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The measurement of the light reflected by the surface of the wafer during process is one of the most widely used endpoint detection methods for the chemical mechanical polishing of metals. In spite of the many advantages in the process, the endpoint traces are seen just as a drop in reflectivity that marks the end of the process. In this paper, we analyze the optical endpoint traces commonly obtained during tungsten CMP and we focus our attention on different features that characterize the curve shape. Using both measurements and theoretical simulations of reflectivity of the film stack involved, we describe such characteristics and connect them to the layers being polished and to the lifetime of CMP pad installed on the equipment. In particular, we focus on a secondary peak of intensity that can be seen when the thickness of titanium nitride under tungsten increases, and on the darkening of pad window as the lifetime increases. Finally, we simulate the endpoint traces by using internal library values of optical properties of materials involved, achieving a good match between real and simulated curve.