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