According to its function, scanning electron microscope consists of six parts: electro-optical system, signal detection and amplification system, scanning system, image display and recording system, vacuum system and power supply system (Figure 5- 1). The electron beam emitted by the electron gun and converged by the electromagnetic lens is scanned to the surface of the solid sample by the scanning coil in a raster manner, and is incident within a range of several microns deep in the sample. After these high-energy electrons interact with atoms in the sample, physical signals such as secondary electrons, backscattered electrons and X-rays are generated in the sample.
Electrons with energy less than 50e V emitted by a solid sample under the action of incident electrons are called secondary electrons (often abbreviated as SE). The energy of most secondary electrons is between 3 and 5 ev. Backscattered electrons (often abbreviated as BE) are incident electrons reflected by solid sample atoms, so they are sometimes called reflected electrons, and their energy is equal to or close to that of incident electrons.
Figure 5- 1 Structure of scanning electron microscope (power supply system not shown)
Scanning electron microscope imaging is similar to closed-circuit television. Physical signals such as secondary electrons and backscattered electrons generated in the sample can be collected by the detector point by point and line by line, respectively. The physical signals are processed in sequence and in proportion and sent to the grid of the cathode ray tube to modulate its brightness, thus displaying the image of the sample. The scanning of the electron beam on the sample surface in the lens barrel of the scanning electron microscope is synchronous with the scanning of the electron beam on the imaging plane in the cathode ray tube. Therefore, the image on the cathode ray tube corresponds to the sample object point by point and line by line. Due to the differences in morphology, composition and structure of different parts of the sample surface, the number of excited secondary electrons and backscattered electrons is different, thus forming different bright and dark images reflecting the surface characteristics of the sample on the cathode ray tube. Therefore, the image of SEM is a contrast image, not a color image. Early SEM images were simulated images and recorded by photographic negatives. In recent years, images have been digitized and can be stored and displayed by computers.
Because of the low energy of secondary electrons, the secondary electrons generated in the sample above the surface 10nm are almost completely absorbed by neighboring atoms, and cannot escape from the sample and are detected by the detector. Therefore, the information reflected by the secondary electron image is completely the characteristics of the sample surface, and it is the most used image in the scanning electron microscope (Figure 5-2).
The characteristics of SEM images are as follows: ① The magnification range is large, and its effective magnification can range from tens to hundreds of thousands of times, which basically summarizes the magnification range from magnifying glass, optical microscope to transmission electron microscope. ② High resolution, large depth of field and strong stereoscopic impression. The resolution of its secondary electron image reaches 3nm, which is about 5 orders of magnitude higher than that of optical microscope. Under the same magnification, the depth of field of SEM image is 10 ~ 100 times larger than that of optical microscope.
Figure 5-2 Scanning Electron Image of Strawberry Pyrite
The basic requirements of scanning electron microscope for samples are as follows: ① The samples must be dry and clean solids, which will not deform or deteriorate under the bombardment of high-energy electron beams and can withstand the pressure of vacuum. ② The sample must be conductive. Non-conductive samples can be sprayed with a conductive film on the surface. In recent years, some non-conductive samples can also be observed at a low acceleration voltage of several hundred volts. Therefore, optical films, films without cover glass and cross sections can be observed by scanning electron microscope. There is no strict requirement on the sample size. The observation area is about 1cm2, and the sample height is less than 1cm2.
In recent years, most scanning electron microscopes are equipped with X-ray energy spectrometer, and sometimes they can be equipped with electron backscattering diffraction components, which can also analyze the composition and structure of micro-regions in situ while observing images. For details, please refer to the relevant parts in the third and fourth sections of this chapter.