New Back-Illuminated CMOS Sensor from Sony Promises More Sensitivity

by David Kender
Published on Jun 11, 2008 9:04 AM



June 11, 2008 – Sony unveiled a new sensor technology today, a CMOS that uses back-illumination rather than traditional front-illumination. Sony reports that the technology will offer nearly twice the sensitivity of its existing CMOS sensors. The company has bulit a prototype back-illuminated sensor with five effective megapixel resolution, capable of 60 frames per second.  While Sony did not specify which products would feature the sensors, the press release indicates that the technology could be used in both camcorders and still cameras.  
 
The difference between front-end and back-end illumination is relatively simple in principle, and the explanation is aided by a series of Sony-provided illustrations.
 
In a traditional front-end illumination sensor, the layers are ordered by:

-    On-chip lens (to focus the light into the receptor)
-    Color filter (to determine if that receptor will be for red, green, or blue)
-    Metal Wiring (including transistors and wiring that sends electrons down the rows and columns for processing)
-    Photo-Diode (the photon collection area)

 
Back-side illumination reverses the order of the last two layers so that incoming light is not blocked by the metal wiring and transistors. The idea of a back-illuminated sensor is not new, but previous designs resulted in increased noise and image degradation, according to Sony. The company claims that its unique photo-diode structure and on-chip lenses compensate for this loss, and can achieve a higher sensitivity of +6dB and a reduced random noise of -2dB.

   
 Front-illuminated image*
    Back-illuminated image*
 
This announcement did not come in tandem with any specific product launch, but Sony is expected to release new camcorders and cameras some time this summer. It is unknown if the back-illuminated CMOS technology will be ready by that time.

*Image provided by Sony. This is not a Camcorderinfo.com test image.