A semiconductor structure for back side illumination (BSI) pixel sensors is provided. Photodiodes are arranged within a semiconductor substrate. A composite grid includes a metal grid and a low refractive index (low-n) grid. The metal grid includes first openings overlying the semiconductor substrate and corresponding to ones of the photodiodes. The low-n grid includes second openings overlying the semiconductor substrate and corresponding to ones of the photodiodes. Color filters are arranged in the first and second openings of the corresponding photodiodes and have a refractive index greater than a refractive index of the low-n grid. Upper surfaces of the color filters are offset relative to an upper surface of the composite grid. A method for manufacturing the BSI pixel sensors is also provided.