## 3 Scanning Electron Microscopy
### Fundamentals of SEM  
The resolution limit is defined as the minimum distance at which two different structures can be separated and distinguished as independent objects. For example, the resolution of the human eye is about 0.1 mm, while the resolution limit of light microscopy is about 2000 A. The SEM, with the right settings, may well reach the range of 1 to 10 nm. This improvement in resolution allows much more information to be acquired from the images, which will lead to better performance in the process of detecting and classifying defects in general [17].  
SEM technology uses a focused electron beam to scan along the surface of the sample, generating a wide range of signals which are fused and converted into a visual signal with the help of a cathode ray tube. Two categories of electron-sample interactions are distinguished: elastic interactions that result from the deflection of the incident electron by the sample's atomic nucleus or by electrons in the outer shell of similar energy without significant loss of energy, and inelastic interactions that occur through various interactions between the incident electrons and the sample's electrons and atoms, resulting in the transfer of substantial energy to that atom by the electron in the primary beam. The elastic interaction generates back-scattered electrons (BSE) and the inelastic interaction generates secondary electrons (SE).  
Those electrons, as well as other signals such as X-rays, Auger electrons and cathodoluminescence, are used to form and analyse the image of the sample. For example, BSE provides compositional and topographic information in SEM, SE provides mainly topographic information, X-rays are used to obtain chemical information of the sample, Auger electrons, which have very low energies, are only used in surface analysis, and cathodoluminescence is a mechanism for energy stabilisation. Some key parameters, such as the beam electron energy and the atomic number of the specimen, determine the characteristics of what is called the region of primary excitation. Higher energies lead to deeper penetration lengths, but surface resolution decreases. Higher atomic numbers lead to a lower depth, since the higher the number of particles, the easier it is to stop the penetration of the electrons. Therefore, the goal is to find a balance and, depending on the requirementsof the scenario, determine the optimal beam energy. A schematic figure of an SEM device, including all its components, can be appreciated in Figure 3.  
The basic components of an SEM device are the electron gun, the lenses, the scan coil and the secondary electron detector. Those elements are going to be briefly described along the following lines. The electron gun task is to produce a stable electron beam with high current and directed to a small spot. There are several types of electron guns such as tungsten electron guns, lanthanum hexaboride guns (LaB6) and field emission guns. Other key components are the lenses. Two kinds of lenses are distinguished: the condenser lenses where the electron beam is converged in a parallel stream, and the objective lenses, which are used to focus the beam into a probe point. In order to form an image, the scan coil deflects the beam throughout the x and y axis to cover all the sample. Finally, the secondary electron detector, another crucial component, collects the secondary electron which, as we mentioned before, has the important role of providing fine topographic information.