Back Side Illumination

Technology for low lighting imaging

Back-Side Illumination, as the definition suggests, is referring to the part of the sensor which is actually exposed to the light. In standard CMOS sensor design, the photosensitive part is the substrate of the silicon. Above it is the rest of the pixel circuit including the wires, which are carrying the signals. The wires and the junctions in the top layer of the silicon are positioned in a way to let the light pass through and reach the photosensitive substrate but, despite this, some photons are bounced back by the wires, thus the sensor will not take all the available light.

To implement the BSI technology, the silicon is mounted upside down in the package in order for the substrate to be directly exposed to the light. This process is not so simple because the bottom part of the chip needs to be treated by applying a very thin photosensitive material.

A BSI sensor gets  a lot more light than a standard sensor which makes it perfect for all those applications where the sensor is placed in dark environment.

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