What is This New Backside Illumination on the New iPhone?

Recently Apple announced the new iPhone and a number of the hardware changes were targeted at the camera. Thanks to the infamous lost iPhone fiasco we expected the 5 mega-pixel camera and flash, but Jobs announced the camera had backside illumination. That left us puzzled so we did a some research on camera chips.

What is backside illumination?

First stop giggling... OK... I'll admit I think they could have found a better name. Some of the jokes I saw on Twitter seemed to be especially crude about this feature. Backside illumination is a trick of getting better digital photos by getting more of the available light to where it is measured.

First an overview on digital cameras

The key part of all digital cameras is a chip called the CCD (charge coupled device) which senses the light falling on its surface. The light affects the charge a grid of millions of tiny capacitors created on the silicon when the chip is made (making chips is called fabrication and involves a lot of etching and depositing of thin layers and different materials in precisely controlled ways). For simplicity just think about black and white CCD's where the array of capacitors corresponds to the array of pixels which make up a digital image (color sometimes uses a lot more tricks). The chip has additional circuitry that measures the charge of all of the capacitors which is basically how the light levels of the image are read. To be able to read all of these capacitors there are minutely thin wires running over the top. They're not actually wires but a thin layer of aluminum or copper that is effectively sprayed on to the chip and then carefully eaten away using acids to leave connecting traces, which are pretty much metal wires bonded to the chip.

Why backside illumination?

All those wires and other parts of the capacitors sit on top of a square silicon tile (called the die) with the capacitors at the bottom. The circuitry and wires don't obstruct it too much as it works but it does block some light and scatter some in to surrounding capacitors which reduces the quality of image captured especially with limited lighting. Back to that square silicon tile. Silicon is the main ingredient of regular glass (aka silicon dioxide) but the stuff used in making chips is a super pure silicon crystal and transparent. With backside illumination the issues with traditional CCD's of the capacitors being on the bottom is quite literally turned upside down and now you shine the image on what was the bottom of the silicon tile and let the light shine through the silicon to the capacitors. This way you avoid all the wires and get more light to the capacitors.

With this new camera chip Apple has shown some beautiful looking pictures but they key performance area will be low lighting. Capturing scenes like the last light of a fading sunset. The earlier iPhones (Original and 3G) had very basic fixed focus 2 mega-pixel cameras which struggled in low lighting. The 3GS brought auto focus and auto white balance that improved performance but still struggled with such scenes, often distorting colors. Now with the new camera chip the iPhone should take vastly improved photos, coupled with the in-built flash to handle low lighting indoors.