International Solid-State Circuits Conference 2013 (3)

Paper presented by L. Braga (FBK, Trento) : “An 8×16 pixel 92kSPAD time-resolved sensor with on-pixel 64 ps 12b TDC and 100MS/s real-time energy histogramming in 0.13 um CIS technology for PET/MRI applications”. After the introduction about PET and various sensor options for PET, the author gave details about his own sensor architecture, called a mini Si-PM. One of the known limitations of SPADs is their small fill factor, but apparently by making the fill factor larger, also the dark-count rate and the yield are worsen more-than-linearly with the area. So instead of using one large SPAD, many smaller SPADs are arranged in a parallel structure. All these smaller SPADs are combined in a large OR-tree, but to avoid too much overlap of the dead times of all the SPADs, several monostables are incorporated in the OR-tree. This results in a spatial as well as temporal compression with 300 ps pulses every time one of the SPADs in the OR-tree is fired. One level higher in hierarchy, the 8×16 pixels are connected in a large H-tree to avoid differences in delays. The overall chip contains 92k SPADs (8 x 16 pixels each having 720 SPADs with 42.6 % fill factor, all connected to one single active time-to-digital converter).

During the presentation several measurements were shown to illustrate the working of this large SPAD chip.

Paper presented by C. Niclass (Toyota) : “A 0.18 um CMOS SoC for a 100m range, 10 fps 200×96 pixel Time of Flight depth sensor”. This chip has a novel idea to discriminate the ToF signal from the background by spatiotemporal correlation of photons. The idea is based on recording the time of arrival of each photon (background + ToF signal) and by making a kind of histogram in the time domain of these arrival times, the ToF signal can be discriminated from the background. The sensor is only 96 pixel in height, and to extend the vertical resolution, a scanning method with a rotating polygon mirror with 6 facets is used. The sensor itself contains a “row” of ToF detectors, as well as a “row” of standard intensity detectors. The ToF pixels are based on SPADs, information about the standard intensity detectors is not given.

The complete chip is relatively large : 4.7 mm x 6.7 mm, while the pixels only take up about 2 x 0.15 mm x 1.6 mm (guess !). So a huge amount of the chip area is used for memory, DSP, TDC, etc. Evaluation results show a very high accuracy of the distance measurements. According to the presenter, this chip is outperforming all other state-of-the-art technologies.

Paper presented by O. Shcherbakova (University of Trento) : “3D camera based on linear-mode gain-modulated avalanche photodiodes”. The technology described in this paper tries to improve the existing 3D sensors w.r.t. power consumption, frame rate and precision. The ToF method applied is making use of the continuous wave ToF. The heart of the sensor is the photodetector plus the demodulator of the signal and these are based on avalanche photodiodes. The device is fabricated in 0.35 um CMOS 1P4M, pixels are 30 um x 30 um with a fill factor of 25.7 %. The demodulation contrast reported is pretty high : 80 % at 200 MHz and 650 nm, maximum frame rate 200 fps. The precision of the depth sensing is 1.9 cm at 2m distance and 5.7 cm at 4.75 m distance. Worthwhile to mention : this paper had a live demonstration during the demo-session, the only one of the image sensor session.

Albert, 23-01-2013.

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