| CPC G03F 7/70633 (2013.01) [G03F 7/70625 (2013.01); G03F 7/70641 (2013.01); G03F 7/70683 (2013.01)] | 15 Claims |

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1. A method comprising:
illuminating a target with measurement radiation comprising at least a first wavelength to obtain resultant scattered radiation having been scattered by the target, the target comprising a mediator periodic structure and at least a first target periodic structure each formed in a respective different layer on a substrate, a pitch of at least the mediator periodic structure being below a single diffraction limit defined by a collection numerical aperture and a wavelength of the measurement radiation, such that the scattered radiation comprises double diffracted radiation, the double diffracted radiation comprising radiation having undergone two sequential same-order diffractions of opposite sign;
collecting the scattered radiation within the collection numerical aperture; and
determining a parameter of interest from the scattered radiation.
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