In a 40nm 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 Grid Seal