This ray diagram illustrates the “scanning” aspect of scanning transmission electron microscopy (STEM), with a displaced scanning electron probe indicated with green rays. In STEM, the electron probe is scanned across the sample using electromagnetic beam deflection coils. One set of deflection coils can only deflect the beam by a certain angle. However, by using double deflection coils that consist of upper and lower deflection coil sets, two consecutive angular deflections can be applied, to create a pure beam shift (or alternatively pure beam tilt) on the sample. This beam shift is used to displace the STEM electron probe, scanning it across the sample. From the available technical information from JEOL and Thermo Fisher Scientific, I have concluded that the relevant beam deflection coils are mounted in the condenser lens system, just above the condenser minilens position. In STEM mode (where the condenser minilens is deactivated), a pure beam shift on the sample can then be achieved by using the deflection coils to “rock” the beam through the center of the front focal plane of the pre-field objective lens that forms the focused probe on the sample—as shown in this ray diagram.
The STEM detectors are conjugated to the back focal plane of the objective lens (post-field), in order to record the intensities of different scattering angles in the diffraction pattern. While the beam is scanned, the diffraction pattern does not move (to first approximation). It therefore remains centered on the detectors. This property is equally well derived from the ray diagram. The central ray of the incident probe on the sample is parallel to the optic axis and, as such, must pass through the optic axis center in the OL post-field back focal plane. This produces a centered direct beam disc in the back focal plane and, in turn, a centered diffraction pattern at the first diffraction plane and detector plane.