Backside-illuminated Sensor
A sensor design in which the wiring layer is moved behind the photodiodes so light reaches them unobstructed.
In depth
In a conventional front-illuminated design, light must pass through a layer of metal interconnects before reaching the silicon, which blocks some of it and worsens the problem at oblique angles. Flipping the wafer over removes that obstruction, improving sensitivity and corner performance — gains that matter most on small sensors and high pixel counts.
Examples in practice
Backside illumination is why modern phone sensors perform far better in low light than their pixel size would suggest.