Semiconductor device and method for fabricating the same

Abstract

A method for fabricating a semiconductor device includes the steps of: forming a first insulating film on a semiconductor substrate; removing part of the first insulating film; forming a second insulating film having a leakage current density higher than that of the first insulating film on a region where the part of the first insulating film has been removed on the semiconductor substrate; forming an undoped semiconductor film on the first and second insulating films; implanting an impurity into part of the undoped semiconductor film, thereby defining semiconductor regions of a first conductivity type dotted as discrete islands; forming a third insulating film on the semiconductor regions of the first conductivity type and the undoped semiconductor film; and removing part of the third insulating film by wet etching. At least the second insulating film is formed under the semiconductor regions of the first conductivity type.

What is claimed is:  
     
         1 . A method for fabricating a semiconductor device, comprising the steps of: 
 forming a first insulating film on a semiconductor substrate;    removing part of the first insulating film;    forming a second insulating film having a leakage current density higher than that of the first insulating film on a region where the part of the first insulating film has been removed on the semiconductor substrate;    forming an undoped semiconductor film on the first and second insulating films;    implanting an impurity into part of the undoped semiconductor film, thereby defining semiconductor regions of a first conductivity type dotted as discrete islands;    forming a third insulating film on the semiconductor regions of the first conductivity type and the undoped semiconductor film; and    removing part of the third insulating film by wet etching, wherein at least the second insulating film is formed under the semiconductor regions of the first conductivity type.    
     
     
         2 . The method of  claim 1 , further comprising the step of implanting an impurity into the undoped semiconductor film to define a semiconductor region of a second conductivity type such that the semiconductor region of the second conductivity type is adjacent to the semiconductor regions of the first conductivity type, 
 wherein the step of forming the third insulating film is the step of forming the third insulating film on the semiconductor regions of the first conductivity type, the semiconductor region of the second conductivity type and the undoped semiconductor film, and    at least the second insulating film is formed under the semiconductor region of the second conductivity type.    
     
     
         3 . The method of  claim 1 , wherein the average density of leakage current from the semiconductor regions of the first conductivity type to the outside of the first and second insulating films has an absolute value of 1×10 −10  (A/mm 2 ) or more in at least one polarity when the potential difference between the semiconductor regions of the first conductivity type and the outside of the first and second insulating films has an absolute value of 1.5 V.  
     
     
         4 . The method of  claim 2 , wherein the average density of leakage current from the semiconductor region of the second conductivity type to the outside of the first and second insulating films has an absolute value of 1×10 −10  (A/mm 2 ) or more in at least one polarity when the potential difference between the semiconductor region of the second conductivity type and the outside of the first and second insulating films has an absolute value of 1.5 V.  
     
     
         5 . The method of  claim 1 , wherein the wet etching is performed with a chemical solution containing fluorine ions.  
     
     
         6 . A method for fabricating a semiconductor device, comprising the steps of: 
 forming a first insulating film on a semiconductor substrate;    forming an undoped semiconductor film on the first insulating film;    implanting an impurity into part of the undoped semiconductor film, thereby defining semiconductor regions of a first conductivity type dotted as discrete islands;    forming, at least on the semiconductor regions of the first conductivity type, a second insulating film having a leakage current density higher than that of the first insulating film; and    removing part of the second insulating film by wet etching.    
     
     
         7 . The method of  claim 6 , further comprising the step of implanting an impurity into the undoped semiconductor film to define a semiconductor region of a second conductivity type such that the semiconductor region of the second conductivity type is adjacent to the semiconductor regions of the first conductivity type, 
 wherein the step of forming the second insulating film is performed at least on the semiconductor regions of the first conductivity type and the semiconductor region of the second conductivity type.    
     
     
         8 . The method of  claim 6 , wherein the average density of leakage current from the semiconductor regions of the first conductivity type to the outside of the second insulating film has an absolute value of 1×10 −10  (A/mm 2 ) or more in at least one polarity when the potential difference between the semiconductor regions of the first conductivity type and the outside of the second insulating film has an absolute value of 1.5 V.  
     
     
         9 . The method of  claim 6 , wherein the wet etching is performed with a chemical solution containing fluorine ions.  
     
     
         10 . A method for fabricating a semiconductor device, comprising the steps of: 
 forming a first insulating film on a semiconductor substrate;    removing part of the first insulating film;    forming a second insulating film having a leakage current density higher than that of the first insulating film on a region where the part of the first insulating film has been removed on the semiconductor substrate;    forming an undoped semiconductor film on the first and second insulating films;    implanting at least one impurity into part of the undoped semiconductor film, thereby defining a semiconductor region of at least one conductivity type;    removing part of the semiconductor region of at least one conductivity type and the undoped semiconductor film, thereby forming a patterned semiconductor region of at least one conductivity type;    forming a third insulating film on the first and second insulating films such that the third insulating film covers the patterned semiconductor region of at least one conductivity type; and    removing part of the third insulating film by wet etching,    wherein at least the second insulating film is formed under the patterned semiconductor region of at least one conductivity type.    
     
