Two-photon induced photopolymerization using photoinitiator with an intramolecular radical quenching moiety for nanolithography.

We have investigated the photoinitiating properties of the photoinitiator 3,6-bis[2-(4-nitrophenyl)-ethynyl]-9-(4-methoxy-benzyl)-carbazole (BNMBC), which has an intramolecular radical quenching group "p-methoxybenzyl," in the substrate lines fabrication of two-photon induced photopolymerization (TPIP). Another photoinitiator 3,6-bis[2-(4-nitrophenyl)-ethynyl]-9-benzyl-carbazole (BNBC) with similar chemical structure but not radical quenching group was studied for comparison. Their photopolymerization properties were studied with resins in which BNBC and BNMBC were used as photoinitiators with a molar ratio of 0.02%, respectively. The linewidth of polymer lines fabricated by TPIP of the photoresist that contained BNMBC could be decreased to 65% of those using BNBC. Besides, we introduced a radical quencher, phenyl methyl ether (PhOCH3), to BNBC and further studied the photopolymerization properties by using BNBC, BNBC/PhOCH3 and BNMBC as photoinitiators with a molar ratio of 0.1%. The results further indicated that BNMBC was effective to confine the radical diffusion in polymerization due to the radical quenching effect of moiety. This intramolecular radical quenching moiety exhibited more effective confining effect of radical diffusion compared to the intermolecular radical quencher PhOCH3. Furthermore, arbitrary complex three-dimensional (3D) microstructure was achieved using the photoresist with photoinitiator BNMBC. The approach could open up broad prospect for improving the resolution in TPIP.