In 40nm Back-Side Illuminated (BSI) CMOS Image Sensors, the Upper Vertical Grid acts as a highly opaque light shield to prevent optical crosstalk between adjacent pixels (Engineering Practice).Following lithographic patterning, the W Etch step transfers the photoresist mask pattern into the underlying tungsten layer to form highly vertical grid walls A2.Tungsten is selected for its exceptional optical density and refractory stability, but its successful integration requires a strictly anisotropi