This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.
In STEM mode, the probe size, or called beam size and probe diameter, of modern microscopes can be defined as the full-width-at-half-maximum (FWHM) which contains 50-75% of the beam current. In this case, the probe is first focused at the eucentric height, and then focused on the viewing screen or CCD camera using the second condenser lens, C2. There are two common methods for evaluating the probe size:
i) The probe is directly recorded on a CCD camera or a negative film at high magnification, and then the FWHM is measured.
ii) The probe is evaluated and defined by the periodic structure of a crystal sample with a certain atomic spacing. However, it should be noted that the results often strongly depend on the sample, including its thickness, surface condition and orientation during imaging due to the channeling effect.
In principle, the probe size should be independent of the dimension of the C2 aperture. However, in reality, the C2 aperture affects the probe size, as high convergence angles (> 10−2 rad) are normally used to form very small probes, resulting in wide tails in the probe that are cut off by the aperture.
In TEM mode, the beam size is determined by the current of the 1st condenser lens (C1), and is affected by the convergence angle that is controlled mainly by the size of the C2 aperture and further adjusted by the objective lens prefield (or condenser mini-lenses).