  Extreme Ultraviolet (EUV) lithography is the newest technology that will be
used in the semiconductor industry for printing circuitry in the sub-20 nm
scale. Low-energy electrons (LEEs) produced upon illumination of resist
materials with EUV photons (92 eV) play a central role in the formation of the
nanopatterns. However, up to now the details of this process are not well
understood. In this work, a novel experimental approach that combines
Low-Energy Electron Microscopy (LEEM), Electron Energy Loss Spectroscopy
(EELS), and Atomic Force Microscopy (AFM) is used to study changes induced by
electrons in the 0-40 eV range in thin films of molecular organometallic EUV
resists known as tin-oxo cages. LEEM-EELS spectroscopic experiments were used
to detect surface charging upon electron exposure and to estimate the electron
landing energy. AFM post-exposure analyses revealed that irradiation of the
resist with LEEs leads to the densification of the resist layer associated to
carbon loss. The same chemical processes that yield densification render the
solubility change responsible for the pattern formation in the lithographic
application. Remarkably, electrons as low as 1.2 eV are able to induce chemical
reactions in the Sn-based resist. Based on the thickness profiles resulting
from LEE exposures in the 3-48 mC/cm 2 dose range, a simplified reaction model
is proposed where the resist undergoes sequential chemical reactions, yielding
first a sparsely cross-linked network, followed by the formation of a denser
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 less than 10 LEEs per molecule on average are
needed to turn the material insoluble and thus render a pattern.
