DFT Study on CO₂- and H₂O-Enhanced Solubility Contrast in Tin-Oxo Cage Photoresists

The tin-oxo cage is a promising photoresist due to tin's large extreme ultraviolet (EUV) photon cross-section and small size. CO2 and H2O play a crucial role in increasing the solubility contrast of tin-oxo cages in deep ultraviolet (DUV) and EUV lithography. However, the internal reaction mechanisms remain largely unknown. This work systematically investigated the cleavage of carbon–tin bonds in tin-oxo cages by density functional theory (DFT) upon diverse irradiation (DUV and EUV). Subsequently, the interactions of the resulting intermediates with CO2/H2O (or their hybrid) were examined, revealing that hydroxyl compounds (from the dissociation of H2O) serve as pivotal precursors to trigger new cross-linking reactions (forming Sn–O–Sn and Sn–OCO–Sn products) and enhance the solubility switch. These theoretical results provide insight into the atomic-scale mechanisms of small molecules improving photoresist properties, offering new perspectives for the design and optimization of novel photoresist materials.