Photoacid Generators

Photoacid generators (PAGs, for short) are compounds that undergo reactions or dissociation to generate specific acids under the light illumination. The acids they produce can cause the acid-sensitive resin to decompose or crosslink, thus increasing the dissolution difference between exposed and non-exposed parts of resins. In the microelectronics industry, PAG serves as a key component of photocurable polymer formulations and chemically amplified (CA) photoresists. The chemical amplification concept was proposed in the 1980s, and then CA resists were developed. Currently, commercial 193 nm and 248 nm resists are primarily CA resists. And the main components of CA resists include polymer resins, PAGs, and additives (such as basic additives, dissolution inhibitors, etc.)

PAG Types

Common PAGs include diazonium salt compounds, triazine compounds, onium compounds, and sulfonate ester compounds. Our focus here is on the PAGs used for the KrF resists and ArF resists.

• PAGs for KrF resists

PAGs play a key role in KrF resists whose exposure wavelength is 248 nm, and the design of PAGs should meet the following requirements: easy to synthesize, low toxicity, good solubility, suitable spectral properties for the exposure wavelength, etc. PAGs that produce sulfonic acids are commonly used in KrF resists, which are further divided into ionic and non-ionic types. The former is usually a diaryliodonium salt or a triarylsulfonium salt, while the latter is a nitrobenzyl ester or sulfonate compound. Ionic PAG has worse solubility than non-ionic PAG, but its photosensitivity is stronger than that of non-ionic PAG. Introducing chemical groups at specific positions can significantly improve the thermal stability of non-ionic PAG. Fig. 1 shows several non-ionic PAG structures ^[1].

Fig. 1 Structures of several non-ionic PAG ^[1].

• PAGs for ArF resists

The resins of ArF resist do not contain phenolic hydroxyl groups, so electrons cannot be transferred to PAG for sensitization to generate acid. Besides, the efficiency of PAG in ArF resist is lower than that in KrF resist. These two factors determine that ArF resists need PAGs with higher sensitivity, higher acid production efficiency, and stronger acidity. At present, the commonly used PAGs for ArF resists are sulfonium salts, iodonium salts, and N-hydroxysuccinimide sulfonates. Fig. 2 shows some representative PAG structures ^[2].

Fig. 2 Some representative PAGs for 193 nm lithography ^[2].

Some fluorine-free PAGs have also been researched, for the improvement of environmental awareness. Fig. 3 shows two PAGs used in ArF resists that have good stability, excellent light transmission, and photolithographic properties ^[3].

Fig. 3 Flurorine-free 193 nm PAG ^[3].

Alfa Chemistry is a leading global supplier of photoresists. We adhere to the tenet of "customer first" and are committed to providing customers with high-quality PAGs to solve their photolithography problems. Please click on the link at the top of the page to learn more about PAGs. If you have any questions, please contact us.

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