An IC structure includes a semiconductor fin, first and second gate structures, and an isolation structure. The semiconductor fin extends from a substrate. The first gate structure extends above a top surface of the semiconductor fin by a first gate height. The second gate structure is over the semiconductor fin. The isolation structure is between the first and second gate structures, and has a lower dielectric portion embedded in the semiconductor fin and an upper dielectric portion extending above the top surface of the semiconductor fin by a height that is the same as the first gate height. When viewed in a cross section taken along a longitudinal direction of the semiconductor fin, the upper dielectric portion of the isolation structure has a rectangular profile with a width greater than a bottom width of the lower dielectric portion of the isolation structure.