Semiconductor device and method of fabrication thereof

Abstract

A method includes the steps of: introducing insulation film into a trench to provide a trench isolation; planarizing the trench isolation to expose a passivation film; and removing the passivation film and depositing a second silicon layer on a first silicon layer and the trench isolation; and in the step of depositing the first silicon layer the first silicon layer is an undoped silicon layer and in the step of depositing the second silicon layer the second silicon layer is a doped silicon layer or an undoped silicon layer subsequently having an impurity introduced thereinto or the like and thermally diffused through subsequent thermal hysteresis into the first silicon layer.

1 . A method of fabricating a semiconductor device, comprising the steps of:. 
 forming a gate insulation film on a semiconductor substrate;    forming a first silicon layer on said gate insulation film;    forming a passivation film over said first silicon layer;    employing a single mask and thus etching said passivation film, said first silicon layer, said gate insulation film and said semiconductor substrate to provide a trench;    filling said trench with insulation film to provide an isolating insulation film;    planarizing said isolating insulation film to expose said passivation film;    removing said passivation film and forming a second silicon layer on said first silicon layer and said isolating insulation film; and    patterning said second silicon layer, wherein in the step of forming said first silicon layer an undoped silicon layer is formed as said first silicon layer and in the step of forming said second silicon layer a doped silicon layer or an undoped silicon layer is formed as said second silicon layer and subsequently an impurity is introduced into said undoped silicon layer and said impurity included in said second silicon layer is thermally diffused through a subsequent thermal process into said first silicon layer.    
   
   
       2 . The method according to  claim 1 , wherein said impurity introduced into said second silicon layer is thermally diffused through said subsequent thermal process into said first silicon layer to allow said first silicon layer to have a maximum impurity concentration higher than said second silicon layer.  
   
   
       3 . The method according to  claim 1 , wherein as said first silicon layer's undoped silicon layer, an undoped amorphous silicon film having a thickness of 15 to 50 nm is formed.  
   
   
       4 . The method according to  claim 1 , wherein as said first silicon layer's undoped silicon layer, an undoped polysilicon film having a thickness of 30 to 70 nm is formed.  
   
   
       5 . The method according to  claim 1 , wherein said second silicon layer has introduced therein said impurity of one of phosphorus and boron of a concentration of 0.5×10 20  to 5×10 20  atoms/cm 3 .  
   
   
       6 . A semiconductor device comprising: 
 a semiconductor substrate;    isolating insulation films each filled a trench in said semiconductor substrate and having a portion protruding above said semiconductor substrate;    a gate insulation film formed between said isolating insulation films on and in contact with said semiconductor substrate;    a first polysilicon layer formed on and in contact with said gate insulation film; and    a second polysilicon layer formed on said first polysilicon layer, wherein said first polysilicon layer is smaller in crystal grain size than said second polysilicon layer.    
   
   
       7 . The semiconductor device according to  claim 6 , wherein said first polysilicon layer has a crystal grain size of at most 70 nm.  
   
   
       8 . The semiconductor device according to  claim 6 , wherein said first polysilicon layer has a thickness of 15 to 70 nm.  
   
   
       9 . The semiconductor device according to  claim 6 , wherein said first and second polysilicon layers contain an impurity allowing said first and second polysilicon layers to be conductor and said impurity's concentration has a maximum value located in said first polysilicon layer.  
   
   
       10 . The semiconductor device according to  claim 9 , wherein said impurity's concentration has a distribution extending from said second to first polysilicon layers continuously and having a relative maximum in said first polysilicon layer.  
   
   
       11 . The semiconductor device according to  claim 9 , wherein said first polysilicon layer contains one of phosphorus and boron as said impurity, and a concentration of said impurity has a maximum value of at least 1×10 20  to 1×10 21  atoms/cm 3 .