Principles and applications of scanning electron microscopy (SEM)
Characteristics of Scanning Electron Microscope
Compared with optical microscope and transmission electron microscope, scanning electron microscope has the following characteristics:
(i) It is capable of directly observing the structure of the sample surface, and the size of the sample can be as large as 120mm x 80mm x 50mm.
(ii) The sample preparation process is simple, without having to cut into thin slices.
(iii) The sample can be translated and rotated in three degrees of space in the sample chamber, so that the sample can be observed from various angles.
(iv) The depth of field is large, and the image is rich in three-dimensional sense. The depth of field of SEM is hundreds of times larger than that of optical microscope and tens of times larger than that of transmission electron microscope.
(E) image magnification range is wide, the resolution is also relatively high. Can be magnified a dozen times to hundreds of thousands of times, it basically includes from the magnifying glass, optical microscope until the transmission electron microscope magnification range. Resolution between the optical microscope and transmission electron microscope, up to 3nm.
(vi) The damage and contamination of the sample by the electron beam is small.
(vii) While observing the morphology, other signals emitted from the sample can be used for micro-regional compositional analysis.
Structure and working principle of scanning electron microscope
(a) Structure 1. barrel
The barrel includes the electron gun, condenser mirror, objective lens and scanning system. Its role is to produce a very fine electron beam (diameter of about a few nm), and make the electron beam in the sample surface scanning, while excitation of a variety of signals.
Electron Signal Collection and Processing System
In the sample chamber, the scanning electron beam interacts with the sample to produce a variety of signals, including secondary electrons, back-scattered electrons, X-rays, absorbed electrons, Auger electrons, and so on. In the above signals, the most important is the secondary electrons, which are the outer electrons in the sample atoms excited by the incident electrons, generated in the region of a few nm to tens of nm below the surface of the sample, and its generation rate depends mainly on the morphology and composition of the sample. The scanning electron microscope image is commonly referred to as the secondary electron image, which is the most useful electron signal for studying the surface morphology of the sample. Detection of secondary electrons detector (Figure 15 (2) of the probe is a scintillator, when the electrons hit the scintillator, 1 in which the light is generated, this light is transmitted to the photomultiplier tube by the light pipe, the light signal that is converted into a current signal, and then by the preamplifier and the video amplification, the current signal is converted into a voltage signal, and finally be sent to the gate of the picture tube.
