Back-illuminated sensor
A back-illuminated sensor is a type of digital camera sensor that uses a novel arrangement of the imaging elements to increase the amount of light captured and thereby inprove low-light performance. First introduced for consumer use by Sony in a 5 Mpx 1.75 µm CMOS sensor in 2009, the technology has been introduced by several other vendors, and is a major selling point of the camera in the latest iPhone.[1]
A traditional digital camera sensor consists of an array of individual picture elements. Each element is constructed in a fashion similar to the human eye, with a lens at the front, sensors at the back, and wiring in between. The wiring interfers with the light passing though the system, which reduces its effectiveness. This odd layout is due to mechanical limitations in both cases - in the eye the rear surface of the retina is needed to supply blood, in the case of a camera sensor, mechanical limiations of the front of the detectors requires an active matrix to be placed on their front surface. The matrix and its wiring reflects or absorbs some of the incoming light, thereby reducing the signal that is available to be captured.[2]
Moving the active matrix transistors to the back of the photosensitive layer normally leads to a host of problems cross-talk, which causes noise, dark current, and color mixing between adjacent pixels. These problems could be solved through improved manufacuring processes, but only at the cost of lower yields, and consequently higher prices. They found a use in niche roles where their better low-light performance was important, like industrial sensors, security cameras and astronomy uses.[3]
However, industry observers noted that the ability for a back-illuminated sensor to theoretically cost less than a similar front-illuminated version. The ability to collect more light meant that a similarly-sized sensor array could offer higher resolution without the drop in low-light performance otherwise associated with the megapixel race. Alternately, the same resolution and low-light capability could be offered on a smaller chip, lowering costs. Key to attaining these advantages would be an improved process that addressed the yield problems, largely through improving the uniformity of an active layer on the front of the detectors.[3]
Sony's work on new photodiode materials and processes allowed them to introduce the first consumer back-illuminated sensor as their CMOS-based "Exmor R" in August 2009.[2] According to Sony, the new material offered +8 dB signalling. When combined with the new back-illuminated layout, the sensor improved low-light performance by as much as two times.[2]
Competitors followed suit, and by the end of the year most companies were offering a version in their high-end products. In 2010, back-iilumincation came to the low-end of the market when Apple included a 5 Mpx detector in the latest revsion of the mobile phone, the iPhone 4.[1]
References
Notes
Bibliography
- (Sony), "Sony unveils 'Exmor R' back-illuminated CMOS technology", Sony press release, 6 August 2009
- P.K. Swain and David Cheskis, "Back-Illuminated Image Sensors Come to the Forefront", Photonics, August 2008
- (Apple), "iPhone 4: Camera", Apple Inc., 7 June 2010