Platen with multiple sensors for eddy current monitoring

Abstract

A chemical mechanical polishing apparatus includes a platen to support a polishing pad, a carrier head to hold a substrate such that a layer on the substrate contacts the polishing pad, an actuator that controls a radial position of the carrier head over the platen, an eddy current monitoring system, and a controller. The eddy current monitoring system includes a first plurality of eddy current sensors supported by the platen and arranged in a first ring at a first distance from an axis of rotation of the platen and a second plurality of eddy current sensors supported by the platen and arranged in a second ring at a larger second distance from the axis of rotation of the platen. The controller is configured to control the actuator such that the second plurality of sensors sweep only across an edge portion of the substrate held by the carrier head.

What is claimed is: 
     
         1 . An apparatus for chemical mechanical polishing, comprising:
 a platen having a surface to support a polishing pad;   a carrier head to hold a substrate such that a layer on the substrate contacts the polishing pad;   an eddy current monitoring system including a first plurality of eddy current sensors supported by the platen and arranged in a first ring at a first distance from an axis of rotation of the platen and a second plurality of eddy current sensors supported by the platen and arranged in a second ring at a larger second distance from the axis of rotation of the platen such that each sensor of the first and second pluralities of sensors intermittently sweep below the substrate held by the carrier head and each respective sweep of a sensor generates a respective signal trace that includes a sequence of signal values, and wherein each respective sensor is configured to generate a magnetic field that intermittently impinges the substrate; and   a controller configured to
 receive each respective signal trace from the first and second pluralities of sensors, 
 for each respective signal trace, convert the sequence of signal values to a corresponding thickness trace that includes sequence of thickness values for different locations on the substrate, thus generating a sequence of thickness traces, 
 for each respective thickness trace in the sequence of thickness traces, identify a plurality of minima in the respective thickness trace, 
 calculate a sequence of layer thickness values over time based on the plurality of minima from the respective traces in the sequence of thickness traces, and 
 at least one of detect a polishing endpoint or adjust a polishing parameter that affects the polishing process based on the sequence of layer thickness values. 
   
     
     
         2 . The apparatus of  claim 1 , wherein a second number of eddy currents sensors of the second plurality of eddy current sensors is greater than a first number of eddy currents sensors of the first plurality of eddy current sensors. 
     
     
         3 . The apparatus of  claim 2 , wherein the second number is two or three times the first number. 
     
     
         4 . The apparatus of  claim 1 , wherein the first plurality of sensors are distributed at equal first angular intervals around the axis of rotation of the platen, and the second plurality of sensors are distributed at equal second angular intervals around the axis of rotation of the platen. 
     
     
         5 . The apparatus of  claim 4 , wherein the second angular interval is smaller than the first angular interval. 
     
     
         6 . The apparatus of  claim 4 , wherein at least some sensors of the second plurality of sensors are angularly aligned with sensors of the first plurality of sensors. 
     
     
         7 . The apparatus of  claim 1 , wherein the controller is configured to control an actuator that controls a radial position of the carrier head over the platen. 
     
     
         8 . The apparatus of  claim 7 , wherein the controller is configured to control the actuator such that the second plurality of sensors sweep only across an edge portion of the substrate held by the carrier head. 
     
     
         9 . The method of  claim 1 , wherein calculating the sequence of layer thickness values comprises, for each sweep, averaging minima from a sweep. 
     
     
         10 . The method of  claim 9 , wherein calculating the sequence of layer thickness values comprises calculating a sequence of layer thickness values for each of a plurality of zones on the substrate, and wherein calculating the sequence of layer thickness values for a respective zone from the plurality of zones comprises, for each sweep, averaging minima from a sweep for minima from the respective zone. 
     
     
         11 . An apparatus for chemical mechanical polishing, comprising:
 a platen having a surface to support a polishing pad;   a carrier head to hold a substrate such that a layer on the substrate contacts the polishing pad;   an eddy current monitoring system including a first plurality of eddy current sensors supported by the platen and arranged in a first ring at a first distance from an axis of rotation of the platen and a second plurality of eddy current sensors supported by the platen and arranged in a second ring at a larger second distance from the axis of rotation of the platen such that each sensor of the first and second pluralities of sensors intermittently sweep below the substrate held by the carrier head and such that each respective sweep of a sensor generates a respective signal trace that includes a sequence of signal values, wherein each respective sensor is configured to generate a magnetic field that intermittently impinges the substrate, and wherein a number of sensors in the second plurality of sensors is exactly two or three times a number of sensors in the first plurality of sensors.   
     
     
         12 . The apparatus of  claim 11 , wherein the first plurality of sensors are distributed at equal first angular intervals around the axis of rotation of the platen, and the second plurality of sensors are distributed at equal second angular intervals around the axis of rotation of the platen. 
     
     
         13 . The apparatus of  claim 12 , wherein at least some sensors of the second plurality of sensors are angularly aligned with sensors of the first plurality of sensors. 
     
     
         14 . An apparatus for chemical mechanical polishing, comprising:
 a platen having a surface to support a polishing pad;   a carrier head to hold a substrate such that a layer on the substrate contacts the polishing pad;   an actuator that controls a radial position of the carrier head over the platen;   an eddy current monitoring system including a first plurality of eddy current sensors supported by the platen and arranged in a first ring at a first distance from an axis of rotation of the platen and a second plurality of eddy current sensors supported by the platen and arranged in a second ring at a larger second distance from the axis of rotation of the platen such that each sensor of the first and second pluralities of sensors intermittently sweep below the substrate held by the carrier head and each respective sweep of a sensor generates a respective signal trace that includes a sequence of signal values, and wherein each respective sensor is configured to generate a magnetic field that intermittently impinges the substrate; and   a controller configured to
 control the actuator such that the second plurality of sensors sweep only across an edge portion of the substrate held by the carrier head, 
   receive each respective signal trace from the first and second pluralities of sensors,
 for each respective signal trace, convert the sequence of signal values to a corresponding thickness trace that includes sequence of thickness values for different locations on the substrate, thus generating a sequence of thickness traces, and 
 at least one of detect a polishing endpoint or adjust a polishing parameter that affects the polishing process based on the sequence of thickness traces. 
   
     
     
         15 . The apparatus of  claim 14 , wherein a second number of eddy currents sensors of the second plurality of eddy current sensors is greater than a first number of eddy currents sensors of the first plurality of eddy current sensors. 
     
     
         16 . The apparatus of  claim 14 , wherein the first plurality of sensors are distributed at equal first angular intervals around the axis of rotation of the platen, and the second plurality of sensors are distributed at equal second angular intervals around the axis of rotation of the platen. 
     
     
         17 . The apparatus of  claim 16 , wherein the second angular interval is smaller than the first angular interval. 
     
     
         18 . The apparatus of  claim 14 , wherein at least some sensors of the second plurality of sensors are angularly aligned with sensors of the first plurality of sensors.