Crosstalk mapping in CMOS SPAD arrays

Optical crosstalk is one of the main factors limiting the performance of Single-Photon Avalanche Diode (SPAD) arrays and Silicon Photomultipliers (SiPMs). In this paper, a set of crosstalk measurements on a CMOS SPAD pixel array with 50µm × 75µm pitch and 51.6% Fill Factor, designed for direct particle detection, is reported. Measurements were performed on dies with different thickness: 280µm, 50µm and 25µm. The dependence of crosstalk on excess bias voltage and distance between pairs of pixels is presented and its overall effect on the Dark Count Rate (DCR) distribution of the array is discussed. Results show a strong dependence of crosstalk coefficient on substrate thickness, confirming the role of photon reflection on the wafer backside surface. In thinned devices, the silicon substrate acts as a planar waveguide, increasing the transmission efficiency of secondary photons and greatly enhancing long-distance optical crosstalk.