In a nanoscale Backside Illuminated (BSI) CMOS Image Sensor, the sensing active area is illuminated from the backside, requiring the metal interconnects and bond pads to be accessed through a complex stack of backside optical layers A2. The "Optical Pad 3 Etch" is a critical intermediate step in a sequential etch cascade designed to progressively breach the upper grid seal layer and various optical confinement layers to expose the underlying connection pad P1. It immediately follows the Upper Gr