A back-illuminated image sensor, also called backside illumination (BSI) sensor

A back-illuminated sensor contains the same elements as  Traditional Sensors

A kind of digital imaging sensor which makes use of a innovative arrangement of the image elements to enhance the volume of light acquired and thereby increase low-light operations. This technique was used in specialized roles such as cameras for low-light security plus astronomy sensors, although was extremely complex to manufacture and required additional refinement before it could become used on a wider scale.

Sony became the first manufacture to diminish these issues along with the cost sufficiently enough to build a 5 megapixel 1.75 Ám BI CMOS image sensor in 2009 that met general consumer prices. BI sensors made by OmniVision Technologies have subsequently been installed in consumer electronics for other electronic manufactures such as the HTC' EVO 4G, and as a foremost selling point of the Apple iPhone 4 camera

The traditional, front-illuminated sensor in a digital camera is made up using a matrix of distinctive photo elements where every element is built similar to a human eye, containing a lens in the front, with wiring in the center, plus photo detectors at the rear. This traditional sensor orientation puts the active matrix at the front surface and makes manufacturing make straightforward. However, the matrix along with its wiring, reflect part of the inbound light, and only letting the photocathode layer receive the incoming light that is left and the resulting reflection diminishes the signal that's left to be captured.

BSI Diagram

A back-illuminated image sensor contains all the same elements, however orients the wiring to the back of this photocathode layer by reversing the silicon wafer while being manufactured and consequently thinning its back side allowing that light to bump into the photocathode layer while avoiding going through the layer with wiring This adjustment improves the odds of this input photon capture from around 60% to more than 90%. Moving the wiring to the back of the light sensors becomes akin to the difference in a Cephalopod eye or an vertebrate eye.

Re-orienting these active matrix transistors to the back of a photocathode layer can create a host of issues, like cross-talk, which creates noise, color mixing among adjacent pixels and dark current. Also thinning allows the silicon wafer to become more fragile. These issues are solvable by improving the manufacturing processes, although only at the expense of reduced yields, with consequently increased prices. Despite these problems, early BI sensors initiated niche role uses where their improved performance in low-light was important. Early uses were for security cameras, industrial sensors, astronomy systems and microscope cameras.

Observers in the industry noted that the back-illuminated image sensor could in theory become less expensive than a comparable front-illuminated sensor. With the capability to gather more light means that a sensor array of similar size could deliver increased resolution without the reduction in low-light implementation otherwise connected to the megapixel race. As an alternative, the identical resolution along with low-light performance could be provided on a smaller size chip, reducing costs. Fundamental to attaining these increased benefits would be to improve the process that deals with the yield issues, largely by enhancing the uniformity of this active layer to the front side of the detectors

A significant step in adopting the BI sensors occurred in 2007 when OmniVision Technologies tested their initial sensors using this technique. However, these sensors, did not come into widespread use because of their high costs. Sony's work on improved photo diode processes and materials which allowed them to announce their first back-illuminated consumer sensor in their "Exmor R" CMOS-based sensor in August of 2009.

According to Sony, this new material delivered -2 dB noise and +8 dB signaling. When joined with their new back-illuminated design, the sensor improved upon low-light accomplishment by two times as much Competitors soon followed suit, and before the end of that year the majority of companies were providing a version within their high-end devices. OmniVision continues to push this technology within their product lines.

In contrast, the recently announced iPhone 4s uses a Sony manufactured sensor. The HTC 4G EVO is another example which uses an 8 megapixel, 1.4 Ám pixel BSI sensor made by OmniVision. Sony put their own Exmor R sensor into operation in 2011 in the Sony Ericsson Xperia Arc, their flagship smartphone.

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New Article Feb 25, 2012

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