backsideilluminated

Backside-illuminated (BSI) is a method used in image sensors in which the light-sensitive region is illuminated from the side opposite the wiring and transistor layers. In conventional front-illuminated sensors, metal interconnects on the light-facing surface absorb and scatter photons, reducing sensitivity. BSI mitigates this by removing or relocating most of the metal circuitry away from the light path, allowing more light to reach the photodiodes.

Manufacturing involves thinning the silicon wafer from the front, forming the photodiodes on the back, and fabricating the readout circuitry on the back side or under the active layer. The result is a clearer path for photons. Additional steps include back-side passivation, micro-lenses, and anti-reflection coatings to maximize photon collection and uniform response.

Advantages include higher quantum efficiency, especially at small pixel pitches, improved low-light performance, and greater effective fill factor. This makes BSI popular for compact and mobile cameras, where light collection per pixel is critical. It also enables higher dynamic range and better color accuracy under challenging lighting.

Challenges include increased manufacturing complexity and cost, which can affect yield. Processing quality of the back surface controls dark current and fixed-pattern noise, so advanced passivation and thinning techniques are required. BSI sensors may also need careful microlens and color filter design to maximize light capture.

BSI sensors were researched in earlier decades and have become widely adopted in CMOS image sensors since the mid-2000s, becoming standard in many smartphones and other compact imaging devices.