Endpoint detection in chemical mechanical polishing using multiple spectra

Abstract

A computer implemented method includes obtaining at least one current spectrum with an in-situ optical monitoring system, comparing the current spectrum to a plurality of different reference spectra, and determining based on the comparing whether a polishing endpoint has been achieved for the substrate having the outermost layer undergoing polishing. The current spectrum is a spectrum of light reflected from a substrate having an outermost layer undergoing polishing and at least one underlying layer. The plurality of reference spectra represent spectra of light reflected from substrates with outermost layers having the same thickness and underlying layers having different thicknesses.

1 . A computer implemented method comprising:
 obtaining at least one current spectrum with an in-situ optical monitoring system, the current spectrum being a spectrum of light reflected from a substrate having an outermost layer undergoing polishing and at least one underlying layer;   comparing the current spectrum to a plurality of different reference spectra, the plurality of reference spectra representing spectra of light reflected from substrates with outermost layers having the same thickness and underlying layers having different thicknesses; and   determining based on the comparing whether a polishing endpoint has been achieved for the substrate having the outermost layer undergoing polishing.   
     
     
         2 . The method of  claim 1 , wherein determining whether the polishing endpoint has been achieved includes calculating differences between the current spectrum and the reference spectra. 
     
     
         3 . The method of  claim 2 , wherein determining whether the polishing endpoint has been achieved includes determining whether at least one of the differences has reached a threshold value. 
     
     
         4 . The method of  claim 3 , wherein the at least one of the differences is a smallest difference from the differences. 
     
     
         5 . The method of  claim 3 , wherein the at least one of the differences is a median difference from of the differences. 
     
     
         6 . The method of  claim 2 , wherein determining whether the polishing endpoint has been achieved includes activating an endpoint detection algorithm when at least one of the differences has reached a threshold value. 
     
     
         7 . The method of  claim 6 , wherein determining whether the polishing endpoint has been achieved includes generating a difference trace that includes multiple points, each point representing a smallest of the differences calculated for a revolution of the platen. 
     
     
         8 . The method of  claim 7 , wherein the endpoint detection algorithm includes determining whether the difference trace has reached a minimum. 
     
     
         9 . The method of  claim 8 , wherein determining whether the difference trace has reached a minimum includes calculating a slope of the difference trace. 
     
     
         10 . The method of  claim 7 , wherein the endpoint detection algorithm includes determining whether the difference trace has risen to a threshold value above the minimum. 
     
     
         11 . The method of  claim 1 , wherein the reference spectra are generated empirically. 
     
     
         12 . The method of  claim 1 , wherein the reference spectra are generated from theory. 
     
     
         13 . The method of  claim 1 , further comprising obtaining a plurality of current spectra at different times. 
     
     
         14 . The method of  claim 13 , wherein the plurality of current spectra include a sequence of current spectra from a plurality of sweeps of the in-situ optical monitoring system across the substrate. 
     
     
         15 . The method of  claim 13 , wherein the plurality of current spectra include a plurality of current spectra from a same sweep of the in-situ optical monitoring system across the substrate. 
     
     
         16 . The method of  claim 14 , further comprising comparing the plurality of current spectra from the same sweep to the plurality of reference spectra to generate a plurality of differences between the current spectra and the reference spectra. 
     
     
         17 . The method of  claim 15 , further comprising, determining a smallest of the plurality of differences and using the smallest of the plurality of differences to determine whether a polishing endpoint has been achieved. 
     
     
         18 . A computer program product, encoded on a tangible program carrier, operable to cause data processing apparatus to perform operations comprising:
 obtaining at least one current spectrum with an in-situ optical monitoring system, the current spectrum being a spectrum of light reflected from a substrate having an outermost layer undergoing polishing and at least one underlying layer;   comparing the current spectrum to a plurality of different reference spectra, the plurality of reference spectra representing spectra of light reflected from substrates with outermost layers having the same thickness and underlying layers having different thicknesses; and   determining based on the comparing whether a polishing endpoint has been achieved for the substrate having the outermost layer undergoing polishing.