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In this work we analyze the optical endpoint traces commonly obtained during tungsten CMP and used as process control method. In spite of the many advantages in the process, the endpoint traces are seen just as a drop in reflectivity that signals the end of the process. We go beyond this simple interpretation and we focus our attention on different features that characterize the curve shape. Using both measurements and theoretical simulations of reflectivity of the stack being polished, we describe such characteristics and connect them to the layers being polished and to the lifetime of CMP pad installed on the equipment.