The Key Role of Very-Low-Energy-Electrons in Tin-based Molecular Resists for Extreme Ultraviolet Nanolithography

Extreme Ultraviolet (EUV) lithography (13.5 nm) is the newest technology that allows high-throughput fabrication of electronic circuitry in the sub-20 nm scale. It is commonly assumed that low-energy electrons (LEEs) generated in the resist materials by EUV photons are mostly responsible for the solubility switch that leads to the nanopattern formation. Yet, reliable quantitative information on this electron-induced process is scarce. In this work, we combine Low-Energy Electron Microscopy (LEEM), Electron Energy Loss Spectroscopy (EELS), and Atomic Force Microscopy (AFM) to study changes induced by electrons in the 0-40 eV range in thin films of a state-of-the-art molecular organometallic EUV resist known as tin-oxo cage. LEEM-EELS spectroscopy uniquely allows to correct for surface charging and thus to accurately determine the electron landing energy. AFM post-exposure analyses revealed that irradiation of the resist with LEEs leads to the densification of the resist layer due to carbon loss. Remarkably, electrons with energies as low as 1.2 eV can induce chemical reactions in the Sn-based resist. Electrons with higher energies are expected to cause electronic excitation or ionization, opening up more pathways to enhanced conversion. However, we do not observe a substantial increase of chemical conversion (densification) with the electron energy increase in the 2-40 eV range. Based on the dose-dependent thickness profiles, a simplified reaction model is proposed where the resist undergoes sequential chemical reactions, first yielding a sparsely cross-linked network, then a more densely cross-linked network. This model allows us to estimate a maximum reaction volume on the initial material of 0.15 nm3 per incident electron in the energy range studied, which means that about 10 LEEs per molecule on average are needed to turn the material insoluble and thus render a pattern. Our observations are consistent with the observed EUV-sensitivity of tin-oxo cages.