A semiconductor structure includes a plurality of sub-array structures separated from each other by a plurality of isolation structures. The semiconductor structure further includes a three-dimensional array of memory cells. The memory cells include a plurality of cell groups disposed in the sub-array structures, respectively. The semiconductor structure further includes a plurality of conductive structures. Each of the conductive structures includes a plurality of conductive columns correspondingly disposed in each of the isolation structures along an extending direction of the isolation structures. The conductive columns penetrate through the each of the isolation structures. Each of the conductive columns has a circular cross section.