A technological process aimed at realizing passivated porous silicon (PS) layers as the thermo-insulating material for thin as well as thick film gas sensor applications is reported and discussed. Oxidized PS (OPS) layers (5 to 35 μm thick) have been realized on p-Si substrates using the Si anodization process followed by PS thermal oxidation. The thick SiO 2 layer obtained from PS has the same stoichiometry as standard thermal silicon dioxide; unfortunately, stress introduced by the oxidation process induces significant wafer warpage, which inhibits further technological processing. Passivation of the PS layer can be achieved by a nitridation process executed in a rapid thermal system (RTS) in ammonia. In this case, the Si rods are covered by a thin oxynitride layer, which stabilizes the PS structure without introducing large stress. Nitrided PS membranes (25 - 30 μm thick) that are coplanar with the surrounding bulk Si and have good mechanical stability have been obtained.