Introduction to Scanning Electron Microscope
The electron beam emitted by the electron source is focused through the electron optical path of the electromagnetic lens. The diameter of the electron source is reduced to a nano-scale electron beam spot. The scanning coil in the electron optical lens barrel synchronized with the scanning of the display controls the electron beam and must be tiny on the sample surface. Within the area, scan point by line. The electron beam interacts with the sample, and the signal with imaging contrast emitted from the sample is collected point by point by an appropriate image detector, and the signal passes through the preamplifier and video amplifier, and is modulated on the display by a modulation and demodulation circuit. The brightness of the display pixels forms an image that we humans observe and reflects the two-dimensional shape of the sample or other understandable contrast mechanism images. Since the pixel size of the image display is much larger than the size of the electron beam spot, (0.1mm / 1nm = 100,000 times) and the pixel size of the display is less than or equal to the normal resolution of the human eye, so the image on the display is equivalent to the corresponding small The area is enlarged. By adjusting the deflection magnetic field of the scanning coil, the size of the scanning area of ​​the electron beam on the surface of the sample can be controlled. In theory, the scanning area can be infinitely small, but the limit of the effective magnification of the displayed image is the limit of the resolution of the scanning electron microscope.
Analog image scanning system: the analog signal of each pixel on the sample directly modulates the brightness of the display pixel corresponding to the cathode ray tube. Since it takes a few seconds or tens of seconds / frame to generate a high-quality image, the analog electron microscope uses a slow afterglow tube terminal to display a In order to facilitate the observation of the image on the CRT, a dark room is required. The operator can adjust the instrument parameters according to certain procedures, such as image focusing, moving the sample stage to search for the area of ​​interest, adjusting the magnification, brightness contrast, eliminating astigmatism, etc. The best image quality. The analog image output uses a high-resolution camera tube, which is directly recorded on the film point by point with a SLR camera, and then the photos are processed. Since 1985, analog image electron microscopy has been replaced by digital electron microscopy.
Digital image scanning system: the imaging signal emitted by each pixel on the sample is amplified by the pre-amplifier and video amplifier after being detected by the image detector, and the signal is directly digitized, and then stored in the frame memory of the image acquisition card to form Digital image data. The image data can be read by the electron microscope operating software. The operator adjusts and controls the image on the graphical interactive interface (GUI) and stores the adjusted digital image in the hard disk of the computer.
Analog control is the control signal is not directly controlled by the computer software, and the actuator is directly controlled by the operation panel key knob, etc., which is manually controlled, and the control accuracy is determined by the operator observing the change of the instrument panel. For example, high-voltage power supply, scanning coil, detector power , Electron gun control, magnetic lens control, sample stage motion control, etc.
Digital control is that all control commands are input to the interactive control software, and then the computer sends precise digital commands to the actuator. Most of the parameters of the scanning electron microscope can be automatically adjusted by the software, such as automatic focusing, brightness contrast, automatic astigmatism, automatic Electron gun launch saturation, automatic electron gun centering, automatic objective diaphragm closing axis, etc. If the scanning electron microscope detects samples of the same shape and material, the automatic program control can achieve the best image quality without any intervention of the operator, which has been widely used in the semiconductor chip manufacturing line. Due to the many types of sample shapes and materials faced by laboratory scanning electron microscopes, the imaging mechanism is very different. Program control is often not satisfactory. To obtain good image quality, operators often need to adjust and intervene in the parameters of the electron microscope. The working parameters of the electron microscope can be saved as an executable control file, which can be called at any time when analyzing the same type of sample in the future.
With the development of electronic technology and computer technology, not only have digital images been realized, but all functions of the electron microscope have been digitally controlled. The modern SEM electrical control system is highly integrated, the structure of SEM is becoming more and more compact, and the automation function is getting higher and higher, which greatly improves the man-machine operating environment.
Brief introduction of the use of scanning electron microscope
The most basic function is to perform high-resolution morphological observation on the surface of various solid samples. Large depth of field images are the characteristics of SEM observations, such as: biology, botany, geology, metallurgy, etc. The observation can be the surface of a sample, or a cut surface, or a section. Metallurgists have been excited to see the original or worn surface directly. It is very convenient to study the defects of oxide surface, crystal growth or corrosion. On the one hand, it can more directly check the fine structure of paper, textiles, natural or prepared wood, and biologists can use it to study the structure of small fragile samples. For example: pollen particles, diatoms and insects. On the other hand, it can take strong three-dimensional photos corresponding to the sample surface.
