What are the factors that have an impact on the resolution of a transmission electron microscope?
A. Incident electron beam spot diameter: the limit of the resolving power of the SEM. Generally, the minimum spot diameter of hot cathode electron gun can be reduced to 6nm, and field emission electron gun can make the spot diameter less than 3nm.
B. Expansion effect of the incident electron beam in the sample: the degree of diffusion depends on the incident beam electron energy and the atomic number of the sample. The higher the energy of the incident beam and the lower the atomic number of the sample, the larger the volume of the electron beam, the larger the area of the signal generated with the diffusion of the electron beam, thus reducing the resolution.
C. imaging method and the modulation signal used: when the secondary electron modulation signal, due to its low energy (less than 50 eV), the average free range is short (10 ~ 100 nm or so), only in the surface layer of the depth range of 50 ~ 100 nm of the secondary electrons can escape from the sample surface, the occurrence of the number of scattering is very limited, basically did not expand laterally, so the resolution of the secondary electron image is approximately equal to the beam spot diameter. When the backscattered electrons are used as the modulation signal, the backscattered electrons can escape from the deeper region of the sample (about 30% of the effective depth) due to their high energy and penetrating ability. In this depth range, the incident electrons have been quite wide lateral expansion, so the backscattered electron image resolution is lower than the secondary electron image, generally in the 500 ~ 2000nm or so. If the absorption of electrons, X-rays, cathodoluminescence, beam sensing conductance or potential as a modulation signal of other modes of operation, due to the signal from the entire electron beam scattering region, the resulting scanning image of the resolution is relatively low, generally in the l,000 nm or l,000 nm or more than vary.
