This specific Pre Litho Cleaning step occurs immediately after Optical Pad 3 CMP and Post CMP Cleaning, and right before the Mid Vertical Grid Trench Photo step (Engineering Practice).While Post CMP Cleaning primarily removes bulk slurry abrasives and gross organic residues from the planarized surface P4, this Pre Litho Cleaning ensures the surface is chemically conditioned and absolutely free of nanoscale defects or moisture prior to photoresist application P1.Unlike earlier pre-litho cleans (s