ions or parasitic beamlets in the u201c outside u201d half of the ion beam 30 may be avoided fig4 illustrates an exemplary method for reducing contamination based on a half moon shaped mask 404 in accordance with an embodiment of the present disclosure in this top view of a wafer 402 the half moon shaped mask 404 is positioned to block the u201c outside u201d half of the wafer 402 a ribbon beam 40 which is slightly wider than the wafer 402 is generated and extends horizontally across the wafer 402 during a first phase of ion implantation the wafer 402 and the mask 404 may be translated vertically with respect to the ion beam 40 typically the first phase may be completed with two full scans of the wafer 402 by the ion beam 40 e g by translating the wafer 402 and the mask 404 up and down upon completion of the first phase the ion beam 40 may be turned off or otherwise kept off the wafer 402 and the wafer 402 may be rotated 180 u00b0 around an axis 42 then a second phase of the ion implantation may be performed wherein the half wafer that was masked.