Scanning Electron Microscopy

A scanning electron microscope (SEM) focuses an electron beam to a sharp probe, with its diameter, which depends on the acceleration voltage and the aberration coefficients of the probe-forming lens, determining SEM resolution. This electron beam is scanned over the specimen and signals arising from a variety of beam-specimen interactions are recorded to form images using different detectors positioned in the specimen chamber. Secondary electrons, detected with the Everton-Thornley detector, reveal the topography and electrical properties; back-scattered electrons provide information about the average atomic number and local crystallography of the specimen. Ferromagnetic materials alter the trajectory of secondary (Type I) and back-scattered (Type II) electrons to provide magnetic contrast. The magnetic polarization of the secondary electrons can also be analyzed directly (SEMPA) to image domains. The electron beam also excites characteristic X-rays for chemical microanalysis. Luminescent specimens produce light (Cathodoluminescence); these photons provide information on the electronic structure, particularly the defect states, of the specimen. Environmental SEMs, with differential pumping, image the specimen in a gaseous environment and/or under hydration for biological materials. A SEM combined with a focused ion beam (FIB) column is used for nano-fabrication, including preparation of electron-transparent TEM specimens.