Image sensor enables better images, thinner cellphones

Avago Technologies has unveiled its latest addition to its CMOS image sensor family, claiming the 1/5-inch optical format, 1.3Mpixel device is smaller and offers better image quality than other 1.3Mpixel sensors in the industry. This also marks Avago's entry into the emerging small form factor (SFF) market.

Avago said the ADCC-3100 is the industry's first 1/5-inch, 1.3Mpixel SFF CMOS image sensor with advanced image processing including spot metering, bad cluster detection and auto illuminant detection capabilities for cellphones. These SFF CMOS sensors also enable handset OEMs and camera module assemblers to easily upgrade VGA camera phones or develop ultra-thin camera phones.

A key difference between the ADCC-3100 and other megapixel devices is size. Extremely small—smaller than the tip of a pencil's eraser—the ADCC-3100 fits into a 6-by-6-by-4.5mm low-profile camera module. While the ADCC-3100 is offered in a 6-by-6mm module, many competitors are still offering an 8-by-8mm module with a 1/4-inch optical format, said Feisal Mosleh, director of worldwide marketing and support for the mobile imaging business at Avago Technologies.

However, the interesting thing about the Avago image sensor is that instead of addressing the issue of higher resolution, which can impact design complexity, size and cost—and are all key concerns for cellphone makers—the ADCC-3100 addresses the need for higher image quality with enhanced features at the same resolution in a smaller form factor. Mosleh said, "It's not about more megapixels; it's about better images."

"The race to 2-, 3-, 4- and 5Mpixels is slowing down because the larger size and greater memory required are adding too much cost and complexity to the equation and handset manufacturers and carriers aren't willing to add that kind of cost," Mosleh said.

For example, a 1Mpixel camera module will cost about $6 to $7; add a few more dollars for a 2Mpixel and a 3Mpixel will cost double at $12, Mosleh said. "It's not just the expense of the module, you'll need triple the memory, a more substantial bus architecture to carry large amounts of data, a more powerful chip for manipulation of data—all in a larger package. In addition, it will require more power and generate more heat."

Size vs. image quality
Typically, the camera module is the largest z-height device in the phone. The problem with decreasing the x and y dimensions, which allows for a lower profile, is worse image quality, said Mosleh. So Avago was faced with trying to counterbalance the poor image quality as a result of the smaller size to get the same quality as a larger megapixel device. Avago believes it has achieved both—a higher image quality and a small form factor—with the ADCC-3100.

There is a big push to make smaller, thinner and more sleek phones, said Mosleh. However, the requirements for improving resolution, which delivers better image quality, poses a number of challenges, he said. "Adding more complexity, you're going to need more memory, more storage space, larger data busses, more expensive lens and packaging, and a larger 10 x 10 module."

As a result, Avago decided to create a new megapixel product that meets the requirements for better image quality while retaining a small size and not creating some of the issues—more complexity and higher costs—often associated with improving resolution, Mosleh added.

The ADCC-3100 sensor is capable of 15fps at full 1280-by-1024pixel SXGA resolution, and 30fps in 640-by-480pixel VGA mode. Other key specifications include an SNR of 40dB at 20lux and responsivity of 1V/Lux.s. It also offers excellent low-light sensitivity at 5lux. But this doesn't tell the whole story.

Many enhanced features of the ADCC-3100, which include capturing more light with fewer pixels for better image quality in combination with a smaller package, are due to the device's on-chip image processing and the company's enhanced performance (EP) pixel and array architecture and eighth-generation I-Pipe processing.

At the heart of the ADCC-3100 is Avago's EP pixel and array architecture and I-Pipe processing, which significantly increases image quality for photos. EP reduces dark current and noise, and reduces lens shading effects to offer low-light CMOS sensor performance that is said to rival CCD sensors. In addition, with 2.2-by-2.2Ωn pixels, the ADCC-3100's 3D pixel e-field shaping makes each pixel ultra-light sensitive. The key benefits: lower noise, lower crosstalk and improved sensitivity. The product extends pixel sensitivity through 8-by-8pixel binning compared with 1-by-2 binning typically offered by the competition.

Multiplexed pixels
One key feature of Avago's EP pixel architecture is that the pixels are multiplexed (where the fourth transistor is shared between adjacent pixels) to improve sensitivity. Other associated benefits include lower power consumption, less heat generation and smaller die size. Key features of the I-Pipe include auto thumbnails and fast reload to avoid missing shots. It also features fast auto focus, dual flash support, shutter lag amelioration, and an advanced auto illuminant detector using the second-generation plausible illuminant. In addition, it provides spot metering for better subject exposure and focus, enhanced sensor averaging for better color, bad cluster correction for better yields, color variant correction to ensure consistency from die to die, and capacitive networking sampling to reduce fixed pattern noise, which saves pixel array space and reduces noise, Mosleh said.

The on-chip image processing eliminates the need for an additional space-consuming, power-hungry companion chip and is said to reduce system processing overhead, design complexity and time to market.

The ADCC-3100 CMOS camera-on-a-chip is already sampling with volume production slated for December 2006. Pricing is expected in the sub-$2 range.

- Gina Roos
eeProductCenter