Two epitaxial layers to reduce crosstalk in an image sensor

Abstract

An image sensor includes a substrate of a first conductivity type having an image area with a plurality of photosensitive sites, wherein a portion of the charge generated in response to light is collected in the pixel; and a subcollector of a second conductivity spanning the image area that collects another portion of the generated charge that would have otherwise diffused to adjacent photosensitive sites.

1 . An image sensor comprising:
 (a) a substrate of a first conductivity type having an image area with a plurality of photosensitive sites, wherein a portion of the charge generated in response to light is collected in the photosensitive sites; and   (b) a subcollector of a second conductivity spanning the image area that collects another portion of the generated charge that would have otherwise diffused to adjacent photosensitive sites.   
   
   
       2 . The image sensor as in  claim 1 , wherein the subcollector is at a deeper depth than the photosensitive sites. 
   
   
       3 . The image sensor as in  claim 1 , wherein the image sensor is an active pixel sensor. 
   
   
       4 . The image sensor as in  claim 1  further comprising a first epitaxial layer disposed on the substrate in which the subcollector is formed. 
   
   
       5 . The image sensor as in  claim 4  further comprising a second epitaxial layer in which the photosensitive sites are disposed. 
   
   
       6 . The image sensor as in  claim 4 , wherein the image sensor is an active pixel sensor. 
   
   
       7 . The image sensor as in  claim 1  further comprising one or more contacts that connect to the subcollector to reverse bias the subcollector with respect to the substrate. 
   
   
       8 . A method for making an image sensor, the method comprising the steps of:
 (a) providing a substrate having a first epitaxial layer;   (b) implanting a subcollector in the first epitaxial layer;   (c) implanting a subcollector contact region in the first epitaxial layer that connects to the subcollector; and   (d) growing a second epitaxial layer on the first epitaxial layer.   
   
   
       9 . The method as in  claim 8  further comprising the step of diffusing the subcollector and the subcollector contact region. 
   
   
       10 . The method as in  claim 8  further comprising the step of providing one or more implants in the second epitaxial layer for providing connections to the subcollector contact region. 
   
   
       11 . The method of  claim 8  further comprising the step of providing a plurality of pixels each containing an active element for creating an active pixel sensor.