The present disclosure relates to a stacked SPAD image sensor with a CMOS Chip and an imaging chip bonded together, to improve the fill factor of the SPAD image sensor, and an associated method of formation. In some embodiments, the imaging chip has a plurality of SPAD cells disposed within a second substrate. The CMOS Chip has a first interconnect structure disposed over a first substrate. The imaging chip has a second interconnect structure disposed between the second substrate and the first interconnect structure. The CMOS Chip and the imaging chip are bonded together through along an interface disposed between the first interconnect structure and the second interconnect structure.