reducing the size of the C2 aperture, but this is at the expense of the total probe current.
Furthermore, the spatial resolution of STEM measurements is also determined by the specimen thickness, due to cone shape of the probe as shown in Figure 2238. For instance, if we have the experimental conditions below:
i) A specimen is in thickness of 600 nm.
ii) The incident electron spot size is ~1 nm.
iii) The convergence angle of the incident electron is 10 mrad.
Then, the diameter of the electron at the bottom surface of the specimen is about 10 nm. Therefore, the spatial resolution will be about 10 nm in diameter. Note that the specimen thickness is more critical when the semiconvergence angle is large, for instance, under C_{s}corrected STEM condition.
Figure 2238. Schematic illustration of incident and scattered electrons in STEM. α is the semiconvergence angle, while β is the semicollection angle. t is the specimen thickness.
