US Patent 9067295 Monitoring retaining ring thickness and pressure control

A chemical mechanical polishing apparatus includes a carrier head including a retaining ring having a plastic portion with a bottom surface to contact a polishing pad, an in-situ monitoring system including a sensor that generates a signal that depends on a thickness of the plastic portion, and a controller configured to receive the signal from the in-situ monitoring system and to adjust at least one polishing parameter in response to the signal to compensate for non-uniformity caused by changes in the thickness of the plastic portion of the retaining ring.