Patent ID: 7585202

Claim:
A computer-implemented method for process control in a chemical mechanical polishing, comprising: measuring an initial thickness of a layer of a product substrate at a metrology station; determining a target amount to remove from the initial thickness and a selected target thickness; determining an endpoint criterion based on the amount to remove, wherein the endpoint criterion includes a fractional number of interference cycles; monitoring the substrate with an in-situ monitoring system during polishing to detect an actual thickness removed from the substrate, wherein monitoring includes directing a light beam onto the substrate and measuring reflected light, the reflected light forming an interference signal that changes during polishing resulting in interference cycles; polishing the substrate for a number of interference cycles; calculating an average duration of previous interference cycles; calculating a second duration equal to a fraction of the average duration, the fraction equal to a difference between the fractional number and the number of interference cycles; polishing past an interference peak for the second duration; and stopping polishing when the endpoint criterion indicates that the target amount to remove is detected.