Upside-down CMOS sensor may spark digital revolution

California-based company OmniVision Technologies has designed a new CMOS sensor for digital imaging, claiming this innovation will not only improve image quality but will also shrink the size of image-capturing devices.

The silicon substrate sits on the surface of a BSI sensor, unlike FSI, which is below layers of metal. (Click to enlarge)

The conventional sensor has layers of metal on top of the silicon substrate (which absorbs the light). These layers may not only deflect the light on the sensor, they could also block it. The company has coined the term "front side illumination (FSI)" image sensors for these CMOS chips.

What the engineers at OmniVision did was to reverse the order of the layers in the new sensor, meaning that light will hit the silicon first, which is now on the surface of the chip and supported by the metal infrastructure below. This new technology is known as backside illumination (BSI), and was joint-developed with the firm's long-time partner, Taiwan Semiconductor Manufacturing Corporation.

According to OmniVision, the advantage of using BSI is getting better performance out of the sensor. It commented that the sensitivity per unit area will be improved, and since light strikes the silicon directly, it will be able to deliver better low-light shots. Also, the angle which light can reach the sensor is increased and this enables the use of lenses that are shorter in height. This, in turn, can slim down the next generation of digital cameras and camera-phones even more.

We all know that images taken at high ISO sensitivity with a point-and-shoot in dim environments often result in noisy pictures, and the small sensor size is to blame. Digital SLR sensors, on the other hand, utilise a larger light-sensitive chip, and this produces better low-light shots. But Michael Chan, a senior regional manager of OmniVision, told CNET.com.au's sister site CNET Asia that a compact shooter using a BSI sensor can perform better than a dSLR. He also mentioned that the overall cost of producing one BSI sensor is significantly lower than to manufacturing a typical larger sensor. So end users can expect greater performance from compact digicams in the future at the same, if not lower, price.

OmniVision is looking to implement the BSI sensor in image-capturing devices by the end of this year, although it didn't mention which manufacturer is on-board with the plan.