Automatic Gain Control

A computer-program product, tangibly stored on machine-readable medium, the product comprising instructions operable to cause a chemical mechanical polisher to perform a method comprising:

• commencing a polishing step in which a film on a substrate is polished by a chemical mechanical polisher that includes a polishing pad and an in-situ monitoring system, polishing being effected by causing the film to be in contact with the polishing pad while there is relative motion between the film and the polishing pad;
• during the polishing step, causing the in-situ monitoring system to monitor the film being polished through a first portion of the polishing pad;
• during the polishing step, generating a first electronic signal from a detector in the in-situ monitoring system with a gain for the in-situ monitoring system set to a first value, the detector sensitive to a characteristic of the film being polished;
• and
• during the polishing step, evaluating whether the first electronic signal satisfies one or more constraints and, when the first electronic signal is evaluated not to satisfy the one or more constraints, adjusting the gain for the in-situ monitoring system to a second value different from the first value so that the first electronic signal would satisfy the one or more constraints.

A chemical mechanical polisher, comprising:

• a polishing pad that includes a first portion;
• a light source and a light detector;
• and
• a controller operable to perform a calibration method comprising:
• commencing a polishing step in which a film on a substrate is polished by the polisher, polishing being effected by causing the film to come into contact with the polishing pad while there is relative motion between the film and the polishing pad;
• during the polishing step, causing the light source to emit light and directing light emitted from the light source through the first portion and to a surface of the film being polished;
• during the polishing step, receiving, at the light detector, light reflecting from the surface of the film being polished and passing through the first portion;
• during the polishing step, generating a first electronic signal based on the light received at the light detector;
• and
• during the polishing step, evaluating whether the first electronic signal satisfies one or more constraints and, when the first electronic signal is evaluated not to satisfy the one or more constraints, adjusting a gain for the light detector so that the first electronic signal would satisfy the one or more constraints.