Patent ID: 7775852

Claim:
An apparatus for chemical mechanical polishing (CMP) a wafer, the apparatus comprising; a platen; a polishing pad having a polishing surface, a bottom surface contacting the platen, and an aperture; a solid light transmissive element positioned in the aperture, a bottom surface of the solid light transmissive element positioned above a plane defined by a surface of the platen the polishing pad contacts, the solid light transmissive element being more transmissive to light than the polishing surface; a polishing head which during processing holds the wafer against the polishing pad; and an optical monitoring system comprising a detector and a light source arranged to direct a light beam at least during part of the polishing operation through the light transmissive element so that the light beam impinges on the wafer and reflects back through the light transmissive element to the detector and thereby provides for in-situ monitoring of the polishing process.