A method of producing a microelectronic device in a substrate including a first semiconductor layer, a first dielectric layer, and a second semiconductor layer, including: etching a trench through the first semiconductor layer, the first dielectric layer, and a part of the second semiconductor layer, defining one active region, and such that, at the level of the second semiconductor layer, a part of the trench extends under a part of the active region; deposition of one second dielectric layer in the trench; etching the second dielectric layer such that remaining portions of the second dielectric layer forms portions of dielectric material extending under a part of the active region; deposition of a third dielectric layer in the trench such that the trench is filled with the dielectric materials of the second and third dielectric layers and forms an isolation trench.