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The use of optical metrology techniques for process control is now widespread. These techniques are fast and nondestructive, allowing higher throughputs than non-optical techniques like electron microscopies or AFM. We present here new developments using complete Mueller polarimetry in the back focal plane of a microscope objective to characterize overlay for microelectronic industry. Based on fundamental symmetries in the physics of periodic structures and polarized light and redundancies in the angle-resolved Mueller images we define estimators which vary linearly with the overlay. As a result, overlay measurement is sensitive to both the direction and sign of the overlay, and it does not require any detailed modeling of the target structures, provided two independent targets with known overlay values are available in close locations on the wafer. Realistic simulations on optimized structures suggest that accuracies in the order of 1 or 2 nm or better should be achievable. Moreover, with high NA objectives the proposed technique can be implemented with targets with lateral sizes as small as a few μm. Experimental results of both grating line profiles and overlay determinations will be presented. The samples, elaborated at LETI, have been accurately characterized by optical imaging AIM techniques and state-of-the-art AFM. The latest developments on the device itself as well as the advantages, possibilities and limitations of this new metrology technique will be discussed.

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 1 . 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 / 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 / = 0 the absolute value of Mueller matrix elements is invariant by matrix transposition. This symmetry breaks down when G z 0. As a result, we can define the following overlay estimator matrix: ( _0 _ _0 _ t . The simulations show that matrix element E 14 is the most sensitive to the overlay error. In the experiments the scalar estimator of E 14 was defined by averaging the pixel values over specifically chosen mask. The scalar estimator is found to vary essentially linearly with G for the overlay values up to 50 nm. Our technique allows entering quite small overlay marks (down to 5 µm wide). Th e only one target measuremen t 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. Keywords: Overlay metrology, Mueller matrix polarimetry, polarimetric imaging, conoscopic mode imaging.

International audience; Angle resolved Mueller polarimetry implemented as polarimetric imaging of a back focal plane of a high NA microscope objective has already demonstrated a good potential for CD metrology. Here we present the experimental and numerical results indicating that this technique may also be competitive for the measurements of overlay error delta. A 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-mu m wide. When the overlay error is delta is equal to 0, absolute values of elements of real 4x4 Mueller matrix M are invariant by matrix transposition. Otherwise this symmetry breaks down. Consequently, we define the following overlay estimator matrix as E = |M| - |M|(t). The simulations show that matrix element E-14 is the most sensitive to the overlay error. The scalar estimator of E-14 was calculated by averaging the pixel values over a specifically chosen mask. This estimator is found to vary linearly with d for overlay values up to 50 nm. Our technique allows entering small overlay marks (down to 5-mu m wide). Only one target measurement is needed for each overlay direction. The actual overlay value can be determined without detailed simulation of the structure provided two calibrated overlay structures are available for each direction.