The exposed wafers are developed in dipping baths or in spray processes. While dipping baths allow the development of multiple wafers at a time, in spray development one wafer is processed after another. As in resist coating processes the wafer is placed on a chuck and sprayed with developing chemicals while rotating at low rpm. Subsequent the wafer development process is stopped with water.
Some advantages of the spray development in contrast to dipping baths are:
- smallest structures can be developed
- the chemical is renewed steadily
- the amount of chemicals is much less
Depending on the type of resist - negative/positive - exposed areas are solubly or insolubly in developing chemicals and a patterned wafer remains after development. The exposure causes a chemical reaction in the resist thus that the sensitizer forms an acid which is neutralized by the developer like follows:
Illustration of the resist after development
Because potassium hydroxide or sodium hydroxide leave residuals on the wafer, chemicals without metal ions, like TMAH (tetramethylammonium hydroxide), are used. An additional annealing (hard-bake) hardens the resist to be resistant to subsequent etch processes or ion implantation.