Introduction to Scanning Electron Microscopy

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I. Definition of Scanning Electron Microscope A scanning electron microscope (SEM for short) is a complex system that integrates electron optics technology, vacuum technology, precision mechanical structures, and modern computer control technology. It is mainly used to observe the morphology of nanoparticles, their dispersion in the matrix, and measure particle size. SEM can usually only provide morphological information at the micron or sub-micron scale. When a high-energy incident electron beam strikes the sample surface, due to the interaction between the incident electrons and the sample, over 99% of the energy of the incident electrons is converted into thermal energy of the sample. Approximately 1% of the energy of the incident electrons is used to generate various useful forms of information from the sample, including secondary electrons, transmitted electrons, Auger electrons, X-rays, etc. Different information reflects the different physical and chemical properties of the sample itself. The function of a scanning electron microscope is to use different information detectors, based on the mechanisms by which various types of information are generated, in order to enable selective detection of the images produced by the microscope. II. Composition of the scanning electron microscope The scanning electron microscope is mainly composed of an electron optical system and a display unit. The electro-optical system consists of an electron gun, several magnetic lenses, scanning coils, and a sample chamber (an illumination system, which does not require an imaging system) ； The display unit includes the components for signal collection, amplification, processing, display, and recording (i.e., photography). III. Imaging principle of the scanning electron microscope: Under the action of high acceleration voltage, the electrons emitted by the electron gun are focused into a fine electron beam through multiple electromagnetic lenses. Under the action of the scanning coil above the final lens, the electron beam performs a raster scan across the surface of the sample. The interaction between the incident electrons and the sample generates various types of information such as secondary electrons, backscattered electrons, and X-rays. The two-dimensional intensity distribution of this information varies depending on the characteristics of the sample surface. By converting the information collected by various detectors into video signals in a sequential and proportional manner, and then transmitting these signals to a synchronously scanning cathode ray tube to modulate its brightness, a scanned image that reflects the condition of the sample surface can be obtained. If the signals received by the detector are digitized, that is, converted into digital signals, they can then be further processed and stored by a computer. Table 1: Main signals in a scanning electron microscope and their functions
Signal category | Function
Secondary electrons | Morphological observation
Backscattered electrons | Composition analysis, crystallographic studies
Characteristic X-rays | Micro-area composition analysis
Auger electrons | Composition analysis

IV. Characteristics of the scanning electron microscope
The scanning electron microscope is primarily used for observing thick samples with large height differences and rough surfaces; therefore, it features a deep focus effect in its design. It is generally used to analyze fracture surfaces as well as natural surfaces that have not been artificially treated. The main features are as follows: It is possible to directly observe the original surface of large-sized specimens, with these specimens having dimensions of up to 120×80×50 mm. The specimens can be translated and rotated in three dimensions within the specimen chamber, allowing them to be viewed from various angles. The specimen preparation process is simple – there is no need to cut the specimens into thin slices. The image depth of field is large, providing a strong sense of three-dimensionality. The depth of field of a scanning electron microscope is several hundred times greater than that of an optical microscope, and several dozen times greater than that of a transmission electron microscope. It also offers a wide range of magnification options along with high resolution. It can provide magnification of ten to several hundred thousand times; it essentially covers the range of magnification from magnifying glasses and optical microscopes to transmission electron microscopes ； Its resolution lies between that of an optical microscope and a transmission electron microscope, reaching 3 nm; high-resolution images can be obtained of thick samples. The electron beam causes less damage and contamination to the sample. Dynamic observations are possible (such as dynamic stretching, compression, bending, heating, and cooling), and various types of information corresponding to the sample’s morphology can be acquired. Additional functions, such as micro-area composition and crystallographic analysis, can be added without compromising the capabilities of a scanning electron microscope. V. The domestic and international status of scanning electron microscopes. The design concepts and working principles of scanning electron microscopes were proposed as early as 1935. In 1942, Britain was the first to develop a scanning electron microscope for laboratory use, but its practical value was limited due to the poor resolution of the images and the long exposure times required. Thanks to the efforts of scientists around the world, and especially with the continuous