SEM working principle: the detection of backscattered electrons

Emission of backscattered electrons

When the primary beam hits the surface of a sample, the incident electrons can interact with the nuclei of the atoms and their trajectories are deviated, as shown.

If the conditions are just right, the incident electron can be scattered back and emerge on the surface of the sample, preserving its high energy. Typically, heavier elements, because of their bigger nuclei, can deflect incident electrons more strongly than lighter elements. Hence, heavy elements like silver, which has the atomic number Z=47, appear bright in a SEM image compared to light elements, such as silicon, that has atomic number Z=14, because more backscattered electrons are emitted from the sample surface.

An example of the different BSE contrast between silver and silicon in shown here. This image shows a region of a solar cell, where the white area is silver and the dark area is silicon.

Physics of a semiconductor detector 

But how are the backscattered electrons detected in a SEM?

The detection of BSEs is often carried out by solid state (or semiconductor) detectors. These consist of a doped semiconductor material (typically silicon) and are placed directly above the sample, as shown in Figure 2. The principle of semiconductor detectors is based on the generation of electron-hole pairs in a semiconductor by the incident BSE electrons. In simpler words: BSE electrons that hit the detectors excite the silicon electrons, creating an electron-hole pair.

To form an electron-hole pair in silicon, an energy of 3.6 eV is required and the number of electron-hole pairs that are generated is proportional to the energy of the incident electrons. Moreover, semiconductor detectors are only sensitive to electrons with high energy, which is the reason why they are only used for the detection of backscattered electrons.

The electronic circuit of semiconductor detectors in an SEM 

The free electrons and pairs that are generated from incident backscattered electrons can be separated before their recombination, generating a current. This current can be measured by an electronic circuit that can be schematically described as an operational amplifier with an input resistance and a feedback resistor, as shown here.

Here the detector is shown as a charge collection current generator (Icc), in parallel with the resistance and capacitance of the depletion layer formed at the p-n junction of the doped silicon (Rd and Cd), in series with the internal bulk resistance of the semiconductor (Rs). Because the amplifier can become unstable for large values of RF/Re, an additional capacitance is added in the feedback loop to prevent the amplifier from oscillating.