A method of forming a substrate comprises providing a receiver substrate and a donor substrate successively comprising: a carrier substrate, a sacrificial layer, which can be selectively etched in relation to an active layer, and a silicon oxide layer, which is arranged on the active layer. A cavity is formed in the oxide layer to form a first portion that has a first thickness and a second portion that has a second thickness greater than the first thickness. The cavity is filled with a polycrystalline silicon filling layer to form a second free surface that is continuous and substantially planar. The receiver substrate and the donor substrate are assembled at the second free surface, and the carrier substrate is eliminated while preserving the active layer and the sacrificial layer.