Low voltage SEM

These improvements allow the microscopes to operate at low voltage, but as shown in Section 

There are several important differences in image formation at low voltages in the SEM, and these differences are due to the small penetration of the incident beam. The range of electrons, R, can be expressed as R = aE , where E is the electron energy and a and n are constants. According to the general empirical expression of Kanaya and Okayama [33, 28] if R is in /xm, 

Secondary electrons produced by a high energy beam also have a broader distribution in space, because the emerging backscattered electrons (BSE) produce secondaries as they leave the specimen. These secondaries are called SE2, to distinguish them from SEl, which are second- 

The secondary electron image appearance is different at low voltage. One reason is that the interactions are all at the surface, so that all of the image features are much more sensitive to the exact state of the surface, contamination and all. The other reason is that the effect of tilting the surface is different. At high voltage the secondary emission is strongly increased when the sample is tilted, because part of the interaction 

If the detector is to one side of the sample, then its collection efficiency will be greater for surfaces tilted towards it than away from it. This is true both at high and at low beam voltages, even 

Operation at low voltages has been described in detail [60] and in general texts on SEM [38, 59]. For minimum charging, the beam energy should be near to the crossover energy E2 (see Section 3.2.2.2). This energy depends on the sample, and Table 3.5 shows a summary of data for many polymers from Butler et al. [61]. However, it is difficult to predict E2 for a new material from this data. 

Two similar detectors, one on each side of the sample chamber, will reduce the central field [80, 81] but it may be better to use a semi-in-lens design with a through-lens detector (see Fig. 3.17C). In this case the field that extracts the secondary electrons is the same one that is used to form the probe, so cannot disturb it. 

A common detector for backscattered electrons at high energy is the passive scintillator with a large area and large collection angle. However, at 10keV, the scintillator will not work well, as the electrons do not have enough 

X-ray microanalysis is still possible at low beam voltages, but it may not be easy. There are the advantages of better spatial resolution and reduced absorption correction, as the volume generating the x-rays is small and near the surface. The disadvantages are that the x-ray intensity produced is small, and not all elements can be analyzed. In principle, elements of higher atomic number can be identified using L- and M-shell x-rays, but these are much more complicated than the simple K-shell emissions. 

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The first corrected electron-optical SEM was developed by Zach [10]. Eor low-voltage SEM (LVSEM, down to 500 eV electron energy instead of the conventional energies of up to 30 keV) the spot size is extremely large without aberration correction. Combining and correction and a electrostatic objective lens, Zach showed that a substantial improvement in spot size and resolution is possible. The achievable resolution in a LVSEM is now of the order of 1-2 mn. More recently, Krivanek and colleagues succeeded in building a corrected STEM [53,M]. 

Low-voltage SEM is valuable when applied to frozen uncoated samples because it allows the observation of food in the near-native (frozen hydrated) state at a high magnification, particularly by field emission SEM, which is best suited for this type of work. Since the samples do not need to be coated with metal, they may be fractured continuously over the course of the observation. 

The (quadratic) superposition of effective probe diameter dp (Fig. 79), where dg and ds dominate for high electron energies and thermionic guns, and da and d dominate for low-voltage SEM and field-emission guns. This superposition results in a minimum probe size between 1 and 10 nm at optimum apertures of tens of milliradians. An anticipated increase in resolution (decrease of electron probe diameter) requires a compensation of the chromatic aberration by using a combination of elec-tro.static and magnetic multipoles. 

The value of low voltage SEM is shown, for example in Fig 5.24 of a PE biaxially blown film [139]. The film was cut, pulled until it necked and imaged uncoated at 800 eV. Fig 5.24A is an undeformed region showing lamellae. Fig. 5.24B is an image of the transition toward deformation. 

Fig. 5.24 Low voltage SEM of cin uncoated PE biaxially blown film, pulled until it necked and imaged in an undeformed region showing lamellar deformation (B) and in a region showing necking (C). (Reproduced with permission, T. Reilly, impublished [139]).

The HRSEM is very often operated at low beam voltage, and the technique may be specifically referred to as high resolution low voltage SEM. As discussed in the previous section, the... 

It is the combination of high surrounding gas pressure and no requirement for conductive coating that makes these instruments uniquely useful. Experiments that demand its use include the dynamic observation of reactions involving a solid and a gas or liquid, such as oxidation or other corrosion, as well as observation of wet specimens. However, if specimen charging is the only problem, a low voltage SEM may be a more practical solution. 

Wallheinke et al. [312] studied the location of different compatibilizers at the interface in blends of thermoplastic polyurethane (TPU) and polypropylene (PP) by SEM and TEM. Selective dissolution of the TPU matrix in dimethylformamide with 1% dibutylamine resulted in a solution of the TPU with PP/EC particles that was filtered, dried, and the membrane with particles imaged by low voltage SEM. In the uncompatibilized blend, the surfaces of the PP particles are very smooth. Therefore, the surface coverage by the compatibilizers can be detected easily. Compared with TEM, using SEM on separated particles has the advantage of a fast and easy preparation without microtoming and staining. 

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