What Is a Photoacid Generator?
Photoacid generators (PAGs) are compounds that release acid upon exposure to light, such as ultraviolet (UV) light. This acid then catalyzes reactions, commonly used in photocation polymerization initiators and photoresists for photolithography. Sulfonium salts are typical PAGs, with epoxy resins as a frequent curing material. The reaction initiation by PAGs is controlled by specific light irradiation, not occurring under normal room lighting conditions.
Applications of Photoacid Generators
PAGs have widespread applications, notably in curing photo-curable resins and photolithography, with growing demand due to advances in resin applications and photolithographic technology.
1. Curing of Photo-Curable Resins
In the curing process of photo-curable resins, PAGs trigger polymerization reactions upon light exposure, hardening the resin. This technology is applied in coatings, can paints, and 3D printing materials, with epoxy resins being a common choice for their strong bonding properties and resistance to corrosion and electricity.
2. Photolithography
PAGs serve as photoresist materials in photolithography, critical for semiconductor and LCD production. They produce acid when exposed to patterned light, etching a substrate surface to create a desired pattern. Recent developments include chemical amplification types, where the generated acid acts as a catalyst for further reactions.
Principle of Photoacid Generators
PAGs consist of onium compounds, such as sulfonium and iodonium ions, that absorb light and subsequently generate acid. Upon light absorption, these compounds undergo a process where the cationic part releases an electron, and the anionic part gains an electron, leading to acid production. This acid initiates polymerization reactions, transforming monomers into polymers or other target compounds.
Types of Photoacid Generators
1. Onium Compound Type
Onium compound PAGs, including sulfonium and iodonium salts, are preferred for UV-curable resins and visible light-curable resins, respectively, based on their light absorption characteristics.
2. Nonionic Type
Nonionic PAGs, such as imidosulfonates and oximesulfonates, offer faster curing for UV-curable resins compared to onium PAGs, expanding the range of applications in resin technology.