Overlay measurements by Mueller polarimetry in the back focal plane

Angle resolved Mueller polarimetry implemented as polarimetric imaging of the back focal plane of a high NA microscope objective has already demonstrated a good potential for CD metrology. In this paper we present the experimental and numerical results which indicate that this technique may also be competitive for measurements of the overlay error δ between two gratings at different levels. Series of samples of superimposed gratings with well controlled overlay errors have been manufactured and measured with the angle resolved Mueller polarimeter. The overlay targets were 20 μm wide. When overlay error δ = 0 the absolute value of Mueller matrix elements is invariant by matrix transposition. This symmetry breaks down when δ ≠ 0. As a result, we can define the following overlay estimator matrix: E = |M| - |M|^t. The simulations show that matrix element E₁₄ is the most sensitive to the overlay error. In the experiments the scalar estimator of E₁₄ was defined by averaging the pixel values over specifically chosen mask. The scalar estimator is found to vary essentially linearly with δ for the overlay values up to 50 nm. Our technique allows entering quite small overlay marks (down to 5 μm wide). The only one target measurement is needed for each overlay direction. The actual overlay value can be determined without detailed simulation of the structure provided the two calibrated overlay structures are available for each direction.