Scanning electron microscopy - Principles and equipment

Scanning Electron Microscopy (SEM) is a powerful technique for observing surface topography. It is based primarily on the detection of secondary electrons emerging from the surface under the impact of a very fine brush of primary electrons, which sweeps across the observed surface and enables images to be obtained with a separating power often below 5 nm and a great depth of field.

SEM also uses the other interactions of primary electrons with the sample: emergence of backscattered electrons, absorption of primary electrons, as well as the emission of X-ray photons and sometimes photons close to the visible range. Each of these interactions is often indicative of surface topography and/or composition.

The instrument forms a very fine (down to a few nanometers), quasi-parallel brush of electrons, strongly accelerated by adjustable voltages from 0.1 to 30 kV, focuses it on the area to be examined and progressively scans it. Appropriate detectors - specific electron detectors (secondary, backscattered, sometimes absorbed...) - complemented by photon detectors, enable significant signals to be collected as the surface is scanned, and various significant images to be formed.

The present article [P 865] recalls the interactions between imaging sources and the constitution of the current instrument. The article [P 866] explains image formation, contrast sources, recent instrument developments and various applications.