Principle and application of Scanning electron microscope
Compared with optical microscopy and transmission electron microscopy, scanning electron microscopy has the following characteristics:
（1） Able to directly observe the surface structure of the sample, with sample sizes up to 120mm × 80mm × 50mm.
（2） The sample preparation process is simple and does not require cutting into thin slices.
（3） The sample can be translated and rotated in three dimensions in the sample chamber, so it can be observed from various angles.
（4） The depth of field is large, and the image is rich in three-dimensional sense. The depth of field of scanning electron microscopy is several hundred times larger than that of optical microscopy and several tens of times larger than that of transmission electron microscopy.
（5） The magnification range of the image is wide, and the resolution is also relatively high. It can be magnified from tens to hundreds of thousands of times, and it basically includes the amplification range from a magnifying glass, optical microscope to a transmission electron microscope. The resolution is between optical microscopy and transmission electron microscopy, reaching up to 3nm.
（6） The damage and contamination of the sample by electron beams are relatively small.
（7） While observing the morphology, other signals emitted from the sample can also be used for micro zone composition analysis.
The structure and working principle of scanning electron microscopy
（1） Structure 1. Mirror tube
The lens barrel includes an electron gun, a condenser, an objective, and a scanning system. Its function is to generate a very fine electron beam (with a diameter of about a few nanometers), and to make the electron beam scan on the surface of the sample, while stimulating various signals.
2. Electronic signal collection 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 electron, X-ray, absorption electron, Auger electron, etc. Among the above signals, the most important one is the Secondary electrons, which is the outer electron in the sample atom excited by the incident electron, generated in the area of several nm to tens of nm below the sample surface, and its production rate mainly depends on the morphology and composition of the sample. Generally speaking, the scanning electric image refers to the Secondary electrons image, which is the most useful electronic signal for studying the surface morphology of samples. The probe of the detector for detecting Secondary electrons (Fig. 15 (2)) is a scintillator. When the electron hits the scintillator, 1 generates light in it. This light is transmitted by the photoconductor to the Photomultiplier tube, and the optical signal is converted into a current signal. 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.
