Timing and probability of crosstalk in a dense CMOS SPAD array in pulsed TOF applications
As the distance between neighboring devices in large CMOS single-photon avalanche diode (SPAD) arrays is reduced for improving the density, increased crosstalk becomes an important issue, limiting the maximum practical fill factor of the array. In this study, the temporal correlation of crosstalk events, as well as the crosstalk probability, and their dependence on parameters, such as the illumination wavelength and intensity, and the distance between SPADs, are investigated via measurement of a ~45%-fill factor CMOS SPAD array fabricated using 0.35-µm high-voltage CMOS technology. The SPADs have 24 µm × 24 µm square-shaped active areas, and all devices share a common deep-N-well cathode. On-chip time-to-digital converters with 65-ps resolution are used to measure the timing of crosstalk events in “coincidence measurements.” For the crosstalk measurements, the internal noise in one SPAD is used to produce crosstalk events in the neighboring devices. The measurement results indicate both optical and electrical crosstalk with the crosstalk events, having a specific temporal distribution. The crosstalk probability in the first two adjacent pixels is found to be 0.3% and 0.01%, with a distribution having full widths at half maximum (FWHMs) of 700 and 400 ps, respectively. In pulsed time-of-flight measurements, when one SPAD is triggered with external short-pulsed (FWHM of approximately 200 ps) illumination, extra correlated noise in the adjacent SPADs added to the crosstalk noise, increasing the correlated noise considerably. This additional noise was a secondary effect of the absorbed laser photons deep in the substrate.