What are the factors affecting the resolution of transmission electron microscopy?
A. Incident electron beam spot diameter: it is the limit of the resolving power of scanning electron microscope. Generally, the minimum beam spot diameter of the hot cathode electron gun can be reduced to 6nm, and the field emission electron gun can make the beam spot diameter smaller than 3nm.
B. Expansion effect of the incident electron beam in the sample: the degree of diffusion depends on the energy of the incident beam electrons and the atomic number of the sample. The higher the energy of the incident beam, the smaller the atomic number of the sample, the larger the volume of the electron beam, and the area where the signal is generated increases with the diffusion of the electron beam, thereby reducing the resolution.
C. Imaging method and modulation signal used: when the secondary electron is used as the modulation signal, due to its low energy (less than 50 eV) and short mean free path (about 10-100 nm), only the surface layer of 50-100 nm The secondary electrons in the depth range can only escape from the sample surface, and the number of scattering occurs is very limited, and basically does not expand laterally. Therefore, the resolution of the secondary electron image is approximately equal to the diameter of the beam spot. When the backscattered electrons are used as the modulation signal, the energy of the backscattered electrons is relatively high and the penetrating ability is strong, so they can escape from a deeper area in the sample (about 30% of the effective depth). In this depth range, the incident electrons have a fairly wide lateral expansion, so the resolution of the backscattered electron image is lower than that of the secondary electron image, generally around 500-2000nm. If other operation modes such as absorption of electrons, X-rays, cathodofluorescence, beam-induced conductance or potential are used as modulation signals, since the signals come from the entire electron beam scattering area, the resolution of the obtained scanning images is relatively low, generally at l 000 nm Or above l0000nm.
