1. An image sensor chip comprising: a first wafer including an image sensor having a plurality of sub-pixels, each of the plurality of sub-pixels being configured to detect at least one photon and to output a sub-pixel signal according to a result of the detection; and a second wafer including an image processor configured to process sub-pixel signals for each of the plurality of sub-pixels, and to generate image data; wherein the first wafer and the second wafer are formed in a wafer stack structure.