Towards arrays of smart-pixels for time-correlated single photon counting and time of flight application

We present a novel “smart-pixel” able to detect single photons and to measure and record in-pixel the time delay between a START pulse (e.g., laser excitation, cell stimulus, or LIDAR flash) and a STOP pulse given by the detection of a single photon (e. g., fluorescence decay signal or back reflection from an object). This smart-pixel represents the basic building block of SPAD arrays aimed at time-correlated single photon counting (TCSPC) applications (like FLIM, FCS, FRET), but also at photon timing and direct Time-of-Flight (dTOF) measurements for 3D ranging applications (e.g., in LIDAR systems). The pixel comprises a Single-Photon Avalanche Diode (SPAD) detector, an analog sensing and driving electronics, and a Time-to-Digital Converter monolithically designed and manufactured into the same chip. We report on the design and characterization of prototype circuits, fabricated in a 0.35 μm standard CMOS technology containing complete conversion channels, smart-pixels and ancillary electronics with 20 μm active area diameter SPAD detectors and related quenching circuitry. With a 100 MHz reference clock, the TDC provides a time-resolution of 10 ps, a dynamic range of 160 ns and very high conversion linearity.