Archive for June, 2011

Birthday Party

Monday, June 20th, 2011

Last week Friday Caeleste celebrated its 5th anniversary with a Scientific Seminar.   Bart Dierickx invited 6 speakers in the domain of X-ray vision, radiation tolerance, particle detections.  These where:
 
- Evi Bongaers (SkyScan, Kontich, Belgium), “The use of micro-CT in material science”
- Jeroen Hostens (SkyScan, Kontich, Belgium) on “Micro-CT in biological science”
- Albert Theuwissen (undersigned), on “Cosmic radiation damage in image sensors”.  (I promised myself that this was going to be the last time I was giving this presentation.)
- Erik Heijne (CERN, Geneva, Switzerland), on “From microscope to attoscope: silicon eyes at the CERN LHC”
- Pablo Fajardo (E.S.R.F., Grenoble, France), on “Detectors for high energy X-ray synchrotron radiation applications”
- Claire Bourgain (V.U.B., UZ-Brussel, Belgium), on “The diagnostic relevance of color X-ray”.  It seems to be really true that there is color information in X-rays.  It helps to better interpret X-ray mammograms.
After the Seminar we enjoyed a good barbecue on the roof of Caeleste’s office.

Congratulations to Bart and his team.  Hopefully we seen each other again on the next birdthday party.

Albert 20-06-2011

Day 4 of the International Image Sensor Workshop in Hokkaido, Japan

Tuesday, June 14th, 2011

The last day of the workshop contained only two sessions : the first one on global shutter pixels and the second one on high-speed and ADCs.

It is clear that a lot of effort is done in the direction of global shutter pixels.  The rolling shutters of the CMOS imagers is still a disadvantages compared to the global shutter of the CCDs.  Nevertheless the performance of the global shuttered CMOS pixels is constantly improving.  The world’s first global shutter in combination with back-side illumination was presented.  The pixel is an 8-transistor cell and to realize the global shutter, the information is stored in the voltage domain on an external-to-the-silicon capacitor.  This leads to a very high shutter efficiency of 1/110,000.  This presentation did perfectly fit into the scope of the workshop, being : discussing work in progress.  Although the shutter efficiency is already pretty high, the back-side technology presented needs further improvement.  But is should be encouraged that people are willing to show their results in a very early stage of their projects.  This leads to interesting discussions ! 

Completely opposite to-work-in-progress was the next presentation based on a product of a 1.2 Mpixel global shutter sensor with automatic gain selection.  This pixel makes use of 5 transistors and the in-pixel storage node is realized as a MOS-capacitor with storage in the charge domain.  The shutter efficiency is not as good as the one reported in the previous work, but on the other hand, the pixel is also much smaller, being 3.75 µm.  It was claimed that this is the smallest pixel in the industry with a global shutter. 

Also interesting was the presentation in which a high-speed column-parallel CMOS image sensor was developed with a SA-ADC based on the PTC.  First time I see the “PTC” in the definition/name of an ADC.  A similar idea was also implemented by one of my PhD students : apply a very fine ADC step when needed (= low-light levels) and allow a very coarse ADC step when allowed (= noise dominated by photon shot noise).  The ADC is capable of resolving 16 bits at the lowest segment with a conversion time of only 2.475 µs.  Papers on multiple windowing, global pipelined shutter CMOS devices, 2-stage pipelined cyclic ADCs  followed.   Apparently dividing the ADC workload in multiple steps seems to be the way to go for high-speed applications.  This was also described in the last two papers of the workshop.

Overall this was a great workshop !  An high-level technical program, superb organization and great service.  Thanks very much to our Japanese friends who were responsible for the organization of the workshop under extremely difficult conditions.  CONGRATULATIONS and THANKS VERY MUCH to Nobu, Junichi, Shoji and all others that contributed to the success of the workshop !

Tomorrow no more news.

Albert, June 14th, 2011

Day 3 of the International Image Sensor Workshop in Hokkaido, Japan

Sunday, June 12th, 2011

Peter Seitz (CSEM) opened with his invited talk on “Single Photon Imaging”.  He started with giving us a definition of single photon detection.  Looks very straight forward, but apparently it is not because together with the detection of incoming photons,

-          you may miss some of them and/or,

-          you think you detect a photon but in reality the electron detected may be generated through dark current.

