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.