When using scanning electron microscopy to observe samples, attention will be paid to resolution, lining, depth of field, the authenticity of the morphology, the need for other analysis, etc., and different shooting conditions are required between different points of interest, and sometimes even contradict each other.
Today we will talk about how to choose the appropriate acceleration voltage based on the sample type and the concerns.
Issues to be aware of for regular shooting
Usually, electron microscope users observe conventional samples, which are not as ideal as standard samples of resolution, the samples are more complex, and sometimes the focus is not the same. Therefore, we choose the appropriate electron microscopy conditions according to the sample type and the concerns we are concerned about.
Focus on resolution, lining, depth of field, realism of morphology, other analytical needs, etc., different focus points require different electron microscopy conditions, sometimes even contradictory. Therefore, we must clarify the purpose of shooting and look for the most suitable electron microscopy conditions, rather than rushing to pursue large multiples.
The operating conditions of electron microscopy include many, acceleration voltage, beam spot, working distance, diaphragm size, chiaroscuro contrast, detector selection and so on.
This issue will introduce you to the choice of acceleration voltage.
Any electron microscope is accelerated voltage higher resolution, but does not mean that any specimen is the larger the voltage is better. The choice of voltage is one of the most important of the various operating conditions in electron microscopy. There are various factors that need to be considered, and there are contradictions between various factors, and at this time, it is also necessary to properly consider comprehensively or take other measures.
Sample damage and factors of charge
The selected acceleration voltage must not cause significant irradiation damage or charge to the specimen, otherwise the observed image is not the true morphology of the specimen. If there is a charge generated, the voltage needs to be reduced below V2, which was elaborated in the previous charge effect and will not be repeated here.
For samples with good electrical and thermal conductivity such as metals, it can be observed with a higher voltage, such as 10kV and above; for some specimens with not very good conductivity but relatively stable, a medium acceleration voltage can be used, such as about 5kV; for some samples that are easy to damage, such as polymer materials, biological materials, etc., a lower voltage, such as 2kV or less, may be required.
Electronic yield factors
For single-phase materials, because there is no difference in composition, we choose the largest range of electronic production V1 ~ V2, but for mixed phase materials, we hope that there is a good degree of composition lining at the same time, such a picture appears to be better lining, the amount of information is also the largest, often we will also think that such a picture is the clearest. Therefore, we need to choose an area with a large difference in secondary electronic production for shooting.
As shown in Figure 5-13, the left figure is the secondary electron yield of carbon and gold, the middle picture is the secondary electron image of the gold particles at 1kV, and the right picture is the secondary electron image under 200V. Obviously, the yield of carbon and gold is the same at 200V, so the image taken at this time only shows a difference in morphology, and the difference in the composition of carbon and gold will not appear no matter how the chiaroscuro contrast is adjusted. At 1kV, the difference in electronic yield between carbon and gold is maximized, so in addition to the morphological lining, it also shows excellent compositional lining.
Fig. 5-13 SE image of gold and carbon at electron yield (left) and 1kV (middle) and 200V (right).
For some metal materials, there is often a relatively large difference in yield at higher acceleration voltages, while for some low atomic number specimens, lower voltages tend to have a greater difference in electronic yield.
As shown in Figure 5-14, the specimen is a carbon-silver mixture. The image on the left is a 5kV SE image and the right image is a 20kV SE image. At 5kV, it not only shows better composition lining than 20kV, but also better indicates details.
Fig. 5-14 SE image of carbon-silver hybrid material at 5kV (left) and 20kV (right).
As shown in Figure 5-15, the specimen is a cross-section of a copper-clad aluminum wire, the left figure is a 5kV SE image, and the right figure is a 20kV SE image. At 20kV, the copper layer of the outer ring and the aluminum layer inside can be better distinguished.
Fig. 5-15 SE image of copper-clad aluminum conductor section at 5kV (left) and 20kV (right).
For some phases that are themselves very different, if you can find the voltage corresponding to the maximum difference in secondary electron production, you can also distinguish them. Of course, some yields do not have a reference curve and need to be found after many attempts.
For example, Fig. 5-16, the specimen is an intrinsic semiconductor film on a doped semiconductor substrate, and the difference in electron yield reaches a maximum of 1kV, and the image corresponding to 1kV can distinguish between the two layers of film, while other voltages do not have too good lining.
Fig. 5-16 Contrast of lining of semiconductor films at different voltages
Balance of lining
Although the maximum lining of the components can be achieved by the selection of the acceleration voltage mentioned in the previous point, sometimes this condition is not the voltage that observes the best morphology.
At this point, we need to consider whether to focus on morphology or component lining, using secondary electrons for observation, or using backscattered electrons for observation, or using compromise methods for observation. This requires the operator to choose according to the problem that the electron microscope photos want to illustrate.
