A method of determining a structure parameter of a target on a substrate is disclosed. Illuminating a first region of the target with a first beam of coherent radiation and measuring a diffraction intensity pattern. Shifting the relative position between the target and the projection system to offset a second region to be illuminated from the first region. Illuminating the second region, offset from and overlapping with the first region, and measuring a diffraction intensity pattern. Repeating until the whole portion of interest of the target has been illuminated once, with adjacent illumination spots having some physical overlap. Retrieving phase information from the measured intensity patterns. Modeling the target to calculate a modeled diffraction intensity pattern and modeled phase information. Determining the structure parameter of the target by comparing the measured diffraction intensity patterns and the retrieved phase to the calculated modeled diffraction intensity pattern and the modeled phase information.