In-situ metrology system and method

Abstract

At least one dimensional characteristic of a workpiece is measured, in situ, while at least one surface of the workpiece is subjected to a finishing operation. The measurements are obtained using an interferometer that generates interfering wavefronts reflected from the front and rear surfaces of the workpiece. Variations in the optical thickness of the workpiece can be determined from the resulting interferogram. The resulting optical thickness data can be used directly and/or combined with other pre-acquired data about the workpiece to obtain information concerning a desired dimensional characteristic of the workpiece. This dimensional characteristic may be transmitted wavefront error, work surface smoothness, and/or work surface profile. These measurements then can be used to terminate the finishing process at an optimal time and/or to control the operation of the surface-finishing machine. The difference in temperature between a workpiece's work surface and the surface opposed to it can also be extracted from the metrology results when measured in conjunction with a witness sample or monitor plug which demonstrates a differing thermal deformation characteristic due to a different thickness or having a differing coefficient of thermal expansion. Preferably, the witness sample or monitor plug should be made from a material that is relatively insensitive to temperature changes with respect to the workpiece. This temperature differential then can be used to control the temperature of a finishing process parameter such as slurry temperature. A finishing process that incorporates in situ metrology can dramatically reduce the surface finishing process time as compared to conventional finishing processes.