In this chapter, the measurement results of contact angles and sliding angles of our dual structures will be reported. Measured reflection spectra of our dual structures will also be shown.
4-1 Measurement of Contact Angle and Sliding Angle
As mentioned in Chapter 1, an intuitive way to describe the wettability of a solid surface is by measuring its contact angle. For self-cleaning property, sliding angle is more important than contact angle. Here the two angles will be measured. The setup of the contact angle analyzer is shown in Fig. 4-1 Contact angle analyzer, which is home made. The home made setup for sliding angle measurement is shown in Fig. 4-2 and Fig. 4-3. The setup for sliding angle measurement was consisted of a laser pointer and a concentric ruler. As shown in Fig. 4-3 (a), the laser pointer was calibrated by a wafer floating on water. The laser light shot to the wafer floating on water and then shot back to the laser pointer. This process made sure that the laser light was parallel to a plumb line. As schematically shown in Fig. 4-3 (b), the sample was placed at a stage that can tilt small angles. A 20 μl water drop was placed on the sample and the stage started to tilt, once the water drop slide and the distance from the laser pointer and the reflected laser light on the concentric ruler was recorded. The sliding angle
can be calculated by Eq. (4.1).
= (arctan(d/L))/2 (4.1) The setup of sliding angle measurement shown in Fig. 4-3 is used to measure low sliding angle (<5o). When the sliding angle is larger than 5o, another setup of sliding angle measurement is used, which is shown in Fig. 4-4 Setup for sliding angle measurement.
The measured contact angles and sliding angles of dual structures are listed in Table 4.1. The shapes of the 5 dual structures are also shown. From Table 4.1, the 5 types of dual structures are all superhydrophobic. The sliding angles of these dual structures were all lower than 5°. The sharp peaks of the dual structures are smaller than 75 nm in diameter. In comparison to the dual structures which have sharp peaks, the simple structures with larger top diameters have larger sliding angles. The measured contact angles and sliding angles of 3 simple structures are listed in Table 4.2. The structure shapes are shown in Fig. 4-6.The sliding angles of the 3 simple structures are all larger than 18o. The top diameters of the simple structures are larger than 140 nm. The top diameter of the three structures which are shown in Fig. 4-6 (a), (b), and (c) are 150, 160, and 220 nm, respectively. From the results, the sharp peaks of the subwavelength structures did help to have low sliding angles.
Fig. 4-1 Contact angle analyzer.
Fig. 4-2 Setup for sliding angle measurement for <5o.
Fig. 4-3 Setup for sliding angle measurement (a) First calibration (b) Then sliding angle measurement.
Fig. 4-4 Setup for sliding angle measurement >5o
Table 4.1 The contact angles and sliding angles of dual structures. The corresponding structure shapes are shown in the figures below.
Contact angle Sliding angle
A 157° 4.5°
B 155° 2.4°
C 162° 2.7
D 157° 3°
E 162° 1.8°
Fig. 4-5 Dual structure A, B, C, D, and E.
Table 4.2 The contact angles and sliding angles of simple structures. The corresponding structure shapes are shown in the figures below.
Contact angle Sliding angle
I 150o 18o
II 146o 20o
III 137o 37o
Fig. 4-6 Simple structures.
4-2 Measurement of Reflectance
The reflectance spectra of the dual structures were measured by our home made reflectometer shown in Fig. 4-7.
The measured reflectance spectra of our 5 dual structures are shown in Fig. 4-8.
The structure shapes of the 5 dual structures are shown in Fig. 4-5.
The dual structure A has the highest reflectance because it has negatively tapered side wall. The dual structure D and E have higher reflectance than that of dual structure B and C. The increased reflectance of dual structure D was due to its low fill factor. The increased reflectance of dual structure E was due to its vertical side wall.
The dual structure B has lower reflectance than that of dual structure A, D, and E and its average reflectance is 3.83%. The optical response of dual structure B is similar to the optical response of the structure shapes shown in Fig. 4-9 (b). The three shapes shown in Fig. 4-9 (b) are described by Eq. (2.3) and v is 4 here. From the v value, it can be understood that the increased reflectance of dual structure B over 500 to 700 nm is caused by the obvious transition between top part and bottom part.
The dual structure C has the lowest average reflectance. The reflectance of dual structure is lower than 5% at 400 to 700 nm and its average reflectance is 1.8%. The slope of the side wall slightly changes in the middle of the structure. The reflectance spectrum of dual structure C and the reflectance spectra of the better 2 structure
shapes proposed in Chapter 2 are shown in Fig. 4-10 (c). The reflectance spectra of the 3 structures are all lower than 5% from 400 to 700 nm but the spectra are different.
This may be due to the peak of the shapes proposed in Chapter 2 is sharper than the peak of dual structure C. Moreover, the uniformity of dual structure C is not so perfect and causes the structure shape deviation between each pillar. The deviation may also cause the reflectance spectrum to be changed. The sample picture of dual structure C is shown in Fig. 4-11. The dark red color indicates that there is scattering lights from the sample.
Fig. 4-7 Setup for reflectance measurement.
Fig. 4-8 Measured reflectance spectra of 5 dual structures and flat silicon.
Fig. 4-9 (a) Dual structure B (b) Reflectance spectra of dual structure B and simulated SWSs. The shapes shown in the up-left corner were used for simulation and the structure height for simulation is 540 nm. The 3 shapes were described by Eq. (2.3) and v was 4 here.
Fig. 4-10 (a) Structure shapes for simulation and the structure height is set to 405 nm (b) Dual structure C (c) Simulated reflectance spectra of Fig. 4-10 (a) and flat silicon and measured reflectance of dual structure C.
Fig. 4-11 Sample picture of dual structure C.