Color filter array alignment mark formation in backside illuminated image sensors

Abstract

A backside illuminated image sensor includes a sensor layer comprising photosensitive elements of the pixel array, an epitaxial layer formed on a frontside surface of the sensor layer, and a color filter array formed on a backside surface of the sensor layer. The epitaxial layer comprises polysilicon color filter array alignment marks formed in locations corresponding to respective color filter array alignment mark openings in the frontside surface of the sensor layer. The color filter array is aligned to the color filter array alignment marks of the epitaxial layer. The image sensor may be implemented in a digital camera or other type of digital imaging device.

1 . An image sensor having a pixel array configured for backside illumination, comprising:
 a sensor layer comprising a plurality of photosensitive elements of the pixel array;   an epitaxial layer formed on a frontside surface of the sensor layer, the epitaxial layer comprising polysilicon color filter array alignment marks formed in respective ones of the color filter array alignment mark openings in the sensor layer, wherein the epitaxial layer is formed on an unetched portion of the frontside surface of the sensor layer while the polysilicon color filter array alignment marks are being formed simultaneously; and   a color filter array formed on a backside surface of the sensor layer, wherein the color filter array is aligned to the color filter array alignment marks of the epitaxial layer.   
     
     
         2 . The image sensor of  claim 1 , further comprising a dielectric layer formed on a frontside surface of the epitaxial layer. 
     
     
         3 . The image sensor of  claim 1 , wherein the color filter array alignment mark openings terminate in the sensor layer. 
     
     
         4 . The image sensor of  claim 1  further comprising an oxide layer arranged between the color filter array and the backside surface of the sensor layer. 
     
     
         5 . The image sensor of  claim 4 , wherein the color filter array alignment mark openings extend through the sensor layer and terminate in the oxide layer. 
     
     
         6 . The image sensor of  claim 4 , wherein the color filter array alignment mark openings extend through the sensor layer and terminate at a frontside surface of the oxide layer. 
     
     
         7 . The image sensor of  claim 1 , wherein at least a given one of the polysilicon alignment marks has a cross-sectional shape that is substantially trapezoidal. 
     
     
         8 . The image sensor of  claim 1 , wherein the image sensor comprises a CMOS image sensor. 
     
     
         9 . A digital imaging device comprising:
 an image sensor having a pixel array configured for backside illumination; and   one or more processing elements configured to process outputs of the image sensor to generate a digital image, wherein the image sensor comprises:
 a sensor layer comprising a plurality of photosensitive elements of the pixel array; 
 an epitaxial layer formed on a frontside surface of the sensor layer, the epitaxial layer comprising polysilicon color filter array alignment marks formed in respective ones of the color filter array alignment mark openings in the sensor layer, wherein the epitaxial layer is formed on an unetched portion of the frontside surface of the sensor layer while the polysilicon color filter array alignment marks are being formed simultaneously; and 
 a color filter array formed on a backside surface of the sensor layer, wherein the color filter array is aligned to the color filter array alignment marks of the epitaxial layer. 
   
     
     
         10 . The digital imaging device of  claim 9 , wherein the imaging device comprises a digital camera.