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 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.
Highlights
As the miniaturization of electronic components in computer chips continues, novel nanopatterning technologies are necessary to attain a cost-effective high-volume manufacturing.[1]
The mechanism responsible for the solubility change of TinOH promoted by Extreme ultraviolet (EUV) photons was proposed in previous works.[8−10,31] Here, we investigate how low-energy electrons (LEEs) directly distance away from the photon absorption point where induce changes in the solubility properties of this material as a electrons induce solubility changes depends on the electron mean free path.[25]
Understanding interactions of low-energy electrons (LEEs) with photoresist materials and the energy dependence of those interactions presents an essential contribution to estimate, and eventually control, the efficiency of the photoresist as well function of electron energy and dose within a relevant energy window for extreme ultraviolet lithography (EUVL) (0−40 eV).[15,32−34] The design of our LEE microscopy (LEEM) experimental setup allows us to evaluate the effect of LEEs on the photoresist using in situ and ex situ approaches
Summary
As the miniaturization of electronic components in computer chips continues, novel nanopatterning technologies are necessary to attain a cost-effective high-volume manufacturing.[1]. Among the variety of materials that are being investigated for EUVL applications, metal−organic materials, called inorganic resists, are considered the most promising Their main advantage is that the incorporation of metallic elements enhances EUV absorptivity.[7] In particular, Sn-containing materials have attracted much attention as they can yield nanopatterns at relatively low doses.[8−10] Yet, a lack of detailed understanding of the chemical processes occurring upon the absorption of EUV photons hinders the rational design of efficient resists. When an EUV photon is absorbed by the resist, primary and secondary electrons (SEs) with energies in the 0−80 eV range are produced.[11,12] These electrons play a central role in the chemical transformations that photoresists undergo They can induce molecular bond scissions,[13,14] which change the photoresist structure and its solubility properties, thereby enabling pattern formation.[11,15−21] very few studies of the electron. The efficiency of electron-induced reactions contributes to the overall sensitivity of the photoresists.[18,19] At the same time, the so-called electron blur in the final nanopattern the maximum
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