In-situ detection of thin-metal interface using optical interference

An invention is disclosed for an optical endpoint detection system that utilizes optical interference to determine when a metal layer has reached a thin metal zone during a CMP process. A portion of a surface of a wafer is illuminated with broad band light source. Then, reflected spectrum data corresponding to a plurality of spectrums of light reflected from the illuminated portion of the surface of the wafer is received. An endpoint is then determined based on optical interference occurring in the reflected spectrum data, which is a result of phase differences in light reflected from different layers of the wafer, and occurs when the top metal layer is reduced to the thin metal zone.