A method of producing a microelectronic device in a substrate comprising a first semiconductor layer, a dielectric layer and a second semiconductor layer, comprising the following steps: etching a trench through the first semiconductor layer, the dielectric layer and a part of the thickness of the second semiconductor layer, thus defining, in the first semiconductor layer, one active region of the microelectronic device, ionic implantation in one or more side walls of the trench, at the level of the second semiconductor layer, modifying the crystallographic properties and/or the chemical properties of the implanted semiconductor, etching of the implanted semiconductor such that at least a part of the trench extends under a part of the active region, —filling of the trench with a dielectric material, forming an isolation trench surrounding the active region and comprising portions extending under a part of the active region.