The fabrication of a semiconductor fixed structure defining a volume, for example of a MEMS micro electro-mechanical system includes, determining thicknesses beforehand depending on the functional distances associated with elements. At least one element is formed on a substrate by thermal oxidation of the substrate so as to form an oxide layer followed by selective etching of the oxide layer so as to define the volume in an etched portion by baring the underlying substrate so as to define the element in an unetched portion, and later oxidation of the substrate so as to form an oxide layer, in order to obtain the elements at the functional distances.