Patent Document ID: 9946167
Application ID: 14824696
Patent Flag: 1

Claim One:
1. A method comprising: using a lithographic process to form a periodic structure on a substrate; a first measurement comprising detecting a first image of the periodic structure, while illuminating the structure with a first beam of radiation, the first image being formed using a first selected part of diffracted radiation; a second measurement comprising detecting a second image of the periodic structure, while illuminating the structure with a second beam of radiation, the second image being formed using a second selected part of the diffracted radiation which is symmetrically opposite to the first part, in a diffraction spectrum of the periodic structure; using a difference in intensity values derived from the detected first and second images together to determine a property of the periodic structure; and controlling a spatial light modulator to apply a varying non-binary optical attenuation over the first and second selected parts of the diffracted radiation prior to detecting the first and second images respectively.