CCD vs. CMOS: Facts and Fiction
|First posted Oct 21, 2007|
Last update Apr 17, 2008
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Webmasters note: I have written several times asking permission to repost this article with no responce. I therefore take it that they don't mind. I belive this is an exelent article and am posting it here for the benifit of thoes who take a true intrest in this techonology, and to try to correct some of the mis-information which is common in this feild. While the article is a number of years old now, its information is still valid and worthwhile.
Reprinted from the January 2001 issue of PHOTONICS SPECTRA © Laurin Publishing Co. Inc.
CCD vs. CMOS: Facts and Fiction
by Dave Litwiller
Choosing an imager means considering not only the chip, but also its manufacturer and how your application will evolve.Much has been made in the past five years of the potential for CMOS imagers and of the impending demise of the incumbent image-sensing technology, CCDs. Strong claims by the proponents of a resurgent CMOS technology have been countered by equally forceful claims by CCD defenders. In a pattern typical of battling technologies (both with significant merits but also lacking maturity in some regards), users have become leery of performance representations made by both camps. Overly aggressive promotion of both technologies has led to considerable fear, uncertainty and doubt.
For the foreseeable future, there will be a significant role for both types of sensor in imaging. The most successful users of advanced image capture technology will be those who consider not only the base technology, but also the sustainability, adaptability and support. They will perform the best long term in a dynamic technology environment that the battle between CCDs and CMOS promises to deliver.
Both image sensors are pixelated metal oxide semiconductors. They accumulate signal charge in each pixel proportional to the local illumination intensity, serving a spatial sampling function.
When exposure is complete, a CCD (Figure 1) transfers each pixel’s charge packet sequentially to a common output structure, which converts the charge to a voltage, buffers it and sends it off-chip. In a CMOS imager (Figure 2), the charge-to-voltage conversion takes place in each pixel. This difference in readout techniques has significant implications for sensor architecture, capabilities and limitations.
Eight attributes characterize imagesensor performance:
Both image chip types are equally reliable in most consumer and industrial applications. In ultrarugged environments, CMOS imagers have an advantage because all circuit functions can be placed on a single integrated circuit chip, minimizing leads and solder joints, which are leading causes of circuit failures in extremely harsh environments.
CMOS image sensors also can be much more highly integrated than CCD devices. Timing generation, signal processing, analog-to-digital conversion, interface and other functions can all be put on the imager chip. This means that a CMOS-based camera can be significantly smaller than a comparable CCD camera.
The user needs to consider, however, the cost of this integration. CMOS imagers are manufactured in a wafer fabrication process that must be tailored for imaging performance. These process adaptations, compared with a nonimaging mixed-signal process, come with some penalties in device scaling and power dissipation. Although the pixel portion of the CMOS imager almost invariably has lower power dissipation than a CCD, the power dissipation of other circuits on the device can be higher than that of a CCD using companion chips from optimized analog, digital and mixed signal processes. At a system level, this calls into question the notion that CMOS-based cameras have lower power dissipation than CCDbased cameras. Often, CMOS is better, but it is not unequivocally the case, especially at high speeds (above about 25-MHz readout).
The other significant considerations in system integration are adaptability, flexibility and speed of change. Most CMOS image sensors are designed for a large, consumer or near-consumer application. They are highly integrated and tailored for one or a few applications. A system designer should be careful not to invest fruitlessly in attempting to adapt a highly application-specific device for a use to which it is not suited.
CCD image sensors, on the other hand, are more general purpose. The pixel size and resolution are fixed in the device, but the user can easily tailor other aspects such as readout speed, dynamic range, binning, digitizing depth, nonlinear analog processing and other customized modes of operation.
Even when it makes economic sense to pay for sensor customization to suit an application, time to market can be an issue. Because CMOS imagers are systems on a chip, development time averages 18 months, depending on how many circuit functions the designer can reuse from previous designs in the same wafer fabrication process. And this amount of time is growing because circuit complexity is outpacing design productivity. This compares with about eight months for new CCD designs in established manufacturing processes. CCD systems can also be adapted with printed circuit board modifications, whereas fully integrated CMOS imaging systems require new wafer runs.
Which costs less?
One of the biggest misunderstandings about image sensors is cost.
Many early CMOS proponents argued that their technology would be vastly cheaper because it could be manufactured on the same high-volume wafer processing lines as mainstream logic and memory devices. Had this assumption proved out, CMOS would be cheaper than CCDs.
However, the accommodations required for good electro-optical performance mean that CMOS imagers must be made on specialty, lowervolume, optically adapted mixed-signal processes and production lines.
This means that CMOS and CCD image sensors do not have significantly different costs when produced in similar volumes and with comparable cosmetic grading and silicon area. Both technologies offer appreciable volumes, but neither has such commanding dominance over the other to establish untouchable economies of scale.
CMOS may be less expensive at the system level than CCD, when considering the cost of related circuit functions such as timing generation, biasing, analog signal processing, digitization, interface and feedback circuitry. But it is not cheaper at a component level for the pure image sensor function itself.
The larger issue around pricing, particularly for CMOS users, is sustainability. Many CMOS startups are dedicated to high-volume applications. Pursuing the highest-volume applications from a small base of business has meant that these companies have had to price below their costs to win business in commodity markets. Some start-ups will win and sustain these prices. Others will not and will have to raise prices. Still others will fail entirely.
CMOS users must be aware of their suppliers’ profitability and cost structure to ensure that the technology will be sustainable. The customer’s interest and the venture capitalist’s interest are not well-aligned: Investors want highest return, even if that means highest risk, whereas customers need stability because of the high cost of midstream system design change.
Increasingly, money and talent are flowing to CMOS imaging,in large part because of the high-volume applications enabled by the small imaging devices and the high digital processing speeds. Over time, CMOS imagers should be able to advance into higher-performance applications.
For the moment, CCDs and CMOS remain complementary technologies — one can do things uniquely that the other cannot. Over time, this stark distinction will soften, with CMOS imagers consuming more and more of the CCD’s traditional applications. But this process will take the better part of a decade — at the very least.
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Apr 17, 2008