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Backside Illumination

BSI stands for backside Illumination. The backside illumination technology adapts a radically different method to conventional frontside illumination (FSI) pixel structure. The purpose of using backside illumination technology is to provide CMOS image sensor a better way to improve its light sensitivity, color reproduction and image quality.

The basic idea of BSI involves turning the whole image sensor upside down and applying the color filter (RGB) and micro lenses to the substrate instead of metal layers. Such technique overcomes the traditional FSI problems, where lights travel to the metal layers through front side of the image sensor to cause a reduced fill factor and cross talk between pixels.

VisEra_Color_Filter_and_Micro_Lens_Backside_Illumination

Advantages of BSI Sensors for Lens and Module Design

BSI technology provides camera module and lens designers three options over traditional FSI pixel designs:

Better support for larger CRAs, enabling thinner modules.
Better support for larger CRAs, enabling lenses with larger apertures (faster lenses).
Support for variations in CRAs, enabling high-quality zoom lens designs.

VisEra The wider optical path of BSI enables new camera designs: FSI pixel / BSI pixel thinner module / BSI pixel wider aperture (faster lens) / BSI pixel zoom lens (variable lens)

Figure above: The wider optical path of BSI enables new camera designs
Reference: OmniVision Technologies, Inc. June 2009