An integrated circuit structure includes a lower interconnect structure, a first semiconductor fin, a lower gate structure, first source/drain structures, an upper gate structure, and an upper interconnect structure. The first semiconductor fin is above the lower interconnect structure. The lower gate structure is under the first semiconductor fin and extends across the first semiconductor fin. The first source/drain structures are in the first semiconductor fin and on opposite sides of the lower gate structure. The first source/drain structures forms a lower transistor with the lower gate structure. The upper gate structure is above the first semiconductor fin and extends across the first semiconductor fin. The upper gate structure forms an upper transistor with the first source/drain structures. The upper interconnect structure is above the upper gate.