Semiconductor device and method for fabricating the same

Abstract

The semiconductor device comprises a silicon substrate 14 having a step formed in the surface which makes the surface in a flash memory cell region 10 lower than the surface in a peripheral circuit region 12 ; a device isolation region 20 a formed in a trench 18 in the flash memory cell region 10 ; a device isolation region 20 c formed in a trench 24 deeper than the trench 18 in the peripheral circuit region 12 ; a flash memory cell 46 including a floating gate 32 and a control gate 40 formed on the device region defined by the device isolation region 20 a ; and transistors 62, 66 formed on the device regions defined by the device isolation region 20 c.

1 . A semiconductor device comprising: 
 a semiconductor substrate having a step in a surface which makes a surface of a first region lower than a surface of a second region;    a first device isolation region formed in a first trench formed in the semiconductor substrate in the first region;    a second device isolation region formed in a second trench formed in the semiconductor substrate in the second region, the second trench being deeper than the first trench;    a memory cell including a floating gate formed on a first device region defined by the first device isolation region, with a first insulation film formed therebetween, and a control gate formed on the floating gate with a second insulation film formed therebetween; and    a transistor formed on a second device region defined by the second device isolation region.    
     
     
         2 . A semiconductor device according to  claim 1 , wherein 
 a surface of the semiconductor substrate in the first region is removed.    
     
     
         3 . A semiconductor device according to  claim 1 , further comprising 
 a semiconductor layer formed on the semiconductor substrate in the second region which forms the step in the surface of the semiconductor substrate.    
     
     
         4 . A semiconductor device according to  claim 1 , wherein 
 a height of an upper surface of the floating gate and a height of an upper surface of the semiconductor substrate in the second region are substantially the same.    
     
     
         5 . A semiconductor device according to  claim 2 , wherein 
 a height of an upper surface of the floating gate and a height of an upper surface of the semiconductor substrate in the second region are substantially the same.    
     
     
         6 . A semiconductor device according to  claim 3 , wherein 
 a height of an upper surface of the floating gate and a height of an upper surface of the semiconductor substrate in the second region are substantially the same.    
     
     
         7 . A semiconductor device according to  claim 1 , wherein 
 the floating gate includes a main portion formed on the semiconductor substrate with the first insulation film formed therebetween, and a sidewall portion formed on a side wall of the main portion.    
     
     
         8 . A semiconductor device according to  claim 2 , wherein 
 the floating gate includes a main portion formed on the semiconductor substrate with the first insulation film formed therebetween, and a sidewall portion formed on a side wall of the main portion.    
     
     
         9 . A semiconductor device according to  claim 3 , wherein 
 the floating gate includes a main portion formed on the semiconductor substrate with the first insulation film formed therebetween, and a sidewall portion formed on a side wall of the main portion.    
     
     
         10 . A method for fabricating a semiconductor device comprising the steps of: 
 forming a first conduction film to be a floating gate on a semiconductor substrate in a first region with a first insulation film formed therebetween, the first insulation film having etching characteristics different from those of the semiconductor substrate;    forming a mask having a first opening formed in the first region and a second opening formed in a second region on the first conduction film and the semiconductor substrate in the second region;    etching the first conduction film, the first insulation film and the semiconductor substrate exposed in the first opening while etching the semiconductor substrate exposed in the second opening to form a first trench in the semiconductor substrate in the first region while forming a second trench deeper than the first trench in the semiconductor substrate in the second region, the first conduction film being patterned to be the floating gate;    forming a first device isolation region in the first trench defining a first device region while forming a second device isolation region in the second trench defining a second device region; and    forming a control gate on the floating gate with a second insulation film formed therebetween while forming a gate electrode of a transistor on the second device region.    
     
     
         11 . A method for fabricating a semiconductor device according to  claim 10 , further comprising before the step of forming the first conduction film, the step of 
 forming a step in a surface of the semiconductor substrate so that a surface in the first region is lower than a surface of the second region.    
     
     
         12 . A method for fabricating a semiconductor device according to  claim 11 , wherein 
 in the step of forming the step in the surface of the semiconductor substrate, the surface of the semiconductor substrate in the first region is selectively etched to form the step in the surface of the semiconductor substrate.    
     
     
         13 . A method for fabricating a semiconductor device according to  claim 11 , wherein 
 in the step of forming the step in the surface of the semiconductor substrate, the surface of the semiconductor substrate in the first region is selectively oxidized to form an oxide film, and the oxide film is removed to form the step in the surface of the semiconductor substrate.    
     
     
         14 . A method for fabricating a semiconductor device according to  claim 11 , wherein 
 in the step of forming the step in the surface of the semiconductor substrate, a semiconductor layer is selectively grown on the surface of the semiconductor substrate in the second region to form the step in the surface of the semiconductor substrate.    
     
     
         15 . A method for fabricating a semiconductor device according to  claim 11 , wherein 
 in the step of forming the first conduction film, the first conduction film is formed so that a height of an upper surface of the semiconductor substrate in the second region is substantially the same as a height of an upper surface of the first conduction film in the first region.    
     
     
         16 . A method for fabricating a semiconductor device according to  claim 12 , wherein 
 in the step of forming the first conduction film, the first conduction film is formed so that a height of an upper surface of the semiconductor substrate in the second region is substantially the same as a height of an upper surface of the first conduction film in the first region.    
     
     
         17 . A method for fabricating a semiconductor device according to  claim 13 , wherein 
 in the step of forming the first conduction film, the first conduction film is formed so that a height of an upper surface of the semiconductor substrate in the second region is substantially the same as a height of an upper surface of the first conduction film in the first region.    
     
     
         18 . A method for fabricating a semiconductor device according to  claim 10 , further comprising after the step of forming the first device isolation region and the second device isolation region and before the step of forming the control gate, the step of 
 forming a sidewall portion of a second conduction film on a side wall of the floating gate.    
     
     
         19 . A method for fabricating a semiconductor device according to  claim 11 , further comprising after the step of forming the first device isolation region and the second device isolation region and before the step of forming the control gate, the step of 
 forming a sidewall portion of a second conduction film on a side wall of the floating gate.    
     
     
         20 . A method for fabricating a semiconductor device according to  claim 18 , wherein 
 in the step of forming the sidewall portion, the second conduction film is buried between adjacent ones of a plurality of the floating gates and is etched to be left selectively on the side wall of the floating gates.