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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. 2 teledyneimaging.com THE FUTURE IS BRIGHT FOR CCD SENSORS Scientific applications include optical electron microscopy. 3 TELEDYNE IMAGING 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. 4 teledyneimaging.com THE FUTURE IS BRIGHT FOR CCD SENSORS “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. 5 TELEDYNE IMAGING 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. 6 teledyneimaging.com THE FUTURE IS BRIGHT FOR CCD SENSORS 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
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