Scanning Electron Microscopy (SEM)

Stylianos Aspiotis, Jörg Fromm

Scanning electron microscopy (SEM) (Hitachi S-520, FEI Quanta FEG 250) is a type of electron microscope fundamentally different from the optical one, as SEM has the capability of producing high-resolution images of a sample surface. An electron beam is focused on a ~ 1 nm spot on the sample surface, transfering energy to that spot. The bombardment of the sample surface with electrons, which are called primary electrons, results in the displacement of electrons belonging to atoms of the specimen surface. Thus, various types of signals are produced, such as secondary electrons (SE) and back-scattered electrons (BSE) (EMPA). In particular, SE, that can only “escape” from the very superficial layer (in a nanometre scale) of the sample surface, are collected by a detector, which converts them into a signal. For the production of a typical SEM image, the electron beam needs to scan the desired area, producing in that way many signals. The produced signals can deliver valuable information about the surface topography (in a 3D format, as the density of the sample can affect these signals) and composition of the studied sample.