In the 40nm BSI CMOS Image Sensor flow, the Optical Pad 3 Deposition step serves as a critical dielectric encapsulation and planarization layer above the lower on-chip lens (OCL) structures A2. While Optical Pad 1 and 2 generally serve as foundational anti-reflective layers or initial interstitial gap-fillers for the light shield grid, Optical Pad 3 is specifically deposited over the finalized lower OCL to establish a flat optical baseline for top-level integration (Engineering Practice). This d