Acoustic monitoring of cmp retaining ring

A chemical mechanical polishing apparatus, comprising:

• a platen supporting a polishing pad;
• a carrier head to hold a surface of a substrate against the polishing pad, the carrier head including a retaining ring for holding the substrate;
• an acoustic sensor supported on the platen;
• a motor to generate relative motion between the platen and the carrier head so as to polish the substrate and such that the acoustic sensor travels in a path below the carrier head and the retaining ring;
• and
• a controller configured to select portions of a signal from the acoustic sensor corresponding to the acoustic sensor being below the retaining ring and to generate an alert or to modify a polishing parameter based on the selected portions of the signal from the acoustic sensor.

A method of a chemical mechanical polishing apparatus, comprising:

• bringing a surface of a substrate into contact with a polishing pad;
• generating relative motion between the substrate and the polishing pad so as to polishing the substrate and using a carrier head to hold the substrate against lateral motion;
• during polishing sweeping an acoustic sensor in a path below the carrier head and the retaining ring;
• and
• analyzing a signal from the acoustic sensor and determining a characteristic of the retaining ring based on the signal.