A mask or set of masks is disclosed in which outward projections are placed on either side of at least one aperture. An ion beam is then directed through the mask toward a workpiece. An ion collecting device or an optical system is then used to measure the alignment of the mask to the ion beam. These projections serve to increase the sensitivity of the system to misalignment. In another embodiment, a blocker is used to create a region of the workpiece that is not subjected to a blanket implant. This facilitates the use of optical means to insure and determine alignment of the mask to the ion beam.