Patent ID: 6261152
Filing Date: 2001-07-17
Classification: B24B,G01B

Abstract:
A system for thickness monitoring of a semiconductor wafer having a front surface, a back surface opposite the front surface, an interface surface between said front and back surfaces, and a layer thickness measured between said interface and front surfaces, the system comprising:an assembly incorporating a laser source configured to generate (a) a first laser beam at substantially a first optical frequency and (b) a second laser beam at substantially a second optical frequency different from the first frequency, said semiconductor wafer being disposed in a first optical path over which said first laser beam is propagated as said wafer undergoes chemical-mechanical polishing along said front surface;a rotatable carrier having at least a partially optically transparent portion attached to said back surface with said first optical path passing through said optically transparent portion; anda measurement photodetector (a) disposed in said first optical path to receive light of said first laser beam after that light of said first laser beam has reflected off at least said front surface, (b) configured to receive light of said second laser beam controlled to avoid being significantly reflected off said front surface, and (c) having an output terminal that provides a measurement output signal which varies (c1) with said layer thickness and (c2) with time as a function of the difference between the frequencies of said laser beams.