A SEM microscope is a type of microscope that uses a beam of electrons combined with detectors to view very small areas. The device is usually referred to as a SEM, as the letters are an acronym for the microscope’s correct name - Scanning Electron Microscope. This type of microscope is extremely powerful, and has an average useful resolution between 7 nm and 3 nm, though lower resolutions have been achieved.
SEMs work by interpreting data from detectors when a beam of electrons is directed at a specimen. The electron beam is generated by a filament inside the SEM’s electron gun, then travels down the column towards the specimen. While in the column, the path of the electrons is moved, condensed, blocked, and/or altered by various parts in order to improve the imaging. The column opens into the specimen chamber, where the beam of electrons strikes the specimen. Electrons that are released or reflected by the specimen will then strike detectors that are in the specimen chamber. The results of the strikes are then used to create highly magnified images of the specimen.
The electrons released by a specimen in a SEM can be detected in many different ways; the three most common, however, are through backscatter, secondary, and x-ray imaging. Backscatter electrons (BSE) tend to penetrate deeply into the surface of a specimen, and images produced through their detection can more easily show contrast in materials within the substance. Secondary electrons are used to produce images of a specimen’s surface, and can result in stunning 3-D representations. X-ray detectors can tell what elements make up a specific part of a specimen, and are often used in forensics. Other detection methods also exist, and include cathodoluminescence and auger detection.
The “S” in SEM stands for Scanning, one aspect that differentiates the SEM from other types of electron microscopes. Instead of using a fixed beam of electrons, the SEM uses a beam that moves over the desired area in what is known as a raster pattern (rastering). Rastering provides many benefits, and is one of the reasons images produced from the secondary detector have almost a 3-D quality.
SEMs are used in many different areas of research, but are probably most famous for the roles they play in forensic science. One method for testing for gunshot residue involves swabbing the back of a suspect’s thumb, webbing, and trigger finger; the swab is then analyzed using backscatter detection, with areas of interest being examined with x-ray detection to determine what they are made of. Backscatter detection can also be used to examine the surface composition of an object, and anomalous areas can be tested using x-ray detection to find undesirable materials such as lead.