Next he gave a very nice overview of several techniques that can be used for single photon detection, including good old vacuum tubes, hybrid solutions with vacuum and solid-state state, and finally all kind of solid-state devices.  All these techniques look pretty familiar, but if you see them all gathered on one sheet you are surprised to see how much work has been done in this field, but apparently (and for us fortunately) the holy grail has not yet been found. 

Another very informative part of the talk was the link between dark current and light sensitivity.  To detect light a certain bandgap for our semiconducting material is needed, to make the sensors more sensitive to longer wavelengths a smaller bandgap is a necessity, and the latter will also increase the dark current.  Something we probably all know, but it was the first time to have this seen in a graph of dark current versus bandgap energy (ideal curve together with published data).

At the end of the talk it was all clear that there does not exist a single solution for single photon detection in all applications.  The talk concluded with a flow chart for the selection of the appropriate photosensor technology with single-photon resolution, depending on parameters and specification of the application.

Then it was SPAD-time !  Several papers showed new device structures and new technologies to overcome the classical drawbacks of SPADs, being for instance their limited quantum efficiency (pretty low in the red part of the spectrum) and their limited fill factor (due to guards and circuits in every pixel).  It is clear that SPADs are rapidly improving as well as expanding their application field.  On the other hand, all papers presented came from European academics (one with ST’s support).  So when will the big imaging companies jump on the SPADs ?

After the SPAD the real big guys showed up : sensors of multiple cm2 instead of mm2.  A few examples of silicon tiles (these can no longer be called dies), all CMOS, are :

-          20.2 x 20.5 mm2, 300 mm wafer-sized, monochrome,

-          23 x 25.9 cm2, 4 sensors butted with very small butting gaps, RGB,

-          61 mm x 63 mm, for electron detection.

All these huge sensors are making use of stitching technology to make silicon devices that are much larger than the stepper’s reticle size.  Stitching seems to be common practice these days and even available at the CMOS foundries.  Apparently everyone is already that familiar with stitching that no one is any longer referring to the original work in the field of stitching for imagers …

The technical part of the day ended with two interesting papers on medical topics :

-          a first attempt to do colour imaging for X-rays (energy detection), and

-          single grain TFTs and photodiodes intended for large area X-ray detectors.

During the workshop banquet three important announcement were made :

-          the best poster award was won by Mikio Ihama and co-workers from FujiFilm for the poster : “CMOS Image Sensor with an Overlaid Organic Photoelectric Conversion Layer : Optical Advantages of Capturing Slanting Rays of Light”.

-          a newly established Exceptional Service Award was presented to Vladimir Koifmann for the creation and the editorship of the Image Sensors World blog,

-          the Walter Kosonocky Award for best paper published in 2009 and 2010 was handed out to Hayato Wakabayashi and his co-authors from Sony Corporation for the paper entitled : “A ½.3 inch 10.3 Mpixel 50 frames/s Back-Illuminated CMOS Image Sensor”.  This work was presented at the 2010 IEEE Internatinal Solid-State Circuits Conference.

Many congratulations to the winners of the three awards.  Hopefully our community keeps up the excellent work and will be able to publish their results obtained.

Tomorrow more news.

Albert, June 12th, 2011

Day 2 of the International Image Sensor Workshop in Hokkaido, Japan

Saturday, June 11th, 2011

Day 2 started with a session on Time-of-Flight sensors.  ToF seems to be the logical choice for depth sensing, but also for fluorescence detection.  The session kicked-off with a kind of overview paper presented by Robert Henderson (although he did not contribute to the paper, he gave an excellent talk!).  In this overview three different techniques to perform ToF were compared :

-          Buried channel demodulator, which suffers from the fact that a special technology is needed,

-          Current assisted photonic demodulator, which suffers from power consumption, but was mentioned to be the best choice for low cost applications,

-          The pixel with in-pixel switched cap circuitry, suffering from its complex pixel structure but seems to be the preferred choice in the case of industrial and/or security applications.

Another interesting observation with ToF is their move towards pinned photodiode pixels as well.  Apparently most foundries that supply the silicon for these ToF, all have pinned photodiodes available these days.  In other cases where still photogates are used, the photogates can be biased negative to get the same accumulation at the interface and to lower the dark issues.

