Carborane-Based Photoacid Generators: New Superacids For 193nm and EUV Lithography

Photogenerated carborane superacids have been found to be effective as catalysts for chemically amplified resists in 193 nm and EUV. It was found unexpectedly that despite the documented high acid strength of the 1-carboranes, the photospeed of resist formulations containing them was significantly lower than that of corresponding perfluoro-substituted sulfonic acids. This behavior is attributed to the high stability of the intermediate carbocation/acid anion complex, which for perfluoroalkane sulfonates is a transition state but an isoable intermediate for the 1-carborane acids. The long residence time of the carborane acid catalyst in this intermediate reduces its availability and causes the lower photospeed; at the same time, this phenomenon leads to lower diffusion rates as a result of the immobility of the bulky intermediate complex. Coupled with the high transparency of carborane anions in EUV, it is expected that the use of carborane PAGs for EUV formulation optimization will offer new opportunities to escape the photospeed/resolution/LER triangle trade-off that has limited progress in EUV photoresists.