The principle and structure of scanning electron microscope
Scanning electron microscope, the full name of scanning electron microscope, English is scanning electron microscope (SEM), is an electronic optical instrument used to observe the surface structure of objects.
1. The principle of scanning electron microscope
The fabrication of scanning electron microscopes is based on the interaction of electrons with matter. When a beam of high-energy human electrons bombards the surface of a material, the excitation region produces secondary electrons, Auger electrons, characteristic X-rays and continuum X-rays, backscattered electrons, transmission electrons, and electromagnetic radiation in the visible, ultraviolet, and infrared regions. . At the same time, electron-hole pairs, lattice vibrations (phonons), and electron oscillations (plasmons) can also be generated. For example, the collection of secondary electrons and backscattered electrons can obtain information on the microscopic morphology of the material; the collection of X-rays can obtain information on the chemical composition of the material. Scanning electron microscopes work by scanning a sample with an extremely fine electron beam, exciting secondary electrons on the surface of the sample. The first-order electrons are collected by the detector, converted into optical signals by the scintillator there, and then converted into electrical signals by photomultiplier tubes and amplifiers, which control the intensity of the electron beam on the phosphor screen, and display the scanned image in synchronization with the electron beam. The images are three-dimensional images that reflect the surface structure of the specimen.
2. The structure of scanning electron microscope
(1) Lens barrel
The lens barrel includes the electron gun, condenser lens, objective lens and scanning system. Its role is to generate an extremely fine electron beam (about a few nanometers in diameter) that scans the surface of the sample while exciting various signals.
(2) Electronic signal acquisition and processing system
In the sample chamber, the scanning electron beam interacts with the sample to generate a variety of signals, including secondary electrons, backscattered electrons, X-rays, absorbed electrons, Russian (Auger) electrons, and more. Among the above-mentioned signals, the most important are secondary electrons, which are outer electrons excited by incident electrons in the sample atoms, and are generated in the region several nanometers to tens of nanometers below the sample surface. The generation rate is mainly determined by the morphology and composition of the sample. The scanning electron microscope image usually refers to the secondary electron image, which is the most useful electronic signal to study the surface topography of the sample. The probe of the detector that detects the secondary electrons is the scintillator. When the electrons hit the scintillator, light is generated in the scintillator. This light is transmitted through the light pipe to the photomultiplier tube, which converts the light signal into a current signal, which is then passed through Preamplification and video amplification convert the current signal into a voltage signal, which is finally sent to the grid of the picture tube.
(3) Electronic signal display and recording system
Scanning electron microscope images are displayed on a cathode ray tube (picture tube) and recorded by a camera. There are two kinds of picture tubes, one is used for observation and has a lower resolution and is a long afterglow tube; the other is used for photographic recording and has a higher resolution and is a short afterglow tube.
(4) Vacuum system and power supply system
The vacuum system of the scanning electron microscope consists of a mechanical pump and an oil diffusion pump. The power supply system provides the specific power required by each component.
3. The purpose of scanning electron microscope
The most basic function of scanning electron microscopes is to observe the surfaces of various solid samples at high resolution. Large depth of field images are a feature of scanning electron microscope observations, such as: biology, botany, geology, metallurgy, etc. Observations can be sample surfaces, cut surfaces or cross-sections. Metallurgists are happy to see pristine or worn surfaces directly. Easily study oxide surfaces, crystal growth or corrosion defects. On the one hand, it can more directly examine the fine structure of paper, textiles, natural or processed wood, and biologists can use it to study the structure of small, fragile samples. For example: pollen particles, diatoms and insects. On the other hand, it can take three-dimensional pictures corresponding to the surface of the sample. Scanning electron microscopy has a wide range of applications in the study of solid materials, and is comparable to other instruments. For complete characterization of solid materials, scanning electron microscopy.
