  Erbium doped low symmetry Y$_2$SiO$_5$ crystals attract a lot of attention in
perspective of quantum information applications. However, only doping of the
samples during growth is available up to now, which yields a quite homogeneous
doping density. In the present work, we deposit Er$^{3+}$-ions by the focused
ion beam technique at Yttrium sites with several fluences in one sample. With a
photoluminescence study of these locally doped Er$^{3+}$:Y$_2$SiO$_5$ crystals,
we are able to evaluate the efficiency of the implantation process and develop
it for the highest efficiency possible. We observe the dependence of the ion
activation after the post-implantation annealing on the fluence value.
