Abstract: The invention concerns an ion beam apparatus, by means of which defects in a substrate can be recognized, and repaired under continuous control. For this purpose, the ion beam apparatus, in its beams path, after the ion source, is equipped with a mask exhibiting a preferrably circular hole and between ion source and mask with a controllable lens for the purpose of modification of the divergence angle under which the beam strikes the mask. The aperture of the mask is imaged upon the substrate. In this way the intensity of the ion beam may be varied for use in inspecting a substrate for defects and subsequently removing the detected defects.