Backscattered electrons (BSEs) are high-energy electrons that are produced by the elastic scattering of the primary beam electrons with the atom nuclei. The yield of BSEs, that is the ratio of the number of emitted BSEs and the amount of primary beam electrons, depends on the atomic number: the higher the atomic number, or the heavier the element, the brighter the contrast. In the Phenom SEM, BSEs are detected using four-quadrant semiconductor detectors placed above the sample. In this blog, we will explain what a semiconductor detector is and how backscattered electrons are detected in a scanning electron microscope.
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.
Noise of semiconductor detectors in SEM
The main sources of noise in semiconductor detectors are the shot noise of the incident BSE current and the noise of the preamplifier. Shot noise in the primary beam is caused by the fluctuations in the amount of emitted primary beam electrons. The number of primary electrons hitting the specimen in a certain amount of time is statistically distributed and follows the Poisson distribution¹. The shot noise introduced by the incident electron beam will be increased during SE and BSE emission. The noise in the preamplifier is thermal noise, called also Johnson-Nyquist noise, and it is proportional to the Boltzmann’s constant and the temperature of the resistor.
Learn everything you need to know about scanning electron microscopy
A large number of areas make use of different types of microscopes and technology: from X-ray microscopy, optical microscopy, scanning probe microscopy to scanning acoustic microscopy.
How do these types vary from one another and what are the unique characteristics of scanning electron microscopy? In the SEM working principle whitepaper, you'll learn about the essentials of microscopy with a special focus on electron microscopy: