2.1. 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*, F*, and O* 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.
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*, SF*, S*, and O*, were generated by the ionization of the SF
6 and O
2 gases. When these atomic groups met the test sample, the F* 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*, F*, and Si atoms underwent passivation reaction and produced Si
xO
yF
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. 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.
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.
2.2. Static Contact Angle of the Sample Surfaces
The static contact angle of the sample surfaces was measured by the contact angle measuring instrument XG-GAMB1 (Xuanyichuangxi Industrial Equipment Co. Ltd., Shanghai, China). Ultrapure water with a volume of 2 μL was used as the experimental working medium, and the environmental temperature was set at 20 °C. The static contact angle measuring instrument was used to measure the contact angle of the sample surface using the height–width method. To reduce accidental error of the sample surface damage or surface structure non-uniformity, the contact angle was analyzed in at least three regions for each sample. The static contact angle results are shown in
Figure 6.
Figure 6 exhibits a significant increase in the contact angle of the sample surface prepared by PIII technology. In addition, the hydrophilic surface was transformed into a hydrophobic surface (the contact angle was greater than 90°).
Figure 3 and
Figure 4 present a combination of the SEM and AFM surface morphologies, wherein sample 2 exhibited minimal changes in its surface morphology as compared to sample 1, whereas the contact angle exhibited significant changes because the intensity of the etching reaction was not enough to form an obvious microstructure on the surface of the sample at a low gas mixing ratio. However, the reaction continued to change the chemical properties of the surface of the sample, thereby generating a change in the wetting properties of the surface. An increase in the mixing ratio of the gases generated an increase in the value of the static contact angle of the surface, though this growth rate of contact angle gradually decreased. A combination of the previous figures with
Figure 5 exhibited a simultaneous increase in the area ratio and roughness of the sample following an increase in the mixing ratio of the gases from 1:1 to 3:1. When the gas mixture ratio was greater than 3:1, the roughness exhibited a slight decrease, even when following an increase in the convex/concave area ratio. Therefore, the increased surface roughness (Ra) and area ratio (r) generated an increase in the contact angle. When the mixing ratio of the gases was less than 3:1, an increase in both the roughness and area ratio resulted in a significant increase in the contact angle. In addition, a gas-mixing ratio greater than 3:1 inhibited the increase of the contact angle due to the decrease in the roughness, thereby decreasing the growth rate of the contact angle under the condition of a large mixing ratio.
2.3. Theoretical Value of Static Contact Angle
The structural parameters corresponding to the theoretical model of the contact angle were extracted from the SEM and AFM characterization results. In addition, the static contact angles of the sample surface were calculated by the Wenzel and Cassie–Baxter theoretical models, respectively. According to the Wenzel theory, the Wenzel’s contact angle (
) on the microstructural surfaces can be calculated based on Equation (2), where r was obtained by the AFM characterization results and the Young’s contact angle was 54°, which was obtained from the experiment with sample 1. Photoshop software (Adobe, Inc., CA, USA) was applied to extract the pixels of the different shades of gray from
Figure 3. The proportion of the pixels in the lighter color region to the total number of pixels in the whole image was calculated, wherein the projection area of the protruding structure accounted for the percentage of the total projection area of the picture, namely, the solid–liquid fractional surface area (
). According to the Cassie–Baxter theory, the Cassie–Baxter’s contact angle (
) on the microstructural surfaces can be calculated based on Equation (3) for a Young’s contact angle of 54°.
Table 2 presents the microstructural characteristic parameters of the sample surfaces and the contact angles of the corresponding theoretical models.
Variations in the contact angle of experimental measurements (
), the theoretical Cassie–Baxter contact angle (
), and the theoretical Wenzel contact angle (
) with the mixing ratio of the gases are presented in
Figure 7, wherein the graph indicates that the trend of the theoretical Wenzel’s contact angle (
) is the opposite of that of the contact angle of the experimental measurement (
). The presence of microstructures generated an obvious increase in the area ratio (r) of the sample surface, thereby resulting in a significant reduction in the contact angle. It was obvious that Equation (2) was not suitable for predicting the static contact angle of the sample. The trend of the theoretical Cassie–Baxter’s contact angle (
) was the same as that of the contact angle of the experimental measurement (
), wherein an increase of the static contact angle caused an increase of the mixing ratio of the gases. When the mixing ratio of the gases was 1:1, the Young’s contact angle in Equation (3) still used the contact angle of the untreated silicon surface, which did not consider the change in the Young’s contact angle due to the chemical reaction. Therefore, a significant difference was observed between
and
when the mixing ratio of the gases was 1:1, though this was not observed in the theoretical model. However, when the mixing ratio of the gases was greater than 3:1,
continued to increase due to both the continuous decrease of
and the increased deviation between
and
. On one hand, these results indicated that the calculation method of
greatly influenced the contact-angle calculation results. On the other hand, it also indicated that
was not the only factor that affected the static contact angle. According to the analysis of the contact angle of the experimental measurements in the last section, the contact angle was co-influenced by the roughness and area ratio. However, the Cassie–Baxter theory unified all the influence factors of the contact angle to parameter
. The present study could not get the exact
of the microstructural surface as it had a volcanic shape, which was also the limitation of the Cassie–Baxter theory when predicting the contact angle of microstructural surfaces.