Following Optical Pad 3 CMP, the wafer surface is heavily contaminated with residual slurry abrasives, pad debris, and organic chemical by-products P2.In the BSI CIS flow, achieving an atomically clean and defect-free optical pad surface is mandatory before the subsequent Pre Litho Cleaning and Mid Vertical Grid Trench Photo steps A1.Any residual nanoparticles or metallic contaminants at this stage will act as physical obstacles or light-scattering centers during lithography, compromising the de