A process for fabricating an integrated circuit includes, in a stack of layers including a silicon substrate overlaid with a buried insulating layer overlaid with a silicon layer, etching first trenches into the silicon substrate, depositing a silicon nitride layer on the silicon layer to fill the first trenches and form first trench isolations, forming a mask on the silicon nitride layer, etching second trenches into the silicon substrate, in a pattern defined by the mask, to a depth greater than a depth of the first trenches, filling the second trenches with an electrical insulator to form second trench isolations, carrying out a chemical etch until the silicon layer is exposed, and forming a FET by forming a channel, a source, and a drain of the field effect transistor in the silicon layer.