Photoresist is a light sensitive material used in the process of Photolithography to form a patterned coating on the wafer 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 photoresist is based on a mixture of Diazonaphthoquinone(DNQ) and Novolac 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. DNQ-novolac resists are developed by dissolution in a basic solution (usually 0.26N tetra-methyl ammonium hydroxide in water).