Aberration correcting optical system

Abstract

A projection system for projecting a pattern of a mask onto a substrate. The projection system includes a projection optical system disposed between the mask and the substrate, and an optical element for correcting aberration produced in the projection optical system. The optical element has different refracting powers in two orthogonal directions or has a refracting power in one direction of two orthogonal directions and no refracting power in the other of the two orthogonal directions. The optical element is disposed between the mask and the substrate, in which an optical axis of the optical element is inclined with respect to an optical axis of the projection optical system.

1 - 20 . (canceled)  
   
   
       21 . A projection system for projecting a pattern of a mask onto a substrate, said projection system comprising: 
 a projection optical system disposed between the mask and the substrate; and    an optical element for correcting aberration produced in said projection optical system, said optical element having different refracting powers in two orthogonal directions or having a refracting power in one direction of two orthogonal directions and no refracting power in the other of the two orthogonal directions, and said optical element being disposed between the mask and the substrate, wherein an optical axis of said optical element is inclined with respect to an optical axis of said projection optical system.    
   
   
       22 . A projection system according to  claim 21 , wherein said projection system comprises a plurality of said optical elements, and 
 wherein one of said plurality of optical elements is used selectively to change the aberration.    
   
   
       23 . A projection system according to  claim 21 , wherein said optical element is mainly composed of a transparent material of one of quartz and fluorite.  
   
   
       24 . A projection system according to  claim 21 , wherein the or each surface of said optical element, having a refracting power, has a refractive power not greater than 3×10 −7  mm −1 .  
   
   
       25 . A projection exposure apparatus, comprising: 
 an illumination system for illuminating a mask; and    a projection system as recited in  claim 21  for projecting a pattern of the mask onto a substrate.    
   
   
       26 . A device manufacturing method, comprising: 
 a process for transferring a device pattern onto a substrate by use of a projection exposure apparatus as recited in  claim 25.