Exposure Mechanism of Positive Photoresist

Photoresist is mainly composed of photosensitive agents, resins, solvents, additives (plasticizers, surfactants, photosensitive stabilizers, etc.). Resin is the basic skeleton of photoresist and is not sensitive to light. The solvents mainly include PGMEA,EGMEAD, etc., which are used to dilute the photoresist and adjust its viscosity. Photosensitive agents are mainly photochemical reactions to light energy.

The exposure principle of positive photoresist?

The above picture, for example, is the reaction equation of a positive photoresist:

1, the first step: the decomposition reaction of the photoreceptor
The photoinitiator used in this photoresist is triphenyl sulfonium chloride. Under ultraviolet light (hv) irradiation, triphenyl sulfonium chloride is decomposed into triphenyl sulfonium cation and hexafluoroantimonium anion (SbF6-), acid ion and free radical.
2, the resin deprotection reaction:
The resin before exposure is a polymer containing chemical protection groups, and the resin is deprotected under the catalysis of acid.
The protective groups (such as the Boc group, shown below) are catalyzed to be removed by the acid to produce products soluble in the developer (such as hydroxystyrene and carbon dioxide).

Since the developer is an alkaline solution, which can react with the generated acid ions, and the decomposition of the resin is easily dissolved in water, the exposed part is more easily removed, leaving the unexposed part.