Several IISW2015 papers dealt with the attempt to obtain a large conversion gain to bring down the noise floor (expressed in noise equivalent electrons) of the imagers. With a noise floor down to 0.25 electrons, a “standard” CIS could be applied in single electron detection. [Deliberately I do not call it “single photon detection” because we never have a QE of 100 %, so by definition we do not detect every photon. The intention of the conversion gain engineering is to detect a single electron present in the PPD and/or at the FD node.]
Tohoku University demonstrated how to extract the various components of the floating diffusion capacitance, and to further lower this capacitance. Their mainn focus was lowering the concentration of the FD junction and working without LDD at the drain side of the source follower. A conversion gain of 243 uV/electron is reported. [LDD’s are normally introduced to reduce the effect of hot carriers, what about the hot carriers in this structure without LDD ?] In a second paper of the same group, the LDD-less FD structure was implemented in a real device. To overcome the limitation of the small full well capacity with a large conversion gain, the pixel has applied the LOFIC technique in the pixel.
Dartmouth School of Engineering published their work on Multi-Bit Quanta Image Sensors by showing a measurement histogram indicating that single electron detection was realized. The sensor used in the experiment had a conversion gain of 242 uV/electron (just 1 uV/electron lower than Tohoku Univ. !). The paper is suggesting that a conversion gain of 1 mV/electron may be realized in the near future. Is was not mentioned how this can be done. But for sure very advanced CIS technologies of 65 nm or less are needed.
Also worthwhile to mention is the work of CEA, in which they use a p-type in-pixel readout structure to obtain a conversion factor of 185 uV/electron. This is still not large enough to perform single electron detection, but is moving in the right direction.
Caeleste presented a small test array based on the pixel that was presented by the same group at ISSCC a couple of years ago. The p-type source follower is swept between accumulation and inversion to make the 1/f noise uncorrelated between various multiple sampling moments. Apparently there are still problems to solve in this structure, but besides that, a conversion factor around 400 uV/electron was reported for s 180 nm CIS technology.
A bit in the same direction as the papers described above, is the work reported by ON Semiconductor (former Truesense Imaging, former Kodak) describing an EM-CCD. The overall concept is not new, but after TI and E2V, ON Semi is the next one to put EM-CCDs on the market. With the EM concept, the primary goal is not to reach a large conversion gain, but to reach very low (equivalent) noise levels. To continue along the EM-line, E2V published their work on EM-CMOS, fabricated in a 0.18um process.
Albert, 02-07-2015.