THICK OXIDIZED POROUS SILICON LAYER AS A THERMO-INSULATING MEMBRANE FOR HIGH-TEMPERATURE OPERATING THIN-FILM 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-fil m gas sensor applications, is reported and discussed. OPS layers (5-35 mu m thick) have been realised on p-Si substrates using the Si anodis ation process followed by the PS thermal oxidation. A phosphorus impla ntation of patterned Si substrates has been performed to take advantag e of the selective anodisation of p(+) vs. n(+) Si. A PS layer with me soporous structure has been selected as starting material, and the mor phology of the obtained PS layer has been investigated by TEM observat ions. The surface and in-depth stoichiometry of the OPS layer has been evaluated using Rutherford backscattering and EDS microanalysis techn iques, combined with wet etching steps to partially remove the OPS lay er. Cross-sections of the OPS layer have been observed by SEM, in orde r to get information on the uniformity of the PS layer thickness and o n 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 al ternative to the thermal oxidation of the PS layer to realise thermo-i nsulating membranes of PS, mechanically stable and coplanar with the s urrounding bulk Si, is presented. (C) 1998 Elsevier Science S.A. All r ights reserved.