Patent Document ID: 9140998
Application ID: 13294057
Patent Flag: 1

Claim One:
1. An inspection apparatus comprising: an illumination arrangement operable to deliver conditioned beams of radiation to a substrate for use in measurements; a detection arrangement operable during such measurements to detect respective images of the substrate using radiation diffracted from the substrate, the illumination arrangement and the detection arrangement forming a measurement optical system; a stop arrangement within the detection arrangement, wherein the illumination arrangement and stop arrangement together are configured to select which part of a diffraction spectrum of the diffracted radiation contributes to each image; a controller configured to control the apparatus to produce first and second images of a periodic structure on the substrate, wherein: the first image is detected in a first measurement using a first selected part of the diffracted radiation, the second image is detected in a second measurement using a second selected part of the diffracted radiation, which is symmetrically opposite to the first part in the diffraction spectrum of the periodic structure without rotating the substrate; and wherein the detection arrangement further comprises a spatial light modulator operable to apply a varying optical attenuation over the selected part of the diffracted radiation prior to forming the first and second images respectively; and a computational arrangement arranged to use a difference in intensity values derived from the first and second images to determine an asymmetry-related parameter.