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One very common positive photoresist used with the I, G and H-lines from a mercury-vapor lamp is based on a mixture of diazonaphthoquinone (DNQ) and novolac resin (a phenol formaldehyde resin). DNQ inhibits the dissolution of the novolac resin, but upon exposure to light, the dissolution rate increases even beyond that of pure novolac.
The term comes from Swedish "lack" for lacquer and Latin "novo" for new, since these materials were envisioned to replace natural lacquers such as copal resin. Typically novolaks are prepared by the condensation of phenol or a mixture of p- and m-cresol with formaldehyde (as formalin). The reaction is acid catalyzed.
Positive photoresists are composed of a novolac resin, ethyl lactate solvent, and Diazonaphthaquinone (DQ) as the photoactive compound. [9] Positive photoresist reacts with light to cause the polymer to break down and become soluble in a developer solution. Positive resist has better resistance to etchant than negative photoresist.
Such photoresists are used in the manufacture of semiconductors. [2] [3] [4] In this application DNQs are mixed with Novolac resin, a type of phenolic polymer. The DNQ functions as a dissolution inhibitor. During the masking/patterning process, portions of the photoresist film are exposed to light while others remain unexposed.
Phenol-formaldehyde resins, as a group, are formed by a step-growth polymerization reaction that can be either acid- or base-catalysed.Since formaldehyde exists predominantly in solution as a dynamic equilibrium of methylene glycol oligomers, the concentration of the reactive form of formaldehyde depends on temperature and pH.
SU-8 is a commonly used epoxy-based negative photoresist. Negative refers to a photoresist whereby the parts exposed to UV become cross-linked, while the remainder of the film remains soluble and can be washed away during development. As shown in the structural diagram, SU-8 derives its name from the presence of 8 epoxy groups.
Bisphenol A and novolac based cyanate esters are the major products; bisphenol F and bisphenol E are also used. The aromatic ring of the bisphenol can be substituted with an allylic group for improved toughness of the material. Cyanate esters can also be mixed with bismaleimides to form BT-resins or with epoxy resins to optimize the end use ...
In one example, researchers incorporated PPGs into a silica-based sol-gel. [62] In a second example, a hydrogel was synthesized to include protected Ca 2+ ions. [ 63 ] [ 64 ] Finally, PPGs have been utilized to cross-link numerous photodegradable polymers , which have featured linear, multi-dimensional network, dendrimer, and branched structures.