A Si wafer implanted with a known dose of P was used to determine the minimum P detection limit and corresponding minimum raster size and optimum ROI for a range of Cs + primary ion currents (Fig. , S4 & S5). The detection limit for P improved from 10 17 atoms per cubic centimeter (at./cm 3 ) for 10 pA Cs + with a 0.6 x 0.6 µm 2 ROI from a 2 x 2 µm 2 raster, to 10 14 at./cm 3 for 8 nA Cs + with a 12 x 12 µm 2 ROI from a 40 x 40 µm 2 raster. We found a consistent relationship between detection limit, minimum raster size and relative ROI size (Fig. ). The detection limit was lower for higher beam currents because of the larger dynamic range between the peak of the P implant and the tail, but the raster size had to be increased to obtain the minimum detection limit because the primary beam diameter increased with beam current .