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SOI unit has been conducting research in the field of CMOS Single-Photon Sensors since 2004. High-sensitivity detection capability is enabled by special detectors called Single-Photon Avalanche Diodes (SPAD), also known as Geiger-mode avalanche photodiodes. The main advantages of these detectors, if compared to conventional photodiodes, are the absence of readout noise, the excellent timing resolution, in the picosecond range, and their digital output.
SPAD detectors have been integrated in several CMOS technology nodes, from 0.8μm downto 0.13μm CIS processes. Pixel-level integrated readout channels with intrinsic timing capabilities, i.e. Time-to-Amplitude converters, Time-to-Digital converters and time-gated digital and analog counters, have been designed to record number and time stamps of the detected photons.
Main technological achievements:
- High fill-factor time-resolved SPAD pixel: 20.8% fill factor was obtained for a 25um pixel pitch featuring sub-nanosecond time-gating resolution
- High spatial resolution and high frame-rate SPAD imager: a 160x128-pixel SPAD-based image sensors with in-pixel 55ps TDC was co-developed by SOI within the framework of Megaframe project
- Silicon Photo-Multipliers (SiPM) array: an array of 8x16 digital SiPM pixel was co-developed by SOI within the framework of SPADnet project
The main challenges that SOI is pursuing in this research field are:
- Improve SPAD characteristics: Dark Count Rate, Photon Detection Efficiency, timing resolution, afterpulsing
- Shrink pixel size and improve of Fill Factor of SPAD-based pixel arrays
- Realize high-resolution single-photon image sensors