The latest generation of CMOS and charge-coupled device (CCD) image sensors features wider spectral bandwidths, higher sensitivity levels, lower noise operation, and smaller form factors. Better fabrication processes help lower costs. And, novel architectures are injecting greater flexibility and versatility into circuit designs.
As a result, imaging sensors now find homes in mobile phones, notebook and laptop PCs, digital still cameras, video games, toys, medical devices, automobiles, security, industrial, and many other applications. According to IC Insights, CMOS and CCD imagers will see a compound annual growth rate (CAGR) of 14% over the next five years. Both types of sensors are finding wide use, but prognostications for CMOS imagers are particularly strong. Forecasts show that they will garner a 73% market share by 2012, up from 58% last year.
Like most electronic devices, performance and cost will continue to be the main issues for CMOS and CCD imagers. Although CMOS imagers are predicted to garner more applications, there’s still a need for CCD imagers in applications that require high performance levels. It isn’t simply a question of which type of imager is better. Depending on the application, a CMOS or CCD sensor may be the best choice based on performance and cost parameters.
CMOS imagers, which are generally less expensive than CCD imagers, no doubt can be found in many consumer electronics items that stress low cost. Their performance is on the rise— they’ve been making inroads into automotive safety applications while encroaching into the CCD imaging arena, where performance levels are acceptable but at a lower cost.
CCDs feature the higher performance needed in industrial and machine-vision inspection applications, as well in security systems and scientific and military aerospace applications. They can also be found in niche apps like astronomy and the medical realm. Cost is dropping, too, while still making impressive leaps in performance that’s outstripping CMOS imagers (see “CCDs: Performance That Can’t Be Beat”).
CMOS PERFORMANCE IS RISING
CMOS imagers are meeting many system requirements determined by multiple application parameters, such as wider bandwidths and global-shuttering capability. And new design and manufacturing proposals and implementations will drive their performance even higher.
One novel idea from NASA’s Jet Propulsion Laboratory (JPL) substantially reduces imager diffusion crosstalk. The researchers propose adding two implants in each CMOS pixel that would affect vertical isolation between the MOSFETs and the pixel photodiodes used in their imager (Fig. 1). They argue that this separation makes it possible to optimize both the MOSFET and the photodiode performance, eliminating or vastly reducing crosstalk and noise, while increasing sensitivity, spatial resolution, and color fidelity.
Image synchronization and operation under often difficult and unfavorable conditions, particularly in machine-vision automated inspection applications, is a big challenge facing CMOS imager designers. The industry has traditionally relied on CCD imagers using interline-transfer techniques to deliver high-speed shuttering for crisp images.
Recent CMOS imager advances have enabled these sensors in machine-vision applications. With parallel outputs, windowing, and on-chip integration, some CMOS image sensors now offer capabilities that rival those of CCD imagers for some machinevision applications.
For instance, CMOS sensors from Cmosis feature globalshuttering capability. Thanks to its pipelined global-shutter pixel technology, imaging systems can capture the next frame during the readout process. Cmosis achieves this by incorporating a storage node in each of the image sensor’s pixels, to which the signal is transferred after the image capture step.
The storage node has an extremely low parasitic light sensitivity. Each pixel can be read out with low noise and with a wide dynamic range. The firm developed fast analog-to-digital converters (ADCs) located in the sensor’s pixel columns.
Dalsa Corp. has come up with interline-transfer CMOS imagers that can also deliver high-speed shuttering capability. These sensors provide the sensitivity, signal capacity, noise performance, and dynamic range that’s required in many machinevision applications.
Photonfocus employs its patented LinLog technology in the A1312 CMOS imager for fast shuttering capability and a wide dynamic range of up to 120 dB. The sensor features 8- by 8-µm pixels in a 1312- by 1028-pixel format and operates at 110 frames/s with full resolution.
As CMOS imager pixel sizes shrink, maintaining imager performance and image quality becomes a tougher task. One option has been backside illumination. Working with Taiwan Semiconductor Manufacturing Corp. (TSMC), OmniVision believes it has found the key with the OmniBSI approach (Fig. 2). The company is able to produce 8-Mpixel devices from a 1.4-µm process for mobile phones.
Sony has also seen success with backside illumination. The company has produced a 5-Mpixel device on a 1.75-µm process for mobile phones, digital cameras, and camcorders. And STMicroelectronics, working with France’s CEA Leti and Tracit Technologies, has demonstrated the feasibility of manufacturing 3-Mpixel CMOS imagers on a 1.45-µm process using backside illumination.
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