Highly efficient readout integrated circuit for dense arrays of SPAD detectors in time-correlated measurements

In the last years, Time Correlated Single Photon Counting (TCSPC) has become the technique of choice in fluorescence lifetime measurements, given its remarkable sensitivity, accuracy and timing resolution. Nevertheless, a major drawback of this technique lies in the relatively long acquisition time. In order to overcome this issue, many multichannel systems have been proposed in literature, but the presence of many independent acquisition chains gives rise in principle to a huge data rate at the output, which cannot be processed in real time by a PC. Typically adopted solutions involve a limitation of the maximum detection frequency of each channel, so the measurement speed of currently available systems has not increased accordingly with the number of acquisition chains and is still limited well below the saturation of the transfer rate towards the elaboration unit. We present a completely different approach: starting from the maximum manageable data rate imposed by the transfer towards the PC, a proper number of high-performance external channels has been chosen to be shared among a much larger number of Single Photon Avalanche Diode (SPAD) detectors. Then, at each excitation period a dynamic routing mechanism performs a selection among the whole set of detectors carrying a valid signal and routes them towards the external channels. The selection logic relies on a pixelated architecture and on 3D-stacking techniques to connect each SPAD to its dedicated electronic, leading to a minimization of the number of interconnections crossing the integrated system.