Semiconductor device having protective layer on field oxide

Abstract

Disclosed is a manufacturing method capable of easily manufacturing a semiconductor device exhibiting a high reliability but no decrease in a field isolation voltage due to an influence by overetching. Field oxide is formed on a silicon substrate by a LOCOS method. Polysilicon is deposited on the surface of the field oxide and on the surface of a silicon nitride layer formed on the silicon substrate when forming the field oxide layer. The polysilicon layer is deposited thicker than a thickness of the silicon nitride layer. The polysilicon layer deposited on the silicon nitride layer and on the field oxide is removed by polishing like a CMP method, whereby the surface of the silicon nitride layer is exposed. A structure having the polysilicon layer existing on only the surface of the field oxide is obtained by removing the silicon nitride layer. The polysilicon layer functions as a protective layer for the field oxide, thereby preventing the field oxide layer 34 from being etched when in overetching.

What is claimed is:  
     
         1 . A method of manufacturing a semiconductor device, comprising: 
 a step of forming an oxidation proof layer including an aperture on a silicon substrate;    a step of forming a field oxide for a device isolation thermally oxidizing silicon at the aperture;    a step of depositing a protective layer thicker than a thickness of said oxidation proof layer on said oxidation proof layer and on said field oxide, said protective layer being composed of such a selective removable material as to establish a condition under which said oxidation proof layer is selectively removed;    a step of making said protective layer residual on only the surface of said field oxide by removing a part of said protective layer deposited in said depositing step till the surface of the said oxidation proof layer is exposed; and    a step of removing said oxidation proof layer.    
     
     
         2 . A method of manufacturing a semiconductor device according to    claim 1   , wherein said protective layer is composed of polysilicon.  
     
     
         3 . A method of manufacturing a semiconductor device according to    claim 1    or    2   , wherein said step of removing the part of said protective layer is a step of executing a polishing process based on CMP (Chemical Mechanical Polishing).  
     
     
         4 . A semiconductor device comprising: 
 a field oxide for a device isolation; and    a layer formed on the surface of said field oxide, said layer being composed of such a selective removable material as to establish a condition under which a silicon nitride layer is selectively removed.    
     
     
         5 . A semiconductor device according to    claim 4   , wherein said selective removable material is polysilicon.