Method of fabricating a single photon avalanche diode imaging sensor

A method of image sensor system fabrication, the method comprising:

• forming a first doped semiconductor layer on a substrate, wherein the first doped semiconductor layer is grown with a first doping concentration;
• forming a second doped semiconductor layer on the first doped semiconductor layer, wherein the second doped semiconductor layer is grown with a second doping concentration, and wherein the second doped semiconductor layer is of an opposite majority charge carrier type as the first doped semiconductor layer;
• forming a plurality of avalanche photodiode pixels, wherein the plurality of avalanche photodiode pixels include a doped contact region disposed in the second doped semiconductor layer, and wherein the doped contact region is of a same majority charge carrier type as the second doped semiconductor layer and has a greater doping concentration than the second doped semiconductor layer;
• forming a guard ring region which extends entirely through the second doped semiconductor layer, wherein the guard ring region is of an opposite majority charge carrier type as the second doped semiconductor layer, and wherein the guard ring region separates individual pixels in the plurality of avalanche photodiode pixels;
• and
• connecting a logic wafer to the second doped semiconductor layer, wherein the second doped semiconductor layer is disposed between the logic wafer and the first doped semiconductor layer.