Nonvolatile semiconductor memory device

Abstract

In a memory cell, a gate oxide film is formed on a surface of semiconductor substrate and a first floating gate is formed on the gate oxide film. An insulating film is formed on a first floating gate and a second floating gate is formed on the insulating film. The first and second floating gates constitute a floating gate in the memory cell. An insulating film between the first floating gate and the second floating gate acts as an etching stopper when a polysilicon constituting the second floating gate is etched.

What is claimed is:  
     
         1 . A nonvolatile semiconductor memory device comprising: 
 a memory cell having a floating gate formed over a semiconductor substrate with a gate insulating film formed therebetween and a control gate insulated from the floating gate and source/drain regions formed in the semiconductor substrate at those areas situated on both sides of the floating gate; and    an insulating film provided inside the floating gate.    
     
     
         2 . A device according to  claim 1 , wherein the insulating film is comprised of one of a silicon oxide film and silicon nitride film.  
     
     
         3 . A device according to  claim 2 , wherein the insulating film is in a range of above about 10 angstrom but below about 50 angstrom.  
     
     
         4 . A nonvolatile semiconductor memory device comprising: 
 a first conductive film formed over a semiconductor substrate with a gate insulating film formed therebetween;    a first insulating film formed on the first conductive film;    a second conductive film formed on the first insulating film, the second conductive film and first conductive film constituting a floating gate;    a second insulating film formed on the second conductive film;    a third conductive film formed on the second insulating film and constituting a control gate; and    source/drain regions formed in the semiconductor substrate at those areas situated on both sides of the floating gate.    
     
     
         5 . A device according to  claim 4 , wherein the first insulating film is comprised of one of a silicon oxide film and silicon nitride film.  
     
     
         6 . A device according to  claim 5 , wherein the first insulating film is in a range of above about 10 angstrom but below about 50 angstrom.  
     
     
         7 . A method for manufacturing a nonvolatile semiconductor memory device comprising the steps of: 
 forming a first polysilicon film over a semiconductor substrate with a gate insulating film formed therebetween;    forming a first insulating film on the first polysilicon film;    forming a second polysilicon film on the first insulating film;    forming a second insulating film on the second polysilicon film;    forming a third polysilicon film on the second insulating film;    forming a mask material on the third polysilicon film;    a first etching process for etching the third polysilicon film by using mask material as a mask, the first etching process forming a control gate;    a second etching process for etching the second polysilicon film to the first insulating film by using mask material as a mask, the second etching process forming a first floating gate; and    a third etching process for etching the first polysilicon film to the gate insulating film by using mask material as a mask, the third etching process forming a second floating gate.    
     
     
         8 . A method according to  claim 7 , wherein the first insulating film is formed by one of oxidizing the first polysilicon and depositing an insulating material.  
     
     
         9 . A method according to  claim 7 , wherein the second etching step has a greeter selection ratio to the first insulating film.  
     
     
         10 . A method according to  claim 9 , wherein the first insulating film is comprised of one of a silicon oxide film and silicon nitride film.  
     
     
         11 . A method according to  claim 10 , wherein the first insulating film is in a range of above about 10 angstrom but below about 50 angstrom.  
     
     
         12 . A method for manufacturing a nonvolatile semiconductor memory device, comprising the steps of: 
 sequentially forming a gate insulating film, first polysilicon film and first insulating film over a semiconductor substrate;    forming a trench in the semiconductor substrate through a gate insulating film, first polysilicon film and first insulating film;    depositing a second insulating film over a whole surface to bury the trench with the second insulating film;    removing the second insulating film by etching to allow the second insulating film to remain in the trench at a height level lower than the surface of the first insulating film;    removing the first insulating film;    forming a third insulating film on the first polysilicon film;    forming a second polysilicon film on the third insulating film;    forming a fourth insulating film on the second polysilicon film;    forming a third polysilicon film on the fourth insulating film;    forming a mask material on the third polysilicon film;    a first etching process for etching the third polysilicon film by using mask material as a mask, the first etching process forming a control gate;    a second etching process for etching the second polysilicon film to the third insulating film by using mask material as a mask, the second etching process forming a first floating gate; and    a third etching process for etching the first polysilicon film to the gate insulating film by using mask material as a mask, the third etching process forming a second floating gate.    
     
     
         13 . A method according to  claim 12 , wherein the third insulating film is formed by one of oxidizing the first polysilicon film and depositing an insulating material.  
     
     
         14 . A method according to  claim 12 , wherein the second etching step has a greater selection ratio to the third insulating film.  
     
     
         15 . A method according to  claim 14 , wherein the third insulating film is comprised of one of a silicon oxide film and silicon nitride film.  
     
     
         16 . A method according to  claim 15 , wherein the third insulating film is in a range of above about 10 angstrom but below about 50 angstrom.  
     
     
         17 . A nonvolatile semiconductor memory device comprising: 
 a plurality of first conductive films formed over a semiconductor substrate with a gate insulating film formed therebetween;    a trench formed between those adjacent first insulating films and in the semiconductor substrate;    a first insulating film for element isolation which is provided on the trench;    a second insulating film formed on the first conductive film;    a second conductive film formed on the second insulating film and, together with the first conductive film, constituting a floating gate;    a third insulating film formed on the second conductive film;    a third conductive film formed on the third insulating film and constituting a control gate; and    source/drain regions formed in the semiconductor substrate at those areas situated on both sides of the floating gate.    
     
     
         18 . A device according to  claim 17 , wherein the second insulating film is comprised of one of a silicon oxide film and silicon nitride film.  
     
     
         19 . A device according to  claim 18 , wherein the second insulating film is in a range of above about 10 angstrom but below about 50 angstrom.