US Patent 9299732 Stacked chip SPAD image sensor

An example imaging sensor system includes a Single-Photon Avalanche Diode (SPAD) imaging array formed in a first semiconductor layer of a first wafer. The SPAD imaging array includes an N number of pixels, each including a SPAD region formed in a front side of the first semiconductor layer. The first wafer is bonded to a second wafer at a bonding interface between a first interconnect layer of the first wafer and the second interconnect layer of the second wafer. An N number of digital counters are formed in a second semiconductor layer of the second wafer. Each of the digital counters are configured to count output pulses generated by a respective SPAD region.