How to Separating BSEs (Backscattered Electrons)
from SEs (Secondary Electrons) in SEM
- Bias-Induced Contrast/Intensity Effect in SEM -
- Practical Electron Microscopy and Database -
- An Online Book -

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This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.

 

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To separate the BSEs (backscattered electrons) from the SEs (secondary electrons), a bias of ±50 eV can be applied to the sample by modifying a Faraday-cup sample stage as shown in Figure 4440a. This method can also be used to examine whether the SEM image intensity is a result of BSEs or secondary electrons, or both. As we know that SEs have energy of < 50 eV and most of them are emitted with less than 10 eV energy so that a small positive bias on the specimen will suppress the emission to be collected by the detector. On the other hand, BSEs almost have the same energy as that of primary electrons so that the small positive bias will not affect the image intensity from BSEs. Furthermore, the bias can be a variable in order to investigate the energy distribution of SEs.

Schematic diagram showing the applied bias used to separate the contrast effect of BSEs from that of BEs

Figure 4440a. Schematic diagram showing the applied bias used to separate
the contrast effect of the BSEs from that of the SEs.

By studying the change in intensity of the SEM image of uranium on carbon film as a function of applied bias, Zhu et al [1] noticed that at + 10 eV bias, the intensity droped nearly 80% and at + 50 eV, only 10% intensity remains, suggesting that 90% of SEs and 10% of BSEs contributed to the image, independent of the atomic number Z. They also statistically found that the ratio of SEs and BSEs is in the range of 85 - 90% to 15 - 10% for 200 kV electrons.

Figure 4440b shows the  schematic illustration of HD 2700C electron microscope which can be used to simultaneously record SE (secondary electron) image using SE and backscattered electrons (BSE), bright- field (BF) STEM image using transmitted electrons (TE) scattered in the forward direction, and annular dark-field (ADF) STEM image using transmitted electrons scattered at large angles. The three images were obtained from Pd/C catalysts in the same area of the carbon support. It can be seen that the BF, ADF, and SE images are complementary. Furthermore, the positive bias of 10 kV in the Hitachi SE detector above the sample is applied to collect low-energy electrons generated at the surface of the specimen for ultrahigh-resolution  SE imaging. The positive electric bias of 50 eV on the specimen can suppress the escape of the SEs (with ≤ 50 eV) from the surface, but allow the backscattered electrons with higher energies reaching the detector.

Schematic illustration of HD 2700C electron microscope

Figure 4440b. Schematic illustration of HD 2700C electron microscope. [2]

 

 

 

 

 

 

 

 

 

 

[1] Y. Zhu, H. Inada, K. Nakamura, and J. Wall, Imaging single atoms using secondary electrons with an aberration-corrected electron microscope, Nature Materials, 8 (2009) 808.
[2] H. Inada, D.Su, R. F. Egerton, M.Konno, L.Wu, J.Ciston, J.Wall, Y.Zhu, Atomic imaging using secondary electrons in a scanning transmission electron microscope: Experimental observations and possible mechanisms, Ultramicroscopy 111(2011)865–876.

 

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