The present disclosure provides an image sensor device and a method for manufacturing the image sensor device. An exemplary image sensor device includes a substrate having a front surface and a back surface, a plurality of sensor elements disposed at the front surface of the substrate. Each of the plurality of sensor elements is operable to sense radiation projected towards the back surface of the substrate. The image sensor also includes a high-k dielectric grid disposed over the back surface of the substrate. The high-k dielectric grid has a high-k dielectric trench and sidewalls. The image sensor also includes a color filter and a microlens disposed over the high-k dielectric grid.