Practical Electron Microscopy and Database

The probe forming aperture in STEM is often compared to the objective aperture in parallel beam illumination in CTEM. The effect of aperture size on the CTF for HAADF STEM imaging is a much more complex issue. The point spread function is proportional to the square of the probe wavefunction and not the wavefunction itself (as in Bright field CTEM). The response to increasing the aperture size is non-linear and affects all spatial frequencies, not just the highest. The phase errors at large angles to the optic axis essentially are mixed in to the lower frequencies, degrading both contrast and image localization.

Figure 4187 shows the schematic comparison between contrast transfer functions (CTFs) for a parallel-beam CTEM and a STEM. For STEM imaging, a proper probe-forming aperture should be applied to match the need of the spatial frequency marked by the green arrow.