Laser interferometry endpoint detection with windowless polishing pad for chemical mechanical polishing process

Abstract

A multi-platen chemical-mechanical polishing system is used to polish a wafer. The wafer is polished at a first station. During polishing, an endpoint is detected. The endpoint is detected by generating optical radiation by a first light source. The first optical radiation travels through a translucent area in a surface of a first platen and travels through a first polishing pad. After being reflected by the wafer, the optical radiation returns through the first polishing pad through the translucent window to a first optical radiation detector. The first polishing pad has a uniform surface in that no part of the surface of the first polishing pad includes transparent material through which non-scattered optical radiation originating from the first light source can pass and be detected by the first optical radiation detector. Optical radiation that travels through the first polishing pad and is detected by the first optical radiation detector is haze scattered by inclusions within the first polishing pad. Non-scattered light is absorbed by the first polishing pad. The wafer is also polished at a second station. During polishing a final endpoint is detected. The final endpoint is detected by generating optical radiation by a second light source. The second optical radiation travels through a translucent area in a surface of a second platen and travels through a window embedded in a second polishing pad. After being reflected by the wafer, the optical radiation returns through the window embedded in the second polishing pad, through the translucent area in the surface of the second platen, to a second optical radiation detector.