5th CMOS Imager Workshop, Duisburg, May 4-5, 2010.
DAY 1
Holger VOGT (Fraunhofer IMS, Duisburg, Germany) : “Devices and technologies for CMOS Imaging”
The first talk on the first day gave an good introduction to the workshop. In the first part of the talk several CMOS detectors were reviewed (photodiode, buried photodiode, pinned photodiode and photogate pixels). Special attention was given to the effect of emptying the pixels at higher speed and how to introduce a lateral drift field in the pixels. At the end of the talk several projects and topics were illustrated that form part of the IMS research portfolio. Examples are :
– Colour by metal grids,
– Colour by depth sensing in the silicon,
– The low noise double modified internal gate pixel,
– SPADs,
– BACKSPAD (back-side illuminated SPAD) and,
– Uncooled Bolometers.
Nice opening of the workshop because of the wide overview, with a bit of publicity for the Fraunhofer IMS institute. But they deserve it, because they are the organizer of the workshop.
Lindsay GRANT (ST Microelectronics, Edinburg, UK) : “CMOS image sensors and technology”
In the mean time I have heard several presentation of Lindsay, and they all come down to a very broad overview of the CMOS technologies needed for mobile imaging. And if you have heard a few of them, you get a very good insight in how rapidly this technology is evolving. The topics addressed in this talk are too many to list here, but (for me) the main ones are :
– 0.9 um pixel size on the roadmap, 1.1 um in demo,
– Progress in pixel modeling (optical and device physics),
– Pixel optics,
– Colour improvements,
– Back-side illumination and crosstalk,
– SNR performance metric.
In his last sheet he tried to show us : “What’s next ?” In short :
– The pixel race continues,
– Front-side illumination will remain cost/performance competitive,
– Sensor image quality assessment will continue to a topic in active research.
At the end of the talk Lindsay acknowledged the late Peter Denyer for his inspiring leadership.
Mark ROBBINS (e2v, Chelmsford, UK): “Electron Multiplying CCDs”
The EM-CCD is intended for imaging in a photon starved environment where all sources of noise must be minimized. EM-CCD reduces the effect of charge to voltage conversion noise and noise from the video chain. After a short description of how the EM-CCD works, Mark spent quite a bit of time on the introduction of the noise factor and on the dependency of the gain as a function of temperature and gate voltage. He showed nice results for the EM-CCD in the photon counting mode. In the last part of the talk, the Rose criterion was introduced to quantify the visibility of a feature in a noisy image. The theory was illustrated with images under extreme low light level conditions. As can be expected, the ultimate low-light level sensor will be the EM-CCD in combination with back-side illumination. Interesting to notice that up to this point in the workshop, all speakers were referring to BSI.
Frank ZAPPA (Politecnico di Milano, Milano, Italy) : “SPADs”
In the overview presentation about SPADs, Frank addressed the following topics :
– Single photon counting and timing, such as PMTs, special CCDs (EM-CCD, I-CCD), SSPD, SPAD
– Single photon avalanche diode,
– Circuital modeling, static as well as dynamic,
– Devices structures with focus on planar versus reach-through,
– Processing technologies with focus on custom versus CMOS,
– Circuits : monolithic versus smartchips, detection as well as counting chips,
– Arrays for single-photon imaging.
As a conclusion, Frank stated that SPAD detectors and arrays, microelectronics and instrumentation are available, know-how is present for custom development, and commercial products based on SPADs are available on the market.
Gerhard LUTZ (PNSensor, Munich, Germany) : “Silicon Radiation Detectors”
Sometimes one forgets that there is much more than CCD or CMOS image sensors to detect radiation, but Gerhard put us back with two feeds on the ground. He discussed the basic detection process of radiation in semiconductors, reviewed the basic principles of semiconductor detectors such as the reverse biased diode, the semiconductor drift chamber and the DEPFET detector-amplifier structure. It was quite funny to see the presentation of the good old junction CCDs, never thought that still some products were made out of this technology. But the more you think about, the more intriguing the devices are. The same is true for the DEPFETs. These unique devices are able to satisfy a variety of different requirements depending on specific applications. More sophistic variations of this structure have been invented, their functioning has been proven by simulations and by measurements of finished devices.
Albert THEUWISSEN (Harvest Imaging, Bree, Belgium) : “Noise : you love it or you hate it”
A simulation and evaluation tool is described. The simulation tool accepts the specification of an image sensor as input and creates images. One of the main applications of this simulation software is the study of the various noise sources present in an imager/camera. The artificial images created can be the input for the evaluation tool. But also images generated by a real camera can be used as the input for the evaluation tool. During the presentation an example was shown of the combination simulation-evaluation of images. Also real images generated by a CMOS camera were analyzed. During the talk the main focus was on images created in dark. Even without any light input several important noise contributions can be measured/analyzed. The algorithms applied in the evaluation tool will be part of the new training course that will be offered by Harvest Imaging later this year.
Pierre MAGNAN (ISAE, Toulouse, France) : “Ionization effects in CMOS imagers”
In the first part of the presentation the theory of the different defects and artifacts that can be generated by radiation were discussed. It was clearly shown how complex the physics are behind radiation effects in CMOS image sensors. Attention was given to :
– The generation of electron-hole pairs in the various materials involved,
– Charge transport in the silicon dioxide,
– Charge trapping in silicon dioxide,
– Radiation induces interface traps.
Then the question was answered : and what is going to be the influence on the CIS performance parameters of all these beautiful artifacts ? I can be expected, in the first place the dark current will increase, but also the light response will be changed, unfortunately the light response will become lower.
Pierre ended his talk with some ideas about how to make a design radiation hard.