Abstract: A method includes: forming a masking layer on a backside of a substrate, the substrate including pixel regions having photodetectors, transistors being positioned on or in a frontside of the substrate; forming a mask opening in the masking layer by exposing the masking layer to patterned light, the opening including: mask pixel regions that mask the pixel regions; and mask protrusion regions that extend from the mask pixel regions toward a mask crossroad region; forming a substrate opening in the substrate by etching the substrate through the mask opening; and forming an isolation structure in the substrate opening.