  Thin epitaxial films of metals on insulating substrates are essential for
many applications, as conducting layers, in magnetic devices or as templates
for further growth. In this work, we report on the growth of epitaxial Ru films
on single-crystalline Al$_2$O$_3$(0001) substrates by magnetron sputtering and
their subsequent systematic characterization using Rutherford backscattering
spectrometry of He ions both in random and in channeling conditions. We include
results of a Ru(0001) single crystal for comparison. Analysis of channeling
shows that films thicker than 35 nm grow with (0001) orientation, a
well-defined epitaxial relation with the substrate and a high degree of crystal
quality, comparable to the Ru(0001) single crystal. Thinner films of down to 7
nm in thickness, for which relaxation of epitaxial strain is not complete,
produce a similar degree of dechanneling. The surface of the films can be
prepared in a clean and ordered state in order to allow further epitaxial
growth on top.
