Patent ID: 8823922

Claim:
An overlay measurement apparatus comprising: a polarized light source configured to illuminate a substrate with a polarized light beam; an optical system configured to capture light reflected by the substrate from the polarized light beam, the optical system comprising: a first non-polarizing beam splitter configured to receive the polarized light beam from the polarized light source, direct the polarized light beam to the substrate via a projection system, and receive a reflected light beam from the substrate via the projection system, the reflected beam comprising a zeroth order diffraction component; a second non-polarizing beam splitter configured to receive the reflected light beam from the first non-polarizing beam splitter; a crossed polarizer configured to receive a portion of reflected light beam from the second non-polarizing beam splitter and transmit an in-plane image and crossed-polarized image; a blocking element configured to block the in-plane image; a detector configured to measure an intensity of crossed-polarized image and produce a signal therefrom; and a processing unit configured to receive the signal from the detector and to process crossed-polarized image for overlay metrology measurement using asymmetry data derived from crossed-polarized image.