Principles and Applications of Scanning Electron Microscopy
Features of SEM
Compared with optical microscope and transmission electron microscope, scanning electron microscope has the following characteristics:
(1) The surface structure of the sample can be directly observed, and the size of the sample can be as large as 120mm×80mm×50mm.
(2) The sample preparation process is simple and does not need to be cut into thin slices.
(3) The sample can be translated and rotated in a three-dimensional space in the sample chamber, so the sample can be observed from various angles.
(4) The depth of field is large, and the image is full of three-dimensionality. The depth of field of the scanning electron microscope is hundreds of times larger than that of the optical microscope and dozens of times larger than that of the transmission electron microscope.
(5) The magnification range of the image is wide and the resolution is relatively high. It can be magnified ten times to hundreds of thousands of times, and it basically includes the magnification range from magnifying glass, optical microscope to transmission electron microscope. The resolution is between optical microscope and transmission electron microscope, up to 3nm.
(6) The damage and contamination of the sample by the electron beam are relatively small.
(7) While observing the morphology, other signals from the sample can also be used for micro-component analysis.
The structure and working principle of scanning electron microscope
1. lens barrel
The lens barrel includes electron gun, condenser lens, objective lens and scanning system. Its function is to generate a very thin electron beam (about a few nm in diameter), and make the electron beam scan the surface of the sample, and simultaneously stimulate various signals.
2. Electronic signal collection and processing system
In the sample chamber, the scanning electron beam interacts with the sample to generate various signals, including secondary electrons, backscattered electrons, X-rays, absorbed electrons, Auger electrons, etc. Among the above signals, the most important ones are the secondary electrons, which are the outer electrons in the sample atoms excited by the incident electrons, which are generated in the area of several nm to tens of nm below the sample surface, and the generation rate mainly depends on Morphology and composition of samples. The so-called scanning electron image usually refers to the secondary electron image, which is the most useful electronic signal for studying the surface morphology of the sample. 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 light guide, and the light signal That is, it is converted into a current signal, and then through pre-amplification and video amplification, the current signal is converted into a voltage signal, and finally sent to the grid of the picture tube.
