Extreme Ultraviolet Photoresponse of Organotin-Based Photoresists with Borate Counteranions

Organometallic tin-oxo-hydroxo cage compounds offer a promising photoresist platform for extreme ultraviolet photolithography (EUVL). Their reactivity is dominated by the facile breaking of the tin–carbon bonds upon photon or electron irradiation. As the cage is dicationic, it exists as a complex with anions for charge compensation. In the present work, we explore the n-butyltin-oxo cage with two tetrakis(pentafluorophenyl)borate counteranions (TinPFPB). In contrast to the small counterions that are typically used, the bulky PFPB anion absorbs a substantial fraction (∼30%) of the impinging EUV radiation (13.5 nm, 92 eV), and it has its own reactivity upon photoionization. When thin films of the complex are irradiated with EUV radiation at low doses, a positive-tone development is possible, which is rather unique as all other known tin-oxo cage resists show a negative tone (cross-linking) behavior. We propose that the initial positive tone behavior is a result of the chemical modification of the Sn cluster by fragments of the borate anions. For comparison, we include the tetrakis(p-tolyl)borate anion (TB) in the study, which has similar bulkiness, and its complex with the n-butyltin-oxo cage (TinTB) shows the usual negative tone EUV resist behavior. This negative-tone behavior for our control experiment rules out a hypothesis based purely on the steric hindrance of the anion as the cause of the different EUV reactivity.