Back-illuminated sensor

A back-illuminated sensor, also known as backside illumination (BI) sensor, is a type of digital that uses a novel arrangement of the imaging elements to increase the amount of light captured and thereby improve low-light performance. The technique was used for some time in specialized roles like low-light security cameras and astronomy sensors, but was complex to build and required further refinement to become widely used. Sony was the first to reduce these problems and their costs sufficiently to introduce a 5-megapixel 1.75 μm BI CMOS sensor at general consumer prices in 2009. BI sensors from OmniVision Technologies have since been used in consumer electronics from other manufacturers as in the HTC EVO 4G Android smartphone, and as a major selling point for the camera in Apple's iPhone 4. A traditional, front-illuminated digital camera is constructed in a fashion similar to the human eye, with a lens at the front and photodetectors at the back. This traditional orientation of the sensor places the active matrix of the digital camera —a matrix of individual picture elements—on its front surface and simplifies manufacturing. The matrix and its wiring, however, block some of the light, and thus the photocathode layer can only receive the remainder of the incoming light; the blockages reduces the signal that is available to be captured. A back-illuminated sensor contains the same elements, but arranges the wiring behind the photocathode layer by flipping the silicon wafer during manufacturing and then thinning its reverse side so that light can strike the photocathode layer without passing through the wiring layer. This change can improve the chance of an input photon being captured from about 60% to over 90%, (i.e. a 1/2 stop faster) with the greatest difference realised when pixel size is small, as the light capture area gained in moving the wiring from the top (light incident) to bottom surface (paraphrasing the BSI design) is proportionately larger for a smaller pixel.