  Ruthenium thin films can serve as protective caps for multi-layer extreme
ultraviolet mirrors exposed to atomic hydrogen. Hydrogen permeation through
ruthenium is problematic as it leads to blisters on the mirrors. H has been
shown to exhibit low solubility in bulk Ru, and rapidly diffuses in and out of
Ru. Therefore, the underlying mechanisms of the blistering effect remains
unknown. This work makes use of reactive molecular dynamics simulations to
study the influence of imperfections in a Ru film on the behaviour of H. For
the Ru/H system, a ReaxFF force field was parametrised which reproduces
structures and energies obtained from quantum-mechanical calculations.
Molecular dynamics simulations have been performed with the newly-developed
force field, to study the effect of tilt and twist grain boundaries on the
overall diffusion behaviour of H in Ru. Our simulations show the tilt and twist
grain boundaries provide energetically favourable sites for hydrogen atoms and
act as sinks and highways for H. They therefore block H transport across their
planes, and favour diffusion along their planes. This results in the
accumulation of hydrogen at the grain boundaries. The strong effect of the
grain boundaries on the hydrogen diffusion suggests tailoring the morphology of
ruthenium thin films as a means to curb the rate of hydrogen permeation.
