Yesterday night, March 20th, the ISE 2012 opened with a keynote of Nobukazu Teranishi of Panasonic. Nobu is a world-known top-expert in the field of solid-state imaging. Together with his team at NEC he invented the pinned photodiode and the vertical overflow drain. But yesteday night he talked about “Dark Current and White Blemishes”.
His talk started with a nice overview of the work Nobu and his team did in the early CCD days to reduce the dark current. A lot of very basic technology steps were developed at that time to, firstly understand the behaviour of defects in silicon and secondly to optimize the fabrication technology to reduce these defects. He highlighted all kind of different gettering techniques : internal gettering, external gettering and proximity gettering. If you listen to the story then it seems all so logical what the various gettering steps can do to reduce the amount of stress in the wafers, to attrack impurities and to reduce the dark current. But for sure it must have been a work of many manyears to come to the low dark current levels present in today’s devices.
The last part of his talk was about a new dark current generation model for the pinned photodiodes. Nobu explained that the generation-recombination centers in the p+ top layer and at the interface of the silicon still contribute to the dark current of the PPD. He supported his theory by means of an analytical model and by applying this model to data published by others. Ultimately he came to the conclusion that it still is of utmost importance to keep the dark current as low as possible and one way of doing so it to work with devices that have no LOCOS and no STI to isolate transistors, but that all isolation is done by means of implants. Also this statement seems to be straightforward, but in the CMOS world it appearss to be new, although in the CCD world this technique was already applied a while ago.
Good start of the conference !
Albert, 21-03-2012.
Albert, What would be the physics behind surface dark-current in PPD? If the p+ doping is sufficiently high, I would expect: a) depletions do not touch the surface b) carrier recombination is very high in the p+ layer, with diffusion length of a few nm. This is a trade-off with QE of course. With very good surface passivation, people have realized “nearly” bulk dominated dark current, and current efforts are to reduce this bulk dark current. It will therefore be interesting to know what the new mechanism is. Is it implant induced damage near the interface?
Hi rpk, according to Nobu, the interface states at the Si-SiO2 are not fully in equilibrium. That results in the generation of dark current that can and will be collected by the PPD.