This chip implements Geiger-mode avalanche photodiodes for single-photon detection and is implemented in OnSemi's 500 nm C5 process. The chip was laid out using Electric VLSI. It was fun designing this chip because the photodiodes could not be simulated at all in LTspice so it was a surprise to see how they performed experimentally! I can't discuss the details of this chip so I'm posting only overall chip images. 

Angsuman Roy