Illuminator for dark field inspection

Light from a single source is divided among several illumination arms, each of which directs light via a multimode fiber bundle from the source to the wafer location. The arms are arranged circumferentially around a common illumination region, so that the region is illuminated from several directions. For each arm, light exiting the fiber bundle enters a turning prism, reflects off the hypotenuse of the prism, and is diverged in one dimension by a negative cylindrical surface on the exiting face of the prism. The beam then reflects off an anamorphic mirror and propagates to the illumination region on the wafer. The beam has an asymmetric footprint, so that it illuminates a nearly circular region of the wafer when viewed at normal incidence. The fiber bundle is at the front focal plane in the meridional dimension. The illumination region is at the rear focal plane in both dimensions.