Positioned after Metal 3 CMP and cleaning, ILD 3-1 serves as the critical Cu capping layer, diffusion barrier (DB), and etch stop (ES) for the subsequent Via 3 formation P3.Without this capping layer, the freshly exposed Cu would rapidly diffuse into the subsequent bulk dielectric (ILD 3-2) and oxidize, leading to severe reliability failures such as increased leakage and electromigration P1.This step physically seals the Cu/dielectric interface, utilizing chemically stable amorphous Si-based thi