frontilluminated

Frontilluminated, usually written as front-illuminated, refers to a configuration in imaging sensors and photodiodes where incoming light enters from the same surface that contains the device’s wiring and circuitry. In front-illuminated sensors, the photosensitive region lies under metal interconnects, oxide layers, and protective coatings, so the incident light must pass through these structures before reaching the active layer.

In CMOS and CCD image sensors, front-illumination historically offered straightforward manufacturing and robust performance. However, the overlying metal and dielectric layers can absorb or block part of the light, especially at shorter wavelengths, and the wiring can shade portions of the pixel, reducing quantum efficiency and dynamic range compared with alternatives.

A common alternative is back-illumination, where the light enters from the opposite side. In back-illuminated sensors, the substrate is thinned and the metal wiring is removed from the light path, allowing higher quantum efficiency across a broad spectrum, particularly in the blue region. This makes BI sensors advantageous for high-sensitivity imaging, though they involve more complex fabrication.

Applications of front-illuminated devices include many traditional consumer cameras and embedded imaging sensors where cost, robustness, and established manufacturing processes are prioritized. Advances such as microlenses and surface passivation have mitigated some FI limitations, but BI remains preferred when high sensitivity is required.