CCD vs. CMOS - Teledyne DALSA Inc
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`Imaging » Knowledge Center » Application Notes and Technology Primers » CCD vs. CMOS
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`CCD vs. CMOS
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`Which is better? It's complicated...
`
`Much has been written about the relative advantages of CMOS versus CCD imagers.
`It seems that the debate has continued on for as long as most people can remember
`with no definitive conclusion in sight. It is not surprising that a definitive answer is
`elusive, since the topic is not static. Technologies and markets evolve, affecting not
`only what is technically feasible, but also what is commercially viable. Imager
`applications are varied, with different and changing requirements. Some applications
`are best served by CMOS imagers, some by CCDs. In this article, we will attempt to
`add some clarity to the discussion by examining the different situations, explaining
`some of the lesser known technical trade-offs, and introducing cost considerations
`into the picture.
`
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`CCD and CMOS sensors
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`More Information
`Blog post: Are All CCDs
`Dinosaurs?
`
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`The Evolution of CMOS
`Technology by Behnam
`Rashidian and Eric Fox, 2011
`(PDF)
`
`Teledyne DALSA CCD (left) and CMOS (right) image sensors
`
`In the Beginning...
`CCD (charge coupled device) and CMOS (complementary metal oxide
`semiconductor) image sensors are two different technologies for capturing images
`digitally. Each has unique strengths and weaknesses giving advantages in different
`applications.
`
`Both types of imagers convert light into
`electric charge and process it into
`electronic signals. In a CCD sensor, every
`
`Applications Set Imager Choices
`by Nixon O, in Advanced
`Imaging, July 2008 (PDF)
`
`https://www.teledynedalsa.com/imaging/knowledge-center/appnotes/ccd-vs-cmos/[10/7/2015 11:53:23 AM]
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`CCD vs. CMOS - Teledyne DALSA Inc
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`
`
`2005 Update:
`CMOS vs. CCD: Maturing
`Technologies, Maturing Markets
`by Dave Litwiller, in Photonics
`Spectra August 2005 (421k PDF)
`
`
`CCD vs. CMOS: Facts and
`Fiction by Dave Litwiller, in
`Photonics Spectra, January 2001
`(385k PDF)
`
`pixel's charge is transferred through a
`very limited number of output nodes (often
`just one) to be converted to voltage,
`buffered, and sent off-chip as an analog
`signal. All of the pixel can be devoted to
`light capture, and the output's uniformity
`(a key factor in image quality) is high. In a
`CMOS sensor, each pixel has its own
`charge-to-voltage conversion, and the
`sensor often also includes amplifiers,
`noise-correction, and digitization circuits, so that the chip outputs digital bits. These
`other functions increase the design complexity and reduce the area available for light
`capture. With each pixel doing its own conversion, uniformity is lower, but it is also
`massively parallel, allowing high total bandwidth for high speed.
`
`CCD and CMOS imagers both depend on
`the photoelectric effect to create electrical
`signal from light
`
`CCDs and CMOS imagers were both invented in the late 1960s and 1970s (DALSA
`founder Dr. Savvas Chamberlain was a pioneer in developing both technologies).
`CCD became dominant, primarily because they gave far superior images with the
`fabrication technology available. CMOS image sensors required more uniformity and
`smaller features than silicon wafer foundries could deliver at the time. Not until the
`1990s did lithography develop to the point that designers could begin making a case
`for CMOS imagers again. Renewed interest in CMOS was based on expectations of
`lowered power consumption, camera-on-a-chip integration, and lowered fabrication
`costs from the reuse of mainstream logic and memory device fabrication. Achieving
`these benefits in practice while simultaneously delivering high image quality has taken
`far more time, money, and process adaptation than original projections suggested,
`but CMOS imagers have joined CCDs as mainstream, mature technology.
`
`High Volume Imagers for Consumer Applications
`With the promise of lower power
`consumption and higher integration for
`smaller components, CMOS designers
`focused efforts on imagers for mobile
`phones, the highest volume image sensor
`application in the world. An enormous
`amount of investment was made to
`develop and fine tune CMOS imagers and
`the fabrication processes that
`manufacture them. As a result of this investment, we witnessed great improvements
`in image quality, even as pixel sizes shrank. Therefore, in the case of high volume
`consumer area and line scan imagers, based on almost every performance parameter
`imaginable, CMOS imagers outperform CCDs..
`
`Mobile phones drive CMOS imager volume
`
`Imagers for Machine Vision
`In machine vision, area and line scan imagers rode on the coattails of the enormous
`mobile phone imager investment to displace CCDs. For most machine vision area
`and line scan imagers, CCDs are also a technology of the past.
