Patent ID: 9506743
Filing Date: 2016-11-29
CPC Classification: G01B,G03F,H01L

Claim Text:
1. An apparatus to measure a position of a mark, the apparatus comprising: an objective lens configured to direct radiation on a mark; an optical arrangement configured to receive radiation that is diffracted and specularly reflected by the mark, wherein the optical arrangement is configured to provide a first image and a second image, the first image formed by coherently adding specularly reflected radiation and positive diffraction order radiation and the second image formed by coherently adding specularly reflected radiation and negative diffraction order radiation, wherein intensities or amplitudes of the positive and negative diffraction order radiation are substantially equal to each other; and a detection arrangement configured to detect variation in an intensity of radiation of the first and second images and to calculate from the detected variation a position of the mark in a direction of measurement, wherein the calculated position is a function of the combination of the intensity of the radiation of the first and second images.