A method for forming a semiconductor structure is provided. The method includes patterning a semiconductor substrate to form a first semiconductor fin and a second semiconductor fin adjacent to the first semiconductor fin, and depositing a first dielectric material on the first semiconductor fin and the second semiconductor fin on the semiconductor substrate using an atomic layer deposition process. There is a first trench between the first semiconductor fin and the second semiconductor fin. The method also includes filling the first trench with a flowable dielectric material, and heating the flowable dielectric material and the first dielectric material to form an isolation structure between the first semiconductor fin and the second semiconductor fin.