In the application of scanning electron microscopy, many focus on semiconductor devices and integrated circuits. It can check the actual situation of local surface voltage changes during device operation in detail, because such changes will bring about changes in contrast. Weld cracking and corrosion surface details or interrelationships can be easily observed. Using the beam-induced current, defects inside the semiconductor PN junction can be observed.
The electron beam and the sample area also emit signals related to other properties of the sample material. For example: related to the chemical composition distribution of the sample, backscattered electrons, characteristic X-rays, Auger electrons, cathode fluorescence, sample absorption current, etc .; related to the crystal structure of the sample, detection of backscattered electron diffraction phenomena; and electrical properties of semiconductor materials Related, secondary electron signals, electron beam induced current signals; transmitted electron signals generated when observing thin samples, etc. At present, there are commercially available detectors and devices that can be installed in the scanning electron microscope sample analysis room for detection and qualitative and quantitative analysis of sample material related information.
Scanning electron microscopy is very widely used for the research of solid materials, and no instrument can compare with it. Scanning electron microscopy is essential to describe the comprehensive characteristics of solid materials.
The specific functional uses are summarized as follows:
1. Scanning electron microscope pursues high-resolution topography of solid materials, morphological image (secondary electron detector SEI)-topography analysis (surface geometry, shape, size)
2. Show the spatial variation of chemical composition, based on the phase identification of chemical composition --- chemical composition image distribution, micro area chemical composition analysis
1) Use x-ray energy spectrometer or spectroscopy (EDSorWDS) to collect characteristic x-ray signals, generate element surface distribution map corresponding to sample morphology, or perform qualitative and quantitative analysis of fixed-point chemical composition for phase identification.
2) Using backscattered electrons BSE) based on the contrast of the average atomic number (generally related to relative density) to generate an image of the distribution of chemical constituent phases;
3) Using cathode fluorescence, based on the contrast of the light intensity of certain trace elements (such as transition metal elements, rare earth elements, etc.) excited by the electron beam, a trace element distribution image is generated.
4) Using the sample current, based on the contrast of the average atomic number, a distribution image of the chemical composition phase generated, which is opposite to the back-scattered electron image.
5) Using Auger electrons, qualitative theorem analysis of chemical element distribution on the surface of the sample material at 1 nm,
3. Special applications in semiconductor device (IC) research:
1) Imaging using electron beam induced current EBIC can be used to locate and damage pn junctions in integrated circuits
2) Using sample current imaging, the results can show the opening and short circuit of the metal layer in the circuit, so the resistance contrast image is often used to check the metal wiring layer, polycrystalline wiring layer, metal to silicon test pattern and the conductive form of the thin film resistance .
3) The secondary electron potential contrast image is used to reflect the potential of the sample surface. From it, the level and distribution of the potential on the surface of the sample can be seen, especially for the determination of the hidden open circuit or hidden short circuit of the device.
4. The backscattered electron diffraction signal is used to analyze the crystal structure of the sample material (the arrangement of atoms in the crystal), the analysis of the crystal orientation distribution, and the phase identification based on the crystal structure.
The above are the main functions of the scanning electron microscope. At the same time, the scanning electron microscope can also be used as a micro-operation platform, which can be equipped with nano-manipulators, micro-mechanical probes, ion guns and other devices for ion cutting processing, nano-operation, micro-scale physical and chemical properties measurement; Dynamic observation and the environment where the material is located can be equipped with special sample stage, such as mechanical stretching stage, high temperature sample stage, low temperature sample stage, and the sample analysis room is filled with special gas that can react with the sample physically and chemically.
summary:
Scanning electron microscopes usually have at least one detector (usually a secondary electron detector SEI), many are equipped with additional detectors, such as EDS, BSE, WDS, EBSD, CL, etc., but the special functions of specific instruments strongly depend on A suitable detector, and the installation angle of the detector, that is, the professional design of the sample analysis room, have a great influence on the analysis results. In-depth understanding of various functional mechanisms can obtain the best quality of electron microscopic analysis.
There are many types of SEM models in the market. From an economic point of view, not all models are designed to be versatile. The user must select the appropriate scanning electron microscope according to the observation and analysis needs of the sample.
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