An image sensor (1000; 1000A; 1000B; 1000C; 1000D; 1000E; 1000F; 1000G; 1000H), comprising: a first chip structure including a first substrate and first circuit devices on the first substrate; and a second chip structure on the first chip structure, wherein the second chip structure includes a second substrate having a first surface (201S1) facing the first chip structure and a second surface (201S2) opposing the first surface (201S1), second circuit devices on the first surface (201S1) of the second substrate, anti-reflective layers on the second surface (201S2) of the second substrate, color filters on the anti-reflective layers, microlenses on the color filters, an isolation structure in the second substrate, and photoelectric conversion device regions spaced apart from each other by the isolation structure in the second substrate, wherein the isolation structure has first regions adjacent to side surfaces of the photoelectric conversion device regions and second regions adjacent to each corner of the photoelectric conversion device regions, wherein the isolation structure includes first isolation layers surrounding the photoelectric conversion device regions, respectively, second isolation layers surrounding the first isolation layers, first gap-fill patterns filling at least a portion of a space between the second isolation layers in the first regions, and second gap-fill patterns filling at least a portion of a space between the second isolation layers in the second regions, in a plan view, and wherein the second gap-fill patterns directly contact the first gap-fill patterns, in the plan view. a first chip structure including a first substrate and first circuit devices on the first substrate; and a second chip structure on the first chip structure, wherein the second chip structure includes a second substrate having a first surface (201S1) facing the first chip structure and a second surface (201S2) opposing the first surface (201S1), second circuit devices on the first surface (201S1) of the second substrate, anti-reflective layers on the second surface (201S2) of the second substrate, color filters on the anti-reflective layers, microlenses on the color filters, an isolation structure in the second substrate, and photoelectric conversion device regions spaced apart from each other by the isolation structure in the second substrate, a second substrate having a first surface (201S1) facing the first chip structure and a second surface (201S2) opposing the first surface (201S1), second circuit devices on the first surface (201S1) of the second substrate, anti-reflective layers on the second surface (201S2) of the second substrate, color filters on the anti-reflective layers, microlenses on the color filters, an isolation structure in the second substrate, and photoelectric conversion device regions spaced apart from each other by the isolation structure in the second substrate, wherein the isolation structure has first regions adjacent to side surfaces of the photoelectric conversion device regions and second regions adjacent to each corner of the photoelectric conversion device regions, wherein the isolation structure includes first isolation layers surrounding the photoelectric conversion device regions, respectively, second isolation layers surrounding the first isolation layers, first gap-fill patterns filling at least a portion of a space between the second isolation layers in the first regions, and second gap-fill patterns filling at least a portion of a space between the second isolation layers in the second regions, in a plan view, and first isolation layers surrounding the photoelectric conversion device regions, respectively, second isolation layers surrounding the first isolation layers, first gap-fill patterns filling at least a portion of a space between the second isolation layers in the first regions, and second gap-fill patterns filling at least a portion of a space between the second isolation layers in the second regions, in a plan view, and wherein the second gap-fill patterns directly contact the first gap-fill patterns, in the plan view.