  Implantation of diamonds with helium ions becomes a common method to create
hundreds-nanometers-thick near-surface layers of NV centers for
high-sensitivity sensing and imaging applications. However, optimal
implantation dose and annealing temperature is still a matter of discussion. In
this study, we irradiated HPHT diamonds with an initial nitrogen concentration
of 100 ppm using different implantation doses of helium ions to create 200-nm
thick NV layers. We compare a previously considered optimal implantation dose
of $\sim10^{12}$ to double and triple doses by measuring fluorescence
intensity, contrast, and linewidth of magnetic resonances, as well as
longitudinal and transversal relaxation times $T_1$ and $T_2$. From these
direct measurements we also estimate concentrations of P1 and NV centers. In
addition, we compare the three diamond samples that underwent three consequent
annealing steps to quantify the impact of processing at 1100 $^{\circ}$C, which
follows initial annealing at 800 $^{\circ}$C. By tripling the implantation dose
we have increased the magnetic sensitivity of our sensors by $28\pm5$ %. By
projecting our results to higher implantation doses we show that a further
improvement of up to 70 % may be achieved. At the same time, additional
annealing steps at 1100 $^{\circ}$C improve the sensitivity only by 6.6 $\pm$
2.7 %.
