An image sensor having greatly improved physical and electrical bonding forces between a photodiode and a substrate, and a manufacturing method thereof. The image sensor includes a semiconductor substrate and readout circuitry, a dielectric layer on the semiconductor substrate, a metal line in the dielectric layer, electrically connected with the readout circuitry, an image sensing device including first and second impurity regions on the dielectric layer, a via hole through the dielectric layer and the image sensing device, a hard mask in the via hole, and a lower electrode in the via hole to connect the first impurity region with the metal line.