Lithographic apparatus and a method for determining a polarization property

Abstract

A method and apparatus are provided for measuring the apodization of projection optics for use in a lithographic apparatus, the projection optics having an object plane where, in use, a reticle is placed, a pupil plane, and an image plane where, in use, a wafer is placed. The method includes placing one or more Appropriate apertures in said object plane for creating a substantially uniform light distribution, illuminating the or each aperture and measuring the intensity distribution at a plane which is conjugate to the pupil plane in order to calculate the apodization of the projection optics.

1 . A method of measuring apodization of projection optics for use in a lithographic apparatus, the projection optics having an object plane where, in use, a reticle is placed, a pupil plane, and an image plane where, in use, a wafer is placed, the method comprising: 
 placing one or more apertures in said object plane for creating a substantially uniform light distribution;    illuminating the aperture; and    measuring an intensity distribution at a plane which is conjugate to the pupil plane in order to calculate the apodization of the projection optics.    
   
   
       2 . A method as claimed in  claim 1 , wherein said plane which is conjugate to the pupil plane corresponds to a plane which is out-of-focus with respect to the image plane such that a far field imaging condition applies with respect to said pupil plane.  
   
   
       3 . A method as claimed in  claim 1 , wherein said apertures comprise an array of apertures.  
   
   
       4 . A method as claimed in  claim 3 , wherein the apertures of said array are arranged in a quasi-random or random manner.  
   
   
       5 . A method as claimed in  claim 1 , wherein the or each aperture has a diameter smaller than the wavelength of radiation used.  
   
   
       6 . A method as claimed in  claim 1 , wherein the or each aperture has a diameter approximately equal to the wavelength of the radiation used.  
   
   
       7 . A method as claimed in  claim 1 , wherein the or each aperture has a diameter greater than the wavelength of the radiation used.  
   
   
       8 . A method as claimed in  claim 7 , which further comprises: 
 performing a calibration to establish an intensity profile of light emitted from the aperture or apertures.    
   
   
       9 . A method as claimed in  claim 8 , wherein said calibration is performed off-line.  
   
   
       10 . A method as claimed in  claim 1 , which further comprises: 
 placing a diffuser over the or each aperture to diffuse the light reaching the or each aperture.    
   
   
       11 . Apparatus for measuring apodization of projection optics for use in a lithographic apparatus, the projection optics having an object plane where, in use, a reticle is placed, a pupil plane, and an image plane where, in use, a wafer is placed, the apparatus comprising: 
 one or more apertures located in said object plane for creating a substantially uniform light distribution;    an illumination source for illuminating the or each aperture; and    a sensor for measuring an intensity distribution at a plane which is conjugate to the pupil plane in order to calculate the apodization of the projection optics.    
   
   
       12 . A method as claimed in  claim 11 , wherein said plane which is conjugate to the pupil plane corresponds to a plane which is out-of-focus with respect to the image plane such that a far field imaging condition applies with respect to said pupil plane.  
   
   
       13 . Apparatus as claimed in  claim 11 , wherein said apertures comprise an array of apertures.  
   
   
       14 . Apparatus as claimed in  claim 13 , wherein the apertures of said array are arranged in a quasi-random or random manner.  
   
   
       15 . Apparatus as claimed in  claim 11 , wherein the or each aperture has a diameter smaller than the wavelength of radiation used.  
   
   
       16 . Apparatus as claimed in  claim 11 , wherein the or each aperture has a diameter approximately equal to the wavelength of the radiation used.  
   
   
       17 . Apparatus as claimed in  claim 11 , wherein the or each aperture has a diameter greater than the wavelength of the radiation used.  
   
   
       18 . Apparatus as claimed in  claim 17 , wherein the intensity profile of light emitted from the aperture or apertures is calibrated.  
   
   
       19 . Apparatus as claimed in  claim 11 , which further comprises a diffuser over the or each aperture to diffuse the light reaching the or each aperture.  
   
   
       20 . A lithographic apparatus comprising: 
 an illumination system for conditioning a beam of radiation;    a support structure for supporting a patterning device, the patterning device serving to impart the projection beam with a pattern in its cross-section;    a substrate table for holding a substrate;    projection optics for projecting the patterned beam onto a target portion of the substrate and having an object plane, where, in use, the patterning device is placed, a pupil plane, and an image plane where, in use, the substrate is placed;    one or more apertures located in said object plane for creating a substantially uniform light distribution; and    a sensor for measuring the intensity distribution at a plane which is conjugate to the pupil plane in order to calculate an apodization of the projection optics.    
   
   
       21 . An apparatus as claimed in  claim 20 , wherein said plane which is conjugate to the pupil plane corresponds to a plane which is out-of-focus with respect to the image plane such that a far field imaging condition applies with respect to said pupil plane.