Graded anti-reflective coatings for photolithography

Abstract

A graded anti-reflective coating (ARC) with one or more layers has a bottom layer that is highly absorbing at the lithographic wavelength, and one or more layers between the substrate and the resist layer having inhomogeneous optical constants. The refractive indices are matched across layer interfaces, and the optical constants vary smoothly through the layer thicknesses. In each layer the extinction coefficient and the refractive index have independently selectable values and gradients. This ARC structure provides almost total absorption in the bottom layer and near-zero reflection at the resist interface and all other intermediate interfaces. Layers are preferably of inorganic materials, typically SiO x N y . Because of its highly absorbing bottom layer, an ARC according to an embodiment of the present invention works effectively over diverse substrate materials for a variety of lithographic wavelengths. It provides great latitude of manufacturing tolerances for thicknesses and optical constants.

What is claimed is:  
     
         1 . A graded anti-reflective coating for photolithography at a preselected exposure wavelength, comprising: 
 a first layer overlying a substrate, said first layer having a preselected first thickness, a first top surface, and a first bottom surface interfacing with said substrate;    a second layer overlying said first layer, said second layer having a preselected second thickness, a second bottom surface interfacing with said first top surface, and a second top surface interfacing with a photoresist layer;    said first layer having first optical constants, including a preselected first refractive index and a first extinction coefficient preselected to absorb substantially all radiation of the preselected exposure wavelength entering through said first top surface and propagating through said preselected thickness of said first layer; and    said second layer having second optical constants, including a second refractive index and a second extinction coefficient, each of said second optical constants being preselected to vary smoothly and inhomogeneously through said second thickness of said second layer, such that said second optical constants adjacent said second bottom surface are substantially equal to the corresponding said first optical constants adjacent said first top surface, and such that said second optical constants adjacent said second top surface are substantially equal to the corresponding optical constants of said photoresist layer, thereby providing an optical property gradient through said second thickness of said second layer;    the value and gradient of said second refractive index being selectable and variable independently from the value and gradient of said second extinction coefficient.    
     
     
         2 . The anti-reflective coating of  claim 1 , wherein said first optical constants are substantially homogeneously distributed through said first thickness of said first layer.  
     
     
         3 . The anti-reflective coating of  claim 1 , wherein an optical property gradient is provided through said first thickness of said first layer, the value and gradient of said first refractive index being selectable and variable independently from the value and gradient of said first extinction coefficient.  
     
     
         4 . The anti-reflective coating of  claim 1 , wherein said first layer comprises an inorganic material.  
     
     
         5 . The anti-reflective coating of  claim 4 , wherein said inorganic material is selected from the group consisting of SiO x N y , SiN x , SiO x N y C z , SiO x N y H z , MoSi x O y , and MoSi x O y N z .  
     
     
         6 . The anti-reflective coating of  claim 5 , wherein said first extinction coefficient is greater than 1.5 at an exposure wavelength of 248 nm.  
     
     
         7 . The anti-reflective coating of  claim 5 , wherein said preselected first thickness is in a range from approximately 15 nm to approximately 65 nm.  
     
     
         8 . The anti-reflective coating of  claim 1 , wherein said second layer comprises an inorganic material.  
     
     
         9 . The anti-reflective coating of  claim 8 , wherein said inorganic material is selected from the group consisting of SiO x N y , SiN x , SiO x N y C z , SiO x N y H z , MoSi x O y , and MoSi x O y N z .  
     
     
         10 . The anti-reflective coating of  claim 9 , wherein at an exposure wavelength of 248 nm said second refractive index adjacent said second top surface is greater than approximately 1.5 and said second refractive index adjacent said second bottom surface is greater than approximately 2.0.  
     
     
         11 . The anti-reflective coating of  claim 9 , wherein said preselected second thickness is in a range from approximately 10 nm to approximately 50 nm.  
     
     
         12 . The anti-reflective coating of  claim 1 , wherein said substrate comprises a mosaic pattern of materials selected from the group consisting of aluminum, silicon, polysilicon, silicon oxide, tungsten silicide, and copper.  
     
     
         13 . The anti-reflective coating of  claim 1 , wherein said preselected exposure wavelength is a deep ultraviolet wavelength.  
     
     
         14 . The anti-reflective coating of  claim 13 , wherein said deep ultraviolet wavelength is selected from the group consisting of 157 nm, 193 nm, 248 nm, and 365 nm.  
     
     
         15 . A method of suppressing reflection at a preselected exposure wavelength for photolithography using a graded anti-reflective coating, comprising: 
 propagating radiation of said preselected exposure wavelength through a photoresist layer into a first coating layer underlying said photoresist layer;    further propagating said radiation through said first coating layer into a second coating layer underlying said first coating layer, said second coating layer overlying a substrate; and 
 said radiation being absorbed substantially completely in said second coating layer;  
   said first coating layer having a first refractive index varying smoothly and inhomogeneously through the thickness of said first coating layer, such that said first refractive index adjacent said photoresist layer is substantially equal to the refractive index of said photoresist layer and such that said first refractive index adjacent said second coating layer is substantially equal to the refractive index of said second coating layer adjacent said first coating layer.    
     
     
         16 . The method of  claim 15 , wherein said preselected exposure wavelength is a deep ultraviolet wavelength.  
     
     
         17 . The method of  claim 16 , wherein said deep ultraviolet wavelength is approximately equal to a wavelength selected from the group consisting of 157 nm, 193 nm, 248 nm, and 365 nm.  
     
     
         18 . The method of  claim 15 , wherein said substrate comprises a mosaic pattern of materials selected from the group consisting of aluminum, silicon, polysilicon, silicon oxide, tungsten silicide, and copper.  
     
     
         19 . The method of  claim 15 , wherein the thickness of said first coating layer is less than approximately 50 nm.  
     
     
         20 . The method of  claim 15 , wherein said radiation at an exposure wavelength of 248 nm is transmitted with less than approximately 1.0 percent reflection at an interface between said photoresist layer and said first coating layer and with less than approximately 1.0 percent reflection at an interface between said first coating layer and said second coating layer.