advancement of electronic industry technology, commercial scanning electron microscopes began to be produced in 1956. Thereafter, the Netherlands, the United States, and West Germany also developed various models of scanning electron microscopes, while Japan produced scanning electron microscopes after World War II with the support of the United States. In China, its own scanning electron microscopes were produced in the 1970s. For over half a century, the goal of electron microscopy has been to observe the structures of even smaller objects, finer entities, and even individual atoms, in order to obtain more information about the sample. This includes characterizing amorphous and microcrystalline materials, as well as aspects such as composition distribution, grain shape and size, crystal phases, crystal orientation, grain boundaries, and crystal defects. Such information enables comprehensive analysis and characterization of a material’s microstructure. Over the past few decades, scanning electron microscopes have been widely used in fields such as biology, medicine, chemistry, rare earths science, metallurgy, and nanoscience, contributing to the development of these related disciplines. VI. Types of Scanning Electron Microscopes There are mainly the following types of scanning electron microscopes: analytical scanning electron microscopes, field-emission gun scanning electron microscopes, low-voltage scanning electron microscopes, scanning electron microscopes with extremely large sample chambers, environmental scanning electron microscopes, scanning electroacoustic microscopes, scanning electron microscopes for length/defect detection, scanning electron microscopy for crystallographic orientation imaging, and computer-controlled scanning electron microscopes. 6.1 Analytical Scanning Electron Microscopes and Their Accessories An analytical scanning electron microscope is one that is equipped with various additional instruments to enable the analysis of multiple types of information regarding the sample being tested. The common accessories include the following: Energy dispersive spectrometer accessories, Electron backscatter diffraction instrument accessories, and Spectrometer accessories. 6.1.1 Energy Dispersive Spectrometer Accessories An energy dispersive spectrometer (i.e., X-ray energy dispersive spectrometer, abbreviated as EDS) usually refers to an X-ray energy spectrometer. A spectrometer is primarily used to analyze the composition of micro-regions on a material’s surface. The analysis methods include qualitative analysis at a specific point, quantitative analysis at a specific point, linear distribution of elements, and planar distribution of elements. For example, the composition analysis of inclusions, the diffusion depth of elements in the two phases, and the distribution of elements in multi-phase particles. It is characterized by fast analysis speed, and as an auxiliary tool for scanning electron microscopes, it enables component analysis without affecting image resolution. 6.1.2 Electron Backscatter Diffraction Appendix: Special detectors were developed for backscattered electron diffraction, and computer technology was incorporated to create the technique of backscattered electron diffraction analysis – which is what we commonly refer to as Electron Backscatter Diffraction (EBSD). EBSD is primarily used for phase analysis of single crystals, while also providing pattern quality, confidence indices, and colored grain maps. However, it is subject to various limitations depending on the testing conditions: the single crystal being tested must be intact and free of stress; the sample must be flat and its spatial orientation with respect to the incident electrons 70b must always be maintained; EBSD can only be used to analyze grains larger than a few microns. 6.1.3 Spectrometer accessories The spectrometer (i.e., X-ray wavelength dispersive spectrometer, abbreviated as WDS) was originally developed alongside the invention of the electron probe; it is a core component of the electron probe and is used for composition analysis. The principle of compositional analysis can be expressed by the formula: λ=(d/R)L, where λ is the wavelength of the X-rays generated when an electron beam excites the sample, and d is the interplanar spacing of the diffraction crystal ； R — Radius of the focusing circle of the spectrometer; L — Distance between the X-ray source and the spectroscopic crystal. The element can be determined based on the X-ray wavelength. Therefore, the spectrometer performs component analysis by changing the distance L through the movement of mechanical devices. The spectrometer has a high detection sensitivity, capable of achieving a detection capability of 100x10-6 under the ideal operating conditions of an electron probe. However, spectrometers have strict requirements regarding analysis conditions: the electron beam current must be greater than 0.1 μA; the sample must be extremely flat and can only be placed horizontally. The quantitative analysis of components is accurate, and comparison with standard samples is possible; moreover, the stability of the main unit is very high. 6.2 Field Emission Scanning Electron Microscope (FSEM): It uses a field emission electron gun, which requires a high level of vacuum. In such a high-vacuum environment, the scattering of the electron beam is reduced, thereby improving its resolution further. In recent years, multi-stage vacuum systems have been used, achieving a vacuum level of 10-7 Pa. At the same time, the use of magnetic levitation technology significantly reduces noise and vibration, and also increases the lifespan of the filament. Beam stability within 12 hours