Method of measuring a target, and metrology apparatus

Abstract

Disclosed is a method of measuring a target, and a metrology apparatus. In one arrangement the target comprises a layered structure. The layered structure has a first target structure in a first layer and a second target structure in a second layer. The method comprises illuminating the target with measurement radiation using an illumination profile in the illumination pupil (u) that is offset from an imaginary line (IL) in the illumination pupil passing through the optical axis, to allow propagation to a detection region of the detection pupil of an allowed order (v, v) of a predetermined diffraction order while limiting propagation to the detection region of an equal and opposite order (v', v′) of that predetermined diffraction order. Scattered radiation of plural double-diffracted allowed diffraction orders (w, w) is detected. A characteristic of the lithographic process is calculated using the detected scattered radiation of the predetermined diffraction orders.