**3.2 Electron gun**

One of the most important parts of electron microscopes is electron guns, which we call electron sources. It is very important for imaging that the source can produce electrons continuously and uniformly. This is just like a river that constantly flows into a dam built for a hydroelectric power plant. While tungsten filaments were generally used for the first commercial SEM, FEG guns are now used more commonly and effectively. An important point here is that the tungsten filament does not require a vacuum, while the FEG source is in a vacuum environment [3, 9]. One reason for this is the excellent blasting of the electron flow in a vacuum-free environment. Below we can see the various electron sources (see **Figure 3**).

**Figure 1.** *FEG (field emission gun) scanning electron microscope.*

#### **Figure 2.**

*Schematic view of the scanning electron microscope.*

#### **3.3 Electron sources**

Electron sources are widely used tungsten, LaB6, Cold FEG, Shotky FEG [8]. The most used old model tungsten and FEG electron sources from these sources are indicated in **Figure 4** [8]. Here, tungsten filament electron source is used in older technology devices, while FEG welding is used as a more technological electron source. The advantages and disadvantages of these sources relative to each other are presented in **Table 1**. When **Table 2** is examined, it is clearly seen that FEG electron sources are more useful sources.

As can be seen from the **Table 2**, FEG pistols appear to be advantageous in many respects. In scanning electron microscopes, the most important values for the sample are magnification and resolution.

*Scanning Electron Microscopy DOI: http://dx.doi.org/10.5772/intechopen.103956*

#### **Figure 4.**

*Electron sources (a) tungsten (b) FEG (c) FEG module.*


#### **Table 2.**

*Comparison of electron sources.*

Resolution: It expresses the power of distinguishing two different parts on the viewed surface.

Magnification: Shows the ratio of the imaged area to the scanned area.

These two values are actually a comparative situation, namely qualitative. In fact, the magnification may vary depending on the screen and print size on which it is viewed. Therefore, the main thing in microscopic images is the length bar. The resolution event, on the other hand, depends on the analysis configuration. That is, it depends on the acceleration potential, the working distance (h), the current value and the structure of the sample.

#### **3.4 Interactions of electrons with sample**

Electrons emanating from the gun strike the sample surface with an acceleration potential. Three physical events will occur for these incoming electrons. These are

back scattering, passing through scattering and elastic scattering, respectively. This situation is illustrated in **Figure 5** below.

As can be understood from here, different rays and electrons are formed as a result of the collision of electrons with the surface. Before talking about these electrons, let us look at the depth at which the incoming electrons affect the sample surface (**Figure 6**) [3].

When we look at the electron-sample interaction, shown schematically in **Figure 6**, we see that the interference is in the form of a water drop. Here, high energy electrons form low energy auger electrons as a result of inelastic interference of sample atoms with outer orbital electrons. These auger electrons contain information about the sample surface and form the working principle of auger Spectroscopy [8]. Again, as a result of the interference between the incoming electrons and the orbital electrons, the beam electrons that are thrown out of the orbit or whose energy decreases, move towards the sample surface. These electrons are called secondary electrons. These secondary electrons are collected in the scintillator in the sample chamber and converted into a secondary electron image signal. Secondary electrons come from approximately 10 nm depth of the sample surface. This provides a high resolution topographic image. In addition, inelastic interference occurs between the sample atoms and the electron beam. As a result of these inelastic interactions, characteristic X-rays and continuous radiations occur in the sample. The characteristic radiations generated here are evaluated as wavelength or energy dispersed radiation. This evaluation gives us the chemical composition of the sample, that is, the elemental analysis information. This analysis is called EDX analysis.

The electron beam on the sample also makes elastic interferences with the sample atoms. During this elastic interference, the incident electron beam is deflected by the attractive force of the nuclei of the sample atoms and backscattering occurs. These

**Figure 5.** *Rays and electrons formed as a result of the interaction of the incoming electron beam.*
