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In this paper, a complete optical crosstalk characterization of a Geiger-mode pixelated avalanche detector for particle tracking application is presented. The device is composed of two tiers of CMOS-integrated avalanche detectors bump bonded with pixel-level interconnects, with each layer containing a 16 × 48 detector array. On-chip coincidence detection circuits, designed to discriminate between particle-triggered detection events and dark counts, have been used to acquire direct crosstalk measurements. Crosstalk measurements as a function of excess bias voltage and distance between two different avalanche diodes is presented. The effect of substrate thickness is evaluated measuring dies with three different thicknesses: 280, 50, and 25 μm. Experimental results confirm the role of reflected photons at the bottom surface of the device, with the silicon substrate acting as a 2-D waveguide for thinner samples. The global effect of crosstalk observed when all the pixels in the array are enabled is reported, showing a substantial increase of the Dark Count Rate (DCR) distributions. Pairs of vertically aligned unshielded pixels are employed to characterize intertier optical crosstalk.