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Scanning Electron Microscope A To Z
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Scanning Electron Microscope A To Z - 1

Scanning Electron Microscope A To Z Basic Knowledge For Using The SEM Serving Advanced Technology

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Scanning Electron Microscope A To Z - 3

The Scanning Electron Microscope (SEM) is used for observation of specimen surfaces. When the specimen is irradiated with a fine electron beam (called an electron probe), secondary electrons are emitted from the specimen surface. Topography of the sur- face can be observed by two-dimensional scanning of the electron probe over the surface and acquisition of an image from the detected secondary electrons. The SEM requires an electron optical system to produce an electron probe, a specimen stage to place the speci- men, a secondary-electron detector to collect secondary electrons, an image...

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Scanning Electron Microscope A To Z - 4

An electron microscope generally uses a magnetic lens. When you pass a direct electric current through a coil- wound electric wire, a rotationally-symmetric magnetic field is formed and a lens action is produced on an elec- tron beam. To make a strong magnetic lens (with a short focal length), it is necessary to increase the density of the magnetic line. Thus, as shown in Fig. 3, the surroundings of the coil are enclosed by yokes so that part of the mag- netic field leaks from a narrow gap. A portion with a nar- row gap, called "polepiece," is fabricated with a high accuracy. The main...

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Scanning Electron Microscope A To Z - 5

•Role of the objective lens The objective lens is used for focusing, and this lens is a very important lens that determines the final diameter of the electron probe. If the performance of the objective lens is not good, an optimally-fine electron probe cannot be produced despite all of the efforts before the action of the objective lens. Thus, it is crucial to make the objective lens with the best performance. Specimen Stage In general, the specimen is observed at a high magnification in an electron microscope. Thus, a specimen stage, which stably supports the specimen and moves smoothly,...

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Scanning Electron Microscope A To Z - 6

Secondary Electron Detector The secondary electron detector is used for detecting the secondary electrons emitted from the specimen. Its construction is shown in Fig. 6. A scintillator (fluorescent substance) is coated on the tip of the detector and a high voltage of about 10 kV is applied to it. The secondary electrons from the specimen are attracted to this high voltage and then generate light when they hit the scintillator. This light is directed to a photo-multiplier tube (PMT) through a light guide. Then, the light is converted to electrons, and these electrons are amplified as an...

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Scanning Electron Microscope A To Z - 7

When the specimen surface is two-dimensionally scanned by the electron probe, a SEM image appears on the monitor screen of the display unit. At this time, if the scan width of the electron probe is changed, the magnification of the displayed SEM image is also changed. Since the size of the monitor screen is unchanged, decreasing the scan width increases the magnification, whereas increasing the scan width decreases the magnification. Figure 7 shows this principle. For example, when the size of the monitor screen is 10 cm and the scan width of the electron probe is 1 mm, the magnification is...

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Depth of Focus of SEM In the observation of a specimen with a substantial depth, if the focus is adjusted to the top side, the bottom side may be out of focus. In such a case, if the range between upper and lower image blur is large, it is said that “the depth of focus is large.” Whereas if the range between upper and lower image blur is small, it is said that “the depth of focus is small.” As shown in Fig. 8, when the electron probe is considerably parallel (aperture angle is small), the image stays in focus even if the focus is changed by a large amount. Whereas when the electron probe is...

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Scanning Electron Microscope A To Z - 9

Since the SEM image appears as if you observe an object with the naked eye, you may intuitively understand the features of the object. However, the SEM image often produces a contrast that is difficult to explain. To fully understand the contrast of the SEM image, you must understand the principle of the formation of the SEM image. Interactions of Electrons with Specimens When electrons enter the specimen, the electrons are scattered within the specimen and gradually lose their energy, then they are absorbed in the specimen. This behavior is shown in Fig. 11. The scattering range of the...

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Secondary Electrons When the incident electron beam enters the specimen, secondary electrons are produced from the emission of the valence electrons of the constituent atoms in the specimen. Since the energy of secondary electrons is very small, those generated at a deep region are quickly absorbed by the specimen. Only those generated at the top surface of the specimen are emitted outside of the specimen. This means that secondary electrons are very sensitive to the surface. In addition, as shown in Fig. 14, compared to when the incident electron beam enters perpendicularly to the...

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Scanning Electron Microscope A To Z - 11

Backscattered Electrons Backscattered electrons are those scattered backward and emitted out of the specimen, when the incident elec- trons are scattered in the specimen. They are sometimes called reflected electrons. Since backscattered electrons possess higher energy than secondary electrons, infor- mation from a relatively deep region is contained in the backscattered electrons. The backscattered electrons are sensitive to the composition of the specimen. As shown in Fig. 16, as the atomic number of the constituent atoms in the specimen is larger, the backscattered elec- tron yield is...

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Scanning Electron Microscope A To Z - 12

Edge Effect As shown in Fig. 21, if there exists uneven steps or thin protrusions on the specimen surface, the edges of the steps or protrusions appear bright while the bright parts have a certain width (theoretically, edges should appear as sharp lines). This phenomenon is called the edge effect. Figure 22 shows the schematic diagram of this effect. Even when the electron probe irradiates the area distant from an edge on an uneven step, secondary electrons generated by the electrons diffused in the specimen are emitted from the edge surface as secondary electrons. Electron probe Secondary...

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