Some common semiconductor substrate surfaces, most notably Si, SiO2, SiN, and some other oxide forming layers, are naturally hydrophilic due to absorbed and surface layer water from ambient humidity. Unless it is removed prior to coating photoresist, this water layer will allow developer and etch chemicals to penetrate the resist/substrate interface and cause extreme etch undercutting or complete delamination of the photoresist film. To ensure adhesion during develop and wet etch, these hydrophilic surfaces must be converted to a hydrophobic state. Applying HMDS (hexamethyldisilazane) to dehydrated wafers is one common method for achieving the surface hydrophobicity required to prevent photoresist delamination.

To prime wafers with HMDS, the surface must first be completely dehydrated. Sufficient desorption of water can usually be achieved by baking the substrate at 140 to 160C (this “dehydration bake” is often performed in a vacuum). Dehydrating leaves surface OH groups hence the surface is still hydrophilic.

The surface hydrophobicity can be measured (and monitored) by placing a drop of water on the primed surface and measuring the contact angle as shown. An optical goniometer (often attached to an optical microscope) is used to measure this angle with reasonable precision and repeatability.