A scanning electron microscope (SEM) is a type ofthat images a sample by scanning it with a high-energy beam of en a pattern. The electrons interact with the atoms that make up the sample producing signals that contain information about the sample's surface , composition, and other properties such as
The types of signals produced by an SEM include s electrons (BSE), , light, specimen current and transmitted electrons. Secondary electron detectors are common in all SEMs, but it is rare that a single machine would have detectors for all possible signals. The signals result from interactions of the electron beam with atoms at or near the surface of the sample. In the most common or standard detection mode, secondary electron imaging or SEI, the SEM can produce very high-resolution images of a sample surface, revealing details less than 1 nm in size. Due to the very narrow electron beam, SEM micrographs have a large yielding a characteristic three-dimensional appearance useful for understanding the surface structure of a sample. This is exemplified by the micrograph of pollen shown to the right. A wide range of magnifications is possible, from about 10 times (about equivalent to that of a powerful hand-lens) to more than 500,000 times, about 250 times the magnification limit of the best . Back-scattered electrons (BSE) are beam electrons that are reflected from the sample by . BSE are often used in analytical SEM along with the spectra made from the characteristic X-rays. Because the intensity of the BSE signal is strongly related to the atomic number (Z) of the specimen, BSE images can provide information about the distribution of different elements in the sample. For the same reason, BSE imaging can images of 5 or 10 nm diameter which would otherwise be difficult or impossible to detect in secondary electron images in biological specimens. Characteristic are emitted when the electron beam removes an from the sample, causing a to fill the shell and release energy. These characteristic X-rays are used to identify the composition and measure the abundance of elements in the sample.
