Most photoresists are solutions of organic polymers that are hydrophobic. Like oil and water, hydrophobic materials don’t mix well with hydrophilic materials, such as most semiconductor and metal oxides and nitrides. Thus, resist dispensed on a hydrophilic surface will not spread evenly, and will streak and adhere poorly. However, a hydrophilic surface can be achieved by using an adhesion promoter (primer); the most common is hexamethyldisilazane (HMDS). This is an organosilicon reagent that functionalizes a surface with non-polar trimethylsilane groups, making it hydrophobic.