Metrology system and method

A system to obtain an overlay measurement associated with a patterning process, the system comprising:

• a coherent beam generator configured to generate a coherent beam for illuminating at least of a first overlay pattern and at least part of a second overlay pattern, the first overlay pattern disposed on a first layer of a substrate, the second overlay pattern disposed on a second layer of the substrate, the at least part of the second overlay pattern not overlapping the at least part of the first overlay pattern, and the illumination of the first overlay pattern and the second overlay pattern generating a diffraction signal;
• a detector configured to detect the diffraction signal and generate an interference pattern, in a measurement plane, from the diffraction signal, the interference pattern arising from interference of radiation from the first overlay pattern with radiation from the second overlay pattern;
• and
• a processor system configured to determine an overlay measurement between the first overlay pattern and the second overlay pattern based on the interference pattern, the determination of the overlay measurement between the first overlay pattern and the second overlay pattern comprising:
• obtaining of a value of the position of a first location, in the measurement plane, associated with a first interference fringe of the interference pattern, the first interference fringe being associated with a positive non-zeroth order diffraction of the diffraction signal;
• obtaining of a value of the position of a second location, in the measurement plane, associated with a second interference fringe of the interference pattern, the second interference fringe being associated with a negative non-zeroth order diffraction of the diffraction signal;
• and
• determination, based on the values of the positions of the first and second locations, the overlay measurement between the first overlay pattern and the second overlay pattern.