Upon UV irradiation, the photolysis of diphenyl iodonium
salt generated strong Brönsted acids that initiated cationic polymerization of the epoxy group on the LFAR during the PEB. This reaction was monitored by IR spectroscopy.
As shown in Fig. 2, the absorption peak at
916 cm−1 (Fig. 2(b)) of the epoxy ring disappeared after
UV irradiation and PEB. This cross-linking reaction gave
rise to an insoluble three-dimensional network in the resist
exposed areas.