The fabrication and WVTR of ultrahydrophobic structure

Fluorination was put into use to improve the WVTR of PET as discussed previously due to the lower water solubility and diffusion constant of fluorocarbon layer. To broaden the scope of the research, we would enhance the water-repellency characteristic of PET surface.

It’s suggested that such requirement could be attained by roughening the hydrophobic substance as discussed in section 2.5.

4.4.1 Selectivity of photoresist and PET during O

plasma etching

Lithography process described in section 3.3 defined the shape of pattern, dry etching was then carried out by O2 plasma. Fig. 4-14 showed the relation between the etching time and thickness variation of photoresist detected by n&k analyzer. The etching rate of photoresist was 144 nm/min while the etching rate for PET was 234 nm/min. Selectivity defined as the ratio of photoresist etching rate to PET etching rate was 1.625.

Selectivity=

t photoresis of

rate etching

PET of rate

etching (4-6)

Fig. 4-14 Variation of thickness and etching time for photoresist.

Fig. 4-15 Dependence of thickness and etching time for PET.

4.4.2 Configuration of the regular pattern

Lithography procedure illustrated in section 3.3 had been carried out and followed by the O₂ plasma which would dry etched the photoresist and uncovered area of PET. Fig. 4-16 was the top view and bird's-eye view of the regular pattern without any O₂ plasma treatment.

Lithography process translated the mask information such as the shape and propagations of the pattern to the photoresist. Hence, the configuration of regular pattern here would be accepted to be pillar-like. The height of the pillar shown in Fig 4-16 was about 1.63 µm which depended on the thickness of photoresist, and the diameter of the circular area was 2.2~2.3µm.

We executed the O2 plasma treatment until the photoresist was completely cleaned up. At the same time, PET out of the protection of photoresist also adopted the etching process. Along with the plasma treatment time for 700 second, the contours of the regular pattern were still pillar-like. Due to the faster etching rate of PET, we observed that the height of the pillar increased from 1.63µm to about 1.96µm and the diameter of the pattern was 1.8µm. In summary, we successfully applied the lithography process to fabricate the order regular

pattern, and the original objective to increase the roughness was also satisfied. The final configuration of the pattern maintained pillar-like during the etching process, and its dimension was within the micrometer classification.

Fig. 4-16 The configurations of the regular pattern without O₂ plasma etching (a) top view for large area,(b) the diameter of circular pattern, and (c) bird's-eye view.

Fig. 4-17 The configurations of the regular pattern with O2 plasma etching for 700 seconds with (a) bird's-eye view, and(b) the cross-section view.

4.4.3 Contact angle measurement for fluorinated regular pattern

The same CF₄ plasma treatment for 15 minutes was executed on regular pattern and we measured the contact angle of fluorinated regular pattern. Fig. 4-18 showed the contact angle for the surface which combined the fluorination and regular pattern was 134.7°. This result indicated the surface characteristic had entered the ultrahydrophobic ambits. As discussed in section 2.5, both Wenzel and Cassie- Baxter theories discussed the importance of roughness factor as we concerned the ultrahydrophobic characteristic.[55, 56] However, R. N. Wenzel

assumed that the space between the protrusions on the surface is filled with the liquid while the air is trapped in the asperities supposed in Cassie- Baxter relation. If we put the two theories into together, a θC given by equating the two mathematic functions was considered the threshold value between the two regimes. This yielded:

s

φs= solid fraction contact with liquid r= roughness factor

A comparison between the interfacial energies associated with the Wenzel and Cassie- Baxter situations confirmed that air pocket should be favored only if the Young’s angle θ was larger than θC. [58] Derived from the SEM photography, the configuration of the ordered pattern was pillar-like, and then we could evaluate the roughness factor and solid fraction contact with liquid according to the SEM photography.

Diameter of the pillar≒1.8µm Height of the pillar≒1.96µm

Distance between the neighbor pillar≒5.73µm

Roughness factor= actual surface/geometric surface=1.78 Solid fraction contact with liquid=0.179

Taking the value into Eq. (4-7), we obtained the θC was 120.4°. This result pointed out that ultrahydrophobic characteristic in our case had entered the Cassie- Baxter situations. Double check with the Wenzel equation alone, the maximum theoretical value of the contact angle would be only 121°.

Fig. 4-18 Contact angle measurement of the fluorinated regular pattern.

4.4.4 WVTR of the fluorinated regular pattern

Fig 4-19 showed the comparison of WVTR based on Ca test between the fluorinated regular pattern and flat surface. The WVTR of fluorinated regular pattern was evaluated to be 0.82 g/m²-day. Although the increase in resistance during the initial period was moderated, the resistance raised sharply afterward. Observation by literature research, the water vapor would condense and infiltrate in the cavities of the rough structure. [12,82] Hence, regular pattern generated free surface in which water molecular could absorb on, and this phenomenon was followed by large amount of water molecular diffused through the PET which reacted with the calcium layer and led the resistance increased quickly. Found in the contact angle result that the water-repellency characteristic was increased for fluorinated regular pattern, however, the WVTR of fluorinated regular pattern was worse than the flat surface. Ultrahydrophobic property had been realized in this study, but this behavior was based on the Cassie- Baxter situations, that is, the water drop was braced by carefully constructed pillars. Hence, the ultrahydrophobic feature here didn’t attribute to the lower chemical affinity with water molecular. We revealed that the ultrahydrophobic characteristic sometimes didn’t directly proportion to the lower WVTR. To achieve lower WVTR by such fluorination processes and regular pattern used in this study. We suggested one of the possible

approaches was related to the higher degree of fluorination of PET by different categories of plasma process. Moreover, the application of the regular pattern was still a potential method if it’s possible to fabricate such structure that the distance between the pillars was close enough, and the vapor molecular couldn’t adsorb on the side wall of the pillar.

Fig. 4-19 Water vapor permeation rate measurement of fluorinated regular pattern and flat surface.