Effect of Ambient Conditions on Radiation-Induced Chemistries of a Nanocluster Organotin Photoresist for Next-Generation EUV Nanolithography

Solution-based organometallic nanoclusters are unique nanoscale precursors due to the ability to precisely control their size, shape, structure, and assembly. The interaction of extreme ultraviolet (EUV) or X-ray photons with these organometallic nanoclusters can result in processes that can lead to a change in solubility. This makes these materials prime candidates for next-generation photoresists for EUV nanolithography. In this study, we investigate the interaction of X-ray radiation with a charge neutral, sodium templated, butyl-tin Keggin (β-NaSn13) nanocluster. This nanocluster is used as a model EUV photoresist to better understand the radiation induced solubility transition. Ambient pressure X-ray photoelectron spectroscopy (AP-XPS) was used to characterize the β-NaSn13 thin films, where Sn 3d, O 1s, and C 1s core levels were measured under a range of ambient conditions, including ultrahigh vacuum and 1 mbar of oxygen, water, methanol, or nitrogen. A photon dose array was obtained for each ambient condition to determine their effect on the photon induced chemistries which result in the solubility transition. The resulting contrast curves indicate that an oxygen ambient significantly reduces the required photon dose for the solubility transition relative to UHV, while all other ambients increase the required photon dose for the solubility transition relative to UHV. We performed in situ XPS after postexposure annealing β-NaSn13 thin films in multiple ambients to study the chemistry that occurs after a postexposure bake (PEB). The β-NaSn13 thin films retained a significant amount of aliphatic carbon following the PEB in all the ambients we studied. On the basis of our studies, we propose that the solubility transition for β-NaSn13 thin films occurs through radical hydrogen abstraction and radical–radical coupling reactions. These studies further improve the understanding of photon induced chemistries in a β-NaSn13 model resist and provide mechanistic insights for EUV lithography processing with organometallic nanomaterials.