A method for forming a trench filled with an insulator crossing a single-crystal silicon layer and a first SiO2 layer and penetrating into a silicon support, this method including the steps of forming on the silicon layer a second SiO2 layer and a first silicon nitride layer, forming the trench, and performing a first oxidizing processing to form a third SiO2 layer; performing a second oxidizing processing to form, on the exposed surfaces of the first silicon nitride layer a fourth SiO2 layer; depositing a second silicon nitride layer and filling the trench with SiO2; and removing the upper portion of the structure until the upper surface of the silicon layer is exposed.