Apparatus and method of image enhancement through spatial filtering

Abstract

An image enhancement apparatus and method are disclosed. The apparatus consists of a spatial frequency filter where zero order mask diffraction information is reduced in an alternative pupil plane of the objective lens, specifically just beyond the mask plane. By introducing an angular specific transmission filter into this Fraunhofer diffraction field of the mask, user accessibility is introduced, allowing for a practical approach to frequency filtering. This frequency filtering is accomplished using a specifically designed interference filter coated over a transparent substrate. Alternatively, filtering can also be accomplished in a complimentary region near the wafer image plane or in both near-mask and near-wafer planes.

What is claimed is:  
     
         1 . A filter comprising a substrate and one or more thin films coated on the substrate, 
 wherein the substrate and the films are highly transparent to a selected wavelength of radiation and are relatively different in their respective indexes of refraction for angularly filtering incident radiation.    
     
     
         2 . The filter of  claim 1 , wherein the substrate and the films have extinction coefficients less than 0.1.  
     
     
         3 . The filter of  claim 2 , wherein the extinction coefficient is less than 0.1.  
     
     
         4 . The filter of  claim 1 , wherein the difference between the respective indexes of refraction is in the range of >0.2 at the wavelength of illumination.  
     
     
         5 . The filter of  claim 1 , wherein the difference between the respective indexes of refraction of the films is in the range >0.2 at the wavelength of illumination.  
     
     
         6 . The filter of  claim 5 , wherein the indexes differ by 0.84 or 0.34.  
     
     
         7 . The filter of  claim 3 , wherein the indexes differ by 0.84 or 0.34.  
     
     
         8 . The filter of  claim 1 , wherein near normal radiation is reduced proportionately more than oblique radiation.  
     
     
         9 . The filter of  claim 4 , wherein the difference in filtering between normal radiation and oblique radiation is in the range of 5% to 50%.  
     
     
         10 . The filter of  claim 9 , wherein the difference in filtering between normal radiation and oblique radiation is in the range of 16% to 30%.  
     
     
         11 . The filter of  claim 1 , wherein the substrate and the films comprise organic or inorganic materials.  
     
     
         12 . The filter of  claim 11 , wherein the inorganic materials are selected from the group consisting of the group shown in Table 1.  
     
     
         13 . The filter of  claim 11 , wherein the organic materials comprise polymers.  
     
     
         14 . The filter of  claim 13 , wherein the polymers are selected from the group consisting of a fluoropolymer or nitrocellulose polymers.