Much has been made in thepast five years of the poten-tial for CMOS imagers and

of the impending demise of the in-cumbent image-sensing technology,CCDs.

Strong claims by the proponentsof a resurgent CMOS technologyhave been countered by equally force-ful claims by CCD defenders. In apattern typical of battling technolo-gies (both with significant merits butalso lacking maturity in some re-gards), users have become leery ofperformance representations madeby both camps. Overly aggressivepromotion of both technologies hasled to considerable fear, uncertaintyand doubt.

Imager basicsFor the foreseeable future, there

will be a significant role for both typesof sensor in imaging. The most suc-cessful users of advanced image cap-ture technology will be those whoconsider not only the base technol-ogy, but also the sustainability,adaptability and support. They willperform the best long term in a dy-namic technology environment thatthe battle between CCDs and CMOSpromises to deliver.

Both image sensors are pixelatedmetal oxide semiconductors. Theyaccumulate signal charge in eachpixel proportional to the local illu-mination intensity, serving a spatial

sampling function. When exposure is complete, a CCD

(Figure 1) transfers each pixel’scharge packet sequentially to a com-mon output structure, which con-verts the charge to a voltage, buffersit and sends it off-chip. In a CMOSimager (Figure 2), the charge-to-volt-age conversion takes place in eachpixel. This difference in readout tech-niques has significant implicationsfor sensor architecture, capabilitiesand limitations.

Eight attributes characterize image-sensor performance:

• Responsivity, the amount of sig-nal the sensor delivers per unit ofinput optical energy. CMOS imagersare marginally superior to CCDs, ingeneral, because gain elements areeasier to place on a CMOS image sen-sor. Their complementary transis-tors allow low-power high-gain am-

plifiers, whereas CCD amplificationusually comes at a significant powerpenalty. Some CCD manufacturersare challenging this conception withnew readout amplifier techniques.

• Dynamic range, the ratio of apixel’s saturation level to its signalthreshold. It gives CCDs an advan-tage by about a factor of two in com-parable circumstances. CCDs stillenjoy significant noise advantagesover CMOS imagers because of qui-eter sensor substrates (less on-chipcircuitry), inherent tolerance to buscapacitance variations and commonoutput amplifiers with transistorgeometries that can be easily adaptedfor minimal noise. Externally cod-dling the image sensor through cool-ing, better optics, more resolution oradapted off-chip electronics cannotmake CMOS sensors equivalent toCCDs in this regard.

Reprinted from the January 2001 issue of PHOTONICS SPECTRA © Laurin Publishing Co. Inc.

CCD vs. CMOS:

by Dave Litwiller

Choosing an imager means considering not only the chip, butalso its manufacturer and how your application will evolve.

CCD vs. CMOS

CCD vs. CMOS:

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Figure 1. On a CCD, most functionstake place on the camera’s printedcircuit board. If the application’sdemands change, a designer canchange the electronics withoutredesigning the imager.

Facts and Fiction

• Uniformity, the consistency ofresponse for different pixels underidentical illumination conditions.Ideally, behavior would be uniform,but spatial wafer processing varia-tions, particulate defects and ampli-fier variations create nonuniformi-ties. It is important to make a dis-tinction between uniformity underillumination and uniformity at ornear dark. CMOS imagers were tra-ditionally much worse under bothregimes. Each pixel had an open-loop output amplifier, and the offsetand gain of each amplifier varied con-siderably because of wafer process-ing variations, making both dark andilluminated nonuniformities worsethan those in CCDs. Some peoplepredicted that this would defeatCMOS imagers as device geometriesshrank and variances increased.

However, feedback-based ampli-fier structures can trade off gain forgreater uniformity under illumina-tion. The amplifiers have made the il-luminated uniformity of some CMOSimagers closer to that of CCDs, sus-tainable as geometries shrink.

Still lacking, though, is offset vari-ation of CMOS amplifiers, whichmanifests itself as nonuniformity indarkness. While CMOS imager man-

ufacturers have invested consider-able effort in suppressing darknonuniformity, it is still generallyworse than that of CCDs. This is asignificant issue in high-speed ap-plications, where limited signal lev-els mean that dark nonuniformitiescontribute significantly to overallimage degradation.

• Shuttering, the ability to startand stop exposure arbitrarily. It is astandard feature of virtually all con-sumer and most industrial CCDs,especially interline transfer devices,and is particularly important in ma-chine vision applications. CCDs candeliver superior electronic shutter-ing, with little fill-factor compromise,even in small-pixel image sensors.

