Progress in STED-inspired sub-diffractional lithography of epoxides

The past two decades have seen substantial advancements in direct laser writing by two-photon lithography, enhanced by techniques adapted from stimulated emission depletion (STED) microscopy. 1 Initially, STED-inspired lithography was predominantly applied to radical polymerizations, especially with acrylates and methacrylates, 2 while cationic polymers, particularly epoxides, crucial in semiconductor clean-room technology, did not benefit from this method. Recently, we investigated a resist comprising 3,4-epoxycyclohexylmethyl 3,4 epoxycyclohexanecarboxylate (EPOX) as monomer, triarylsulfonium hexafluoroantimonate salt as starter and 2-isopropylthioxanthone (ITX) as a photo- sensitizer. Using a 780 nm femtosecond laser for two-photon excitation and a 660 nm continuous wave laser for depletion, we achieved 125 nm structure sizes. 3 Utilizing 2-chlorothioxanthone (CTX) instead of ITX, allowed for writing feature sizes down to 83 nm. 4, which is roughly l/10 of the excitation wavelength. This holds promise of delivering feature sizes on par with those obtained in acrylate polymerization5. In this contribution, we will present our latest progress on STED-inspired lithography of epoxides.