The Future Is Bright for CCD Sensors TELEDYNE IMAGING

The Future is Bright for CCD Sensors TELEDYNE IMAGING

CCDs will Continue to Provide a Crucial Imaging Capability for Science

omplementary metal-oxide- CMOS technology accounts for the semiconductor (CMOS) image majority of the visible wavelengths market, sensors now dominate the by revenue as highlighted by Yole Dévelop- Cimaging detector market, but pement Status of the CMOS Image Sensor there are industrial and scientific imaging (CIS) Industry 2018 report. applications where charge-coupled device However, while the economies of scale (CCD) imager sensors is still the preferred and sheer momentum behind CMOS sensor choice, from both technical and commer- development are huge — predominantly in cial perspectives. consumer applications — it is far too early According to market research company, to write off CCD sensors. Research and Markets, the global image As technologies and markets evolve, sensors market is expected to reach $23.2 what is technically feasible and what is billion by 2023 (as of April 2018). commercially viable also evolve, and many Continued market growth is mainly imaging applications in spectroscopy, driven by rising demand for dual-camera microscopy, medium to large ground mobile phones, the development of telescopes and space science will continue low-power and compact image sensors, to be better served by CCDs. and the increased use of image sensing devices in biometric applications.

Teledyne e2v CCD Bruyères detector in handling jig for the ESA PLATO mission.

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Scientific applications include optical electron microscopy.

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Teledyne e2v CIS120 Invented in 1969, Charge-coupled devices CCD AND CMOS BASICS CMOS general purpose where originally designed as a memory CCD and CMOS are both silicon based image sensor for space storage device in the United States at AT&T detector arrays: applications. Bell Labs by Willard Boyle and George E. » Large-format, back thinning is possible Smith, with their potential as imaging de- for both tectors realized shortly afterwards. Notably » Detection efficiency is the same CCD devices became the work-horse detector in ESA and NASA missions for the CMOS have some advantages over CCDs: last thirty years. » Higher readout speed and greater CCD detectors are made from silicon, flexibility which defines their spectral sensitivity. Most » Less prone to some radiation effects often used in UV, Visible or NIR applications, » Simpler interfacing they can also be used for soft-X-ray detection. However, many aspects of CCDs remain Notably Teledyne e2v, in the UK, continues superior to CMOS: to maintain a vertically integrated dedicated » Dynamic Range CCD fab, make technology developments » Longer wavelength (NIR) performance to the design and production of CCDs, » Time Delay and Integration (the Gaia and is committed to the provision of high space telescope would not be practical performance and customized CCD detec- with CMOS) tors and packages on a longterm basis. » Electron Multiplication » Deep understanding of performance and It is worth noting that the ability to reliability through decades of study customize the CCD design is one of the key enabling factors of high-performance Based on the above, it is clear that CMOS imaging. Comparing the merits of both and CCDs have similar performance for CMOS and CCD technology highlights that many parameters but there will always one will not satisfy all requirements over remain critical aspects for which CCDs are the other. the only solution.

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“Teledyne is at the forefront of CMOS sensor design for scientific and space applications, and is the industry standard for CCD imaging, that is how we know when to use CCD and when to use CMOS.”

Both CCD and CMOS imagers convert For Time Delay Integration (TDI) applica- light energy into an electric voltage and tions CMOS has developed to a level that process this into an electronic signal. will likely prevent future CCD TDI develop- In CCD sensors, every pixel’s charge is ments. The summing operation in Teledyne transferred through a very limited number CMOS TDI sensors are now noiseless and of output nodes (often just one) to be can also have CCD type operation embed- converted to voltage, buffered, and sent ded in the CMOS chip giving cost effective off-chip as an analog signal. All of the pixel charge domain operation. can be devoted to light capture, and the Electron multiplication CCDs (EMCCDs) output’s uniformity (a key factor in image multiply the signal charge packet, resulting quality) is high. in a net signal-to-noise ratio (SNR) gain. In CMOS sensors, each pixel has its own EMCCDs can detect signals in applications charge-to-voltage conversion, and the where the signal is so faint that it would sensor often also includes amplifiers, noise- not normally be above the imager noise correction, and digitization circuits, so that floor. This means that EMCCDs are widely the chip outputs digital bits. With each pixel used for the very lowest signal scientific doing its own conversion, uniformity is applications especially where counting of lower, but it is also massively parallel, individual photons is required. allowing high total bandwidth for high speed.

