Indium nitrate hydrate films as potential EUV resists: film formation, characterization, and solubility switch assessment using a 92-eV electron beam

Indium nitrate hydrate films are evaluated as potential extreme ultraviolet (EUV) resists. To study the feasibility of these indium nitrate-based sol–gel precursor films as an EUV resist, the uniformity and stability of these films are examined as a function of metal composition, precursor concentration, chemical sources, precursor dissolution time, solvent drying time, post-application bake conditions, and relative humidity during the deposition. A 0.1 M indium nitrate hydrate solution forms a 20-nm thick resist, which is ideal for EUV lithography. We find two types of defects: macroscale defects that are visible under an optical microscope and nanoscale defects that can only be detected using an atomic force microscope. Both types of defects are affected by humidity during spin coating, dissolution time, and water content in the solvent. Hence, they are likely due to undissolved or re-crystallized indium nitrate hydrate crystals. The spin-coated indium nitrate hydrate films show great stability with no changes in defect density for up to 3 weeks. Using a 92-eV electron beam as a proxy for the EUV source, exposed regions of the film become insoluble upon exposure, acting as a negative-tone resist. Results of operando Fourier-transform infrared spectroscopy and residual gas analysis during the exposure show that the solubility switch is accompanied by the decomposition of nitrate species and the release of water. These results demonstrate the potential of indium nitrate hydrate films as an effective inorganic EUV resist.