On the spatial resolution and nanoscale feature visibility in scanning electron microscopy

This chapter discusses the spatial resolution and nanoscale feature visibility in scanning electron microscopy (SEM). The spatial resolution of the SEM is considered limited by the following main factors: (1) the electron probe diameter (?), (2) the size of the generation volume for the detected signal, and (3) the ratio between the signal (S) and its random variation (noise, N) due to the stochastic nature of the electron-matter interaction. The electron probe diameter depends on the brightness and energy spread of the electron source, the spherical aberration and chromatic aberration of the objective lens, and the electron wavelength that--as a consequence of the small objective aperture angle that is necessary to minimize the aberrations--may give rise to significant diffraction effects. All the backscattered electrons (BSEs) and the secondary electrons (SEs) contribute to the image formation and ignore the spatial distribution of the emitted signals. It is possible to improve the visibility of compositional, topographic, or compositional and topographic details by tuning the range of the interaction volume to the detail sizes.