The most popular positive resists are referred to as DQN, corresponding to the sensitizer material diazonaphthoquinone (DQ) and the matrix material novolac (N). Novolac is a phenolic-formaldehyd resin, i.e., it is a polymer whose monomer is an aromatic ring with two methyl and one phenol group. By itself, it dissolves easily in an aqueous solution. Adding diazonaphthoquinone to novolac in about a 1:1 mixture makes the photoresist almost insoluble in a base solution. This occurs by way of a chemical bonding of the PAC and the resin on the surface of the resist where it is exposed to the developer. The PAC material diazonaphthoquinone is also an organic material consisting of various aromatic and carbon rings. The important part is a weakly bonded nitrogen molecule that is freed by the addition of UV light and leaves a highly reactive carbon site behind. Adding water a second structural transformation happens and the final result is carboxylic acid. This carboxylic acid readily reacts with and dissolves in alkaline solutions like KOH or NaOH diluted with water. Hence, the photoresist is dissolved in irradiated regions but remains almost unchanged in unexposed parts. The development rate can differ by a factor of 100 or more. Another advantage of DQN resists is their high resistance against chemical attack during etching processes. However, big problems arise when using shorter wavelengths. Novolac compounds begin to strongly absorb light just below 250 nm, making KrF-excimer (248 nm) exposures marginally acceptable, but definitely precluding their usage in conjunction with ArF-excimer (193 nm). The development of new DUV resists in combination with advanced resist systems is one of the most demanding problems for further progress in photolithography. Possible approaches will be presented in Section 2.6.4.