Implementing uniform electronicshuttering in CMOS imagers requiresa number of transistors in each pixel.In line-scan CMOS imagers, elec-tronic shuttering does not compro-mise fill factor because shutter tran-sistors can be placed adjacent to theactive area of each pixel. In area-scan (matrix) imagers, uniform elec-tronic shuttering comes at the ex-pense of fill factor because theopaque shutter transistors must beplaced in what would otherwise bean optically sensitive area of each

pixel. CMOS matrix sensor design-ers have dealt with this challenge intwo ways:

A nonuniform shutter, called arolling shutter, exposes different linesof an array at different times. It re-duces the number of in-pixel tran-sistors, improving fill factor. This issometimes acceptable for consumerimaging, but in higher-performanceapplications, object motion manifestsas a distorted image.

A uniform synchronous shutter,sometimes called a nonrolling shut-ter, exposes all pixels of the array atthe same time. Object motion stopswith no distortion, but this approachconsumes pixel area because it re-quires extra transistors in each pixel.Users must choose between low fillfactor and small pixels on a small,less-expensive image sensor, or largepixels with much higher fill factor ona larger, more costly image sensor.

• Speed, an area in which CMOSarguably has the advantage overCCDs because all camera functionscan be placed on the image sensor.

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Figure 2. A CMOS imager convertscharge to voltage at the pixel, andmost functions are integrated into thechip. This makes imager functionsless flexible but, for applications inrugged environments, a CMOScamera can be more reliable.

CCD vs. CMOS

With one die, signal and powertrace distances can be shorter,with less inductance, capacitanceand propagation delays. To date,though, CMOS imagers have es-tablished only modest advantagesin this regard, largely because ofearly focus on consumer appli-cations that do not demand no-tably high speeds compared withthe CCD’s industrial, scientificand medical applications.

• Windowing. One unique ca-pability of CMOS technology isthe ability to read out a portion ofthe image sensor. This allows el-evated frame or line rates forsmall regions of interest. This isan enabling capability for CMOSimagers in some applications,such as high-temporal-precisionobject tracking in a subregion ofan image. CCDs generally havelimited abilities in windowing.

• Antiblooming, the ability togracefully drain localized overex-posure without compromising therest of the image in the sensor.CMOS generally has naturalblooming immunity. CCDs, on theother hand, require specific en-gineering to achieve this capabil-ity. Many CCDs that have beendeveloped for consumer applica-tions do, but those developed forscientific applications generally donot.

• Biasing and clocking. CMOS im-agers have a clear edge in this re-gard. They generally operate with a

single bias voltage and clock level.Nonstandard biases are generatedon-chip with charge pump circuitryisolated from the user unless there issome noise leakage. CCDs typicallyrequire a few higher-voltage biases, but clocking has been sim-plified in modern devices that op-erate with low-voltage clocks.

ReliabilityBoth image chip types are equally

reliable in most consumer and in-dustrial applications. In ultraruggedenvironments, CMOS imagers havean advantage because all circuitfunctions can be placed on a sin-gle integrated circuit chip, mini-

mizing leads and solder joints,which are leading causes of cir-cuit failures in extremely harshenvironments.

CMOS image sensors alsocan be much more highly inte-grated than CCD devices.Timing generation, signal pro-cessing, analog-to-digital con-version, interface and otherfunctions can all be put on theimager chip. This means thata CMOS-based camera can besignificantly smaller than acomparable CCD camera.

The user needs to consider,however, the cost of this inte-gration. CMOS imagers aremanufactured in a wafer fab-rication process that must betailored for imaging perfor-mance. These process adapta-tions, compared with a non-imaging mixed-signal process,come with some penalties indevice scaling and power dis-sipation. Although the pixelportion of the CMOS imager al-most invariably has lowerpower dissipation than a CCD,the power dissipation of othercircuits on the device can behigher than that of a CCDusing companion chips fromoptimized analog, digital and

mixed signal processes. At a systemlevel, this calls into question the no-tion that CMOS-based cameras havelower power dissipation than CCD-based cameras. Often, CMOS is bet-ter, but it is not unequivocally thecase, especially at high speeds (aboveabout 25-MHz readout).

The other significant considera-tions in system integration are adapt-ability, flexibility and speed ofchange. Most CMOS image sensorsare designed for a large, consumeror near-consumer application. Theyare highly integrated and tailored forone or a few applications. A systemdesigner should be careful not to in-vest fruitlessly in attempting to adapta highly application-specific device for a use to which it is not suited.

CCD image sensors, on the otherhand, are more general purpose. Thepixel size and resolution are fixed inthe device, but the user can easilytailor other aspects such as readout

Figure 3. Are they really stars? For anideal detector, each pixel’s response toa photon would be identical, and the“starlight” would be confined to the areaof the star.

