The scanning electron microscope (SEM) is a type of electron microscope capable of producing high resolution images of a sample surface. Due to the manner in which the image is created, SEM images have a characteristic 3-dimensional quality and are useful for judging the surface structure of the sample.
In a typical SEM configuration, electrons are thermionically emitted from a tungsten or LaB6 cathode filiment towards an anode. The electron beam, which typically has an energy ranging from a few keV to 50 keV, is focused by two successive condenser lenses into a beam with a very fine spot size (~ 5nm). The beam then passes through the objective lens, where pairs of scanning coils deflect the beam either linearly or in a raster fashion over a rectangular area of the sample surface. As the primary electrons strike the surface they are inelastically scattered by atoms in the sample. Through these scattering events, the primary beam effectively spreads and fills a teardrop-shaped volume extending about 1 μm into the surface. Interactions in this region lead to the subsequent emission of electrons and x-rays, which are then detected to produce an image.