The disclosed method includes forming an initial layer stack comprising a sacrificial layer of a first semiconductor material and over the sacrificial layer a channel layer of a second semiconductor material, forming a fin structures by patterning trenches in the initial layer stack, forming an anchoring structure extending across the fin structures, and while the channel layers are anchored by the anchoring structure: removing the sacrificial layers by a selective etching of the first semiconductor material, thereby forming a longitudinal cavity underneath the channel layer of each fin structure, and depositing an insulating material to fill the cavities, wherein the insulating material is formed of a flowable dielectric, and subsequently recessing the at least one anchoring structure and the insulating material to a level below the cavities such that the insulating material remains in the cavities to form insulating layers underneath the channel layers of each fin structure.