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Overlay is a critical parameter for any semi-conductor foundry, with a direct impact on the fabrication yield, on the quality, and on the performances of the product. Being able to full the overlay constraints has also a direct in influence on the capacity to scale down and to integrate vertically. In addition, the shift of semiconductor applications into more demanding markets such as spatial and automotive leads to higher specifications for the process control. In the semiconductor manufacturing, the overlay is usually measured optically using dedicated targets in the scribe lines. However, targets differ from the product by their dimensions of an order of magnitude larger and by their position up to a few millimeters far from it. This can lead to residual errors and mismatches in the correction sent to the scanner, thus lowering the fabrication yield and the global product quality. For years, many SEM in-device overlay techniques have been published, however they are generally related to new generation of SEM imaging and metrology equipment and require a new dedicated target. In our case, the ambition is to extend the usage of our current CD-SEM tool park. To do so, an on-device and target-free overlay measurement process has been developed. It is based on sub-pixel contour extractions from CD-SEM images and on the use of the design. From a single image, contours of the two layers of interest are extracted from the SEM image and a custom algorithm calculates the overlay as a difference of the realigned design centroids. This dedicated algorithm allows extracting various overlay measurements from one image and enables a production friendly implementation. To evaluate the performances of this method, it has been applied on the patterns of a SRAM with scanner-programmed overlay. The measurements are compared to the conventional optical measurements. On a basis of thousands on-device measurements, the developed method showed a promising 95% rate of successful measurements. Good correlation between the optical model and the CD-SEM measured overlay on the SRAM has been demonstrated, reaching a coefficient of correlation of 0.99 on a pattern where conventional centroid or CD based overlay are limited. Finally, the flexibility of the method to measure various patterns with an ease of recipe creation is shown. Contour-based metrology offers the capability to extract highly valuable information from SEM images while keeping the layers differentiation. The proposed measurement process, being automated and requiring relatively low human inputs is a promising solution for a SEM on device overlay metrology suitable in a high-volume manufacturing environment.
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