An integrated circuit structure includes a semiconductor substrate, an image sensor extending from a front surface of the semiconductor substrate into the semiconductor substrate, and an isolation structure extending from a back surface of the semiconductor substrate into the semiconductor substrate, wherein the isolation structure includes an air-gap therein. An air-gap sealing layer is on a backside of the semiconductor substrate. The air-gap sealing layer seals the air-gap, wherein the air-gap sealing layer includes a portion exposed to the air-gap.