A planarization structure is formed with a planar upper face enclosing a relief projecting from a planar substrate. The process used deposits a layer of a first material over the reliefs and then forms a layer of a second material with a planar upper face. This second material may be etched selectively with respect to the first material. The second layer is processed so that the protuberances of the first material are uncovered. A planarizing is then performed on the first material as far as the layer of the second material by selective chemical-mechanical polishing with respect to the second material.