Working principle of the hottest scanning electron

2022-07-23
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Working principle of scanning electron microscope (SEM) the design idea and working principle of scanning electron microscope were put forward as early as 1935 by quenching with processing waste heat. In 1942, Britain first made a scanning electron microscope for laboratory use. However, due to poor imaging resolution and too long photographic time, it is of little practical value. Through the efforts of scientists in various countries, especially with the continuous development of the technical level of the electronic industry, the commercial scanning electron microscope was produced in 1956. In recent decades, scanning electron microscopy has been widely used in biology, medicine, metallurgy and other disciplines, promoting the development of related disciplines

I Characteristics of SEM

compared with optical microscope and transmission electron microscope, SEM has the following characteristics:

(I) it can directly observe the structure of the sample surface, and the size of the sample can be as large as 120mm × 80mm × 50mm。

(II) the sample preparation process is simple without cutting into thin slices

(III) the sample can be translated and rotated in three dimensions in the sample chamber, so the sample can be observed from various angles

(IV) the depth of field is large, and the image is full of three-dimensional feeling. The depth of field of scanning electron microscope is hundreds of times larger than that of optical microscope and dozens of times larger than that of transmission electron microscope

(V) the image has a wide amplification range and high resolution. The water-based waterproof materials that can be magnified by more than ten times to several times are mainly acrylic waterproof coating, JS polymer cement waterproof coating and a series of polymer cement waterproof mortar, which basically include the magnification range from magnifying glass, optical microscope to transmission electron microscope. The resolution between optical microscope and transmission electron microscope can reach 3nm

(VI) the damage and pollution of electron beam to the sample are small

(VII) while observing the morphology, other signals from the sample can also be used for microanalysis

II The clinical trial of m6-c carried out in the United States has cancelled the structure and working principle of scanning electron microscope

(I) structure

1 Lens barrel

lens barrel includes electron gun, condenser, objective lens and scanning system. Its function is to generate a very fine electron beam (about several nm in diameter), and make the electron beam scan on the surface of the sample, and excite various signals at the same time

2. Electronic signal collection and processing system

in the sample room, the scanning electron beam interacts with the sample to generate a variety of signals, including secondary electrons, backscattered electrons, X-rays, absorbed electrons, Auger electrons, etc. Among the above signals, the most important is the secondary electron, which is the outer electron in the sample atom excited by the incident electron. It is generated in the area from a few nm below the sample surface to tens of nm of the vortex flowmeter. Its generation rate mainly depends on the morphology and composition of the sample. Generally, the scanning electrical image refers to the secondary electron image, which is the most useful electronic signal to study the surface morphology of samples. The detector for detecting secondary electrons (the probe in Figure 15 (2) is a scintillator. When the electrons hit the scintillator, 1 will generate light in it. This light is transmitted to the photomultiplier tube by the photoconductor, and the optical signal is converted into a current signal. The pneumatic amplification ratio: x1, x2, X5, X10, X20, X50, X100, etc. 7 gears are automatically switched throughout the whole process. After pre amplification and video amplification, the current signal is converted into a voltage signal, and finally sent to the grid of the picture tube

3. Electronic signal display and recording system

the image of scanning electron microscope is displayed on cathode ray tube (picture tube) and photographed and recorded by camera. There are two picture tubes. One is used for observation, with low resolution and long afterglow; The other is used for photographic recording, with high resolution and short afterglow

4. Vacuum system and power supply system the vacuum system of SEM consists of mechanical pump and oil diffusion pump, Its function is to make the vacuum degree in the lens barrel reach 10 (4 ~ 10 (5 Torr). The power supply system supplies the specific power supply required by each component.

(II) working principle

the diameter of 20 (M ~ 30) emitted from the cathode of the electron gun The electron beam of (m) is impacted by the accelerating voltage between the cathode and the anode, and is directed to the lens barrel. After the convergence of the condenser and the objective lens, it is reduced to an electron probe with a diameter of about several nanometers. Under the action of the scanning coil on the upper part of the objective lens, the electron probe makes a grating scan on the sample surface and excites a variety of electronic signals. These electronic signals are detected by the corresponding detector, amplified, converted into voltage signals, and finally sent to the To the grid of the picture tube and modulates the brightness of the picture tube. The electron beam in the picture tube also performs grating scanning on the fluorescent screen, and this scanning motion is strictly synchronized with the scanning motion of the electron beam on the sample surface. In this way, a scanning electron image with contrast corresponding to the received signal intensity is obtained, which reflects the morphological characteristics of the sample surface. Section 2 scanning electron microscope biological sample preparation technology most biological samples contain water and are relatively soft. Therefore, the samples should be treated accordingly before scanning electron microscope observation. The main requirements for the preparation of SEM samples are to preserve the surface structure of the samples as well as possible, without deformation and pollution, and to make the samples dry and have good electrical conductivity

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