Stimulated Emission Depletion Inspired Sub-100 nm Structuring of Epoxides Using 2-Chlorothioxanthone as Photosensitizer.

Until very recently, the enhancement of multiphoton-based optical lithography by stimulated emission depletion (STED) inspired techniques was limited mostly to (meth)acrylates. Epoxides, which play an important role in semiconductor clean-room technology, were basically excluded from capitalizing on STED-inspired lithography, and if they were successfully used in STED-inspired lithography, the achievable structure sizes remained at 125 nm and above. We now found that using 2-chlorothioxanthone (CTX) as a sensitizer for a sulfonium salt acting as the photoinitiator allows for shrinking the structure size down to 83 nm. Compared to the previously used sensitizer 2-isopropylthioxanthone, the triplet lifetime of CTX within the epoxide monomers is supposed to be prolonged by 40%, which renders an optical depletion via excited triplet state absorption more efficient, leading to a sub-100 nm structuring capability.