Acoustic carrier head monitoring

A chemical mechanical polishing apparatus, comprising:

• a platen to support a polishing pad;
• a carrier head comprising a rigid housing, the carrier head configured 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 the substrate;
• an in-situ carrier head monitoring system including a sensor positioned to interact with the housing and to detect vibrational motion of the housing and generate signals based on the detected vibrational motion;
• and
• a controller configured to
• generate a value for a carrier head status parameter based on received signals from the in-situ carrier head monitoring system, wherein the carrier head status parameter indicates an impending fault prior to the fault occurring, and
• change a polishing parameter or generate an alert based on the carrier head status parameter.