Speckle reduction track filter apparatus for optical inspection of

Abstract

An apparatus used to inspect patterned wafers and other substrates with periodic features for the presence of particles, defects and other aperiodic features in which a spatial filter placed in the Fourier plane is used in combination with either broadband illumination, angularly diverse illumination or both. In contrast to prior devices that direct light from a single monochromatic source through a pinhole aperture stop, embodiments are describes that illuminate a patterned substrate using (1) a single monochromatic source with a slit-shaped aperture stop for angularly diverse illumination, (2) a single broadband source with a pinhole aperture stop for broadband illumination, (3) a single broadband source with a slit-shaped aperture stop for both broadband and angularly diverse illumination, or (4) multiple sources with an aperture stop for each source for at least angularly diverse illumination. The spatial filters for these illumination systems are characterized by opaque tracks in an otherwise transmissive filter for blocking the elongated bands produced by diffraction from the periodic features on the illuminates substrate. The filter may be made photographically by exposing high contrast film placed in or near the Fourier plane to the diffracted light from a defect and particle frame substrate having only periodic features. Light scattered from the aperiodic features is able to substantially pass through the filter and be imaged onto, a CCD array, vidicon camera or TDI sensor.