In a BSI (Backside Illuminated) CMOS image sensor flow, backside processing requires opening deep bond pads to connect the sensor to external circuitry (Engineering Practice).The process sequence—Si Etch followed by Pad Oxide Etch, then CESL (Contact Etch Stop Layer) and PMD (Pre-Metal Dielectric) etches—indicates a reverse-order deconstruction of the structural layers from the wafer's backside P1.Early front-end pad oxide etches (such as step #10) are used to prepare active areas by acting as a