`
`The performance advantage of CMOS imagers over CCDs for machine vision merits a
`brief explanation. For machine vision, the key parameters are speed and noise.
`CMOS and CCD imagers differ in the way that signals are converted from signal
`charge to an analog signal and finally to a digital signal. In CMOS area and line scan
`imagers, the front end of this data path is massively parallel. This allows each
`amplifier to have low bandwidth. By the time the signal reaches the data path
`bottleneck, which is normally the interface between the imager and the off-chip
`circuitry, CMOS data are firmly in the digital domain. In contrast, high speed CCDs
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`CCD vs. CMOS - Teledyne DALSA Inc
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`have a large number of parallel fast output channels, but not as massively parallel as
`high speed CMOS imagers. Hence, each CCD amplifier has higher bandwidth, which
`results in higher noise. Consequently, high speed CMOS imagers can be designed to
`have much lower noise than high speed CCDs.
`
`However, there are important exceptions to this general statement.
`
`Near Infrared Imagers
`To image in the near infrared (700 to
`1000nm), imagers need to have a thicker
`photon absorption region. This is because
`infrared photons are absorbed deeper than
`visible photons in silicon.
`
`Most CMOS imager fabrication processes are
`tuned for high volume applications that only
`image in the visible. These imagers are not
`very sensitive to the near infrared (NIR). In
`fact, they are engineered to be as insensitive
`as possible in the NIR. Increasing the
`substrate thickness (or more accurately, the
`epitaxial or epi layer thickness) to improve
`the infrared sensitivity will degrade the ability
`of the imager to resolve spatial features, if the
`thicker epi layer is not coupled with higher
`pixel bias voltages or a lower epi doping
`levels. Changing the voltage or epi doping will
`affect the operation of the CMOS analog and
`digital circuits.
`
`Cracks in silicon solar cells are obvious
`with NIR imaging
`
`CCDs can be fabricated with thicker epi layers while preserving their ability to resolve
`fine spatial features. In some near infrared CCDs, the epi is more than 100 microns
`thick, compared to the 5 to 10 micron thick epi in most CMOS imagers. The CCD
`pixel bias and epi concentration also has to be modified for thicker epi, but the effect
`on CCD circuits is much easier to manage than in CMOS.
`
`CCDs that are specifically designed to be highly sensitive in the near infrared are
`much more sensitive than CMOS imagers.
`
`Ultraviolet Imagers
`Since ultraviolet photons are absorbed very
`close to the silicon surface, UV imagers must
`not have polysilicon, nitride or thick oxide
`layers that impede the absorption of UV
`photons. Modern UV imagers are hence
`backside thinned, most with only a very thin
`layer of AR coating on top of the silicon
`imaging surface.
`
`Although backside thinning is now ubiquitous
`in mobile imagers, UV response is not. To
`achieve stable UV response, the imager
`surface requires specialty surface treatment,
`regardless of whether the imager is CMOS or
`CCD. Many backside thinned imagers
`developed for visible imaging have thick
`oxide layers that can discolor and absorb UV after extended UV exposure. Some
`backside thinned imagers have imaging surfaces that are passivated by a highly
`doped boron layer that extends too deep into the silicon epi, causing a large fraction
`of UV photogenerated electrons to be lost to recombination.
`
`Today's deep submicron lithography
`requires deep UV light for quality
`inspection
`
`UV response and backside thinning are achievable in all line scan imagers, but not all
`area imagers. No global shutter area CCD can be backside thinned. The situation is
`better in CMOS area imagers, though still not without trade-offs. CMOS area imagers
`with rolling shutter can be backside thinned. Conventional CMOS global shutter area
`imagers have storage nodes in each pixel that need to be shielded when thinned, but
`only if these UV sensitive imagers will also be imaging in the visible. In backside
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`CCD vs. CMOS - Teledyne DALSA Inc
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`thinned area imagers, it is not possible to effectively shield part of the pixel from
`incident illumination, without severely degrading the imager’s fill factor (the ratio of the
`light sensitive area to the total pixel area). There are other types of CMOS global
`shutter area imagers that do not have light sensitive storage nodes, but have higher
`noise, lower full well, rolling shutter, or a combination of these.
`
`Time Delay and Integration Imagers
`Aside from area and line scan imagers, there is another important type of imager.
`Time delay and integration (TDI) imagers are commonly used in machine vision and
`remote sensing and operate much like line scan imagers, except that a TDI has
`many, often hundreds, of lines. As the image of the object moves past each line,
`each line captures a snapshot of the object. TDIs are most useful when signals are
`very weak, since the multiple snapshots of the object are added together to create a
`stronger signal.