     
         11 . The method of  claim 10 , wherein the step of defining the semiconductor region of at least one conductivity type includes the step of defining a semiconductor region of a first conductivity type, and then defining a semiconductor region of a second conductivity type such that the semiconductor region of the second conductivity type is adjacent to the semiconductor region of the first conductivity type.  
     
     
         12 . The method of  claim 10 , wherein the average density of leakage current from the semiconductor region of at least one conductivity type to the outside of the first and second insulating films has an absolute value of 1×10 −10  (A/mm 2 ) or more in at least one polarity when the potential difference between the patterned semiconductor region of at least one conductivity type and the outside of the fist and second insulating films has an absolute value of 1.5 V.  
     
     
         13 . The method of  claim 10 , wherein the wet etching is performed with a chemical solution containing fluorine ions.  
     
     
         14 . A method for fabricating a semiconductor device, comprising the steps of: 
 forming a first insulating film on a semiconductor substrate;    forming an undoped semiconductor film on the first insulating film;    implanting at least one impurity into the undoped semiconductor film, thereby defining a semiconductor region of at least one conductivity type;    removing part of the semiconductor region of at least one conductivity type and the undoped semiconductor film, thereby forming a patterned semiconductor region of at least one conductivity type;    forming a second insulating film having a leakage current density higher than that of the first insulating film on the first insulating film such that the second insulating film covers the patterned semiconductor region of at least one conductivity type; and    removing part of the second insulating film by wet etching.    
     
     
         15 . The method of  claim 14 , wherein the semiconductor region of at least one conductivity type includes a semiconductor region of a first conductivity type and a semiconductor region of a second conductivity type.  
     
     
         16 . The method of  claim 14 , wherein the average density of leakage current from the semiconductor region of at least one conductivity type to the outside of the second insulating film has an absolute value of 1×10 −10  (A/mm 2 ) or more in at least one polarity when the potential difference between the patterned semiconductor region of at least one conductivity type and the outside of the second insulating film has an absolute value of 1.5 V.  
     
     
         17 . The method of  claim 14 , wherein the wet etching is performed with a chemical solution containing fluorine ions.  
     
     
         18 . A semiconductor device, comprising: 
 a first insulating film formed on a semiconductor substrate;    a semiconductor film of a conductivity type formed on the first insulating film;    a second insulating film formed on the semiconductor film of the conductivity type such that both ends of the second insulating film are exposed on the semiconductor film of the conductivity type; and    a conductive film formed on each of the ends of the second insulating film on the semiconductor film of the conductivity type,    wherein the second insulating film has a leakage current density higher than that of the first insulating film.    
     
     
         19 . The semiconductor device of  claim 18 , wherein the second insulating film is a silicon oxide film, a silicon nitride film, an undoped silicon oxide film formed under sub atmospheric conditions, a TEOS film formed under a reduced pressure, or a thermal oxidation film.  
     
     
         20 . The semiconductor device of  claim 18 , wherein the second insulating film has a thickness smaller than that of the first insulating film.  
     
     
         21 . The semiconductor device of  claim 18 , wherein the second insulating film has a film density lower than that of the first insulating film.  
     
     
         22 . The semiconductor device of  claim 18 , wherein the conductive film is a metal silicide film or a metal film having a high melting point.  
     
     
         23 . The semiconductor device of  claim 18 , wherein the leakage current density has an absolute value of 1×10 −10  (A/mm 2 ) or more in at least one polarity when the potential difference between the semiconductor film of the conductivity type and the outside of the second insulating film has an absolute value of 1.5 V.  
     
     
         24 . A semiconductor device, comprising: 
 first and second insulating films formed to be in contact with each other on a semiconductor substrate;    a semiconductor film of a conductivity type formed on the first and second insulating films;    a third insulating film formed on the semiconductor film of the conductivity type such that both ends of the third insulating film are exposed on the semiconductor film of the conductivity type; and    a conductive film formed on each of the ends of the third insulating film on the semiconductor film of the conductivity type,    wherein the second insulating film has a leakage current density higher than that of the first insulating film.    
     
     
         25 . The semiconductor device of  claim 24 , wherein the second insulating film has a thickness smaller than that of the first insulating film.  
     
     
         26 . The semiconductor device of  claim 24 , wherein the conductive film is a metal silicide film or a metal film having a high melting point.  
     
     
         27 . The semiconductor device of  claim 24 , wherein the leakage current density has an absolute value of 1×10 −10  (A/mm 2 ) or more in at least one polarity when the potential difference between the semiconductor film of the conductivity type and the outside of the first and second insulating films has an absolute value of 1.5 V.