Optical element, lithographic apparatus including such optical element and device manufacturing method, and device manufactured thereby

Abstract

An optical element includes a top layer which is transmissive for EUV radiation with wavelength in the range of 5-20 nm, and a structure of the top layer is a structure having an rms roughness value equal to or larger than λ/10 for spatial periods equal to or smaller than λ/2. The structure promotes transmission through the top layer to the optical element.

What is claimed is: 
     
         1 . A lithographic apparatus, comprising:
 an illumination system configured to condition a beam of radiation;   a support configured to support a patterning device, the patterning device configured to impart the beam of radiation with a pattern in its cross section;   a substrate table configured to hold a substrate; and   a projection system configured to project the beam of radiation after it has been patterned onto a target portion of the substrate, wherein at least one of the illumination system and the projection system includes an optical element, the optical element comprising
 a layer at least partially transmissive for EUV radiation with a wavelength λ in the range of 5-20 nm, and 
 a top layer transmissive for EUV radiation with the wavelength λ in the range of 5-20 nm, the top layer including a structure having an rms roughness value equal to or larger than λ/10 for spatial periods equal to or smaller than λ/2. 
   
     
     
         2 . An apparatus according to  claim 1 , wherein the rms roughness is equal to or smaller than λ for spatial periods larger than λ/2 and equal to or smaller than 1 μm. 
     
     
         3 . An apparatus according to  claim 1 , wherein the at least partially tarnsmissive layer includes a surface with a structure with an rms roughness value equal to or larger than λ/50 and equal to or smaller than 4*λ for spatial periods equal to or smaller than λ/2. 
     
     
         4 . An apparatus according to  claim 1 , wherein the top layer includes a material selected from Be, B, C, Si, P, S, K, Ca, Sc, Br, Rb, Sr, Y, Zr, Nb, Mo, Ru, Rh, Ag, Ba, La, Ce, Pr, Ir, Au, Pa or U and combinations thereof. 
     
     
         5 . An apparatus according to  claim 1 , wherein the top layer and the structure include Ru. 
     
     
         6 . An apparatus according  claim 1 , wherein the optical element includes a mirror having a mirror surface, the mirror surface includes a protrusion transmissive for EUV radiation with the wavelength λ in the range of 5-20 nm, and at least part of the mirror surface further includes the top layer including the structure. 
     
     
         7 . An apparatus according to  claim 1 , wherein the optical element further comprises a profile having height differences, thereby providing cavities and elevations having a predetermined maximum height difference; and a substantially transmissive layer in the cavities and on the elevations of the optical element, wherein the substantially transmissive layer has a planar surface on which the top layer and structure are provided. 
     
     
         8 . An apparatus according to  claim 1 , wherein the rms roughness value of the top layer is 2 nm or larger for spatial periods equal to or smaller than λ/2. 
     
     
         9 . An apparatus according to  claim 1 , wherein the optical element includes optical filters, optical gratings, mirrors or lenses. 
     
     
         10 . An apparatus according to  claim 1 , wherein the optical element includes a mirror with a mirror surface and a tilted multilayer stack which is tilted with respect to the mirror surface. 
     
     
         11 . A device manufacturing method, comprising:
 providing a beam of radiation;   patterning the beam of radiation with a pattern in its cross-section; and   projecting the patterned beam of radiation onto a target portion of a substrate, wherein the beam of radiation is incident on an optical element including a top layer transmissive for EUV radiation with a wavelength λ in the range of 5-20 nm, the top layer including a structure having an rms roughness value equal to or larger than λ/10 for spatial periods equal to or smaller than λ/2.   
     
     
         12 . A device manufactured according to the method of  claim 11 .