## 1 Introduction
### Basic SEM Operation  
The typical energies of incident electrons originating from electron guns in SEM generally range from 2 keV to 40 keV [11]. SEM instruments can be classified based on the range of energy, which is subject to the type and nature of the sample and analysis, such as low voltage SEM, standard SEM, high-resolution SEM, and field emission SEM. Electron guns are also chosen based on the intended application, and three types of electron guns are usually used for SEM. Type one is the tungsten filament electron gun, which is heated over 2500 \({}^{\circ}\)C, resulting in the thermal emission of electrons from its tip [43; 44]. The second type of electron gun is the lanthanum hexaboride filament, which produces thermionic emissions, with the advantages of a longer working life and brighter beam of electrons from a larger maximum beam current [45; 46]. These electron guns are relatively more expensive than conventional tungsten filament guns. Field emission guns are the third type of electron gun and are known as cold cathode electron emitters, as heating is not involved in the process [47; 48]. It works with the application of a very high electric field to a finely pointed tip, which results in providing the brightest beam with a very small deviation in electron energy. Since field emission guns need 10\({}^{-10}\) Torr of pressure to preserve the tip, the cost of SEM with these guns becomes high [11]. The electron beam is demagnified into a fine probe by two or three electromagnetic condenser lenses. Scan coils are used with fine probes for scanning across the selected surface area of the specimen. The electrons originating from the probe penetrate into the sample in a teardrop-shaped volume (Figure 2). The overall dimensions of this volume are determined by various factors, such as the electron beam energy and atomic masses of the constituent elements present in the specimen. Higher energy and lighter atomic masses of elements tend to result in increased penetration depth inside the sample. The angle of incidence does not significantly affect the penetration depth; rather, it affects the angle of deflection, scattering, and other electron interactions as the beam traverses through the specimen. The production of secondary, Auger, and backscattered electrons takes place due to the interaction of the electron beam with the sample surface. It also accompanies the production of characteristic, continuum, and fluorescent X-rays (Figure 2). Elastic interaction between an electron beam and the sample results in electrons reflecting back, termed backscattered electrons (Figure 2). These electrons are used for generating high-resolution images of the constituent elements present in the specimen. Inelastic collisions result in relatively lower energy electrons originating from the atoms of the sample, which are known as secondary electrons and are helpful in investigating the topography of the specimen surface. Auger electrons are emitted when excited atoms release energy and are characteristic of the sample elements. These electrons help in understanding the elemental composition of the specimen. When the electron beam displaces an electron from the inner shell of an atom, another electron from a higher valence shell takes its place, resulting in a small loss of energy in the form of an X-ray photon. It is considered a characteristic X-ray and helps in investigating the particular element from which it is emitted. The electrostatic force experienced by the high-energy incident electron beam due to the presence of atomic nuclei results in deflections/accelerations/decelerations of electrons. This results in the production of continuum X-rays (also known as Bremsstrahlung or Brems X-rays), with a continuous spectrum ranging from low to high energies (Figure 2). Continuum X-rays do not contribute to primary elemental analysis; however, they help in studying the interaction between the sample and the electron beam.