Novel copolymer, photoresist composition, and process for forming resist pattern with high aspect ratio

Abstract

A novel copolymer includes a repeating unit (B) derived from an unsaturated carboxylic anhydride, a repeating unit (C) represented by Formula (II), and a repeating unit (D) represented by Formula (III).

What is claimed is:  
     
         1 . A novel copolymer comprising: 
 a repeating unit (B) derived from an unsaturated carboxylic anhydride;    a repeating unit (C) represented by Formula (II); and    a repeating unit (D) represented by Formula (III):                          wherein R 1  is a hydrogen atom or a methyl group; and R 2  is an alkyl group having from 1 to 4 carbon atoms.    
     
     
         2 . A novel copolymer according to  claim 1 , wherein said repeating unit (B) is a repeating unit (B-1) derived from a unsaturated cyclic carboxylic anhydride.  
     
     
         3 . A novel copolymer according to  claim 2 , wherein said repeating unit (B-1) is a unit (B-2) represented by Formula (V):  
       
         
           
           
               
               
           
         
       
     
     
         4 . A novel copolymer according to  claim 2 , wherein said repeating unit (B-1) is a unit (B-3) represented by Formula (XVI):  
       
         
           
           
               
               
           
         
       
     
     
         5 . A novel copolymer according to  claim 1 , wherein said repeating unit (D) is a unit (D-1) represented by Formula (VI):  
       
         
           
           
               
               
           
         
       
     
     
         6 . A novel copolymer according to  claim 1 , wherein the content of repeating unit (B) is equal to or more than 15% and equal to or less than 60% of all repeating units constituting said novel copolymer.  
     
     
         7 . A novel copolymer according to  claim 1 , wherein the content of repeating unit (C) is equal to or more than 10% and equal to or less than 40% of all repeating units constituting said novel copolymer.  
     
     
         8 . A novel copolymer according to  claim 1 , wherein the content of repeating unit (D) is more than 0% and equal to or less than 40% of all repeating units constituting said novel copolymer.  
     
     
         9 . A novel copolymer according to any one of  claims 1  to  8 , wherein said novel copolymer has a weight average molecular weight (Mw) in terms of polystyrene of from 7000 to 30000 and a molecular-weight distribution (Mw/Mn, where Mn is a number average molecular weight) of equal to or less than 3.5.  
     
     
         10 . A photoresist composition comprising: 
 a novel copolymer according to  claim 1;     a photosensitive acid generator; and    an organic solvent.    
     
     
         11 . A photoresist composition according to  claim 10 , wherein said photosensitive acid generator is a triphenylsulfonium-based onium salt.  
     
     
         12 . A photoresist composition according to  claim 10 , wherein said organic solvent is propylene glycol monomethyl ether acetate (PGMEA).  
     
     
         13 . A process for forming a resist pattern with a high aspect ratio, said process comprising the steps of: 
 (a) applying a first resist on a substrate and drying the applied first resist to thereby form a first resist layer, applying a photoresist composition of  claim 10  onto the first resist layer and drying the applied photoresist composition to thereby form a second resist layer;    (b) exposing the second resist layer to imaging radiation, subjecting the exposed second resist layer to a heat treatment, and dissolving and removing exposed portions or unexposed portions of the second resist layer by developing in an alkaline aqueous solution to thereby form a resist pattern;    (c) applying a silylation agent onto the formed resist pattern, rinsing the applied resist pattern to thereby enlarge the resist pattern and to form a silylation coating on the resist pattern, said silylation coating being resistant to corrosion induced by oxygen-containing plasma etching; and    (d) etching the first resist layer under the second resist layer with oxygen-containing plasma by using, as a mask, the enlarged resist pattern carrying the silylation coating.