In a 40nm BSI CMOS Image Sensor, opening the backside bond pad requires penetrating a complex stack of optical and dielectric layers (Engineering Practice).Unlike Optical Pad 3 Etch which tackles the uppermost organic or planarization films, and Optical Pad 1 Etch which clears the final barrier immediately above the metal, Optical Pad 2 Etch is specifically positioned to remove the bulk intermediate dielectrics (typically thick SiO2 or SiN) while maintaining strict profile control P2.It follows