Signal processing for finding substrate notch

A computer program product, comprising a non-transitory storage medium encoded with instructions to cause one or more computers to:

• cause an actuator to position a carrier head relative to a substrate with a sensing region of a sensor at an edge of the substrate;
• cause a motor to generate relative motion between the carrier head and the sensor such that the sensing region of the sensor scans along a circumference of the substrate;
• and
• detect an angular position of a notch in the edge of the substrate based on an initial signal from the sensor, including generating a second or higher order derivative signal from the initial signal.

A polishing apparatus, comprising:

• a plurality of stations including a first station that is a polishing station or a transfer station and a second station that is a polishing station;
• a carrier head to hold a substrate, the carrier head movable by an actuator along a path from the first station to the second station;
• a motor to rotate the carrier head about an axis of rotation;
• a sensor to generate a signal that depends on an proportion of a sensing region of the sensor that is covered by a substrate;
• and
• a controller is configured to
• cause the actuator to position a carrier head relative to the substrate with the sensing region of the sensor at an edge of the substrate,
• cause the motor to generate relative motion between the carrier head and the sensor such that the sensing region of the sensor makes multiple scans along a circumference to obtain an initial signal,
• detect an angular position of a notch on the substrate from each respective scan of the multiple scans to generate a plurality of possible angular positions with each possible angular position of the plurality of possible angular positions corresponding to a respective scan, and
• select an angular position from the plurality of angular positions.