Techniques for matching measured spectra to reference spectra for in-situ optical monitoring

Abstract

A method of controlling polishing includes storing a library having a plurality of reference spectra, polishing a substrate, measuring a sequence of spectra of light from the substrate during polishing, for each measured spectrum of the sequence of spectra, finding a best matching reference spectrum using a matching technique other than sum of squared differences to generate a sequence of best matching reference spectra, and determining at least one of a polishing endpoint or an adjustment for a polishing rate based on the sequence of best matching reference spectra. Finding a best matching reference spectrum may include performing a cross-correlation of the measured spectrum with each of two or more of the plurality of reference spectra from the library and selecting a reference spectrum with the greatest correlation to the measured spectrum as a best matching reference spectrum.

1 . A method of controlling polishing, comprising:
 storing a library having a plurality of reference spectra;   polishing a substrate;   measuring a sequence of spectra of light from the substrate during polishing;   for each measured spectrum of the sequence of spectra, finding a best matching reference spectrum using a matching technique other than sum of squared differences to generate a sequence of best matching reference spectra; and   determining at least one of a polishing endpoint or an adjustment for a polishing rate based on the sequence of best matching reference spectra.   
     
     
         2 . The method of  claim 1 , wherein finding a best matching reference spectrum comprises performing a cross-correlation of the measured spectrum with each of two or more of the plurality of reference spectra from the library and selecting a reference spectrum with the greatest correlation to the measured spectrum as a best matching reference spectrum. 
     
     
         3 . The method of  claim 2 , wherein each reference spectrum of the plurality of reference spectra has a stored associated index value, and further comprising determining the associated index value for each best matching spectrum from the sequence of best matching reference spectra to generate a sequence of index values, and fitting a function to the sequence of index values. 
     
     
         4 . The method of  claim 3 , further comprising halting the polishing when the linear function matches or exceeds a target index. 
     
     
         5 . The method of  claim 2 , wherein the substrate includes a second layer overlying a first layer, the first layer having a different composition than the second layer. 
     
     
         6 . The method of  claim 5 , wherein the second layer is a barrier layer and the first layer is a dielectric layer. 
     
     
         7 . The method of  claim 6 , wherein the barrier layer is tantalum nitride or titanium nitride and the dielectric layer is carbon doped silicon dioxide or is formed from tetraethyl ortho silicate. 
     
     
         8 . The method of  claim 5 , wherein the function is fit to a portion of the sequence of index values corresponding to spectra measured after detection of exposure of the first layer. 
     
     
         9 . The method of  claim 1 , wherein finding a best matching reference spectrum comprises performing a sum of enclidean vector distances between the measured spectrum and each of two or more of the plurality of reference spectra from the library and selecting a reference spectrum with the lowest sum as a best matching reference spectrum. 
     
     
         10 . The method of  claim 1 , wherein finding a best matching reference spectrum comprises performing a sum of derivative differences between the measured spectrum and each of two or more of the plurality of reference spectra from the library and selecting a reference spectrum with the lowest sum as a best matching reference spectrum. 
     
     
         11 . The method of  claim 1 , wherein measuring the sequence of spectra of light from the substrate comprises making a plurality of sweeps of a sensor across the substrate. 
     
     
         12 . The method of  claim 8 , wherein each spectrum from the sequence of spectra corresponds to a single sweep of the sensor from the plurality of sweeps. 
     
     
         13 . The method of  claim 1 , wherein the substrate includes a plurality of zones, and a polishing rate of each zone is independently controllable by an independently variable polishing parameter, and further comprising:
 measuring a sequence of spectra from each zone during polishing;   for each measured spectrum in the sequence of spectra for each zone, performing a cross-correlation of the measured spectrum with each of two or more of the plurality of reference spectra from the library and selecting a reference spectrum with the greatest correlation to the measured spectrum as a best matching reference spectrum from the library to generate a sequence of best matching reference spectra; and   adjusting the polishing parameter for at least one zone to adjust the polishing rate of the at least one zone such that the plurality of zones have a smaller difference in thickness at the polishing endpoint than without such adjustment.   
     
     
         14 . A method of controlling polishing, comprising:
 storing a library having a plurality of reference spectra;   polishing a substrate;   measuring a sequence of spectra of light from the substrate during polishing;   for each measured spectrum of the sequence of spectra, performing a cross-correlation of the measured spectrum with each of two or more of the plurality of reference spectra from the library and selecting a reference spectrum with the greatest correlation to the measured spectrum as a best matching reference spectrum to generate a sequence of best matching reference spectra; and   determining at least one of a polishing endpoint or an adjustment for a polishing rate based on the sequence of best matching reference spectra.   
     
     
         15 . A polishing apparatus, comprising:
 a support to hold a polishing pad;   a carrier head to hold a substrate against the polishing pad;   a motor to generate relative motion between the carrier head and the support to polish the substrate;   an optical monitoring system to measure a sequence of spectra of light from the substrate while the substrate is being polished; and   a controller configured to
 store a library having a plurality of reference spectra, each reference spectrum of the plurality of reference spectra having a stored associated index value, 
 for each measured spectrum of the sequence of spectra, find a best matching reference spectrum using a matching technique other than sum of squared differences to generate a sequence of best matching reference spectra; and 
 determine at least one of a polishing endpoint or an adjustment for a polishing rate based on the sequence of best matching reference spectra. 
   
     
     
         16 . The apparatus of  claim 15 , wherein the controller is configured to perform a cross-correlation of the measured spectrum with each of two or more of the plurality of reference spectra from the library and select a reference spectrum with the greatest correlation to the measured spectrum as the best matching reference spectrum. 
     
     
         17 . A computer program product, tangibly embodied in a machine readable storage device, comprising instructions to:
 store a library having a plurality of reference spectra;   polish a substrate;   measure a sequence of spectra of light from the substrate during polishing;   for each measured spectrum of the sequence of spectra, find a best matching reference spectrum using a matching technique other than sum of squared differences to generate a sequence of best matching reference spectra; and   determine at least one of a polishing endpoint or an adjustment for a polishing rate based on the sequence of best matching reference spectra.   
     
     
         18 . The computer program product of  claim 17 , further comprising instructions to perform a cross-correlation of the measured spectrum with each of two or more of the plurality of reference spectra from the library and select a reference spectrum with the greatest correlation to the measured spectrum as the best matching reference spectrum.