backilluminated

Backilluminated, often written back-illuminated, is a term used in imaging technology to describe a sensor design in which the light-sensitive surface faces the incoming light and the wiring and electronics are moved to the opposite side. This contrasts with traditional front-illuminated sensors, where metal interconnects reside on the light-facing surface and can partially block photons. Such sensors are commonly marketed as backside-illuminated (BSI) sensors.

In back-illuminated sensors, the silicon substrate is thinned and flipped so the photodiodes are directly exposed to light. A backside processing sequence places a redistribution layer to route interconnections from the front to the back, followed by passivation, microlenses, and anti-reflective coatings to optimize light capture. The resulting sensors typically achieve higher quantum efficiency and a larger effective pixel fill factor than front-illuminated counterparts, especially at small pixel sizes and in low-light situations.

Advantages include improved sensitivity, better performance in low illumination, and more uniform response across the image. These sensors have enabled higher-quality smartphone cameras and compact digital cameras, as well as scientific and astronomical instrumentation that require high light collection efficiency.

Limitations involve more complex, costly manufacturing and potential challenges in yield and reliability due to backside thinning. Thermal and dark-current characteristics can also differ from traditional sensors, requiring careful design and calibration.