An image sensor including a semiconductor substrate having a first surface and a second surface; and a pixel isolation film extending from the first surface of the semiconductor substrate into the semiconductor substrate and defining active pixels in the semiconductor substrate, wherein the pixel isolation film includes a buried conductive layer including polysilicon containing a fining element at a first concentration; and an insulating liner between the buried conductive layer and the semiconductor substrate, and wherein the fining element includes oxygen, carbon, or fluorine.