Photoresist is a photo-sensitive material applied to the wafer in a liquid state in small quantities. The wafer is spun at ± 3000 rpm which spreads the "puddle" into a uniform layer between 2 and 200 µm thick.

There are two main types of photoresist: negative and positive.

When a negative resist is used, unexposed portions of the photoresist are removed leaving a "negative" image. This method is only capable of producing features down to about 2.0 µm.

Most semiconductor processes today use a positive resist where exposed portions are removed leaving a "positive" image of the mask pattern on the surface of the wafer. Positive resists are more capable of producing the small size of modern device features which are typically below 1.0 µm but may be as small as 0.15 µm.