Lateral shift measurement using an optical technique

A multi-layer semiconductor wafer comprising a patterned area and at least one test site located outside said patterned area, wherein said at least one test site comprises a pair of diffractive structures of predetermined geometries and location relative to each other corresponding to alignment of the layers of the semiconductor wafer within said patterned area thereof, wherein

• the diffractive structures of the pair are arranged such that features of one of diffractive structures of the pair are located either on top of features of the other diffractive structure of said pair, or between features of the other diffractive structure of said pair, and are arranged such that diffractive patterns in the diffractive structures of the pair comprise features aligned along at least one axis with a predetermined non-zero lateral shift between them, which is selected in accordance with a predetermined model of simulated diffraction effects from incident light interaction with laterally shifted diffractive structures of a pair of structures having said predetermined geometries,
• diffraction of the incident light from said test site in the wafer thereby corresponding to a lateral shift defined by said predetermined non-zero lateral shift between the diffractive patterns of the pair of the diffractive structures in the test site and a lateral shift between respective layers of the wafer, thereby enabling direct overlay measurement of the lateral shift between layers in the wafer.