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 first dielectric layer, the first dielectric layer being disposed on a second semiconductor layer, the method comprising:

• etching the substrate to form a trench through the first semiconductor layer, the first dielectric layer, and a part of the thickness of the second semiconductor layer, the trench defining, in the first semiconductor layer, at least one active region of the microelectronic device, and such that, in the second semiconductor layer, at least one part of the trench extends under a part of the at least one active region;
• depositing at least one second dielectric layer against walls of the trench;
• etching a part of the at least one second dielectric layer such that at least a part of remaining unetched portions of the at least one second dielectric layer in said at least one part of the trench extending under the part of the at least one active region form portions of a dielectric material extending under the part of the at least one active region;
• and
• depositing at least one third dielectric layer in the trench such that the trench is filled with at least dielectric materials of the at least one second dielectric layer and the at least one third dielectric layer, thereby forming an isolation trench surrounding the at least one active region.