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

Abstract

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 nanopattern formation. Yet, reliable quantitative information on this electron-induced process is scarce. In this work, we combine LEE 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 uniquely allows to correct for surface charging and thus to accurately determine the electron landing energy. AFM postexposure analyses revealed that irradiation of the resist with LEEs leads to the densification of the resist layer because of 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 and then a more densely cross-linked network. This model allows us to estimate a maximum reaction volume on the initial material of 0.15 nm 3 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.

Unravelling the effect of fluorinated ligands in hybrid EUV photoresists by X-ray spectroscopy

“…During the scattering, more molecules can be ionized so that secondary electrons are generated, which have a broad distribution of kinetic energies in the 0-80 eV range. 14 Therefore, the chemistry induced by EUV photons is the result of the electron-induced chemistry [17][18][19] and the chemical a Advanced Research Center for Nanolithography, Science Park 106, processes that the ionized molecules undergo. Furthermore, the probability of absorbing EUV photons by molecules is mainly determined by their elemental composition rather than by the selection rules based on the characteristics of the frontier molecular orbitals that apply in photochemistry.…”

High Sensitivity Resists for EUV Lithography: A Review of Material Design Strategies and Performance Results

“…Haitjema et al studied the chemical behavior observed in tin oxo cage materials under UV exposure to get clues that could also be of use for determining the EUV imaging chemistry [114]. Further insights on tin oxo cages chemistry at EUV was provided in the recent work by Bespalov where the influence of electron energy was studied [40]. The main results from this study are depicted in Figure 9.…”

“…In this respect, since photoelectrons and secondary electrons resulting from ionizing radiation were recognized to play the main role in solubility changing reactions [ 37 ], attention was drawn to the fundamentals of electron-induced chemistry known from other scientific fields as described for instance in [ 38 , 39 ]. Recently a study on the role of low energy electrons in a tin containing resist was published [ 40 ] and revealed their important role in resist chemistry. Similar studies are expected to dramatically improve the understanding of the resist chemistries at EUV.…”

“…( d ) Schematic representation of the initial Tin-OH molecular material A and of the two insoluble networks B and C. Blue ellipses represent the Sn-based inorganic core and orange bars the butyl chain. Adapted from [ 40 ] ( ), with permission from American Chemical Society, 2020.…”

Isomerization of Na‐Centered Alkyltin Keggin Clusters

“…We are interested in alkyltin clusters for nanolithography, due to the radiation sensitivity of the alkyl ligands, high EUV absorption of Sn, and the precise and small features enabled by molecular clusters [25–27] . The family of alkyltin Keggin ions is unique in that only the β and γ isomers have been identified by single‐crystal x‐ray diffraction.…”