# 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  
The experimental sample was a microstructural surface prepared by plasma immersion ion implantation (PIII) technology on the surface of polished monocrystalline silicon. Figure 2a shows a schematic diagram of a plasma immersion ion implantation (PIII) device, which includes a plasma source, a vacuum chamber, a high-voltage pulse power source, and a worktable on which the sample was placed [29]. The preparation process was implemented as follows--first, the Si slice was placed on the worktable and the high-voltage pulse power source was turned on. At the same time, the SF\({}_{6}\) and O\({}_{2}\) produced a lot of S\({}^{\star}\), F\({}^{\star}\), and O\({}^{\star}\) plasma under the ionization of the plasma source. Finally, plasma impinged the Si surface and initiated a reaction, thereby forming micro/nanostructures on the sample surface. Figure 2: Schematic diagram and preparation schematic of plasma immersion ion implantation (PIII)—(**a**) PIII device and (**b**) process of implantation. The process of PIII preparation included the etching reaction and the passivation reaction. Figure 2b presents the ion implantation process. The mixture of the etching gas (SF\({}_{6}\)) and the passive gas (O\({}_{2}\)) entered the vacuum chamber and the atomic groups, such as F\({}^{\ast}\), SF\({}^{\ast}\), S\({}^{\ast}\), and O\({}^{\ast}\), were generated by the ionization of the SF\({}_{6}\) and O\({}_{2}\) gases. When these atomic groups met the test sample, the F\({}^{\ast}\) entered the inside of the silicon wafer and formed a volatile gas SiF4 with the Si atoms [30]. The sample surface was etched following SiF\({}_{4}\) gas escape from the silicon wafers. The etching reaction caused the silicon surface to be removed. Simultaneously, the O\({}^{\ast}\), F\({}^{\ast}\), and Si atoms underwent passivation reaction and produced Si\({}_{x}\)O\({}_{y}\)F\({}_{z}\). Passivation reaction inhibited etching reaction. When etching reaction and passivation reaction exist simultaneously, the surface of the sample formed inhomogeneous microstructures by the PIII process [29]. In order to control the surface microstructure, the SF\({}_{6}\)/O\({}_{2}\) flow ratio, SF\({}_{6}\) and O\({}_{2}\) total flow rates, and reaction time were controlled during the experiment. In this paper, six different microstructural surfaces were prepared by changing the gas mixture ratio (SF\({}_{6}\)/O\({}_{2}\)), while the reaction time and the total gas flow rate were constant. The experimental samples were numbered 1, 2, 3, 4, 5, 6, and 7 based on the ascending order of the gas mixture ratio. Eight samples were made for each mixed gas ratios. The corresponding relationship between the number and gas ratio is presented in Table 1. Sample 1 is an untreated Si surface, which is considered an ideally smooth surface. The surfaces of the samples were scanned by scanning electron microscopy (SEM) and atomic force microscopy (AFM), and the surface microstructural morphology was obtained. The two-dimensional (2-D) surface morphology of the sample surface was obtained by SEM scanning, of which the distribution of the projection of the convex and concave structures on the surface was displayed well. The overhead views of the microstructural surface observed by SEM at 5000 times magnification are shown in Figure 3, wherein no obvious concave or concave structures on the surfaces of samples 1 and 2 are observed, whereas the surfaces of samples 3-7 exhibit obvious convex and concave structures. These results indicated the minimal presence of SF\({}_{6}\) gas to prepare sample 2, and that the intensity of the etching reaction was far less than that of the passivation reaction. The surface of sample 2 was not significantly etched and was therefore characterized as smooth.