Gradient barrier layer for copper back-end-of-line technology

Abstract

The present invention is directed to a structure of a gradient barrier layer. The gradient barrier with a composite structure of metal/metal salt of different composition/metal such as Ta/Ta x N 1−x /TaN/Ta x N 1−x /Ta (tantalum/tantalum x nitride 1−x /tantalum nitride/tantalum x nitride 1−x /tantalum) is proposed to replace the conventional barrier for copper metallization. The gradient barrier can be formed in a chemical vapor deposition (CVD) process or a multi-target physical vapor deposition (PVD) process. For CVD process, using the characteristics of well-controlled reaction gas injection, the ratio of tantalum (Ta) and nitrogen (N) can be modulated gradually to form the gradient barrier. For the multi-target PVD process, the gradient barrier is formed by depositing multi-layers of different composition Ta x N 1−x films. After subsequent thermal cycle processes such as metal alloy, the inter-layer diffusion occurs and a more smooth distribution of Ta and N is achieved for the gradient barrier. The advantages of forming the gradient barrier include a well-controlled process, a strong adhesion between via and landing metal, more uniform step coverage, and less brittle to reduce crack.

What is claimed is:  
     
         1 . A gradient barrier structure comprising: 
 a first metal layer;    a plurality of layers of a metal salt with different composition; and    a second metal layer.    
     
     
         2 . The structure according to  claim 1 , wherein said first metal layer is selected from the group comprising of tantalum, titanium, and tungsten layer.  
     
     
         3 . The structure according to  claim 1 , wherein said metal salt comprises elements of tantalum (Ta) and nitrogen (N).  
     
     
         4 . The structure according to  claim 3 , wherein said plurality of layers of said metal salt with different composition comprises a plurality of Ta x N 1−x  layers, wherein said x varies in the range between about 0.5 and 1.  
     
     
         5 . The structure according to  claim 3 , wherein said plurality of layers of said metal salt with different composition comprises a plurality of Ta x1 N 1−x1  layers, a TaN (tantalum nitride) layer, and a plurality of Ta x2 N 1−x2  layer, wherein said x1 is decreasing from about 1 to 0.5, and said x2 is increasing from about 0.5 to 1.  
     
     
         6 . The structure according to  claim 5 , wherein said first metal layer is a tantalum layer, and said tantalum layer and said Ta x1 N 1−x1  layers has a total thickness between 10 and 100 angstroms.  
     
     
         7 . The structure according to  claim 5 , wherein said tantalum nitride layer has a thickness between 100 and 200 angstroms.  
     
     
         8 . The structure according to  claim 5 , wherein said second metal layer is a tantalum layer, and said Ta x2 N 1−x2  layers and said tantalum layer has a total thickness between 100 and 200 angstroms.  
     
     
         9 . The structure according to  claim 1 , wherein said second metal layer is selected from the group comprising of tantalum, titanium, and tungsten layer.  
     
     
         10 . A method for forming a gradient barrier on a substrate, said method comprising: 
 forming a first metal layer on said substrate;    forming a plurality of layers of a metal salt with different composition on said first metal layer; and    forming a second metal layer on said plurality of layers of said metal salt with different composition.    
     
     
         11 . The method according to  claim 10 , wherein said step of forming said first metal layer comprises by introducing a first reaction gas to form said first metal layer in a chemical vapor deposition process.  
     
     
         12 . The method according to  claim 11 , wherein said step of forming said plurality of layers of said metal salt with different composition comprises introducing a second reaction gas of varying flow rates to react with said first reaction gas to form said plurality of layers of said metal salt with different composition.  
     
     
         13 . The method according to  claim 12 , wherein said step of forming said second metal layer comprising a step of stopping introducing said second reaction gas to form said second metal by use of said first reaction gas.  
     
     
         14 . The method according to  claim 10 , wherein said first metal layer is a tantalum layer.  
     
     
         15 . The method according to  claim 14 , wherein said step of forming said plurality of layers of said metal salt with different composition comprises forming a plurality of Ta x N 1−x  (tantalum nitride) layers, wherein said x varies in a range between about 0.5 to 1.  
     
     
         16 . The method according to  claim 15 , wherein said step of forming said plurality of layers of said metal salt with different composition comprises: 
 forming a Ta x1 N 1−x1  (tantalum nitride) layer, wherein x1 is less than 1 and greater than 0.5;    forming a TaN (tantalum nitride) layer; and    forming a Ta x2 N 1−x2  (tantalum nitride) layer, wherein x2 is greater than 0.5 and less than 1.    
     
     
         17 . The method according to  claim 10 , further comprising a step of performing a thermal cycle process.  
     
     
         18 . A method for forming a gradient barrier on a substrate, said method comprising: 
 forming a first tantalum layer on said substrate;    forming a plurality of Ta x1 N 1−x1  (tantalum nitride) layers, wherein said x1 gradually decreases from about 1 to 0.5;    forming a TaN (tantalum nitride) layer on said plurality of said Ta x1 N 1−x1  (tantalum nitride) layers;    forming a plurality of Ta x2 N 1−x2  (tantalum nitride) layers, wherein said x2 gradually increases from about 0.5 to 1 ;    forming a second tantalum layer on said plurality of said Ta x2 N 1−x2  (tantalum nitride) layers.