An image sensor device and a method for manufacturing the image sensor device are provided. An image sensor device includes a substrate, sensor elements disposed at a front surface of the substrate, and a dielectric grid disposed over a back surface of the substrate. The dielectric grid includes a first dielectric layer as a bottom portion, a metal pillar, as a core portion of a upper portion, disposed over the first dielectric layer and a second dielectric layer wrapping around the metal pillar. The image sensor device also includes a stack of layers disposed over the back surface of the substrate. Refractive index of each layers increases from top layer to bottom layer. The image sensor device also includes a color filter and a microlens disposed over the back surface of the substrate.