Topographic phase control for overlay measurement

An imaging metrology tool comprising:

• a calibration module operated by one or more processors configured to derive a dependency of an overlay error magnification on a level of defocusing, wherein the dependency is based on target pitch, target asymmetry, amplitude of a first diffraction order, topographic phase, and defocus, and wherein the amplitude of the first diffraction order is based on amplitudes from a plurality of symmetric rectangular areas, the target pitch, and a phase difference,
• controlling the overlay error magnification, using the calibration module, by adjusting a focus position to reduce a factor that is defined by tan(ψ(1)−ψ(0)−2π(1−cos θ1)Δz/λ), wherein Δz is the level of defocusing, λ is an illumination wavelength, θ1 is an illumination angle of the first diffraction order, and ψ(1) and ψ(0) are topographic phases of the first and the zeroth diffraction orders, respectively, and
• an optical system configured to operate at a narrow spectral range, Δλ≤10 nm, at a narrow illumination numerical aperture, NA≤0.1, and at the focus position that corresponds to zero overlay error magnification according to the derived dependency.