An image sensor includes a first chip and a second chip. The first chip includes a first substrate including photoelectric conversion parts and a back-side insulating layer covering a first surface of the first substrate. The second chip, which is disposed adjacent the first chip, includes circuits configured to drive the first chip. The back-side insulating layer may include a fixed-charge layer, a refractive index adjusting layer, and a capping layer, which are sequentially disposed on the first surface of the first substrate. The refractive index adjusting layer may include a first element, a second element, and oxygen. A conduction band minimum of an oxide of the second element may be higher than a conduction band minimum of an oxide of the first element.