Oxidation of macroporous silicon for thick thermal insulation

A thick porous silicon dioxide (SiO2, 60 μm) has been fabricated from macroporous silicon (macro-PS) by electrochemical etching from p-type Si. The macro-PS was produced using hydrofluoric acid–dimethyl sulphoxide (HF–DMSO) as an electrolyte giving a 1–1.5 μm pore size and a porosity of 55–75% for various electrolyte concentrations in the electrolytic cell. Wet oxidation with steam was then carried out at 1050 °C. Using this simple and low cost process a 60 μm porous oxide layer is created in 3 h. The evolution of the porous SiO2 layer was characterized by scanning electron microscopy. Thick porous SiO2 layers is well suited for thermal insulating functions in devices as the thermal conductivity of a porous layer is lower than that of bulk SiO2, making such layers attractive for fabrication of devices such as, for example flow sensors and infrared sensors.