Method of fabricating MOS sensor

Abstract

A method of fabricating a MOS sensor is described. A P-doped region extending into a substrate is formed. A stacked polysilicon structure is formed over the P-doped region. Ions are implanted into the substrate to form an N-doped region extending shallowly into the P-doped region, the stacked polysilicon structure serving as an implantation buffer layer. The stacked polysilicon structure are patterned and etched to form a stacked polysilicon ring over the N-doped region. A metal line is formed for electrically connecting the stacked polysilicon ring with a gate of a MOS transistor.

What is claimed is:  
     
         1 . A method of fabricating a MOS sensor, comprising: 
 forming a P-doped region extending into a substrate;    forming a stacked polysilicon structure over the P-doped region;    implanting ions into the P-doped region to form an N-doped region extending shallowly into the substrate, the stacked polysilicon structure serving as an implantation buffer layer;    patterning and etching the stacked polysilicon structure to form a stacked polysilicon ring partially exposing the N-doped region over the P-doped region; and    forming a metal line for electrically connecting the stacked polysilicon ring with a gate of a MOS transistor.    
     
     
         2 . The method of  claim 1 , wherein the stacked polysilicon structure is formed by the steps of: 
 depositing a first polysilcon film over the P-doped region; and    depositing a second polysilicon film over the first polysilicon film.    
     
     
         3 . The method of  claim 2 , further comprising the steps of patterning and etching the first polysilicon film to form at least a window exposing the P-doped region before depositing the second polysilicon film.  
     
     
         4 . The method of  claim 3 , wherein the second polysilicon film is deposited to cover the first polysilicon film and to make contact with the P-doped region through the window.  
     
     
         5 . A method for electrically connecting a silicon photo-diode with a MOS gate, the method comprising: 
 forming a stacked polysilicon ring and a metal line between the silicon photodiode and the MOS gate.    
     
     
         6 . The method of  claim 5 , wherein the silicon photo-diode has a P-doped region and an N-doped region over the P-doped region.  
     
     
         7 . The method of  claim 6 , wherein the stacked polysilicon ring is formed over the N-doped region.  
     
     
         8 . The method of  claim 6 , wherein the N-doped region is formed by the steps of: 
 depositing a first polysilcon film over the P-doped region;    depositing a second polysilicon film over the first polysilicon film; and    implanting ions into the P-doped region to form the N-doped region extending shallowly into the P-doped region, the first and the second polysilicon films serving as an implantation buffer layer.    
     
     
         9 . The method of  claim 8 , further comprising the steps of patterning and etching the first polysilicon film to form at least a window exposing the P-doped region before depositing the second polysilicon film.  
     
     
         10 . The method of  claim 9 , wherein the second polysilicon film is deposited to cover the first polysilicon film and to make contact with the P-doped region through the window.  
     
     
         11 . The method of  claim 8 , wherein the stacked polysilicon ring is formed by the steps of patterning and etching the first and the second polysilicon films to create the stacked polysilicon ring.  
     
     
         12 . A MOS sensor, comprising: 
 a MOS transistor having a gate;    a metal line connected to the gate of the MOS transistor;    a stacked polysilicon connected to the metal line; and    a silicon photo-diode connected to the stacked polysilicon.    
     
     
         13 . The MOS sensor of  claim 12 , wherein the stacked polysilicon further comprises a first polysilicon film and a second polysilicon film over the first polysilicon film.  
     
     
         14 . The MOS sensor of  claim 13 , wherein the first polysilicon film has at least a window through which the second polysilicon film contacts with the N-doped region.