Diphenyliodonium hexafluoroarsenate

Figure 50, Schematic function of the t-Bocstyrene resist, S is a sensitizer such as diphenyliodonium hexafluoroarsenate which undergoes radiolysis to produce a strong acid (A), The acid attaches the side chain of the poly-(t-Bocstyrene) where it catalyzes acidolysis of the carbonate to liberate CO2 and isobutylene and free the phenolic hydroxyl group to produce poly (p-hydroxystyrene) in the exposed areas of the resist film. The acid A is a catalyst and can cleave many carbonate groups.

The results of these photochemical studies form guidelines for the choice of sensitizers, onium salts and other additives potentially useful in the cationic curing of coatings. The sensitized photochemistry of diphenyliodonium hexafluoroarsenate and triphenylsulfonium hexaflurorarsenate was investigated at 366 nm. Product quantum yields are compared to relative rates of photoinitiated cationic polymerization of an epoxy resin. 

The UV sensitivity of the polyphthalaldehyde-onium salt system is dependent on the concentration and structure of the onium salts. At 10 wt% loading the sensitivity to narrow band width 254 nm radiation is 1-7 mJ/cm2 and is insensitive to the structure of the salts. However, at 2 wt% loading of triphenylsulfonium or diphenyliodonium hexafluoroarsenates or -hexyloxybenzenediazonium tetrafluoroborate much higher doses are required to achieve self-development and the sensitivity decreases with the salt structures in the sequence listed. 

HexylPT films containing diphenyliodonium hexafluoroarsenate are used for optical recording by light irradiation followed by washing with chloroform [135]. 

Photochemical doping involves treating a CP with a dopant which is initially inert but rendered an active dopant by irradiation. These dopants are usable with common CPs. Examples are diphenyliodonium hexafluoroarsenate (in CH2CI2) or triaryl-sulfonium salts (in aqueous medium), both of which are rendered active by UV radiation [125, 126]. 

The reduction energy of diphenyliodonium hexafluoroantimonate was calculated to be -5 kcal/mole (corresponding to -0.2 v vs a Standard Calomel Electrode, SCE). The reduction energy for triphenyl sul fonium hexafluoroarsenate was calculated to be = -28 kcal/mole (corresponding to -1.2 v vs SCE). Details of these calculations were published previously.(8-11,15) Oxidation and reduction potentials of organic compounds are available (14). 

PBOCST is thermally stable to ca. 200°C. Above 200°C, the polymer loses about 45% of its weight as carbon dioxide and isobutene (15). Diphenyliodonium and triphenylsulfonium hexafluoroarsenates are thermally stable to ca. 250° and 300°, respectively. (16,17) Consequently, the resist formulated from PBOCST and these salts is stable to the baking conditions required for formation of high quality spin coated films, and the formulations have a long shelf life when stored at room temperature under yellow light. 

Abstraction of labile hydrogen atoms, for example, from tetrahydrofuran (THF), by photoexcited ketones also yields easily oxidized radicals [60a]. Quantum yields for benzophenone sensitized photolysis of diphenyliodo-nium hexafluoroarsenate in acetonitrile with hydrogen atom donors are shown in Table 9. Diaryliodonium salts are capable of oxidizing electron rich radicals from isopropanol and THF but triarylsulfonium salts are not [96]. The oxidation potential of tetrahydrofuranyl radical is —0.35 V versus SCE [109], apparently sufficient for irreversible reduction of diphenyliodonium cation ( red = approx. —0.7 V versus SCE, see above) but not of triphenyl-sulfonium cation ( red = —1.2V versus SCE). 

---