Revisiting the Role of Carboxylates in Dialkyltin Dicarboxylate EUV Resists: Evidence for Electron-Induced Decarboxylation

Dialkyltin dicarboxylates have been widely employed in the early-stage development of extreme ultraviolet (EUV) metal oxide resists, yet their reaction mechanisms have been inaccurately described. Within the EUV lithography community, carboxylate groups are often regarded as inert "unreactive spacers" that suppress intermolecular collisions without participation in electron-driven reactions. However, in conventional chemistry, it is well established that carboxylate compounds can undergo decomposition upon decarboxylation, releasing carbon dioxide. In this study, we challenge the interpretation of the EUV lithography community by using dibenzyltin diacetate as a model compound. Electron beam irradiation combined with infrared spectroscopy revealed that the characteristic C═O stretching band at 1643 cm^-1, prominent in pristine films, initially increased at low dose but subsequently decreased with further dose increase, indicating carbonation of tin radicals and decarboxylation of acetate groups. Furthermore, quantum chemical calculations on benzylmethyltin diformate and dimethyltin diacetate showed that the acetate group dissociates at a rate that is approximately 68% of the rate at which the benzyl group dissociates, underscoring the appreciable reactivity of carboxylate moieties to alkyl groups. This indicates that carboxylate moieties are not inert but can dissociate almost as readily as alkyl groups. These results demonstrate that carboxylates are not chemically inert under EUV irradiation. Instead, the reduced reactivity of dialkyltin dicarboxylates with bulkier carboxylates arises from increased intermolecular spacing after ligand dissociation, which limits molecular collisions and suppresses subsequent reactions.