Right after Cu CMP, the top surface consists of exposed Cu lines and the previous low-k dielectric P2.Exposed Cu is highly susceptible to oxidation and diffuses rapidly into silicon and adjacent dielectrics, creating deep trap levels that degrade device performance and cause failure P3.Therefore, ILD 4-1 functions primarily as a dielectric diffusion barrier (DB) and copper capping layer (CCL) to seal the exposed Metal 4 layer P2.Additionally, this layer serves as an etch stop (ES) for the subseq