In-situ detection of thin-metal interface using high resolution spectral analysis of optical interference

1. Field of the Invention An invention is provided for detecting an endpoint during a chemical mechanical polishing (CMP) process. A reflected spectrum data sample is received that comprises a plurality of values corresponding to a plurality of spectrums of light reflected from an illuminated portion of a surface of a wafer. The reflected spectrum data sample is decomposed into noise sub-space values and signal sub-space values, and the noise sub-space values are truncated. In addition, outside spectrum data is extrapolated using a linear combination of the values of the reflected spectrum data sample. In this manner, an endpoint can be determined based on optical interference occurring in the reflected spectrum data.