Ionic photoacid generators

Ionic Photoacid Generators. Ease of synthesis, high thermal stability, and good quantum yield have made sulfonium and iodonium salts the most widely used onium salts. Figure 19 depicts some representative examples. 

Ionic Photoacid Generators. The application of onium salts as photochemical sources of catalyst for acid-catalyzed polsmierization was first reported in the 1970s (57). Sulfonium and iodonium salts have become the most widely used members of this group because of ease of synthesis, high thermal stability, and high quantum yield. Representative examples are depicted in structures a-d in Figure 10 (58-61). 

To achieve the best overall resist performance, the optimum PAG for a given resist system, whether ionic or nonionic, must balance the functional properties Hsted eadier in this section. The development of new photoacid generators, and the characterization of their functional properties, ate considered key to the design of resists with increased levels of performance. 

The most widely employed photoactive compound that triggers the chain of chemical reactions is the photoacid generator (PAG), a molecule that upon absorption of a photon undergoes photolysis, producing Bronsted acid and other photoproducts. The acid catalyzes a chemical reaction and it is regenerated at the end of the reaction (Scheme 12.3) [ 1,4,6,11 ]. The PAG may be an ionic organic salt or a nonionic organic compound, vide infra. 

Photoacid generators fall into two main categories ionic and nonionic PAGs. Ionic PAGs such as onium salts have broad sensitivity to DUV, e-beam, and x-ray radiation and can be red-shifted for near-UV apphcations by incorporating appropriate chromophores or through the use of sensitizers. Although triphenylsul-fonium hexafluoroantimonate was employed in the early chemical amplification... 

In contrast to chain scission, chemical amplification (CA) employs an acid catalyzed deprotection or crossfinking reaction, which changes the solubility of the resist in the exposed areas. Therefore, a typical CA resist consists of a matrix polymer and a photoacid generator (PAG). The PAG is converted into a strong acid on absorption of a photon. The acid catalyzes the deprotection or crosslinking reaction and is regenerated (Scheme 2), which explains the amplification effect in the system. A variety of acid generator chemistries are available, both ionic and nonionic, but are not considered here. 

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