Monitoring of acoustic events on a substrate

A chemical mechanical polishing apparatus, comprising:

• a platen to support a polishing pad;
• a carrier head to hold a surface of a substrate against the polishing pad;
• a motor to generate relative motion between the platen and the carrier head so as to polish an overlying layer on the substrate;
• an array of acoustic sensors arranged within the carrier head to receive acoustic signals from the surface of the substrate;
• and
• a controller configured to detect a position on the substrate at which a polishing endpoint has been reached based on received acoustic signals.

A method of polishing, comprising:

• holding a substrate with a carrier head and bringing a surface of a substrate into contact with a polishing pad;
• generating relative motion between the substrate and the polishing pad;
• monitoring acoustic signals from the substrate from a plurality of sensors in the carrier head;
• calculating a position of an acoustic event on the surface of the substrate based on the acoustic signals received from the plurality of sensors. The method of claim 14, wherein calculating the position is based on time-of-flight calculations of received acoustic signals from each of the acoustic sensors of the array.