Abstract: Visible defects are detected on a process semiconductor wafer. Defects are classified according to appearance and an association is kept between classes and apparatuses. When the density of defects in a given class exceeds a control limit the associated apparatus is switched off-line. In an embodiment, the same wafer is inspected repeatedly, each time after a different processing steps and information about the location of detected defects is kept. Defects which occur at a location where defects have already been detected in a previous inspection after an earlier processing step are eliminated from the density which is compared to the control limit.