  Ion implantation is widely used to modify the structural, electrical and
optical properties of materials. By appropriate masking, this technique can be
used to define nano- and micro-structures. However, depending on the type of
mask used, experiments have shown that vacancy-related substrate modification
can be inferred tens of microns away from the edge of the mask used to define
the implanted region. This could be due to fast diffusion of vacancies from the
implanted area during annealing or to a geometric effect related to ion
scattering around the mask edges. For quantum and single-atom devices, stray
ion damage can be deleterious and must be minimized. In order to profile the
distribution of implantation-induced damage, we have used the nitrogen-vacancy
colour centre as a sensitive marker for vacancy concentration and distribution
following MeV He ion implantation into diamond and annealing. Results show that
helium atoms implanted through a mask clamped to the diamond surface are
scattered underneath the mask to distances in the range of tens of micrometers
from the mask edge. Implantation through a lithographically defined and
deposited mask, with no spacing between the mask and the substrate,
significantly reduces the scattering to <5 m but does not eliminate it. These
scattering distances are much larger than the theoretically estimated vacancy
diffusion distance of 260 nm under similar conditions. This paper shows that
diffusion, upon annealing, of vacancies created by ion implantation in diamond
is limited and the appearance of vacancies many tens of micrometers from the
edge of the mask is due to scattering effects.
