Method of providing mask alignment marks

A method of providing mask alignment marks on an active surface of a semiconductor substrate, in which first at least one layer of a material resistant to oxidation is formed on this active surface, after which by a local etching of this layer, zones for isolation by a thick oxide, designated as a field oxide, are defined simultaneously with the alignment marks, and during a later step the zones for the isolation by field oxide and said alignment marks thus defined are subjected to a thermal oxidation, the remainder of the active surface being protected from oxidation by a remaining part of the layer of anti-oxidation material, wherein, after the local etching of the layer of anti-oxidation material and while utilizing the remaining parts of the anti-oxidation layer as a selective mask, substantially flat-bottomed depressions are formed at the surface of the substrate having a given depth at least at locations which contain the alignment marks, which locations are designated as alignment windows, the surface of the substrate is then exposed within said windows, and finally a thermal oxidation step is effected for forming the field oxide, during which the alignment marks are simultaneously covered by an oxide layer of uniform thickness.