Thick oxidised porous silicon layer as a thermo-insulating membrane for high-temperature operating thin- and thick-film gas sensors

A technological process aiming to realise oxidised porous silicon (OPS) layers as thermo-insulating material, for thin- as well as thick-film gas sensor applications, is reported and discussed. OPS layers (5-35 mu m thick) have been realised on p-Si substrates using the Si anodisation process followed by the PS thermal oxidation. A phosphorus implantation of patterned Si substrates has been performed to take advantage of the selective anodisation of p(+) vs. n(+) Si. A PS layer with mesoporous structure has been selected as starting material, and the morphology of the obtained PS layer has been investigated by TEM observations. The surface and in-depth stoichiometry of the OPS layer has been evaluated using Rutherford backscattering and EDS microanalysis techniques, combined with wet etching steps to partially remove the OPS layer. Cross-sections of the OPS layer have been observed by SEM, in order to get information on the uniformity of the PS layer thickness and on the OPS layer profile. Patterned wafers with hundreds of OPS islands show a consistent wafer warpage after the thermal oxidation step. The wafer warpage is a function of the OPS layer thickness. A possible alternative to the thermal oxidation of the PS layer to realise thermo-insulating membranes of PS, mechanically stable and coplanar with the surrounding bulk Si, is presented. (