Being primarily an R&D sensor engineer, the most important paper to me on Thursday morning was the one delivered by Toshiba, entitled : “Dark noise in a CMOS imager pixel with negative bias on transfer gate”, by Hirofumi Yamashita et al.
It is well known that a negative bias on the transfer gate (TG) of a 4T PPD pixel will reduce the dark current and especially the dark current non-uniformities coming from the PPD-TG overlap region. At Delft University we did some research in this field as well. What was reported in the Toshiba paper is the following : if you bias the TX to a negative voltage, the amount of hot pixels becomes smaller, but apparently the amount of warm pixels becomes larger. (Warm pixels have a smaller amplitude than hot pixels).
The hot pixels seem to have an activation energy of 0.55 eV, while the warm ones are characterized by an activation energy of 0.20 eV. So apparently another mechanism is responsible for generating these new and extra warm pixels. Research has revealed that these warm pixels are depending on the difference between the negative voltage on the TG gate and the voltage on the floating diffusion. Conclusion : the warm pixels are generated at the TG-FD transition region, and are generated by means of trap-assisted tunneling through the (transfer-) gate oxide.
Testdevices with a thicker gate oxide as well as with a kind of graded n-doped floating diffusion confirmed the abovementioned model. So the advice is to make sure that the electric fields in the TG-FD region are kept small. This can/is done by splitting the floating diffusion into two parts : the one closest to the TG gets a doping level of 66 % of the rest of the floating diffusion.
This was a very complete paper : description of the problem, formulating a model, checking the model with simulations and teststructures, and ultimately the implementation of the countermeasures.
To me it is still a surprise that such a simple structure as a 4T pixel can keep us, the imaging community, busy after an incredible amount of person-years of R&D already spent on it. Apparently the end is still not there. By itself good news !
Also in the morning session, Morley Blouke gave a nice overview of the early CCD days. He presented several of the projects he worked on. For each project he explained the lessons learned. Nice to get such a look backwards by one of the CCD pioneers. Perfect idea of the conference committee to invite Morley for this. Thanks !
This was my third and last message w.r.t. Electronic Imaging 2011. I reported about just a very limited amount of papers, which does not mean that the other presentations were of less importance, quality or interest.