Harvest Imaging Blog

Day number 4 is a relative short day with only 9 presentations.

Radiation hardening was a topic that came back in 2 papers, one of Brookman Technology and one of ISAE, Toulouse.  In both papers emphasis was made on the tiny details in the lay-out of a pixel to make the pixel more radiation hard.  The devil really sits in the details.  The golden rule remains : keep the electric fields as low as possible !  Surprisingly, there was only one TDI paper presented at the workshop, combined with the single topic on multi-spectral imaging.  It was a report by IMEC of a kind of a follow-up device presented at previous workshops.

Global shutter devices got a separate session : Brillnics with a device that combines GS with HDR in the voltage domain by using a stacking technology, TPSCo with a 2.5 um GS in the charge domain (this paper was a repetition of their IEDM 2018 paper) as well as a 2.8 um GS with near-IR enhanced sensitivity.  A remark made by TPSCo that the next version of their GS shutter device will have 2.2 um pixel pitch.  The work of TPSCo is all front-side illuminated.

ON Semiconductor presented a GS pixel with correction processing by means of a pixel with a single PD, single FD, but double storage node.  The final paper of the workshop was delivered by ams : a BSI pixel with background light suppression intended for structured light applications and object tracking.  A clever pixel, based on 6T (2 resets, 2 transfer gates, 1 source follower, 1 row select and two floating diffusions) and 1C (connected between the 2 floating diffusions) in the pixel, allows to shoot a sequence of multiple frames one after another and only stores the difference between the frames.  Pixel size is just 2.8 um.  Testdevices seem to be available, but measurement results no yet.  Maybe next workshop !?

The IISW2019 was again a big success, many excellent presentations, all high quality papers, good atmosphere, perfect organization.  The interest of the participants can be best judged by the amount of people that are willing to stay till the very last paper is presented.  And it is really remarkable that during all the image sensor workshops, almost everyone really stays till the very end.

Congratulations and thanks to the organization of IISW2019.  See you all again in Europe for the IISW2021.

Albert, 28-06-2019.

What a day today !  Actually a very long day with presentations from the early morning till the late afternoon.  And it still is not over, because there will be the workshop banquet tonight.

It is really a not-to-finish task to give an overview of the talks of today.  A lot of different subjects were presented, for instance high dynamic range, UV sensitive devices, energy harvesting, SPADs, indirect ToF, flicker mitigation, 16-bit analog-to-digital converter, high speed, endoscopes for non-invasive surgery, near-IR imaging with and without silicon detectors, fundus camera, and much more.  Although the amount of paper submissions for this 2019 workshop was lower than in 2017, the quality of the presentations is most probably the best ever.  Congratulations to all speakers !

Albert, 27-06-2019.

Day 2 was a short one.  Only 12 presentations, which could be captured by a title such as : “new pixel architectures and single photon detection”.  Besides the regular papers an invited one was presented about Deep Learning.

A very interesting presentation was given by Francois Roy (ST Microelectronics) about a pixel architecture WITHOUT any implantation in the photodiode.  The pixel was based on the C-DTI technology of ST in which a gate inside the deep-trench isolation is used to bias the interface of the silicon into accumulation and fully deplete the epi-layer.  In this way a pinned photodiode is created in the 3rd dimension, for which a low dark current is expected.  A presentation of a sensor in the same technology was already given at IEDM 2018 and seems to be present in a well-known consumer product.  This IISW2019 paper showed a back-side illuminated version of this technology, in which the back-surface passivation is realized by “charged” dielectric layers on the back.  Also the latter seems to be a trend in BSI CIS.  The reported dark current of this rolling shutter imager : 5 holes/sec at 60 degrees C for a 2 um pixel (2T5), corresponding to a dark current density of 2 pA/cm2 (holes) at 60 degrees C, coming into the sub-pA/cm2 at room temperature.

Personally I am taken with this technology because it is a very nice combination of silicon device physics and silicon processing technology, bringing a lot of the good old CCDs back, but now in CMOS technology.  Back to the future after 50 years !

It is really funny what can be done by these DTIs.  Yesterday a paper of Samsung showed the use of DTI to split a photodiode into two parts to allow the PDAF function within every pixel.  The width of the a single PPD was reduced to 0.61 um.  Today, also Samsung, introduced “dummy” DTIs in a pixel to enhance the scattering of incoming photons and to enhance the quantum efficiency of the pixels.  A quantum efficiency of 43 % was reported for a wavelength of 940 nm.

But the award for the most artistic pixel lay-out goes to Shizuoka University.  The pixel lay-out looks like a cross-section of a aircraft engine.  The pixel has a central photodiode surrounded by 8 taps + 8 floating diffusions in an array of 22.4 um x 22.4 um.  The pixel is used for indirect time-of-flight applications with an inaccuracy of 7.2 mm at a distance of 6.2 m.

Albert, 26/06/2019.

Very brief : on the first day about 10 papers were presented and 33 posters were announced.  I recognized 3 major trends :

1. 0.8 um pixel size is there !  The 3 major suppliers are working on 0.8 um and/or have it available on the market : Sony, OmniVision and Samsung.  A very careful prediction is that in 2021-2022 0.7 um pixel pitch will become available.
2. many people are working on the dominant noise source in a CIS, being the source-follower.  Several papers came with an ownful lot of measurements to explain and further optimize the source-follower noise, including RTS noise.
3. dark current is still a hot topic, and mainly dark current of the floating diffusion node.  The latter is often used as a storage node and then the dark current is really becoming an issue.  A few years ago, a rough comparison between the dark current of the PPD and the dark current of the FD told that there existed a factor of 1000 in dark current density between the two (guess which one is the lowest).  At this moment the dark current of the FD is reduced already more than one order of magnitude compared to what we had a few years ago.  Still some work to do !

Great quality of papers/presentations, very open discussions, very well organized workshop.  More info to follow !!

Regards,

Albert, 25-06-2019.