Embodiments of mechanisms of a backside illuminated image sensor structure are provided. The backside illuminated image sensor structure includes a device substrate having a frontside and a backside and pixels formed at the frontside of the substrate. The backside illuminated image sensor structure further includes a metal element formed in a dielectric layer over the backside of the substrate and a color filter layer formed over the dielectric layer. In addition, the metal element is configured to form a light blocking area in the device substrate and is made of copper.