In advanced backside-illuminated (BSI) CMOS image sensors, the optical stack requires strict physical and optical isolation between adjacent pixels to minimize crosstalk and ensure high spatial resolution P2.Preceding process steps involving tungsten (W) and TiN etching define the metallic light shield and grid structures that provide this critical optical isolation (Engineering Practice).Following the ashing and stripping steps that remove photoresist and post-etch residues, the Upper Grid Seal