Material surfaces can assume different states of surface energy, which determines their ability to repel liquids. If a surface is very strongly repelling any liquid, it is called hydrophobic. The surface state can be easily tested by depositing a small droplet of liquid and observe, in which way the liquid is making contact to this surface. If the droplet remains on the surface in a tall round shape, the angle between surface and the droplet is quite large and the surface is considered to be hydrophobic. In this state, it is very difficult to perform a wet etch on this surface due to the repelling of the etchant.
It is a rather common process to transform the hydrophobic to hydrophilic state with a simple oxygen plasma flash. After the process step, the surface will become hydrophilic and a droplet on the surface will assume a very flat and wide shape and a very shallow contact angle.
Another application requires creating hydrophobic surfaces or a combination of hydrophilic and hydrophobic areas on the same substrate, for instance as pre-process to Ink-Jet printing in the production of OLED displays.