Spectrographic monitoring using index tracking after detection of layer clearing

Abstract

A method of controlling polishing includes storing a library having a plurality of reference spectra, each reference spectrum of the plurality of reference spectra having a stored associated index value, polishing a substrate having a second layer overlying a first layer, 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 to generate a sequence of best matching reference spectra, 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, detecting exposure of the first layer, fitting a function to a portion of the sequence of index values corresponding to spectra measured after detection of exposure of the first layer, and determining at least one of a polishing endpoint or an adjustment for a polishing rate based on the function.

1 . A method of controlling polishing, comprising:
 storing a library having a plurality of reference spectra, each reference spectrum of the plurality of reference spectra having a stored associated index value;   polishing a substrate having a second layer overlying a first layer;   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 to generate a sequence of best matching reference spectra;   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;   detecting exposure of the first layer;   fitting a function to a portion of the sequence of index values corresponding to spectra measured after detection of exposure of the first layer; and   determining at least one of a polishing endpoint or an adjustment for a polishing rate based on the function.   
     
     
         2 . The method of  claim 1 , wherein detecting exposure of the first layer comprises:
 measuring a sequence of groups of spectra of light from the substrate during polishing;   for each group, calculating a value for a dispersion parameter of the spectra in the group to generate a sequence of dispersion values; and   detecting exposure of the first layer based on the sequence of dispersion values.   
     
     
         3 . The method of  claim 2 , wherein calculating the value of the dispersion parameter comprises calculating a difference value for each spectrum in the group to generate a plurality of difference values. 
     
     
         4 . The method of  claim 3 , wherein calculating the value of the dispersion parameter comprises calculating a standard deviation of the plurality of difference values. 
     
     
         5 . The method of  claim 2 , wherein the sequence of groups of spectra includes the sequence of spectra. 
     
     
         6 . The method of  claim 1 , wherein detecting exposure of the first layer comprises monitoring a total reflected intensity from the substrate, a motor torque or a friction between the substrate a polishing pad. 
     
     
         7 . 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. 
     
     
         8 . The method of  claim 7 , wherein each spectrum from the sequence of spectra corresponds to a single sweep of the sensor from the plurality of sweeps. 
     
     
         9 . The method of  claim 7 , wherein making a plurality of sweeps of a sensor across the substrate comprises rotating a platen having a sensor fixed thereto. 
     
     
         10 . The method of  claim 1 , wherein the second layer is a barrier layer. 
     
     
         11 . The method of  claim 10 , wherein the first layer is a dielectric layer having a different composition than the barrier layer. 
     
     
         12 . The method of  claim 11 , wherein the barrier layer is tantalum nitride or titanium nitride and the dielectric layer is carbon doped silicon dioxide or is formed from tetraethyl orthosilicate. 
     
     
         13 . The method of  claim 1 , further comprising halting the polishing when the linear function matches or exceeds a target index. 
     
     
         14 . The method of  claim 13 , further comprising determining a target index difference, determining an index at exposure of the first layer, and calculating the target index by adding the target index difference to the index at exposure of the first layer. 
     
     
         15 . The method of  claim 14 , further comprising receiving a target amount to remove from a user, and wherein determining the target index difference comprises calculating the target index difference from the target amount to remove and a predetermined polishing rate. 
     
     
         16 . The method of  claim 13 , wherein the target index is a value stored prior to polishing of the substrate. 
     
     
         17 . 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:
 storing a target index value for each zone;   measuring a sequence of spectra from each zone during polishing;   for each measured spectrum in the sequence of spectra for each zone, determining a best matching reference spectrum from a library of reference spectra;   for each best matching reference spectrum for each zone, determining an index value to generate a sequence of index values;   for each zone, fitting a linear function to a portion of the sequence of index values corresponding to spectra measured after detection of exposure of the first layer;   for at least one zone, determining a projected time at which the zone will reach the target index value of the at least one zone based on the linear function; and   adjusting the polishing parameter for at least one zone to adjust the polishing rate of the at least one zone of the at least one substrate such that the at least one zone has closer to the target index at the projected time than without such adjustment.   
     
     
         18 . The method of  claim 17 , wherein the polishing parameter is a pressure in a carrier head. 
     
     
         19 . 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 to generate a sequence of best matching reference spectra, 
 determine the associated index value for each best matching spectrum from the sequence of best matching reference spectra to generate a sequence of index values, 
 detect exposure of the first layer from the sequence of spectra, 
 fit a function to a portion of the sequence of index values corresponding to spectra measured after detection of exposure of the first layer, and 
 determine at least one of a polishing endpoint or an adjustment for a polishing rate based on the function. 
   
     
     
         20 . A computer program product, tangibly embodied in a machine readable storage device, comprising instructions 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;   polish a substrate having a second layer overlying a first layer;   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 to generate a sequence of best matching reference spectra;   determine the associated index value for each best matching spectrum from the sequence of best matching reference spectra to generate a sequence of index values;   detect exposure of the first layer;   fit a function to a portion of the sequence of index values corresponding to spectra measured after detection of exposure of the first layer; and   determine at least one of a polishing endpoint or an adjustment for a polishing rate based on the function.