Patent ID: 8867020
Filing Date: 2014-10-21
Classification: G03F

Abstract:
1. A method of measuring asymmetry in a periodic structure formed by a lithographic process on a substrate, the method comprising: using the lithographic process to form a periodic structure on the substrate; a first measurement step comprising forming and 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 part of non-zero order diffracted radiation while excluding zero order diffracted radiation; a second measurement step comprising forming and 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 part of the non-zero order diffracted radiation which is symmetrically opposite to the first part, in a diffraction spectrum of the periodic structure; and using a difference in intensity values derived from the detected first and second images together to determine the asymmetry in the profile of the periodic structure, wherein the first and second measurement steps are performed using different optical paths within a measurement optical system, and at least one correction, applied either to the first and second image intensity values or to the calculated difference between them, for reducing an influence on the determined asymmetry of the difference in optical paths between the first and second measurement steps.