  We report on the systematic characterization of conductive micro-channels
fabricated in single-crystal diamond with direct ion microbeam writing. Focused
high-energy (~MeV) helium ions are employed to selectively convert diamond with
micrometric spatial accuracy to a stable graphitic phase upon thermal
annealing, due to the induced structural damage occurring at the end-of-range.
A variable-thickness mask allows the accurate modulation of the depth at which
the microchannels are formed, from several {\mu}m deep up to the very surface
of the sample. By means of cross-sectional transmission electron microscopy
(TEM) we demonstrate that the technique allows the direct writing of amorphous
(and graphitic, upon suitable thermal annealing) microstructures extending
within the insulating diamond matrix in the three spatial directions, and in
particular that buried channels embedded in a highly insulating matrix emerge
and electrically connect to the sample surface at specific locations. Moreover,
by means of electrical characterization both at room temperature and variable
temperature, we investigate the conductivity and the charge-transport
mechanisms of microchannels obtained by implantation at different ion fluences
and after subsequent thermal processes, demonstrating that upon
high-temperature annealing, the channels implanted above a critical damage
density convert to a stable graphitic phase. These structures have significant
impact for different applications, such as compact ionizing radiation
detectors, dosimeters, bio-sensors and more generally diamond-based devices
with buried three-dimensional all-carbon electrodes.
