Coupling mirror of an optical inspection system

Abstract

A wafer inspection tool comprising an illumination system having: a field of view (FOV); a light source pupil having a size and shape; a central optical axis; and one or more field angle defining a shape of said FOV extending away from said light source pupil; an objective lens arrangement including an objective and a plurality of interchangeable telescopes coupled thereto, the objective lens arrangement being configured to collect light reflected off a plurality of field points on the wafer and to onwardly transmit a light beam formed from the collected light; and a light separator having a first reflective surface with a transmissive region formed therein and a second surface, wherein said transmissive region is arranged to allow therethrough a central portion of said light beam transmitted from said objective lens arrangement corresponding to the brightfield channel while said reflective surface is arranged to reflect a peripheral portion of said light beam transmitted from said objective lens arrangement corresponding to the darkfield channel; a relay module configured to relay said light source pupil to said transmissive region; wherein said transmissive region has a shape defined as a geometric intersection volume between a model of said illumination light and said reflective surface and said second surface; wherein said model includes a plurality of solids each solid having a cross section of said light source pupil and angled to a field angle of said one or more field angle.