5th CMOS Imager Workshop, Duisburg, May 4-5, 2010.
Boyd FOWLER (Fairchild Imaging, Milpitas, CA) : “Scientific CMOS Image Sensor”
The specific sensor architecture and sensor operation was explained, but that part of the talk was very similar to the one presented last year in Toulouse. The most interesting details came when Boyd discussed the performance of the device. He compared the high gain channel with the low gain channel, both in rolling shutter mode as well as global shutter mode. Worthwhile to mention is the mean noise performance of the various modes :
- Read noise of 1.5 e- high gain channel in rolling shutter mode (100 MHz),
- Read noise of 9.2 e- low gain channel in rolling shutter mode (100 MHz),
- Read noise of 1.9 e- high gain channel in rolling shutter mode (290 MHz),
- Read noise of 10.3 e- low gain channel in rolling shutter mode (290 MHz),
- Read noise of 4.8 e- high gain channel in global shutter mode (100 MHz),
- Read noise of 13.3 e- low gain channel in global shutter mode (100 MHz),
- Read noise of 5.7 e- high gain channel in global shutter mode (290 MHz),
- Read noise of 14.7 e- low gain channel in global shutter mode (290 MHz).
The pixel is a 5T cell that can be operated with CDS in the rolling shutter mode, but without CDS in the global shutter mode (digital CDS is performed off-chip). That is the explanation why the rolling shutter mode is that much superior over the global shutter mode. Together with all these numbers, several histograms about noise distribution and dark current distribution are shown. At the end of the talk, images were shown of the colour version of this device, as well as of the BSI-ed version. QE levels over 90 % were reported (if the appropriate anti-reflective coating was applied).
Alex KRYMSKI (Alexima, CA) : “Design of CMOS imagers : Selected Circuits & Architectures”
The focus of the presentation was a series of useful circuits and architectures that were created over the last decade. If you tell people that you will talk about useful circuits, you inherently admit that there exists something like useless circuits as well, and indeed Alex did. He even showed examples of useless circuits.
Examples of useful circuits were : clamping source follower input and output, dynamic source follower, driving rows from both sides, multiple ADCs per column of the imager, multiple busses in the pixel array, pipelining top-bottom and block-memory readout. Together with circuit diagrams, Alex explained the working principles of the circuits and illustrated the concept with products in which these circuits are applied. Also the device for which he and his co-workers received the 2003 Walter Kosonocky Award was highlighted.
Guy MEYNANTS (CMOSIS, Antwerp, Belgium) : “CMOS image sensors for industrial applications”
The outline of Guy’s talk :
- High speed imaging and other requirements for industrial imaging,
- CIS architectures with analog architectures for the fastest, customized imagers, (conclusion about this chapter : analog offered the highest speed so far, mainly obtained by parallelism, but there are limited capabilities to further speed up),
- CIS architectures with on-chip ADC for easy-to-use and easy-to-integrate imagers, (great overview of the various ADC architectures was given),
- Case study : 2 Mpixel imager at 340 fps with global shutter and CDS. A new shutter type implemented with an 8T pixel was shown with a parasitic light sensitivity of 1/60,000. Taken into account a pixel size of 5.5 um, this is an amazing performance.
Walter RUETTEN (Philips Research, Aachen, Germany) : “Solid-State X-ray Imaging”
The talk started with an overview of the various X-ray detection systems : Screen-film system, image intensifiers, fully digital solid state detector systems, photoconductors and scintillators. A parameter that is not that often used in (consumer) digital imaging is the Detective Quantum Efficiency. Although in the medical imaging field, the DQE is a very important characteristic. Walter explained the definition as well as the importance of DQE. A nice example based on numbers and figures (that is what engineers like to see) explained the typical noise issues in an X-ray system.
The second part of the talk was concentrating on monolithic silicon detectors : large area CMOS image sensors, 3 sides buttable that allow to build very large detectors (40 cm x 40 cm). A first test chip of such a detector is available with high speed readout architecture implemented. With this test chip most features of the large area detector can be tested. The very first X-ray experiments show at least the same or even better DAE with higher resolution compared to the commercially available detectors.
What the future going to bring in medical imaging ? A potential next step could be spectral imaging in which the energy of the X-ray can detected (“colour X-ray”). Apparently an interesting “X-ray future” ahead of us.
Werner BROCKHERDE (Fraunhofer IMS, Duisburg, Germany) : “Solid-State ToF Sensors”
3D imaging is a hot topic these days. A typical sensor architecture used to detect the third dimension is ToF : time of flight. ToF can be realized by three approaches : direct time measurement, continuous wave modulation, pulse modulated light. All three principles were explained, compared and benchmarked. At IMS, the pulse modulation technology is explored. A major issue in 3D imaging is the speed of the pixels, measurements need to be done at the “speed of light” and pixels need to be emptied within the same timescale. To come to this point, photogate pixels and pinned photodiode pixels with a built-in lateral drift field are developed. Examples and results of both architectures are shown in the talk.
Ulrich SEGER (Robert Bosch, Germany) : “Imaging sensors for driver assistance applications”
It should be clear that vision systems and intelligent cameras can add a lot of functionality to driver assisted applications. Near IR vision can help a lot in detecting obstacles, etc. Several of these examples are known. In this talk, Ulrich highlighted a couple of issues that were encountered with the CMOS image sensors used. A first example was sun-burn-in. Too heavy sunlight focused on the image sensor could generate some nasty burn-in effects, showing up as FPN. This effect was corrected by changing/adapting the processing of the micro-lenses on top of the image sensor. A second issue was the drift of the FPN after the camera was assembled. This effect had to do with the UV damage introduced during the assembly process.
These two examples show that the harsh automotive environment can put extra constraints and requirements on the imagers/cameras when used for automotive purposes.
Martin WENDLER (Pilz, Ostfildern, Germany) : “Safe CMOS camera system for three-dimensional zone monitoring”
The very last talk of the workshop was focusing on the application of a logarithmic CMOS image sensor with a global shutter to protect a 3D zone of an industrial environment. The safety requirements of such an applications are extremely high, and for that reason a camera is needed with an imager that complies with :
- High dynamic range (120 dB),
- Triggerable global shutter,
- Logarithmic characteristic curve.
Interesting talk to hear the discussion of an image sensor and its requirements presented by the customer.