A process for fabricating a silicon on insulator (SOI) substrate by forming a weakened zone within a semiconductor donor substrate to define a thick layer having a thickness of greater 150 nm and form a boundary between the thick layer and a remainder of the donor substrate, bonding the donor substrate to a semiconductor receiver substrate, with one of the substrates including an oxide layer that is present between the donor and receiver substrates after bonding; detaching a remainder of the donor substrate along the weakened zone to obtain a semifinished SOI substrate comprising the receiver substrate, the oxide layer and the thick layer; and finishing the semifinished SOI substrate by thinning the thick layer to obtain a silicon layer having a thickness is less than that of the thick layer but greater than 150 nm; long annealing the semifinished SOI substrate in a gaseous atmosphere comprising hydrogen and/or argon; and thinning the thin layer to obtain an ultrathin layer with a thickness of 150 nm or less and the finished substrate.