INDUSTRIAL IMAGING ADVANTAGES In certain industrial and scientific imaging CCD201 EMCCD that applications CCD imagers still offer will be used in the significant advantages over CMOS. Coronagraph Instrument for NASA’s One such application is near-infrared WFIRST mission. imaging (700–1,000 nm). CCDs that are specifically designed to be highly sensitive in the near-infrared are much more sensitive than CMOS imagers. At these wavelengths, imagers need to have a thicker photon absorption region, because infrared photons are absorbed deeper than visible photons in silicon. CCDs can be fabricated with thicker epitaxial layers, while maintaining the ability to resolve fine spatial features. In some near-infrared CCDs, this layer can be more than 100 microns thick, compared with the 5–10 micron epitaxial layer of most CMOS imagers.

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Contains modified Copernicus Sentinel data (2019), processed by ESA, CC BY-SA 3.0 IGO

Based on measurements SPACE IMAGING ADVANTAGES CCD imagers offer better QE at the gathered by the Space imaging has several unique challenges, longest wavelengths, higher dynamic range, Copernicus Sentinel-5P and typically requires highly optimized and better uniformity than CMOS imagers mission between January image sensors. The high cost of launching — all of which are critical for space science and April 2019, the image shows high levels of a camera into space means both perfor- and hyperspectral applications. nitrogen dioxide in the mance and heritage are critical. This is why Space-based astronomy usually requires Po Valley in northern CCDs have continued to dominate the very large, even wafer-scale, devices. Italy. The Sentinel-5P’s space market after they have stopped being Currently, only CCDs have the required TROPOMI instrument used for most terrestrial applications. image quality and TRL maturity for such uses Teledyne e2v’s For most space applications very high missions. Although it should be noted that CCD275 detector. electro-optical performance is critical, Teledyne is producing wafer scale CMOS especially when it comes to quantum sensors for a ground based X-ray efficiency (QE), noise, dynamic range (DR), applications. dark signal (or leakage current), uniformity and performance repeatability. Space applications also require radiation resistance, low power dissipation (CMOS typically performs better in this respect), reliability and longterm stability. It can be said for all these latter parameters CMOS meets or exceeds CCD technology.

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Hubble Space Telescope view of Jupiter, taken on June 27, 2019. The image is a composite of separate exposures acquired by the WFC3 instrument.

7 Credits: NASA, ESA, A. Simon (Goddard Space Flight Center) and M.H. Wong (University of California, Berkeley) of (University Wong NASA, ESA, A. Simon (Goddard Space Flight Center) and M.H. Credits: TELEDYNE IMAGING

Silicon wafers entering QUALITY AND PRODUCT intimately optimized over many years. The the furnace as part of a ASSURANCE CCD foundry is used only for CCD production, metalization process. The Teledyne e2v fab facility in the UK offering process stability and repeatability. highlights some unique benefits over the In contrast, CMOS detectors are typically way CMOS foundries operate. The UK facility designed at ‘fab-less’ design houses and includes the CCD semiconductor fabrica- manufactured in commercial semicon- tion line. Silicon wafers enter at one end ductor foundries producing many different and a finished packaged CCD with filters CMOS devices. The main reason being that and mechanical features emerges at the a modern CMOS fabrication plant consti- other. This allows Teledyne e2v to have full tutes a very large investment coupled to knowledge and control over the manufac- a high running cost and hence produce turing processes of the detector, information orders of magnitude higher volumes. that can be shared with strategic partners and This means process and design stability constitutes an essential part of the quality is continuously evolving for CMOS and assurance (QA) and product assurance (PA) presents increased challenges for QA and aspects of using these components in space. PA as well as gaining insight to processing Detector design is also in-house and, like details, key requirements for space qualifi- the manufacturing processes, both have been cation activities.

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Teledyne e2v CCD fab – corridor leading to multiple CCD processing clean rooms.

