Overlay targets with isolated, critical-dimension features and apparatus to measure overlay

Abstract

An optical metrology system is disclosed which is configured to minimize the measurement of specularly reflected light and measure primarily scattered light. The system is similar to prior art beam profile measurements but includes an optical arrangement for increasing the amount of scattered light being monitored in comparison to the amount of specularly reflected light being monitored.

1 . An apparatus for evaluating the geometry of a structure formed on a sample, comprising: 
 a light source for generating a probe beam of radiation;    an objective for focusing the probe beam onto the structure and for collecting light reflected and scattered from the structure;    a detector having a two-dimensional array of photodetecting elements for monitoring the light collected by the objective and generating output signals in response thereto and wherein the numerical aperture of illumination is less than the numerical aperture of detection; and    a processor for evaluating the geometry of the structure based on the output signals and wherein the information that would be attributed to specular reflection is suppressed such that the amount of scattered light being monitored is increased in comparison to the amount of specularly reflected light being monitored.    
   
   
       2 . An apparatus as recited in  claim 1 , wherein the information that would be attributed to specular reflection is suppressed by ignoring signals in the region of the detector which map to the specularly reflected light.  
   
   
       3 . An apparatus as recited in  claim 1 , wherein the information that would be attributed to specular reflection is suppressed by blocking the light attributed to the specularly reflected light from reaching the detector with a blocking member.  
   
   
       4 . An apparatus as recited in  claim 1 , wherein said light source is a laser.  
   
   
       5 . An apparatus as recited in  claim 1 , wherein the numerical aperture of said objective is at least 0.5.  
   
   
       6 . An apparatus as recited in  claim 1 , wherein the numerical aperture of said objective is at least 0.9.  
   
   
       7 . An apparatus for evaluating the geometry of a structure formed on a sample, comprising: 
 a light source for generating a probe beam of radiation;    an objective for focusing the probe beam onto the structure and for collecting light reflected and scattered from the structure;    a detector having a two-dimensional array of photodetecting elements for monitoring the light collected by the objective and generating output signals in response thereto;    an aperture located in the illumination path and dimensioned such that the numerical aperture of illumination is less than the numerical aperture of detection;    a blocking member located in the detection path and dimensioned to suppress at least a portion of the light attributed to the specularly reflected light from reaching the detector; and    a processor for evaluating the geometry of the structure based on the output signals.    
   
   
       8 . An apparatus as recited in  claim 7 , wherein said light source is a laser.  
   
   
       9 . An apparatus as recited in  claim 7 , wherein the numerical aperture of said objective is at least 0.5.  
   
   
       10 . An apparatus as recited in  claim 7 , wherein the numerical aperture of said objective is at least 0.9.  
   
   
       11 . An apparatus for evaluating the geometry of a structure formed on a sample, comprising: 
 a light source for generating a probe beam of radiation;    an objective for focusing the probe beam onto the structure and for collecting light reflected and scattered from the structure;    a detector having a two-dimensional array of photodetecting elements for monitoring the light collected by the objective and generating output signals in response thereto;    an aperture located in the illumination path and dimensioned such that the numerical aperture of illumination is less than the numerical aperture of detection;    a processor for evaluating the geometry of the structure based on the output signals from a subset of said photodetecting elements, the subset being chosen to exclude at least some of the photodetecting elements which are aligned with light which has been specularly reflected from the sample so that the amount of scattered light being monitored is increased in comparison to the amount of specularly reflected light being monitored.    
   
   
       12 . An apparatus as recited in  claim 11 , wherein said light source is a laser.  
   
   
       13 . An apparatus as recited in  claim 11 , wherein the numerical aperture of said objective is at least 0.5.  
   
   
       14 . An apparatus as recited in  claim 11 , wherein the numerical aperture of said objective is at least 0.9.  
   
   
       15 . An apparatus for evaluating the geometry of a structure formed on a sample, comprising: 
 a light source for generating a probe beam of radiation;    an objective for focusing the probe beam onto the structure and for collecting light reflected and scattered from the structure;    a detector having a two-dimensional array of photodetecting elements for monitoring the light collected by the objective and generating output signals in response thereto;    an aperture located in the illumination path and dimensioned such that the numerical aperture of illumination is less than the numerical aperture of detection; and    a processor for evaluating the geometry of the structure based on the output signals and wherein the information that would be attributed to specular reflection is suppressed such that the amount of scattered light being monitored is increased in comparison to the amount of specularly reflected light being monitored.    
   
   
       16 . An apparatus as recited in  claim 15 , wherein said light source is a laser.  
   
   
       17 . An apparatus as recited in  claim 15 , wherein the numerical aperture of said objective is at least 0.5.  
   
   
       18 . An apparatus as recited in  claim 15 , wherein the numerical aperture of said objective is at least 0.9.