Method for fabricating microelectronic devices with isolation trenches partially formed under active regions

A method of producing a microelectronic device in a substrate comprising a first semiconductor layer disposed on a dielectric layer, the dielectric layer being disposed on a second semiconductor layer, the method comprising:

• etching a trench through the first semiconductor layer, the dielectric layer and a part of the thickness of the second semiconductor layer such that another part of the thickness of the second semiconductor layer remains under a bottom wall of the trench, thus defining, in the first semiconductor layer, at least one active region of the microelectronic device;
• performing ionic implantation in one or more side walls of the trench, at a level of the second semiconductor layer, modifying the crystallographic properties, or the chemical properties, or both the crystallographic properties and the chemical properties, of the implanted second semiconductor layer in said side walls;
• etching the implanted second semiconductor layer such that at least a part of the trench extends under a part of the active region;
• and
• filling the trench with a dielectric material, forming an isolation trench surrounding the active region and comprising portions extending under the part of the active region.