Semiconductor image sensor

A BSI image sensor includes a substrate including a front side and a back side opposite to the front side, a plurality of pixel sensors arranged in an array, an isolation grid disposed in the substrate and separating the plurality of pixel sensors from each other, a reflective grid disposed over the isolation grid on the back side of the substrate, an a low-n grid disposed over the back side of the substrate and overlapping the reflective grid from a top view. A depth of the reflective grid is less than a depth of the isolation grid. A width of the low-n grid is greater than a width of the reflective grid.