# The Wettability and Numerical Model of Different Silicon Microstructural Surfaces
## 2 Surface Properties of Silicon Prepared by PIII Technology
### Preparation and Characterization of Microstructural Silicon Surfaces
An increase in the gas mixture ratio generated a gradual increase in the amount of SF\({}_{6}\) gas and enhanced the intensity of the etching reaction, thereby forming an obvious microstructure on the surfaces of samples 3-7. The convex and concave microstructures were randomly distributed on the surface of the sample (the lighter the color, the higher the structure). At the same time, an increase in the SF\({}_{6}\) gas generated a decrease in the shape and area of the local convex structure and an increase in the dispersion, thereby significantly increasing the percentage of the depression structure in the total area as well. The SEM results exhibited an area ratio of the projection on the surface of the convex/concave structures. However, the shape and height of the convex/concave structures could not be clearly displayed through the projection. The surface morphology of the sample was further scanned by AFM, and the three-dimensional (3-D) morphology of the convex/concave structure was obtained, as seen in Figure 4. Similar to the SEM scanning results, the surfaces of samples 1 and 2 were smooth, and the surface of samples 3-7 exhibited an obvious convex structure of a volcanic heap. The AFM results also indicated that an increase in the gas mixture ratio, to a certain degree, did not exhibit any obvious physical changes in the microstructure, as observed in the surface structure of samples 5-7. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Number** & **1** & **2** & **3** & **4** & **5** & **6** & **7** \\ \hline Mixing ratio of the gases (SF\({}_{6}\)/O\({}_{2}\)) & Untreated & 1:1 & 2:1 & 3:1 & 4:1 & 5:1 & 6:1 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Relationship between the sample number and mixture ratio of the gas. The Asylum Research AFM software (version 13.04.77, Oxford Instruments Asylum Research, Inc., CA, USA) was applied to analyze the AFM scanning data and to characterize both the surface roughness (\(Ra\)) and the area ratio (\(r\)) of the total surface area to the projected area of the microstructures. The \(Ra\) was obtained from the statistics of the average height of the convex structure, and \(r\) was obtained from the statistics of the surface area ratio of the convex/concave structure to its projection area at the horizontal plane. Figure 5 presents the relationship between the surface roughness and area ratio of the sample surface and the change in the mixing ratio of gases (SF\({}_{6}\)/O\({}_{2}\)). According to Figure 5, the change in the surface roughness and area ratio of sample 2 was very small when compared to that of sample 1, which further validated the significantly low intensity of the etching reaction to form microstructures on the sample surface under the condition of a low gas mixing ratio. In addition, an increase in the mixed gas proportion first generated a rapid increase and then a slight decrease in the surface roughness, whereas the area ratio of the sample increased continuously. These results indicated that an increase in the mixed gas proportion enhanced the etching reaction and more silicon surfaces participated in the etching reaction, thereby resulting in an increase in the area ratio. However, the surface roughness was determined by both the etching and passivation reactions. An extreme relationship was observed between the maximum roughness and the mixing ratio of the gases. In addition, too large of a mixing ratio reduced the roughness of the microstructural surfaces. Figure 4: AFM image of the sample surface—untreated surface (**a**), gas-treated surface with a mixing ratio of 1:1 (**b**), 2:1 (**c**), 3:1 (**d**), 4:1 (**e**), 5:1 (**f**), and 6:1 (**g**). Figure 3: SEM image of the sample surfaces—untreated surface (**a**), gas-treated surface with a mixing ratio of 1:1 (**b**) 2:1 (**c**), 3:1 (**d**), 4:1 (**e**), 5:1 (**f**), and 6:1 (**g**).