The Scanning Electron Microscope (SEM) is undoubtedly one of the most valuable and powerful analytical tools capable of producing high resolution topographical and morphological images with magnification extending to the nanoscale. Generally speaking, SEM images are simply developed by scanning an electron beam across a surface of a specimen. However, the formation of an image using the SEM technique is technically based on acquiring distinct emitted electrons resulting from the interaction between the incident electrons, known as primary electrons, and the imaged area of the specimen.
Such an interaction will be first briefly discussed in order to understand how these emissions are produced inside the sample and how they can be used to form an SEM image of the sample.[77] Generally when an accelerated finely focused beam of electrons strikes into the surface of a sample a number of emanations are generated such as secondary electrons (SE), Auger electrons, backscattered electrons (BSE), cathodoluminescence and characteristic X-rays. These emitted electrons and photons are only produced from a particular emission volume inside the sample where the size of the emission volume largely depends on the kinetic energy of the
accelerated beam and the atomic number of the sample.[77, 78] Figure 3.6 illustrates a schematic representation of the various produced outcomes due to the interaction between the incident electrons and the imaged material along with the spatial distribution inside the material.[78]
Figure 3.6 A simplified illustration showing various emissions occuring as a beam of electrons strikes a surface of material and the anticipated emission volume inside the material.[78]
These different emissions can be effectively collected, analysed and advantageously applied in many scientific applications such as in the SEM where a variety of very important data can be obtained. They can be classified according to their energies as follows[78]
Auger electrons > SE > BSE > x-rays
When a beam of electrons impinges the surface of a specimen each individual incident electron will go through a scattering effect either
elastically or inelastically. In the case of elastic scattering the kinetic energy and the velocity of the scattered electrons will remain constant but their trajectories will change. However, some of the elastically scattered electrons will bounce back out of the specimen as backscattered electrons (BSE) while the rest will penetrate randomly throughout the sample till their kinetic energies eventually vanish and dissipate as heat.
For the inelastic scattering process the primary electrons may interact with the atoms in the irradiated sample giving rise to several other types of emissions. For example, continuum X-ray (Bremsstrahlung) radiation can be generated when the primary electrons are decelerated by the electromagnetic field (the coulomb field) around the atom. Other effects can also be seen when the primary electron collides with an inner atomic electron, causing it to be completely removed from the atom and simultaneously creating a vacancy in the place of the ejected electron. This vacant position can be filled by another electron from a higher orbital level, leading to either a characteristic X-ray emission or an Auger electron.
Furthermore, low energy secondary electrons can also be produced when the primary electron strikes with a loosely bound electron to the nucleus, which is located in the outer shells of an atom. However, if the secondary electron recombines with a hole, which has been created during the previously mentioned scattering processes, a photon may be emitted at wavelengths in the range of visible or near-infra-red. This photonemission is known as cathodoluminescence. The energy spectrum of the previously mentioned emitted electrons is shown in Figure 3.7. SEs are those which have low energies relative to the energy of the primary electrons typically less than 50 eV. The majority of the SEs lie between 0.5 to 5 eV while the BSEs are more energetic and exhibit a wide range of energy from above 50 eV up to the primary electron energy.[79]
Figure 3.7 An energy spectrum of the different emitted electrons from the sample being imaged.[79]
In general, SEM images can be formed by optionally operating the SEM in one of its different imaging modes based on which kind of emitted electrons are detected and analysed. BSE and SE operational modes are the two main types. Usually, an SEM can be equipped with different detectors either for BSE or SE or both.
The formation of an SEM image based on the SE mode is the most widely used imaging mode since secondary electrons are abundant and their yield (the number of SE per primary electron) is high, which can even exceed unity as can be clearly seen in Figure 3.7. SEs are highly topography-related (i.e., surface texture and roughness) since they are only produced near to the surface of the sample, typically within a few nanometres.[77] This means that SE electrons have very small exit depths and hence any changes
in the surface topography of the sample will alter the sampling depth which in turn will affect the number of the emitted SEs that reaches the detector.
Therefore, the larger the sampling depth the lower the number of SE that can be collected. Hence, bright areas on the image correspond to parts of the sample where large numbers of secondary electrons are emitted relative to other areas on the surface of the sample which would appear darker. This topographic contrast is used to form the SEM images.[77, 80]
In the backscattered electrons imaging mode the number of the emitted BSE largely depends on the atomic number of the atoms that reside on the surface of the imaged sample. Thus, the amount of BSE increase as the atomic number increases which means bright areas indicate regions where higher concentrations of elements of high atomic numbers present. Images formed based on BSE imaging mode differentiate between areas with several chemical composition specifically when the average atomic number in these areas is different. For more details about the various imaging modes refer to references [77 and 80].
Figure 3.8 depicts a cross-section view for an SEM set-up showing its important components. The SEM must be operated under high vacuum conditions to reduce the number of collisions between the electron beam and the atmosphere in the sample chamber. As a result the attenuation effect is minimised and consequently the collection efficiency of the emitted electrons is enhanced. An electron gun produces a beam of electrons which are then accelerated by applying a high voltage between the electron gun and the anode. A number of electromagnetic lenses are used to finely focus the beam onto the surface of the sample. Scanning coils are employed to manipulate the beam and move it across the sample surface to create the image.
Figure 3.8 Schematic cross-section of a typical scanning electron microscope.
In this work an FEI Nova 200 dual-beam SEM device located at King Abdul Aziz City for Science and Technology (KACST) in Saudi Arabia was used to investigate the grown samples. This SEM has a precise focused ion beam (FIB) feature which is valuable in etching and cutting cross-sections of the sample so, more images can be taken.