A device includes a semiconductor substrate having a front side and a backside, a photo-sensitive device disposed on the front side of the semiconductor substrate, and a first and a second grid line parallel to each other. The first and the second grid lines are on the backside of, and overlying, the semiconductor substrate. The device further includes an adhesion layer, a metal oxide layer over the adhesion layer, and a high-refractive index layer over the metal layer. The adhesion layer, the metal oxide layer, and the high-refractive index layer are substantially conformal, and extend on top surfaces and sidewalls of the first and the second grid lines.