Scanning Electron Microscopy (SEM)

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Scanning-electron-microscopes use a high-energy beam of electrons, combined with various detection systems to view very small areas- or to analyze the constitution of a surface (see figure on the right). This type of microscopes are very powerful, and have an average resolution between 7nm and 3nm. Large prints may therefore show magnifications of up to 500’000:1, or even above… !

History of Electron-Microscopy (EM)

The Scanning-Electron-Microscopy (SEM) technology was invented in the early 30ties, by subsequent Noble prize winner Ernst Ruska (1906-1988) and colleague Max Knoll (1897-1969). Their first prototype was a transmisson-electron-microscope (TEM), which uses electrons, instead of light, to produce images of a thin (tissue) layer. Resultant images typically display ultra-high magnifications of cellular compartments and/or viruses in black-and-white.
Ultra-high magnifications of surfaces only became possible several years later, after Manfred von Ardenne (1907-1997) had further developed EM-technology. The SEM was patented in the early 40ties but the first prototype was delayed until after world war II, when it was presented by Sir Charles Oatley and colleagues from the Campbridge University. The first scanning-electron-microscope’s (SEM) became commercially available in the mid 60-ties.

Since then, the progress has never stopped in the field of microscopy and today other innovative technologies allow analysis at the atomic level (e.g. scanning-tunneling-electron-microscope and atomic-force-microscope).

Interaction between primary electron beam and specimen surface

A high-energy (primary) electron beam (up to 30k eV) is thermoionically emitted from a tungsten filament cathode or -cristal tip of the electron gun. Magnetic scanning-coils are then used to adjust and focus the beam onto the specimen. The technology is called “scanning”-electron-microscopy (auf Deutsch: “Raster”-Elektronen-Mikroskopie), because the electron beam is shifted little by little over a rectangular area. Thereby, the area is ‘scanned’ from one pixel to the next.

When primary electrons interact with the sample, they lose energy by random scattering and absorption, within a teardrop-shaped volume of the specimen. This volume may extend from less than 100nm to around 5µm into the surface, the size depending on the specimen’s density. The interactions between primary electrons and specimen lead to emission of secondary electrons and electromagnetic radiation, which can be detected by specific detectors.

SEM Analysis

With a SEM it is possible to analyse different aspects of a specimen by different detectors (SEI/EDX/BSE/WDX), e.g. to gain information about a sample’s surface structure, composition, electrical conductivity, etc. Back-scattered electrons (BSE) are beam electrons that are reflected from the sample. They can be used along with the spectra made from characteristic X-rays to provide information about the distribution of different elements in the sample. On the other hand side, X-rays can be used to identify the composition and measure the abundance of elements in the specimen. Secondary electrons, which are also emitted, can be used to create a topographic image of the specimen’s surface.

Depth of view (DOF)

Due to the very narrow electron beam, SEM scans have a large depth of field, offering a characteristic 3D appearance, which is especially useful for understanding the surface structure of a sample. The DOF can be influenced in a similar way, as with an ordinary camera, by manually controlling aperture and focal distance.

Continue reading how Micronaut images receive their precise color costumes: Coloration (p. 4/4)