Determination of wafer angular position for in-sequence metrology

Abstract

A polishing apparatus includes a carrier head configured to hold a wafer in a first plane, the wafer having a perimeter and a fiducial, a drive shaft having an axis perpendicular to the first plane and configured to rotate the carrier head about the axis, a light source configured to direct light onto an outer face of the wafer at a position adjacent the perimeter of the wafer; a detector configured to detect the light collected from the wafer while the drive shaft rotates the carrier head and the wafer; and a controller configured to receive a first signal indicating an angular position of the drive shaft and receive a second signal from the detector, the controller configured to determine based on the first signal and the second signal an angular position of the fiducial with respect the carrier head.

What is claimed is: 
     
         1 . A polishing apparatus, comprising
 a carrier head configured to hold a wafer in a first plane with an inner face of the wafer abutting a support structure of the carrier head, the wafer having a perimeter and a fiducial defined by removal of a portion of the wafer or by having an optically distinct marking at a specific angular position;   a drive shaft having an axis perpendicular to the first plane, the drive shaft being connected to the carrier head and configured to rotate the carrier head about the axis;   a light source configured to direct light onto an outer face of the wafer at a position adjacent the perimeter of the wafer;   a detector configured to detect the light collected from the wafer while the drive shaft rotates the carrier head and the wafer; and   a controller configured to receive a first signal indicating an angular position of the drive shaft and receive a second signal from the detector, the controller configured to determine based on the first signal and the second signal an angular position of the fiducial with respect the carrier head.   
     
     
         2 . The polishing apparatus of  claim 1 , wherein the detector is a spectrometer and the second signal comprises a spectrum. 
     
     
         3 . The polishing apparatus of  claim 2 , wherein the controller computes an average intensity from the spectrum and matches the average intensity to the angular position of the wafer. 
     
     
         4 . The polishing apparatus of  claim 1 , wherein the controller computes using an algorithm the angular position of the head at which the fiducial is directly over the detector. 
     
     
         5 . The polishing apparatus of  claim 4 , wherein the algorithm is a valley-finding algorithm. 
     
     
         6 . The polishing apparatus of  claim 1 , wherein the drive shaft is configured to rotate at a first angular rate and the detected light is configured to be collected at a second rate. 
     
     
         7 . The apparatus of  claim 6 , wherein the first angular rate is matched to the second rate so that continuous detection of light is made along positions adjacent the perimeter of the wafer. 
     
     
         8 . The apparatus of  claim 1 , wherein the controller is configured to match spectral data to specific spatial positions on the wafer after the position of the fiducial is determined. 
     
     
         9 . The apparatus of  claim 1 , wherein the fiducial is a notch. 
     
     
         10 . The apparatus of  claim 1 , wherein the fiducial is a flat. 
     
     
         11 . A method, comprising:
 holding a wafer in a first plane using a carrier head, an inner face of the wafer abutting a support structure of the carrier head,   rotating the carrier head along an axis perpendicular to the first plane using a drive shaft,   directing light from a light source onto an outer face of the wafer at a position adjacent a perimeter of the wafer,   detecting light collected from the wafer while the drive shaft rotates the carrier head and the wafer;   receiving, at a controller, a first signal indicating an angular position of the drive shaft,   receiving, at the controller, a second signal from the detector, and   determining a position of the fiducial with respect to the angular position of the carrier head, based on the first signal and the second signal.   
     
     
         12 . The method of  claim 11 , further comprising using the controller to compute an average intensity from a spectrum, wherein the detector is a spectrometer and the second signal comprises the spectrum. 
     
     
         13 . The method of  claim 12 , further comprising, matching the average intensity to the angular position of the wafer. 
     
     
         14 . The method of  claim 11 , further comprising computing the angular position of the carrier head, using an algorithm, at which the fiducial is directly over the detector. 
     
     
         15 . The method of  claim 11 , wherein the drive shaft rotates at a first angular rate and the detected light is collected at a second rate, the method further comprising making continuous detection of light along positions adjacent the perimeter of the wafer by matching the first angular rate to the second rate. 
     
     
         16 . The method of  claim 11 , further comprising matching spectral data to specific spatial positions on the wafer after the position of the fiducial is determined. 
     
     
         17 . The method of  claim 16 , wherein the spectral data is collected before the position of the fiducial is determined. 
     
     
         18 . The method of  claim 16 , wherein the spectral data is collected after the position of the fiducial is determined. 
     
     
         19 . The method of  claim 16 , further comprising generating a third signal by matching the spectral data to the specific spatial positions, and feeding the third signal forward to a downstream polishing station configured to polish the wafer. 
     
     
         20 . The method of  claim 16 , further comprising generating a third signal by matching the spectral data to the specific spatial positions, and feeding the third signal backward to an upstream polishing station configured to polish a subsequent wafer.