Fabricating a semiconductor chip with backside optical vias is provided. A silicon wafer is received for processing. The silicon wafer includes an optically transparent oxide layer on a frontside of the silicon wafer. A complementary metal-oxide-semiconductor layer is formed on top of the optically transparent oxide layer. A backside of the silicon wafer is etched to form optical vias in a silicon substrate using the optically transparent oxide layer as an etch-stop.