The surface is said to be hydrophobic when water beads up. The word hydrophobic can be directly translated as a “fear of water” which is a good description of a surface that repels water. Several examples of such surfaces can be found in nature such as lotus leaves, shark skin, and butterfly wings. Nowadays, hydrophobic coatings are used in many applications across several application areas.
Hydrophobic coatings are used in many consumers products. One of the easiest to relate to is outdoor clothing. Different clothing brands have their own version of hydrophobic coatings, but Gore-Tex is probably the best-known one. Gore-Tex material is based on thermo-mechanically expanded PTFE (polytetrafluoroethylene) and other fluoropolymers. Teflon is a brand name for PTFE, and Teflon cookware is another example of consumer products where hydrophobicity is important. The reason why PTFE repels water comes from the high electronegativity of the fluorine atoms. One problem with the hydrophobic coating is that it is prone to wear and tear. Clothes and cooking pans are washed which deteriorates the coating quality over time.
The durability of hydrophobic coatings is one of the biggest challenges. While the hydrophobic chemistry is now quite well understood, it is still a challenge to produce coatings that would tolerate wear and tear. One way to study this is through contact angle measurements. Depending on the surface studied, the wear could mean for example washing of the clothes or the frying pan. In all cases, what typically happens is that while the static contact angle stays fairly constant or decreases slightly over time, the major effect can be seen in contact angle hysteresis. High contact angle hysteresis indicates that the coating is no longer uniform and while droplets with high contact angles are still seen, they don’t roll off the surface as before.
HMDS is used to render the surface of silicon more hydrophobic and thus to match better with the chemistry of the photoresist.
Quality control is needed to detect any possible problems early on and contract angle measurement provides an excellent tool for that.