Image sensor makers are putting their best foot forward with a technique called backside illumination, and it's catching on. Also, Sony details BSI chips for phones.
LONDON--A new type of image sensor that's been flipped front to back is beginning to give photographers a better view of the world.
The new sensors use a technology called backside illumination, and chipmakers including Sony and Samsung are leading the charge to build them into a variety of cameras. And though it's a premium feature today, it's spreading rapidly across the market.
"It's more aggressive than we expected even two years ago," Yole Developpement analyst Jerome Baron said in a talk last week at the Image Sensors Europe conference here.
Image sensors are special-purpose, light-sensitive chips packed with complicated technology. But backside illumination, or BSI, is pretty easy to understand: flip the sensor around so the light it's detecting isn't partially blocked by a bunch of electronics. What used to be the back of the sensor is now facing outward toward the light.
The advantage, illustrated among other places in the iPhone 4 camera, is better light sensitivity. That opens up new options for camera makers.
One option is to offer more pixels without degrading how well each pixel works, yielding more detailed photographs. Another option is better image quality with the same number of pixels, something that's useful when taking photos or videos in dim indoor light.
Either way, it's a compelling option. And though BSI adds new manufacturing costs and challenges, photographers often pay extra for a better image--either to make a mobile phone perform more like a point-and-shoot or to make a higher-end SLR shoot better in dim conditions.
Thus, the increasingly bullish Yole market forecast.
In 2011, backside illumination sensors should account for 20 percent of the $5.6 billion revenue from image sensors built with the mainstream CMOS (complimentary metal oxide semiconductor) process, and are growing fast, according to Yole statistics. By 2015, it should be 70 percent of an $8.2 billion CMOS image sensor business.
There's more, though, said Avi Strum, general manager of the specialty business unit of image sensor designer TowerJazz.
Backside illumination also helps with dynamic range (the ability to capture details in both shadows and bright areas), video frame rate, rapidly transferring data off the chip, and what's called angular response. With BSI's better angular response, pixels on the edge of an image sensor--where light often arrives at an angle--can come closer to the performance of pixels at the center of a sensor, where light more often arrives perpendicularly.
Most sensors today employ front-side illumination, in which transistors and wires necessary to transmit image-sensor data are on the surface of the chip facing the outside of the camera. A lot of recent image sensor work has focused on reducing the number of transistors and on letting multiple pixels on the sensor share the same electronics. That improves the "fill factor," the percentage of each pixel that can be used to gather light.
With backside illumination, though, the fill factor can go all the way up to 100 percent.
"If you have a higher fill factor, you can make higher sensitivity or you can make higher resolution," Baron said. "It's a key revolution."
There's a big manufacturing challenge to BSI, though. Different materials must be bonded together, and layers must be ground away with high precision.
Sony out front with backside illumination
Sony is outgrowing the rest of the pack, in part because of its early adoption of backside illumination in its Exmor R line of BSI sensors, Baron said. "Samsung is moving the same way," he added, and others are following suit. Canon, a pioneer in CMOS image sensors, "is still relevant, but is looking for partners for their future sensors."
Sony's sensors are used not only in its own still cameras and video cameras but also in SLR cameras from Nikon and Pentax. The company isn't afraid to raise expectations about backside illumination and other technologies it plans to use in its sensors.
"The CMOS image sensor will be beyond the human eyes in the future," said Tsutomu Haruta, senior manager of Sony's Semiconductor Business Group, in a talk at the conference.
Sony offers BSI only in video camera sensors today, but it's expanding to mobile devices, too, Haruta said. He showed off two new small sensors, a type 1/2.8 16-megapixel chip with pixels measuring 1.12 microns and a type 1/3.2 8-megapixel model with pixels measuring 1.4 microns. (A type 1/2.8 sensor measures about 6.2x4.6mm, and a type 1/3.2 sensor is even smaller, at about 4.5x3.4mm.)
BSI is chiefly useful in smaller sensors today where pixels are small and noise speckles are more of a problem. But, asked if Sony planned to expand to high-end gear such as SLR cameras or professional video cameras, Haruta indicated that was a possibility. "In the future there are some other areas we will go," he said.
Yole expects BSI sensors will spread both to high-end and low-end cameras.
Image sensors are a vast and important market. Not only does it reach millions of mainstream consumer devices, it's also important to medical instruments, vehicle safety systems, security cameras, and many other markets.
So naturally, there's plenty of research into better image sensor technology. BSI is catching on, but it still suffers a fundamental weakness: silicon-based image sensors aren't as sensitive to green and blue light as it is to red.
One company trying to leapfrog the current image sensor technology InVisage, which is developing a technology it calls QuantumFilm. As with BSI, its light-sensitive layer isn't behind the circuitry. But unlike BSI, it uses precisely sized particles of semiconductor material sensitive specifically to visible light.
How InVisage and other challengers will fare isn't yet certain. What is clear, though, is that there is plenty more change in store for digital photography.