First-principles study of metal and ligand substitution effects on EUV absorption and electron energy loss†

Abstract

Secondary electrons play a vital role in extreme ultraviolet lithography (EUV-L), as low-energy electrons (LEEs) induce the solubility switch of the photoresist via electron-induced reactions. However, optimizing EUV absorption at 92 eV and addressing the relatively long inelastic mean free path (IMFP) of LEEs, which can lead to pattern blurring, remain critical challenges. Here, first-principles calculations based on time-dependent density functional theory (TDDFT) are conducted to evaluate how chemical substitutions in metal and ligand sites affect both EUV absorption and the energy loss function (ELF) of LEEs in oxalate systems. Results highlight that atomic cross-sections alone are insufficient for optimizing photoabsorption, and electronic structure effects must be considered. Analysis of the ELF of LEEs reveals that iodine-containing systems exhibit a higher ELF at low energies, suggesting a reduced IMFP. Additionally, iodine incorporation shows potential to lower the band gap, which may further reduce the IMFP of LEEs in photoresists. These findings underscore the significance of electronic structure effects in EUV-L and demonstrate the value of first-principles calculations in optimizing photoabsorption and electron behavior for next-generation lithography applications.