Platen with multiple sensors for eddy current monitoring

An apparatus for chemical mechanical polishing, comprising:

• a platen having a surface to support a polishing pad;
• a carrier head to hold a substrate such that a layer on the substrate contacts the polishing pad;
• an eddy current monitoring system including a first plurality of eddy current sensors supported by the platen and arranged in a first ring at a first distance from an axis of rotation of the platen and a second plurality of eddy current sensors supported by the platen and arranged in a second ring at a larger second distance from the axis of rotation of the platen such that each sensor of the first and second pluralities of sensors intermittently sweep below the substrate held by the carrier head and each respective sweep of a sensor generates a respective signal trace that includes a sequence of signal values, and wherein each respective sensor is configured to generate a magnetic field that intermittently impinges the substrate;
• and
• a controller configured to
• receive each respective signal trace from the first and second pluralities of sensors,
• for each respective signal trace, convert the sequence of signal values to a corresponding thickness trace that includes sequence of thickness values for different locations on the substrate, thus generating a sequence of thickness traces,
• for each respective thickness trace in the sequence of thickness traces, identify a plurality of minima in the respective thickness trace,
• calculate a sequence of layer thickness values over time based on the plurality of minima from the respective traces in the sequence of thickness traces, and
• at least one of detect a polishing endpoint or adjust a polishing parameter that affects the polishing process based on the sequence of layer thickness values.