The voltage conditions required to obtain a good topography contrast image and an atomic number image are generally different, and there are other ways to solve it appropriately. The best topography lining and the best atomic number lining are taken separately, and later in the electron microscope software, the different photos (the position needs to be exactly the same) are mixed according to a certain proportion to form a picture with both linings.
Effective magnification factor
Generally, electron microscopy has different limit resolutions at different voltages, and its corresponding effective magnification also changes. Shooting electron microscopy photos of specific magnifications, especially high magnification photos, requires the selection of an effective magnification corresponding to the voltage to meet the demand. Otherwise, the image is considered to have a virtual magnification. After the virtual magnification, although the image is also enlarged, but no more information appears, and the virtual enlargement will have more environmental factors.
Therefore, if there is virtual amplification, the acceleration voltage can be increased to increase the effective magnification; if the voltage cannot be changed, the acquisition pixels of the image can be considered to obtain a similar amplification effect. At this time, the environmental factors or sample damage factors are less.
Penetration depth factor
The relationship between accelerated voltage and electron scattering has been described in detail earlier. The higher the acceleration voltage, the greater the energy, and the greater the scattering region of the electrons. A greater proportion of secondary or backscattered electrons are emitted from deeper depths. Therefore, although the larger acceleration voltage has better resolution in the horizontal direction, it ignores a lot of surface details of the specimen; while the low voltage, although the horizontal resolution is relatively poor, has better sensitivity to the depth direction and can reflect more morphological details on the surface.
As shown in Figure 5-17, the specimen is a surface modified silica ball, and no surface details can be seen at 5 kV voltage, while obvious particles can be observed at 2 kV. As shown in Figure 5-18, the performance of nanoparticle powders at different voltages, because the particles are seriously agglomerated, so the agglomerate particles cannot be well distinguished at 5kV voltage, and the particle size is larger, while relatively finer particles can be observed at 1kV.
Fig. 5-17 Image of a SiO2 ball at 5kV (left) and 1kV (right).
Fig. 5-18 Image of nanoparticles at 5 kV (left) and 1 kV (right).
When the acceleration voltage is reduced to an ultra-low level of about 200V, the action area of the electron beam becomes very small, and the conventional edge effect or tip effect can be basically removed, as shown in Figure 5-19.
Fig. 5-19 Voltage of about 200V can eliminate the edge effect
However, this is not to say that the lower the voltage, the richer the surface detail, the better. Sometimes for some special samples, it is desirable to have a higher penetration depth. For example, for some coating materials, if you want to see the coating effect, you need the electron beam to penetrate deeper and reflect the information inside the coating. If only a low voltage is used, only the surface details of the cladding layer can be seen, and the desired effect cannot be achieved.
As shown in Figure 5-20, the figure taken at 5 kV on the right is more helpful for us to observe the effect of graphene coating the sphere than the 2 kV on the left. Some of the things of interest to the sample are embedded in the matrix, and a higher voltage is also required to achieve a certain penetration effect and observe the characteristics of interest.
Fig. 5-20 Penetration effect of graphene coating under different voltages
In addition, low voltages are extremely sensitive to surfaces because of their low penetration, and of course, surface contamination is also more clear. Sometimes we don't want to get too many pollution pictures, at this time to increase the voltage, enhance the electron beam penetration capacity will also reduce the negative impact of surface pollution, as shown in Figure 5-21.
Fig. 5-21 Comparison of surface pollutants at 1 kV (left) and 15 kV (right).
Therefore, the penetration depth should be selected according to the actual needs, low voltage helps to show more surface details, high voltage helps to observe the internal situation of the sample, and acceleration voltage is selected according to actual needs.
Signal-to-noise ratio factor
The acceleration voltage becomes smaller, although the electron yield of many substances is higher, but because the signal of the electron beam becomes weaker, the overall secondary electron and backscattered electron signals are also weakened. Generally speaking, high voltages have a better signal-to-noise ratio than low voltages.
For some cases where the signal-to-noise ratio is not good, the signal-to-noise ratio can be improved by increasing the voltage appropriately; however, if the acceleration voltage cannot be changed for other reasons, the signal-to-noise ratio can be improved in a variety of ways, such as: increasing the beam beam, selecting a larger aperture diaphragm, reducing the scanning speed, using line integration or surface accumulation, reducing the working distance, appropriately increasing the bias of the detector, or tilting the sample.
Other accessory factors
Scanning electron microscopy is not only for taking images, but is often equipped with other analytical accessories. Different accessories also have certain requirements for voltage. In carrying out other analyses, priority may be given to the need for annex work.
Source: Material base
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