The structure of the FIM patterns for uncoated tungsten samples is well known [34,35]. For the sample discussed in this article, a high-resolution FIM image revealing the atomic structure was obtained at 7.5 kV, as shown in Figure 4. The image shows the surface atomic distribution, highlighting the different facets of polycrystalline tungsten at the center of the field of view.

For the coated tungsten samples, the detected FEM behavior was characterized by a switch-on phenomenon and a focused single bright spot. This behavior was previously explained by Mousa in 1986, who described the creation of crystallized channels that allow electrons to pass to and from vacuum to the tungsten surface [2]. The difference in the FEM behavior between the uncoated and coated tungsten samples is presented in Figure 5, as observed in previous studies [16,26].

In this study, the tip was connected to a high-voltage power supply. The applied voltage was gradually increased until the first ion emission was observed at 5 kV. The resulting FIM data were divided into three phases. The first phase (erosion phase), which spans the voltage range of 5–7.2 kV, is illustrated in Figure 6. In this set of figures, the yellow-highlighted regions (bright spots) represent the imaging of the molecular distribution of the resin layer, while the red regions (dull spots) indicate the atomic distribution of the tungsten surface, as seen through the resin layer.

The appearance of bright spots (yellow-highlighted regions) was associated with ${Ne}^{+}$ ions generated through direct ionization by organic molecules at the resin surface. Because the surface of the epoxy layer is uneven, the ionization intensity at the protrusions is the highest at this stage. This was accompanied by erosion of the epoxy layer. In this context, brighter spots indicate a higher density of emitted ${Ne}^{+}$, whereas larger spots, often with multiple connected circular regions (or spots with tails), suggest the imaging of more atoms within the same molecule. In contrast, the dull spots (red-highlighted regions) resulted from the indirect ionization of ${Ne}^{+}$. This occurs in surface regions with a thin resin layer, where electrons from Ne atoms tunnel through the resin molecules to reach the tungsten surface atoms. This tunneling process facilitates the localization and imaging of tungsten atoms, albeit with a lower density of emitted ${Ne}^{+}$. Because molecules are composed of multiple atoms, the production of ${Ne}^{+}$ ions increases with the number of ionization spots supplied. Moreover, molecules have more atoms available for capturing and imaging. This explains the differences in size and brightness between the yellow and red regions, which correspond to the imaged elements, thus providing support for the proposed theory. A schematic of this process is shown in Figure 7.

The second phase of the results was obtained within the voltage range 7.2–9.6 kV. At this stage, erosion areas develop and form channels that facilitate the transport of tungsten ions. In this range, the FIM images revealed only the atomic distribution of the tungsten surface through the resin layer, as evidenced by the dull spots in Figure 8. The bright spots in this case are smaller than those described earlier, indicating that they represent tungsten surface atoms, but with a more intense ionization process for the Ne gas.

The third and last phases of the results were obtained in the voltage range 10.0–15.0 kV. At this stage, the formation of conductive channels increased and depended on the topography and the thickness of the epoxy layer. The FIM images (Figure 9) show new active resin surface regions that contribute to the Ne ionization process. Again, the bright large spots are related to the resin surface molecules, whereas the small bright and dull spots are related to the tungsten surface atoms. In addition, Figure 9A–C show blurred large bright spots, which are believed to be obtained for inner resin surface molecules that were imaged by the tunneling ionization process, where the ${Ne}^{+}$ ions were ionized by losing their electrons when tunneled to the inner surface molecule, and the gradient in brightness is related to the intensity of the ionization of Ne gas.

At some regions, where the epoxy layer was very thin, it was possible to image the tungsten surface atoms when the ${Ne}^{+}$ ions are created through tunneling currents. Ne electrons were charging the resin molecules, which are in turn discharged through the close tungsten atom. This process helps to locate these atoms in addition to the inner resin surface molecules, as seen from the blurred spots in Figure 9D–F.

Achieving brighter and more concentrated emission spots in the case of imaging the resin surface is related to the high concentration of N10e gas ions in small areas within the resin surface molecules, allowing for the creation of a large density of ${Ne}^{+}$ at these spots owing to an intense thermal transition of the Ne gas electrons to the resin surface. These electrons can easily flyover above the reduced potential energy barrier (PEB), which is reduced because of two factors: The first is because of a lower local work function value for the epoxy coating layer (2.97 eV), which reduces the height of the PEB and so also the vacuum level. Second, when an external electrostatic field is applied in the space between the two electrodes, the PEB’s shape changes to a reduced image-rounded PEB, which is known as the Schottky–Nordheim SN-PEB. The top of this SN-PEB can be reduced by increasing the intensity of the electrostatic field, and when applying extremely intense fields, the top of the SN-PEB will be lower than the Fermi level of the used material, allowing the electrons to thermally transfer from above the reduced SN-PEB [36]. This can help increase the density of ${Ne}^{+}$ to be created at small spots, and then emitted at higher densities, providing brighter spots on the imaging screen.