In photolithography, the wafer is uniformly coated with a thick light-sensitive liquid called photoresist. Portions of the wafer are selected for exposure by carefully aligning a mask between an ultraviolet light source and the wafer.

In the transparent areas of the mask, light passes through and exposes the photoresist.

Photoresist hardens and becomes impervious to etchants when exposed to ultraviolet light. This chemical change allows the subsequent developer solution to remove the unexposed photoresist while leaving the hardened, exposed photoresist on the wafer.

The wafer is subjected to an etch process (either wet acid or plasma dry gas etch) to remove that portion of the nitride layer that is not protected by the hardened photoresist. This leaves a nitride pattern on the wafer in the exact design of the mask.

The hardened photoresist is then removed (cleaned) with another chemical.