"Photolithography, also termed optical lithography or UV lithography, is a process used in microfabrication to pattern parts of a thin film or the bulk of a substrate. It uses light to transfer a geometric pattern from a photomask to a light-sensitive chemical "photoresist", or simply "resist," on the substrate. A series of chemical treatments then either engraves the exposure pattern into, or enables deposition of a new material in the desired pattern upon, the material underneath the photo resist. For example, in complex integrated circuits, a modern CMOS wafer will go through the photolithographic cycle up to 50 times. " - Wikipedia
Low Budget Maskless DLP Projector Lithography for Home Chip Labs
The ambient light in labs set up for photolithography are heavily UV and DUV filtered (yellow lighting) so that photoresists are not exposed by mistake while processing.
"With positive photoresists, UV light strategically hits the material in the areas that the semiconductor supplier intends to remove. When the photoresist is exposed to the UV light, the chemical structure changes and becomes more soluble in the photoresist developer. These exposed areas are then washed away with the photoresist developer solvent, leaving the underlying material. The areas of the photoresist that aren’t exposed to the UV light are left insoluble to the photoresist developer. When working with positive photoresists in the semiconductor manufacturing industry, you receive an identical copy of the pattern, which is exposed as a mask on the wafer." Source
"With negative resists, exposure to UV light causes the chemical structure of the photoresist to polymerize, which is just the opposite of positive photoresists. Instead of becoming more soluble, negative photoresists become extremely difficult to dissolve. As a result, the UV exposed negative resist remains on the surface while the photoresist developer solution works to remove the areas that are unexposed. This leaves a mask that consists of an inverse pattern of the original, which is applied on the wafer." Source