`
`
`
`TDI imagers combine multiple exposures synchronized with object motion
`
`Currently, CCD and CMOS TDIs sum the multiple snapshots differently. CCDs
`combine signal charges, while CMOS TDIs combine voltage signals. The summing
`operation is noiseless in CCDs, but not in CMOS. When a CMOS TDI has more than
`a certain number of rows, the noise from the summing operation adds up to the point
`that it becomes impossible for even the most advanced CMOS TDI to have less noise
`than a modern CCD TDI.
`
`One path forward for CMOS TDIs is to emulate CCD TDIs by having CCD-like pixels
`which can then sum charges. We’ll call this the charge domain CMOS TDI. Charge
`domain CMOS TDIs are technically feasible, but would require significant investment
`to develop, fine tune, and perfect. Unlike CMOS area and line scan imagers, the
`economics do not favour charge domain CMOS TDIs. Mobile phones need neither
`TDIs nor charge summing. Hence, there is no coattail for CMOS TDIs to ride on.
`
`Electron Multiplication
`Electron multiplication CCDs (EMCCDs)
`are CCDs with structures to multiply the
`signal charge packet in a manner that
`limits the noise added during the
`multiplication process. This results in a
`net signal-to-noise ratio (SNR) gain. In
`applications where the signal is so faint
`that it is barely above the imager noise
`floor, EMCCDs can detect previously
`indiscernible signals.
`
`EMCCDs are useful for very low signal
`applications, typically in scientific imaging
`
`Compared to CMOS, EMCCDs are most
`advantageous when the imager does not
`need to image at high speed. Higher
`speed operation increases the read noise in CCDs. Hence, even with the SNR
`improvement from the EMCCD, the difference between an EMCCD and a CMOS
`imager may not be much, especially when compared to scientific CMOS imagers that
`are specifically designed to have very low read noise. High speed EMCCDs also
`dissipate significantly more power than conventional imagers.
`
`Low noise CMOS imagers may not have the NIR, UV, or TDI integrating advantages
`of a CCD. Consequently, because the signal can be much weaker, even when the
`read noise is comparable to what an EMCCD can achieve, an EMCCD solution may
`still be better overall.
`
`Cost Considerations
`
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`CCD vs. CMOS - Teledyne DALSA Inc
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`So far, we have focused on the performance
`differences between CMOS and CCD
`imagers. It would be naive to assume that
`business decisions are based on
`performance trade-offs alone. What matters
`more to many business decision-makers is
`value, or the performance received for the
`price paid.
`
`The cost picture can be complicated, so we
`will focus only on a few important points.
`
`Leverage, volume, yield, and the
`number of devices per wafer all affect
`cost
`
`First, leverage is key. At the risk of stating the obvious, imagers that are already on
`the market will cost much less than a full custom imager, regardless of whether it is a
`CMOS or a CCD imager. If customization is necessary, unless the change is minor, it
`is generally cheaper to develop a custom CCD than it is to develop a custom CMOS
`imager. CMOS imager development is generally more expensive because CMOS
`uses more expensive deep submicron masks. There is also much more circuitry to
`design in a CMOS device. As a result, even in applications where a custom CMOS
`imager clearly has better performance, the value proposition can still favor a custom
`CCD.
`
`Secondly, volume matters. Although the cost to develop a new CMOS imager is
`higher, CMOS imagers that can leverage from larger economies of scale will have
`lower unit cost. With high volumes, a low unit cost can be financially more important
`than a low development cost.
`
`Third, supply security is important. It is very costly to be left with a product that is
`designed around an imager that is discontinued. In spite of a better value proposition,
`it may be wiser to choose the company which is best able to produce the imager –
`CMOS or CCD – long term.
`
`Conclusion
`
`Choosing the correct imager for an
`application has never been a simple
`task. Varied applications have varied
`requirements. These requirements
`impose constraints that affect
`performance and price. With these
`complexities at play, it is not surprising
`that it is impossible to make a general
`statement about CMOS versus CCD
`
`imagers that applies to all applications.
`
`CMOS area and line scan imagers outperform CCDs in most visible imaging
`applications. TDI CCDs, used for high speed, low light level applications, outperform
`CMOS TDIs. The need to image in the NIR can make CCDs a better choice for some
`area and line scan applications. To image in the UV, the surface treatment after
`backside thinning is key, as is the global shutter requirement. The need for very low
`noise introduces new constraints, with CMOS generally still outperforming CCDs at
`high readout speeds. The price-performance trade-off can favor either CCD or CMOS
`imagers, depending on leverage, volume, and supply security.
`
`
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`CCD vs. CMOS - Teledyne DALSA Inc
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