In the 40nm BSI CMOS Image Sensor BEOL flow, the "ILD 5-1 SiCN Etch" step serves as the critical etch-stop breakthrough prior to via metallization P1.Following the bulk ILD 5-2 oxide etch, the underlying SiCN layer acts as a barrier that prevents the oxide etch chemistry from inadvertently attacking the underlying Metal 5 copper interconnects A1.By selectively removing this SiCN layer at the bottom of the Via 5 structures, the process exposes the copper surface to establish electrical continuity