1. Basic structure and principle of scanning electron microscope
Scanning electron microscope is basically composed of electron optical system, signal receiving and processing display system, power supply system and vacuum system. Figure 13-2- 1 is the structural principle block diagram of the first two parts. The photoelectric part only has focusing lenses, and their functions are completed by the signal receiving, processing and display system.
Figure 13-2- 1 basic structure diagram of scanning electron microscope.
In the scanning electron microscope, the electron beam emitted by the electron gun is focused by three electromagnetic lenses, forming a diameter of 20 microns to 25? Electron beam. The scanning coil placed on the upper part of the final lens can make the electron beam scan on the sample surface in a raster manner. Under the action of electron beam, the sample excites various signals, and the intensity of the signals depends on the surface morphology of the sample, the composition of the excitation region and the crystal orientation. The detector near the sample receives the excited electrical signal, which is transmitted to the grid of cathode ray tube (CRT) after signal processing and amplification system to modulate the brightness of CRT. Because the electron beam in the picture tube and the electron beam in the lens barrel are synchronously scanned, the brightness of the picture tube is modulated by the intensity of the electrical signal excited by the sample, and the signal intensity collected from any point on the surface of the sample corresponds to the corresponding brightness on the picture tube screen, so the brightness corresponding to different sample states is bound to be different. Therefore, the obtained image must be a reflection of the sample morphology. If the wavelength and energy of characteristic X-rays are collected by a spectrometer and an energy spectrometer and placed obliquely above the sample, the composition analysis can be carried out.
It is worth noting that the incident electron beam scans point by point on the surface of the sample, which seems to be recorded point by point. Therefore, all kinds of signals excited by each point of the sample can be selected and recorded, and can be displayed on several adjacent picture tubes or X-Y recorders at the same time, which brings great convenience to the comprehensive analysis of the sample.
Secondly, the interaction between high energy electron beam and sample.
And stimulate all kinds of information from the sample. For gem workers, the most commonly used are secondary electrons, backscattered electrons and characteristic X-rays. The mechanism of the above information is different. By using different detectors and selectively receiving some information, the composition analysis (characteristic X-ray) or morphology observation (secondary electrons and backscattered electrons) of the sample can be carried out. This information mainly has the following characteristics:
1. Secondary electron
From 100 to the sample surface? Inelastic collisions occur between low-energy electrons excited in the depth range of about 0 ~ 50 ev (generally about 0 ~ 50 ev). Secondary electron image is the most widely used and highest resolution image in SEM, and the imaging principle is also representative. The high-energy incident electron beam (usually 10 ~ 35 kev) is controlled by the magnetic field of the scanning coil, and the surface of the sample is scanned in space for a certain period of time, so that secondary electrons are excited from the sample. The excited secondary electrons are amplified by the secondary electron collector, scintillator, light pipe, photomultiplier tube and video amplifier into strong enough electrical signals to modulate the brightness of CRT. Because the scanning of the incident electron beam on the sample and the scanning of the electron beam on the picture tube screen are modulated by the same scanning generator, this ensures that any object point in the sample corresponds to any "image point" on the screen in time and space; At the same time, the amount of secondary electron excitation changes with the change of surface roughness of the sample, so a secondary electron image with different brightness reflecting the surface morphology of the sample appears on the screen of the picture tube. Because of the low energy of secondary electrons, in order to collect enough information, the secondary electron detector must be at a positive potential (generally +250 V). Under the action of this positive potential, the secondary electrons emitted from the sample surface in all directions are pulled to the collector (Figure 13-2-2a), which makes the secondary electron image have no image, and the observation is more real and intuitive.
2. Backscattered electrons
The energy of the incident electrons scattered from the depth range of 0. 1 ~ 1 micron on the surface of the sample is approximately equal to that of the original incident electrons, and they collide elastically. The imaging process of backscattered electron image is almost the same as that of secondary electron image, except that different detectors are used to receive different information, as shown in figure 13-2-2.
Figure 13-2-2 lighting effects of secondary electron image and backscattered electron image
(according to S.Kimoto, 1972)
A: Secondary electronic detection method; A': the lighting effect of the secondary electronic image; B: backscattered electron detection method; B': Illumination effect of backscattered electronic images
3. Characteristic X-ray
The excited elemental characteristic X-rays in the sample are released (the emission depth is in the range of 0.5 ~ 5 microns). However, in order to analyze the elemental composition of the sample micro-area, it is necessary to use excited characteristic X-rays. This is the so-called "electron probe analysis", and the method to determine the wavelength of characteristic X-rays is usually called wavelength dispersion method (WDS). The method of measuring characteristic X-ray energy is called energy dispersion method (EDS). Scanning electron microscope (SEM) can be applied to the surface morphology of gemstones, and it often needs energy spectrum (EDS) for composition analysis. EDS is mainly composed of high efficiency lithium drift silicon semiconductor detector, amplifier, multichannel pulse amplitude analyzer and recording system. The characteristic X-rays excited by the sample are incident on the lithium drift silicon semiconductor detector, resulting in electron-hole pairs, which are then converted into current pulses, amplified and separated by a multi-channel pulse height analyzer according to energy levels. From the energy position of these pulses, we can know the types of elements contained in the sample, and from the number of pulses with corresponding energy, we can know the relative content of elements. It is easy to determine the composition of gem minerals by this method.
If equipped with an energy dispersive spectrometer (EDS) on a scanning electron microscope, not only can the gemstone morphology image be made without destroying the sample, but also the composition can be analyzed quickly (as shown in figure 13-2-3, Liao Shangyi, 200 1). Therefore, this is a good way to identify and distinguish similar gem minerals, such as red garnet, ruby, spinel, rubellite and so on. Because of their different compositions, their energy spectrum (EDS) diagrams are quite different. The quantitative analysis of spectrum (WDS) is more accurate than energy spectrum (EDS), but EDS is faster.
Figure 13-2-3 energy spectrum of blue potassium-sodium amphibole
Thirdly, the microscopic morphology was observed by scanning electron microscope.
1. Sample preparation
If you choose powder samples, you need to select the sample table in advance. If it is a block sample, the maximum diameter is generally not more than15 mm. If only morphological images are observed, a slightly larger diameter (39mm) is still acceptable, but the sample must be conductive. If it is a non-conductive sample, it must be covered with a layer of about 200? Is the thickness of carbon still 150? Gold.
2. Get 2. SEM image
Fig. 13-2-4 secondary electronic images of yingshi (a) and blue amphibole jade (b) under scanning electron microscope.
In order to observe the morphology of samples, secondary electron images or backscattered electron images are usually used. Fig. 13-2-4 is the secondary electron image of yingshi (a) and blue amphibole jade (K-Na amphibole b). At the same time, secondary electronic images are more commonly used than backscattered electronic images because of their high resolution and high magnification. Backscattered electron images are often used for component analysis.