In this paper, we introduce the generation mechanism, characteristics and practical applications of two important signal electrons in scanning electron microscopy, namely backscattered electrons (BSE) and secondary electrons (SE). Firstly, the basic generation process of various signals is described from the aspects of elastic scattering and inelastic scattering of electron-sample interaction. Then, the internal relationship and key differences between the generation mechanism, energy and spatial distribution, and information characteristics of backscattered electrons and secondary electrons are analyzed. Finally, the advantages and disadvantages of the two signal electrons in the characterization of different samples are illustrated through practical cases, which provides a theoretical basis for the selection of subsequent SEM analysis parameters.

As an important means for the characterization of micro, interface and nanoscale samples, scanning electron microscopy is widely used in many fields such as materials and biology. Among the various types of detection signals in the scanning electron microscope, backscattered electrons and secondary electrons are the two most commonly used and important signal electrons. Understanding their generation mechanism and characteristics can better guide the subsequent parameter setting, so as to obtain better quality SEM images.

1. Overview of the interaction between the electron beam and the sample

When a high-energy electron beam is incident and scans the surface of a sample, complex elastic and inelastic scattering occurs with the sample atoms. The former mainly refers to the interaction between the electron beam and the nucleus of the sample, and the incident electrons are deflected at a large angle but there is almost no energy loss. The latter mainly refers to the interaction between the electron beam and the outer electrons of the sample atom, and the deflection of the incident electron orbit is relatively small but the energy loss is large. Together, these two complex dynamics result in the gradual loss of kinetic energy of the incident electrons and the generation of various detectable signals.

From the perspective of detection signal classification, the electrons related to elastic scattering are called backscattered electrons; The electrons associated with inelastic scattering are called secondary electrons; In addition, there are continuum and characteristic spectrum X-rays. These signals carry information about the sample topography, elemental composition, crystallization orientation, and more. Backscattered electrons and secondary electrons are the two most widely used and important types of signals in SEM imaging.

Figure Life analogy of elastic collision and inelastic collision

Figure Elastic scattering and inelastic scattering of electron-atom interactions

2. Backscattered electrons and their characteristics

When the incident electrons are scattered at a large angle by the nucleus of the sample and then escape from the sample, it is defined as backscattered electrons. Due to the high probability of interaction with the nucleus of heavy elements, the backscattered electrons are sensitive to heavy elements. In addition, the areal density of atoms with different orientations is also different, which leads to the direction dependence of the backscatter probability. As a result, the backscattered electrons carry information about the elemental distribution and orientation in the sample.

Specifically, the generation of backscattered electrons is related to the following factors:

(1) the distance between the incident electron and the nucleus; Closer to the core, the backscatter probability increases

(2) atomic number; The "resistance" of the nucleus of heavy elements is large, and the probability of backscattering increases

(3) crystal orientation; The backscattering probability is different due to the different areal densities of atoms in different orientations

(4) surface morphology; Bevels or edges reduce the distance of incident electrons and increase the probability of backscattering

Figure Intensities of BSE received in different directions and their superposition algorithms

From the above, it can be seen that the backscattered electrons reflect the elemental distribution and crystal orientation information of the sample.

Correspondingly, some of the key characteristics of backscattered electrons include:

(1) High energy and wide range (50eV - incident electron energy);

(2) The generation depth is large, up to 1 micron depth of the sample;

(3) There is no special bias in the directional distribution.

Figure Mechanism and characteristics of backscattered electron generation

3. Secondary electrons and their characteristics

When the incident electrons transfer part of the kinetic energy to the electrons close to the surface of the sample, leaving them from the nucleus and becoming free electrons, these free electrons are secondary electrons.

Secondary electrons are closely related to the following factors:

(1) The distance between the incident electron and the surface of the sample; There is a high probability of being generated close to the surface

(2) surface topography; The ramp or edge structure increases the probability of secondary electron escape

(3) surface electric field; The local electric field can change the secondary electron trajectory, which can affect the detection efficiency

Figure SE yield as a function of atomic number Z

From the above, it can be seen that the secondary electrons mainly reflect the local surface state of the sample, which is an important signal to evaluate the morphology of the sample.

Some of the key characteristics of secondary electrons also include:

(1) Low energy (<50eV), most of which are below 10eV;

(2) The escape depth is extremely shallow, about 10nm;

(3) There is no clear orientation in the directional distribution.

Figure The mechanism and characteristics of secondary electron generation

From the above, it can be seen that there are no definite directional distribution characteristics of backscattered electrons and secondary electrons. However, there are significant differences in energy value and detection depth: the former has high energy and broad spectrum, while the latter has low energy limitations. The former information comes from the depths, while the latter comes mainly from very shallow surfaces. It is these differences that allow the two to complement the composition and topography of the sample.

Fourth, the practical application and the complexity of the formation mechanism

In simple cases, you can guide the parameter settings according to the above understanding. In practice, however, it is often more complex:

(1) Secondary electrons are also related to the deep composition of the sample;

(2) Backscattered electrons are also related to surface morphology;

(3) the two are mutually transformed, and it is difficult to separate them;

(4) There is also a relationship between signal strength and detection position.

Take the cross-section detection of a fuel cell as an example. The results of secondary electron and backscattered electron imaging in the same region are significant: the latter shows more information about elements and orientations, while the former highlights the topography details.

Figure Imaging of fuel cells using different imaging signals

It can be seen that the formation mechanism and characteristics of secondary electrons and backscattered electrons are not fixed, and are closely related to the change of detection conditions, so they should be selected and optimized according to the specific situation. But in general, secondary electrons are better at morphology, and backscattered electrons are better at composition and structure.

5. Summary

From the above discussion, it can be seen that the generation mechanism and characteristics of backscattered electrons and secondary electrons are not black and white like simple theoretical models. There are many transformations and overlaps between the two, and the characteristics will also be complicated due to the changes in detection conditions and equipment parameters. However, in general, the backscattered electrons are better at reflecting the elements and orientation information of the sample, and the secondary electrons are better at representing the morphology and surface structure of the sample. Understanding the formation process and advantages and disadvantages of the two is of great guiding significance for the design and optimization of subsequent SEM experiments.

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