CMOS, CCDs invade space imagers
Initially intended as memory devices, CCDs resulted in the very first silicon-based image sensors. Willard Boyle and George Smith were awarded the Nobel Prize in Physics in 2009 for their work. Unfortunately, the committee forgot their colleague Michael Tompsett, who was the first to use CCDs as imagers.
Today, advanced imaging technologies are found in satellites orbiting the Earth or in outer space. They map the sky with ever increasing detail, including most recently the first exoplanets. It’s even become commonplace for people to use live satellite images to check whether a thunderstorm will ruin an outdoor barbecue.
CCDs initially ruled the imaging world both for consumer devices as well as for high-end space imagers thanks to their ability to support both large imaging areas and extremely low, dark current and noise. Work on improving light sensitivity, especially through backside illumination, helped avoid reflections at the metals and absorption in the dielectrics present at the front side of the device.
In the mid-nineties, researchers at NASA’s Jet Propulsion Laboratory, among others, started to explore the potential of CMOS technology for image devices. After achieving good results, the race between CCD and CMOS imagers to conquer the consumer market took off.
Today, CMOS image devices have taken the lead in the consumer market due to advanced pixel scaling — now close to one micron in pixel size — and integration of digital logic on the imager chip, which is not possible in a CCD. State-of-the-art CMOS imagers now feature high resolution and excellent optical performance at low cost, thanks to their small die sizes.
While CMOS has become the standard in consumer electronics, the space community is still mainly relying on CCD technology. In part, that’s because backside illumination processing for CMOS imagers is almost uniquely developed for 300mm wafers, whereas high-end space imagers typically use 200mm wafer technologies.
However, the space industry would well be served with a customized high-end backside illuminated CMOS image-processing platform on 200mm wafers. Given the strategic value of Earth observation, it is not surprising that governments on both sides of the Atlantic are currently supporting local initiatives in this area.
Consumers are driving adoption of CMOS imagers, but the space community continues to push the limits in imaging devices. For example, space imagers still maintain a lead in areas such as the detection of non-visible light. I see both CMOS and CCD imagers playing critical roles in both the short and long term, with each taking turns in the spotlight.
Piet De Moor is the program manager for specialty imagers at Belgian nanoelectronics research center IMEC. This article first appeared on sister site www.eetimes.com.
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