Electron Optics for Scanning Electron Microscope (SEM)
Abstract: Spatial resolution is one of the important parameters of the scanning electron microscope (SEM). In this article, a paraxial ray theory for a focusing system in SEM is presented in order to understand the relationship between electron optical components and spatial resolution. It is demonstrated that characteristics of resolution, i.e., diffraction limited, brightness limited, and aberration limited depend upon probe current. The resolution at any given probe current improves when the optimum beam convergence angle at the specimen becomes large. The optimum convergence angle of the beam is determined by inherent aberrations of the objective lens and the acceleration voltage. It can be calculated accurately using a method based on the information theory for an optical image.
Key words: SEM, Resolution, Brightness, Diffraction, Aberration