A very nice invited talk came from Masatoshi Ishikawa : “New application areas made possible by high speed vision”.  The talk had a lot in common with the one he gave last year at the ISSCC forum.  But it was still great to hear it again, also because of the great movies to highlight the capabilities of custom designed high-speed sensors.  In many high-speed applications regular devices are used in combination with very highly sophisticated algorithms.  But the lesson of M. Ishikawa is the following : exchange spatial resolution for temporal resolution and the algorithms will become much more simple.  For normal sized robots a frame rate of at least 1000 fr/s are needed, for micro-machined type of robots 10,000 fr/s is a must.

In one of the papers the use of ToF with a standard CMOS 5T pixel was illustrated.  The sensor originally was not intended for this application, but nevertheless, it is possible to detect depth.  The results are not up to the same quality level as the specialized ToF, but a bit more optimization of the sensor and/or timing could lead to better results.  A very interesting side-effect was mentioned during the Q&A : the ToF mode of this device can be used to measure the delays in the sensor’s metal wiring.

Dark current, dark current  non-uniformities, hot pixels, they all seem to be a joy for ever !  Of course we all knew the positive influence of having an accumulation layer at the interface, but a couple of papers illustrated how you can do this dynamically in CCDs as well as with the transfer gate of the CMOS devices.  This is absolutely the way to go if the sensors need to show dark currents close to their theoretical limit.  Although, it was mentioned in one of the papers that over time, a negative bias of the transfer gate can introduce some nasty aging effects.

Day 2 ended with a 25th anniversary talk of the workshop given by the father of the workshop himself : Eric Fossum.  The talk started with the statement of Bernard of Chartres (1115 AD) “We are like dwarfs on the shoulders of giants, so that we can see more than they, and things at a greater distance, not by virtue of any sharpness of sight on our part, or any physical distinction, but because we are carried high and raised up by their giant size”.  This was a quite nice start to acknowledge the contribution of several great solid-state imaging pioneers that prepared the path on which all of us are walking these days.  The older images and papers that Eric showed brought back good memories of several previous workshops.

Tomorrow more news.

Albert, June 11th, 2011

Day 1 of the International Image Sensor Workshop in Hokkaido, Japan

Friday, June 10th, 2011

This Wednesday the IISW 2011 started in Japan.  After all the issues that happened on March 11th, several people cancelled their contribution and/or trip to Japan, but nevertheless, the technical program of the first day was more than packed.  It is impossible to report about all the papers, also because the afternoon program contained about 30 flash presentations that go together with the poster session.

In the morning session the pixel shrinkage was a kind of main topic.  What could be learned from the presentations and the publications ?

-          All companies are moving towards a kind of common performance level : noise, dark current, QE, SNR of the various technologies seem to converge, although the different companies are using different technologies,

-          CMOS image sensors are deviating more and more from the standard CMOS technology.  In the early days of CMOS it was a strong argument that CMOS could be made in standard, cheap CMOS technologies, but this argument no longer holds.  Another consequence is the second source option for CMOS.  Because everyone is relying on its own technologies, one can forget about second sourcing (except maybe for second sourcing within the same company),

-          Over the last couple of years BSI became an important technology for the CMOS devices, but as usual, a new technology also boosts the developments in the old technology.  It is quite remarkable that several companies tell at the workshop about their light guiding technology that was used in the 1.45 mm pixel FI devices.  In this way they could keep up the lightsensitivity in these small FI pixels, even in some 1.1 mm pixels.  But with the step towards sub-micron pixel pitches, BI seems to be inevitable. 

There was an interesting invited talk on colour filter technology by Hiroshi Tagushi of FujiFilm.  Quite nice paper containing information on the road map of the filter material.  It seems that the photoresist + colour pigment is no longer an option for sub-micron pixels, because simply, there is no space anymore for the lightsensitive resist component in the filter material.  For that reason sub-micron pixels with colour filters will also need a classical lithostep for the filter definition, in combination with a reactive ion etching technology.

Day one ended with the poster session.  Every poster presenter gets a few minutes to introduce his/her work during a flash presentation.  Within 2 hours the amount of information thrown on the audience is incredibly large.  Not just because of the quantity but also because of the quality of the work.  It should be remarked that over the years the overall quality of the content as well as of the presentations given at the workshop has enormously grown .  It clearly demonstrates the importance of this workshop.  Tomorrow more news.

Albert, June 10th, 2011.