Archive for May, 2010

CMOS Imager Workshop, Duisburg, May 4-5, 2010 (2/2)

Wednesday, May 12th, 2010

5th CMOS Imager Workshop, Duisburg, May 4-5, 2010.

DAY 2

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.

 

 

 

CMOS Imager Workshop, Duisburg, May 4-5, 2010 (1/2)

Sunday, May 9th, 2010

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.