Photoresist is a light sensitive material used in several industrial processes, such as photolithography and photoengraving to form a patterned coating on a surface. Photoresists are classified into two groups, positive resists, in which the exposed areas become more sensitive to chemical etching and are removed in the developing process, and negative resists, in which the exposed areas become resistant to chemical etching, so the unexposed areas are removed during the developing process.
One very common positive (I, G, H-line) photoresist is based on a mixture of Diazonaphthoquinone (DNQ) and Novolac resin (a phenol formaldehyde resin). DNQ inhibits the dissolution of the novolac resin, however, upon exposure to light, the dissolution rate increases even beyond that of pure novolac. The mechanism by which unexposed DNQ inhibits novolac dissolution is not well understood, but is believed to be related to hydrogen bonding(or more exactly diazocoupling in the unexposed region). DNQ-novolac resists are developed by dissolution in a basic solution (usually 0.26N tetra-methyl ammonium hydroxide in water).
Deep Ultraviolet (DUV) resist are typically polyhydroxystyrene-based polymers
with a photoacid generator providing the solubility change. However, this material does not experience the diazocoupling.