Endpoint control of multiple substrates of varying thickness on the same platen in chemical mechanical polishing

Abstract

A difference between a first expected required polish time for a first substrate and a second expected required polish time for a second substrate is determined using a first pre-polish thickness and a second pre-polish thickness measured at an in-line metrology station. A duration of an initial period is determined based on the difference between the first expected required polish time and the second expected required polish time. For the initial period at a beginning of a polishing operation, no pressure is applied to whichever of the first substrate and the second substrate has a lesser expected required polish time while simultaneously pressure is applied to whichever of the first substrate and the second substrate has a greater expected required polish time. After the initial period, pressure is applied to both the first substrate and the second substrate.

What is claimed is: 
     
         1 - 13 . (canceled) 
     
     
         14 . A computer-implemented method of polishing, comprising:
 measuring a first pre-polish thickness of a layer of a first substrate at an in-line metrology station;   measuring a second pre-polish thickness of a layer of a second substrate at the in-line metrology station;   simultaneously contacting the first substrate and the second substrate to the same polishing pad, wherein pressures applied to the first substrate and the second substrate are independently controllable;   determining a difference between a first expected required polish time for the first substrate and a second expected required polish time for the second substrate based on the first pre-polish thickness and the second pre-polish thickness;   determining a pressure ratio to apply between the first substrate and the second substrate using the difference between the first expected required polish time and the second expected required polish time such that the first substrate and the second substrate have closer to the same thickness at an intermediate time before a projected endpoint time than without such a pressure difference; and   at least until the intermediate time, applying pressures to the first substrate and the second substrate at the pressure ratio.   
     
     
         15 . The method of  claim 14 , wherein the pressure ratio equals the ratio between the second expected required polish time and the first expected required polish time. 
     
     
         16 . The method of  claim 14 , further comprising storing a first target thickness for the first substrate and a second target thickness for the second substrate. 
     
     
         17 . The method of  claim 14 , further comprising commencing polishing of the first substrate and the second substrate at a substantially same time. 
     
     
         18 . The method of  claim 16 , wherein the first target thickness is equal to the second target thickness. 
     
     
         19 . The method of  claim 18 , wherein the first target thickness and the second target thickness are zero. 
     
     
         20 . The method of  claim 20 , wherein the layer of the first substrate and the layer of the second substrate are metal. 
     
     
         21 . The method of  claim 16 , further comprising subtracting the first target thickness from the first pre-polish thickness to determine a first amount to remove from the first substrate and subtracting the second target thickness from the second pre-polish thickness to determine a second amount to remove from the second substrate. 
     
     
         22 . The method of  claim 21 , further comprising determining the difference between the first expected required polish time and the expected required second polish time based on a polishing rate calculated prior to polishing of the first substrate and the second substrate. 
     
     
         23 . The method of  claim 22 , wherein determining the difference between the first expected required polish time and the second expected required polish time comprises dividing the first amount to remove by the polishing rate to determine the first expected required polishing time, and dividing the second amount to remove by the polishing rate to determine the second expected required polishing time. 
     
     
         24 . The method of  claim 22 , further comprising calculating the polishing rate by determining the polishing rate of at least one substrate polished on the polishing pad before polishing of the first substrate and the second substrate. 
     
     
         25 . The method of  claim 24 , wherein determining the polishing rate comprises calculating a moving average of polishing rates of a plurality of substrates polished on the polishing pad before polishing of the first substrate and the second substrate. 
     
     
         26 . The method of  claim 14 , further comprising:
 monitoring the first substrate and the second substrate during polishing with an in-situ monitoring system;   at a time after the initial period, adjusting the pressure of at least one of the first or second substrate such that the first substrate and the second substrate reach target thickness at closer to the same time than without such adjustment.