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Front side or back side illuminated sensors

Conventional sensors are designed to convert the light falling on the front side of the sensor to electrons. As discussed in the article Pixel size and well capacity not all the sensor surface is sensitive to light as the pixel also needs control electronics limiting the fill factor.

As we see higher resolution sensors come to market utilising the same sensor size, we see pixel sizes reduce in some cases to under 2 microns. This reduction has the effect of reducing this fill factor further as the area for the control electronics becomes more significant.

To maximise sensitivity we are seeing additional optical lightguides added to microlenses to channel as much light as possible to the limited photosensitive area, however a new approach is now being seen in some of the latest small pixel sensors that overturn the pixel design to illuminate the pixel from the back side (Back side illuminated or BSI), so the control electronics do not limit the light sensitive area. Due to the structure of the silicon pixel, light in a BSI sensor has to travel further into the silicon to be converted into electrons and is therefore less efficient at converting the light (lower quantum efficiency or QE).

We therefore see a tipping point of pixel sizes between 1 and 2 μm where a BSI sensor will deliver more signal than a traditional front side illuminated (FSI) pixel. As we see the sensor resolution continue to increase, we will see more sensors using this technique.