Multi-wavelength (MWL) micro diffraction-based overlay (μDBO) is a prominent method for after-develop inspection (ADI) overlay measurements, which is favorable for accuracy and robustness. Continuous-bias DBO (cDBO) is expected to offer robustness improvements against stack variation, asymmetry, and imbalance. In this paper, dual-WL (DWL) cDBO profiles were evaluated to secure the advantages of both of MWL and cDBO applications. The metrics used to evaluate accuracy and robustness of ADI overlay measurements are residual, dynamic precision (DP), and wafer-to-wafer variation of the difference between ADI and after-etch inspection overlay. 70% of DWL profiles had improvements in their residual values comparing with their single-WL (SWL) constituents on Samsung R&D wafers in layer A. On layer B, the best DWL cDBO profiles showed around 5% improved residuals comparing with its SWL constituents. DWL cDBO showed around 30% averaged improved DP compared with SWL counterparts. DP improvements of MWL cDBO are following the expected DP improvements, based on the signal-to-noise ratio improvement with increasing number of signals. Residual improvement with increasing number of WLs is different from the DP improvement, and the best DWL residual improvement is higher than that of SWL measurements with noise reduction techniques applied. This shows that the residual improvement cannot be attributed to the increased number of acquisitions, and that it could be an innate advantage of MWL cDBO.
During metrology overlay recipe setup typically a wide range of different target designs are present to select from. The main goal of recipe setup is to select the most accurate target type-recipe combination at ADI (after develop inspection) without additional external information that can be used for production on a large set of wafers. We will introduce a method based on blind source separation to disentangle the contributions of target asymmetry and the overlay of the targets. Based on this separation, the most accurate target-recipe combination can be selected. On top of selecting the most accurate target-recipe combination, it is important to stabilize the difference between the overlay on device and the overlay as measured on the target. In order to increase that stability we will introduce advanced algorithms in the ADI measurements that use measured asymmetry parameters to correct for inline target asymmetry variation. We will show a metrology to device matching improvement of up to 40% on product wafers.
State of the art after-develop (ADI) overlay is measured with multi-wavelength micro diffraction-based overlay techniques. A micro diffraction-based overlay target consists of two pairs of gratings, with the same pitch in the top and bottom layer. The gratings in the top layer have a bias offset with respect to the bottom layer in the positive or negative direction. When illuminated, +1st and -1st order light is diffracted. The asymmetry in the intensity of these signals contains the overlay information. In this paper, ADI overlay is measured with a new dark-field target design for ADI overlay. Like a micro diffraction-based overlay target, it consists of pairs of gratings in the top and bottom layer. Instead of a bias offset between top and bottom gratings, different pitches are used resulting in a continuous-bias throughout the grating pair. When illuminated the diffracted light contains moiré fringes, in which the overlay is stored in the phases. This technique has improved accuracy and robustness by design, because it is immune to symmetrical process changes like stack height variations and grating imbalance. Additionally, it shows more stable behavior through wavelength, both in signal strength and overlay. These characteristics make it possible, with a single wavelength, to achieve similar or better performance than micro diffraction-based overlay using a multi-wavelength solution, resulting in higher throughput. This is demonstrated on Samsung’s latest memory node where on average an 21% reduction is achieved in the 3sigma of the mis reading correction with a single-wavelength phase-based overlay measurements, compared to multi-wavelength micro diffraction-based overlay measurements.
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