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EETE SEPTEMBER 2012

DESIGN & PRODUCTS OPTOELECTRONICS Merging the benefits of CCD and CMOS for imaging By Enrico Marchesi OvEr thE last dEcadEs, the imager world has seen numer- ous publications on the comparison of ccd and cMOs imaging technologies. Intense discussions and even fierce disputes about the superiority of either approach took place. While these discussions and the development they have spurred may have been fruitful in terms of technological progress, users of such imagers were forced to make a choice and to accept the sub- sequent consequences. There were distinct trade-offs between Fig. 1: epc’s back side illumination concept. the two approaches that needed close consideration and care- ful balancing with the application in mind. of proprietary process design features allow for the definition of From a user’s point of view this wasn’t always desirable. dedicated areas for charge handling within each pixel sepa- Why couldn’t one have their cake and eat it too? and what rately while at the same time keeping the option of true cMOs coherent reasoning did we offer the application engineer for analogue pixel design. and combined with an advanced Back that constraint? after all, both technologies are based on the side Illumination (BsI) concept, this full design freedom comes same semiconductor process technology. there are, of course, without any drawback in the fill factor. reasons that explain to some extent why these two approaches emerged in parallel. Nevertheless, an inherent physical or tech- Starting with quantum efficiency nological separation no longer exists. a pivotal requirement for the process development was the Just over a decade ago, there was no arguing about which maximization of the quantum efficiency over a broad wave- technology had the crown when it came to actual imaging per- length range reaching well into the near infrared. today’s imag- formance. ccds had a 30 year history during which they were ing applications are not confined to a specific wavelength. From specifically tailored to and optimized for imaging applications. classical consumer and performance imaging in the visible technical limitation in process technology kept cMOs well spectrum to Ir operation in industrial applications and further to behind. In the 1990’s, however, cMOs imagers had emerged multispectral and hyperspectral imagers in space, each applica- as candidates with a compelling potential. Being used in large tion typically requires its specific range. Naturally, the more an volumes for memory and logic devices, the cost potential of this imager process is able to cover, the higher the potential for suc- technology was highlighted on the radar of imager companies. cess. Furthermore, as noise is always an issue in imaging, any reduced power consumption and a high degree of integration increase in quantum efficiency will basically improve the noise (soc devices) matched the requirements of the rapidly growing behaviour with all other factors being equal. there’s simply consumer electronics markets. more charge available to create a signal. today it seems that cMOs has won the race. Not all the promises the absorption length of light in were fulfilled as expected, though. The silicon inherently determines the quan- predicted cost advantage was surely tum efficiency. Hence it is important to not met as significant increases in pixel consider the thickness of the depleted and process complexity were necessary detector area as one of the key design in order to catch up with ccd imag- parameters. the choice of a suitable ing performance on one hand and to substrate material contributes among compensate for the detrimental effects other things to a fully depleted detec- on analog signals in sub-micron cMOs tor thickness of 50µm in epc imagers. structures on the other. Neverthe- Figure 1 illustrates this basic concept. less, the advantages still dominate. In epc detectors feature an entirely sen- 2011 cMOs sensors accounted for 92 sitive backside with an optical entrance percent of all area image sensors and window that can be further tuned with Fig. 2: Quantum efficiency of epc detectors. this share is projected to increase to 97 appropriate anti-reflection coatings. The percent by 2015. The difficult choice seems over. devices are thinned to a thickness of 50µm. In effect, the chip is depleted through the entire thickness of 50µm. the resulting Espros Photonics (epc) from Switzerland has developed quantum efficiency is essentially limited only by the material and industrialized its CMOS process to break away with the properties of silicon-based detectors – see figure 2. And unlike traditional separation of cMOs and ccd functionality. Within many conventional BsI imagers, epc imagers do not require this environment, the process does not strictly determine the wire bonding to establish the depletion voltage. the backside border between the charge and voltage domain. a special set contacts are a process-specific feature and all physical con- tacts remain on the frontside of the chip. Enrico Marchesi is head of Marketing & sales at Espros The deployed measures to optimize quantum efficiency are Photonics aG - www.espros.ch not a stringent precondition per se for a successful merge of the 28 Electronic Engineering Times Europe September 2012 www.electronics-eetimes.com


EETE SEPTEMBER 2012
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