RADIATION HARDNESS reset of power cycling. While these can be Image sensors in space encounter high-energy considered ‘soft’ transient errors, heavier protons, electrons, and galactic cosmic particles and energetic protons can cause rays. This causes both gradual deterioration single event latch-ups (SELs) in CMOS in sensor performance and instantaneous circuits, which can damage an image single event effects (SEEs). The long-term sensor permanently. effects are caused by ionization damage in CCDs do not suffer from SEU, SEFI or SEL the insulators (typically oxides and nitrides) because of the lack of CMOS logic circuitry and their interfaces to the active silicon, and parasitic thyristors. Qualifying the and by displacement damage to the silicon radiation hardness of an image sensor is crystal lattice. Both can lead to an undesir- usually a time-consuming job and requires able increase of the dark current, noise and a great level of expertise in device physics charge transfer inefficiency. and operation. Both CCDs and CMOS sensors are subject to radiation damage and whilst CMOS is LONG-TERM AVAILABILITY more resistant to proton radiation, CCDs Most space science missions take decades offer mitigation in this area in other ways. from conception to launch. By contrast, Instantaneous effects include single event commercial technology development upsets (SEU) which can corrupt memory timescales are much shorter. This can create elements in CMOS sensors or interfere with concerns around the long-term availability the normal operation of their logic circuitry. of the preferred manufacturing processes Single event functional interrupt (SEFI) (especially in fast-moving consumer causes major disruption in the operation electronics), and the longevity of the semi- of CMOS circuitry, but is correctable via conductor vendor.

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What’s more industrial and consumer sensor technologies tend to go in con- trasting directions to space and science. Space agencies and scientific organizations can take-heed that there is a longterm supply of the required sensor technologies focused on space and science needs, from Teledyne, to support them.

HERITAGE The image sensor constitutes a critical point of failure and redundancy is generally deemed as impractical. The huge costs of building and launching spacecraft, and the risk of mission loss, mean the space industry is very conservative and risk-averse. Image sensors and cameras with problem-free flight heritage are highly valued, and may often be preferred to more capable but untested alternatives. CCDs have tens of thousands of years of cumulative space heritage while CMOS sensors are only finding use in space in recent times. The investment required to reach reliability comparable CCD is huge.

COMMERCIAL CONSIDERATIONS Of course, the choice of imaging sensor will never come down to just technical superiority: commercial considerations are clearly an important factor. The cost picture for imaging sensors is complicated, but there are a few important points to consider. The FIRST is leverage: imagers that are already on the market will cost much less than a full custom imager, regardless of fundamental technology. Whenever customization is required, it is generally less costly to develop a custom CCD than a custom CMOS imager for all but the most insignificant adaptations. This is because CMOS imagers use more expensive deep submicron masks, and have much more on-chip circuitry. SECOND, volume matters. Although the higher development cost of a new CMOS

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imager can be recouped when production batches over a short period of time. How- (Above and left) CCD fab volumes are high, for small production ever it can become an issue if repeat runs inspection and process runs, the lower development costs of of the same build are required several years control. CCDs can be an important factor. apart; re-qualification may be needed. THIRD, security of supply is vital: it is very costly to be left with a product that is CONCLUSION designed around a sensor that is discontin- The large majority of high-performance ued. Whatever the value proposition, it may applications currently use CCDs thanks to be wiser to choose a sensor source that is their heritage, excellent uniformity and QE. committed to continued development and Furthermore, the development of CMOS manufacture for the longterm. sensors for space imaging will continue. For longer-term projects, the use of However, certain scientific applications, commercial CMOS foundries can be such as astronomy, spectroscopy and problematic, as the manufacturing process microscopy, will also continue to rely on undergoes continual improvement to meet CCD performance. commercial needs. The changes may not It is necessary for instrument designers always be visible from one manufacturing to take note of the different detector opera- run to the next, which may be fine for tion requirements and not to expect CMOS projects requiring just one or two silicon as a direct replacement in all applications.

11 TELEDYNE IMAGING Credit: ESA - P. Carril, 2010 ESA - P. Credit:

Teledyne e2v ultraviolet With some leading manufacturers in enhancements to the signal handling and laser detector technol- recent times ending or planning to end noise performance, increasing the tolerance ogy, CCD69, deployed their CCD production, it will be important to non-ionising radiation, and changes to on ESA’s Aeolus Satellite for industrial and scientific imaging system further increase the UV and NIR quantum to profile the world’s winds. designers to have continued access to CCD efficiency and stability. development expertise and manufacturing Teledyne e2v, in the UK, continues to capability for specialist industrial and develop a longterm, vertically integrated, scientific imaging applications, especially dedicated CCD fab and make technology for space imaging applications. developments to the design and production Despite their maturity, CCDs have not of CCDs. Teledyne is committed to providing reached the end of their development, and the correct standard, customized and high several aspects have been identified which performance CCD and CMOS solutions, could be of benefit for future science investing significantly in CCDs, CMOS and applications and space missions for example. related imaging technologies. Identified improvement areas include