Insight into the evolution upon ionization from tin-oxo cage photoresist and counterions by DFT calculation

Abstract

The tin-oxo cage ([R₁₂Sn₁₂O₁₄(OH)₆]²⁺) is an excellent photoresist candidate in extreme ultraviolet lithography (EUVL). Photoionization that leads to plenty of photoelectrons and secondary electrons is an inevitable process upon irradiation by EUV light. The secondary electrons could cause electron blur and decrease the resolution of the photoresist. The utilization and improvement are limited owing to its poorly understood fundamental chemical process, such as exposure to EUV. In this study, the property of complexes (bare cage, binary complexes, and ternary complexes) formed by the combination of the cage and different counterions (methanesulfonate, tosylate, and triflate) was explored using density functional theory (DFT). Extended transition state-nature orbital of chemical valence (ETS-NOCV), charge decomposition analysis (CDA), and atomic dipole moment corrected Hirshfeld (ADCH) atomic charge analyses were carried out to characterize the interaction process (including ionization, electron attachment, and cleavage of the carbon–tin bond) and inherent mechanism. The number and species of counterions are closely associated with ionization and electron attachment; however, they show an inconspicuous effect on the carbon–tin bond energy. Meanwhile, tin free radical could be the reactivity site of ionization (and electron affinity). The activity of radical species is affected by counterions; this could be attributed to the changes in the charge density by diverse counterions, which could be an important factor in why the counterion affects the pattern. In addition, a superb linearity between the ADCH of tin free radical and IP/AP was found, which indicate that the atomic charge could be a simple and reliable substitution of complicated calculation for IP (or AP) to screen suitable counterions and improve the resolution of photoresist.