CCD vs. CMOS

Choose Your ImagerCMOS imagers offer superior integration,power dissipation and system size at theexpense of image quality (particularly inlow light) and flexibility. They are the tech-nology of choice for high-volume, space-constrained applications where imagequality requirements are low. This makesthem a natural fit for security cameras, PCvideoconferencing, wireless handheld de-vice videoconferencing, bar-code scan-ners, fax machines, consumer scanners,toys, biometrics and some automotive in-vehicle uses.

CCDs offer superior image quality andflexibility at the expense of system size.They remain the most suitable technol-ogy for high-end imaging applications,such as digital photography, broadcasttelevision, high-performance industrialimaging, and most scientific and medicalapplications. Furthermore, flexibilitymeans users can achieve greater systemdifferentiation with CCDs than with CMOSimagers.

Sustainable cost between the two tech-nologies is approximately equal. This isa major contradiction to the traditionalmarketing pitch of virtually all of the solelyCMOS imager companies. G

DALSA is a leader in the design, development, manufacture, and sale of high-performance digital imagingsolutions. DALSA’s image sensor chip and electronic camera products are based on core competencies incharge-coupled device (CCD) technology and CMOS imagers. DALSA sells to original equipment manu-facturers (OEMs) requiring high performance imaging products for their vision systems. Our products arehigh speed, high resolution and highly light sensitive. We serve markets in the United States, Europe, Japanand Asia. For more information contact us at sales@dalsa.com or visit our web site at www.dalsa.com.

speed, dynamic range, binning,digitizing depth, nonlinear ana-log processing and othercustomized modes of operation.

Even when it makes economicsense to pay for sensor cus-tomization to suit an applica-tion, time to market can be anissue. Because CMOS imagersare systems on a chip, devel-opment time averages 18months, depending on howmany circuit functions the de-signer can reuse from previousdesigns in the same wafer fab-rication process. And thisamount of time is growing be-cause circuit complexity is out-pacing design productivity. Thiscompares with about eightmonths for new CCD designs inestablished manufacturingprocesses. CCD systems canalso be adapted with printed cir-cuit board modifications,whereas fully integrated CMOSimaging systems require newwafer runs.

Which costs less?One of the biggest misunder-

standings about image sensorsis cost.

Many early CMOS proponents argued that their technologywould be vastly cheaper be-cause it could be manufacturedon the same high-volume wafer pro-cessing lines as mainstream logicand memory devices. Had this as-sumption proved out, CMOS wouldbe cheaper than CCDs.

However, the accommodations re-quired for good electro-optical per-formance mean that CMOS imagersmust be made on specialty, lower-volume, optically adapted mixed-sig-nal processes and production lines.

This means that CMOS and CCDimage sensors do not have signifi-cantly different costs when producedin similar volumes and with compa-rable cosmetic grading and silicon

area. Both technologies offer appre-ciable volumes, but neither has suchcommanding dominance over theother to establish untouchableeconomies of scale.

CMOS may be less expensive at the system level than CCD, when considering the cost of related cir-cuit functions such as timing gen-eration, biasing, analog signal pro-cessing, digitization, interface andfeedback circuitry. But it is notcheaper at a component level for thepure image sensor function itself.

The larger issue around pricing,particularly for CMOS users, is sus-

tainability. Many CMOS start-ups are dedicated to high-vol-ume applications. Pursuingthe highest-volume applica-tions from a small base ofbusiness has meant thatthese companies have had toprice below their costs to winbusiness in commodity mar-kets. Some start-ups will winand sustain these prices.Others will not and will haveto raise prices. Still others willfail entirely.

CMOS users must be awareof their suppliers’ profitabil-ity and cost structure to en-sure that the technology willbe sustainable. The cus-tomer’s interest and the ven-ture capitalist’s interest arenot well-aligned: Investorswant highest return, even ifthat means highest risk,whereas customers need sta-bility because of the high costof midstream system designchange.

Increasingly, money andtalent are flowing to CMOSimaging,in large part becauseof the high-volume applica-tions enabled by the smallimaging devices and the highdigital processing speeds. Overtime, CMOS imagers shouldbe able to advance into

higher-performance applications. For the moment, CCDs and CMOS

remain complementary technologies— one can do things uniquely thatthe other cannot. Over time, thisstark distinction will soften, withCMOS imagers consuming more andmore of the CCD’s traditional appli-cations. But this process will takethe better part of a decade — at thevery least. G

Meet the authorDave Litwiller is Vice President,

Corporate Marketing at DALSA inWaterloo, Ontario, Canada.

Figure 4. Shuttering is a concern in military targetacquisition applications. A “rolling shutter” can startand stop exposure on a CMOS device, but thetechnique can result in a distorted image.

CCD vs. CMOS