Scatterometry overlay targets as well as target design and measurement methods are provided, which mitigate the effects of grating asymmetries in diffraction based overlay measurements. Targets comprise additional cells with sub-resolved structures replacing resolved coarse pitch gratings and/or comprise alternating sub-resolved structures with coarse pitch periodicity—to isolate and remove inaccuracies that result from grating asymmetries. Measurement methods utilize orthogonally polarized illumination to isolate the grating asymmetry effects in different measurement directions, with respect to the designed target structures.