Front-Side Imager Having a Reduced Dark Current on a SOI Substrate

Abstract

A front-side image sensor may include a substrate in a semiconductor material and an active layer in the semiconductor material. The front side image sensor may also include an array of photodiodes formed in the active layer and an insulating layer between the substrate and the active layer.

What is claimed is: 
     
         1 . A front-side image sensor comprising:
 a substrate comprising a semiconductor material;   an active layer disposed over the substrate;   an array of photodiodes in the active layer; and   an insulating layer between the substrate and the active layer, the substrate being configured to be biased at a voltage lower than a voltage of the active layer during operation so that an interface between the active layer and the insulating layer responds to the voltage by an accumulation of a positive charge at the interface.   
     
     
         2 . The front-side image sensor of  claim 1 , wherein the insulating layer comprises a silicon oxide layer having a thickness in a range to reflect photons in a visible range. 
     
     
         3 . The front-side image sensor of  claim 1 , further comprising an intermediate layer between the insulating layer and the active layer, the intermediate layer having a same conductivity type as the active layer and having a higher doping level than the active layer. 
     
     
         4 . The front-side image sensor of  claim 1 , wherein the substrate is configured to be biased at a voltage lower than a voltage of the active layer during operation. 
     
     
         5 . The front-side image sensor of  claim 1 , wherein the substrate and the insulating layer define a silicon-on-insulator (SOI) substrate. 
     
     
         6 . The front-side image sensor of  claim 1 , further comprising:
 a passivation layer disposed over the active layer;   an array of colored filters disposed over the passivation layer; and   an array of collimating lenses disposed over the array of colored filters.   
     
     
         7 . A method of producing a front-side image sensor comprising:
 forming an active layer over a silicon-on-insulator (SOI) substrate;   forming an array of photodiodes in the active layer;   forming an array of color filters and collimating lenses over the active layer; and   forming a trench isolator extending through the active layer to an insulating layer of the SOI substrate.   
     
     
         8 . The method of  claim 7 , further comprising forming an intermediate layer over the insulating layer, the intermediate layer having a same conductivity type as the active layer and having a doping level higher than the active layer. 
     
     
         9 . The method of  claim 7 , wherein forming the array of photodiodes comprises:
 forming a buried layer having an opposite doping type than the active layer in the active layer.   
     
     
         10 . The method of  claim 9 , further comprising
 forming a transfer gate over the active layer, the transfer gate being coupled to the buried layer.   
     
     
         11 . The method of  claim 7 , wherein the SOI substrate comprises a silicon oxide layer having a thickness in a range to reflect photons in a visible range. 
     
     
         12 . The method of  claim 7 , further comprising:
 forming a passivation layer disposed between the active layer and the array of color filters.   
     
     
         13 . The method of  claim 12 , further comprising:
 forming metal tracks embedded in the passivation layer.   
     
     
         14 . The method of  claim 7 , further comprising lining the trench isolator with a higher doping level than the active layer. 
     
     
         15 . The method of  claim 14 , wherein the lining has the same doping type as the substrate. 
     
     
         16 . A method of operating a front-side image sensor, the method comprising:
 accumulating positive charge at an interface between an active layer and an insulating layer by applying a potential difference across the insulating layer disposed over a substrate comprising a semiconductor material, wherein the active layer is disposed over the insulating layer, wherein an array of photodiodes is disposed in the active layer, wherein the insulating layer is disposed between the substrate and the active layer.   
     
     
         17 . The method of  claim 16 , further comprising providing an intermediate layer over the insulating layer, the intermediate layer having a same conductivity type as the active layer and having a doping level higher than the active layer. 
     
     
         18 . The method of  claim 16 , wherein the array of photodiodes comprises:
 a buried layer having an opposite doping type than the active layer in the active layer.   
     
     
         19 . The method of  claim 18 , further comprising
 providing a transfer gate over the active layer, the transfer gate being coupled to the buried layer.   
     
     
         20 . The method of  claim 18 , further comprising:
 providing an array of color filters and collimating lenses over the active layer;   providing a passivation layer disposed between the active layer and the array of color filters; and   providing metal tracks embedded in the passivation layer.   
     
     
         21 . The method of  claim 18 , providing a trench isolator extending to the insulating layer. 
     
     
         22 . The method of  claim 21 , wherein the trench isolator is lined with a higher doping level than the active layer.