Back to Front - Sony's New Back-Illuminated CMOS Sensor
Reviewed on 17 June 2008

Ground-breaking innovations don't come along every day, in fact, the last few photographic advances have been interesting but it's hard to say if they were truly revolutionary. Sure, face detection has altered the way we take portraits, we don't have to worry about settings and other such details. And smile shutter has completely changed the way we take photographs, we don't even have to press the shutter anymore. Slightly more earth shattering advances would be things like vibration compensation for DSLRs, and especially dust reduction systems which helped improve photographs or made photographer's lives a little easier. But truly revolutionary new technologies?

Well Sony might have just broken new ground when it comes to digital photography by coming up with a new sensor. The back-illuminated CMOS sensor, as it's called, is a radically new sensor that promises to be twice as sensitive as conventional CMOS sensors with better noise control. Sony has achieved this by completely turning the design of the sensor on its head.

Up until now, if anyone was thinking of improving image sensors, the only thing people were concentrating on was miniaturizing the pixels so they could squeeze in more in any given space. Think of all the many cameras that came out with more ridiculous megapixels sizes. However, there is a point where you can't make the pixels any smaller which is why resolutions have stabilised in the 10-12 megapixels range.

Current sensors are called front illuminated. Basically, they are designed with the light sensitive diodes on the bottom, the metal wiring and transistors in the middle then the colour filter over that and the on-chip lens. Light passes through the lens and filter then through the gaps in between the metal wirings before it hits the photo-diodes where it's registered. The trouble with this design is the amount of light that is blocked by the metal wiring and transistors. Light photons are small, and the gaps between the metal wiring are big enough to let them through, but a fair amount of light is still blocked.

In back-illuminated CMOS sensors, the metal wiring and transistors sit on the bottom, while the light sensitive diode sits on top of that. Light passes through the on-lens, the colour filter to hit the diodes. Not much light is lost in that process which means the sensor is much more sensitive to low levels of light. The idea is so simple; you might think why didn't they do that before? Well the idea isn't actually new. This topsy turvy structure isn't without its share of issues. The structure often caused higher noise levels, colour mixture and defective pixels. Because the images weren't perfect, the idea was abandoned. Until now.

Sony has come up with a new unique photo-diode structure and on-chip lens that works better for the back-illuminated CMOS sensor. They've managed to improve the sensor so much that it's more sensitive, with less random noise and defective pixels than even current front-illuminated CMOS sensors. Coupled with the advances in speed and quality of Sony's other sensors, such as Exmor and dual noise reduction technologies, Sony has developed a unique image sensor that promises to improve image quality in all photographs.

Sony is the first to market the back-illuminated CMOS sensor structure for cameras and camcorders. It's not certain when we will be able to try out cameras with this new technology, but since a 5 megapixels prototype is ready, it seems likely that pretty soon we will see this new sensor built into cameras. We have to wait to see whether this new structure is as exciting as it sounds.