Properties and Applications of Electron Microscope
1. The working principle of scanning electron microscope
The scanning electron microscope uses a focused electron beam to scan and image the surface of the sample point by point. The sample is bulk or powder particles, and the imaging signal can be secondary electrons, backscattered electrons or absorbed electrons. Among them, secondary electrons are the most important imaging signal. The electrons emitted by the electron gun with an energy of 5-35keV use the cross spot as the electron source, and form a fine electron beam with a certain energy, a certain beam current intensity and a beam spot diameter through the reduction of the secondary condenser lens and the objective lens. Driven by the scanning coil , scan the surface of the sample according to a certain time and space sequence. The focused electron beam interacts with the sample to generate secondary electron emission (and other physical signals), and the amount of secondary electron emission varies with the surface topography of the sample. The secondary electron signal is collected by the detector and converted into an electrical signal. After being amplified by the video, it is input to the grid of the kinescope, and the brightness of the kinescope that is scanned synchronously with the incident electron beam is modulated to obtain a secondary electron image that reflects the surface topography of the sample.
Second, the scanning electron microscope has the following characteristics
(1) Large samples can be observed (larger diameters can be observed in the semiconductor industry), and the sample preparation method is simple.
(2) The depth of field is large, three hundred times that of an optical microscope, which is suitable for the analysis and observation of rough surfaces and fractures; the image is full of three-dimensional, realistic, easy to identify and explain.
(3) The range of magnification is large, generally 15-200000 times, which is convenient for general survey at low magnification and observation and analysis at high magnification for heterogeneous materials with multi-phase and multi-composition.
(4) It has a considerable resolution, generally 2-6cm
(5) The quality of the image can be effectively controlled and improved by electronic methods, such as the tolerance of image contrast can be improved by modulation, so that the brightness and darkness of each part of the image are moderate. Using a double magnification device or an image selector, images of different magnifications or images of different forms can be observed on the fluorescent screen at the same time.
(6) Analysis of various functions can be carried out. When connected with an X-ray spectrometer, it can perform micro-component analysis while observing the morphology; when equipped with accessories such as an optical microscope and a monochromator, it can observe cathodofluorescence images and perform cathodofluorescence spectrum analysis.
(7) Dynamic tests can be carried out using sample stages such as heating, cooling and stretching to observe phase transitions and morphological changes under different environmental conditions.
three. Application of Electron Microscopy
It is an indispensable tool in material defect analysis, metallurgical process analysis, thermal processing analysis, metallography, failure analysis, etc. For example, a military enterprise has the following requirements for the scanning electron microscope in its bidding document: "This set of equipment is used to analyze and measure the chemical composition of material micro-regions, metallurgical defects, and the internal structure of product materials, and is also used for process changes. Analyze and measure the internal and surface structure of the material, changes in morphology and defects, etc. At the same time, the process can be guided according to the results.
